2. Learning Objectives
• After studying this introduction, you should
• be able to:
• 1. State the mind–brain problem and con-
• trast monism with dualism.
• 2. List three general points that are important
• to remember from this text.
• 3. Give examples of physiological, ontoge-
• netic, evolutionary, and functional explana-
• tions of behavior.
• 4. Discuss the ethical issues of research with
• laboratory animals.
3. Biological Psychology?
• Is the study of the physiological, evolutionary, and
developmental mechanisms of behavior and experience.
It is approximately synonymous with the terms
• Neuroscience includes much that is relevant to behavior
but also includes more detail about anatomy and
chemistry.
4. The Mind–Brain
Problem
◉ One view, called dualism, holds that the mind is separate
from the brain but somehow controls the brain and
therefore the rest of the body.
◉ However, dualism contradicts the law of conservation of
matter and energy, one of the cornerstones of physics.
◉ According to that principle, the only way to influence any
matter or energy, including the matter and energy that
compose your body, is to act on it with other matter or
energy.
4
5. Four Types of Biological
Explanation
◉ Physiological explanation relates a behavior
to the activity of the brain and other organs. It
deals with the machinery of the body—for
example, the chemical reactions that enable
hormones to influence brain activity and the
routes by which brain activity controls muscle
contractions.
6. ◉ The term ontogenetic comes from Greek roots meaning
the origin (or genesis) of being. An Ontogenetic
explanation describes how a structure or behavior
develops, including the influences of genes, nutrition,
experiences, and their interactions.
◉ For example, males and females differ on average in
several ways. Some of those differences can be traced to
the effects of genes or prenatal hormones, some relate to
cultural influences, many relate partly to both, and some
await further research.
7. ◉ An Evolutionary explanation reconstructs the
evolutionary history of a structure or behavior. The
characteristic features of an animal are almost always
modifications of something found in ancestral species.
◉ For example, bat wings are modified arms, and porcupine
quills are modified hairs. In behavior, monkeys use tools
occasionally, and humans evolved elaborations on those
abilities that enable us to use tools even better (Peeters et
al., 2009).
8. ◉ A functional explanation describes why a structure or
behavior evolved as it did. Within a small, isolated
population, a gene can spread by accident through a
process called genetic drift.
◉ For example, a dominant male with many offspring spreads
all his genes, including some that may have been irrelevant
to his success or even disadvantageous. However, a gene
that is prevalent in a large population probably provided
some advantage—at least in the past, though not
necessarily today.
◉
9. ◉ For example, many species have an appearance that
matches their background. A functional explanation is that
camouflaged appearance makes the animal inconspicuous
to predators. Some species use their behavior as part of
the camouflage.
10. ◉ For example, zone-tailed hawks, native to Mexico and the
southwestern United States, fly among vultures and hold
their wings in the same posture as vultures. Small
mammals and birds run for cover when they see a hawk,
but they learn to ignore vultures, which pose no threat to
healthy animals. Because the zone-tailed hawks resemble
vultures in both appearance and flight behavior, their prey
disregard them, enabling the hawks to pick up easy meals
◉ (Clark, 2004).
11.
12.
13. The Use of Animals in
Research
◉ Given that most biological psychologists and
neuroscientists are primarily interested in the
human brain and human behavior, why do they
study nonhumans? Here are four reasons:
14. ◉ 1. The underlying mechanisms of behavior are
similar across species and sometimes easier
to study in a nonhuman species.
15. ◉ 2. We are interested in animals for their own
sake.
16. ◉ 3. What we learn about animals sheds light on
human evolution.
17. ◉ 4. Legal or ethical restrictions prevent certain
kinds of research on humans.
18. Opposition to Animal Research
◉ “Never knowingly harm an innocent” and
“Sometimes a little harm leads to a greater
good.”
20. Neurons and Glia
• Neurons receive information and transmit it to other cells.
• Glia serve many functions that are difficult to summarize,
and we shall defer that discussion until later in this
module.
THE CELLS OF THE
NERVOUS SYSTEM
21. • Cajal wanted to become an artist, but his
father insisted that he study medicine as
a safer way to make a living.
• He managed to combine the two fields,
becoming an outstanding anatomical
researcher and illustrator. His detailed
drawings of the nervous system are still
considered definitive today.
• Italian investigator Camillo Golgi found a
way to stain nerve cells with silver salts.
This method, which completely stains
some cells without affecting others at all,
enabled researchers to examine the
structure of a single cell.
• Cajal’s research demonstrated that nerve cells remain
separate instead of merging into one another (Oddly,
when Cajal and Golgi shared the 1906 Nobel Prize
for Physiology or Medicine, they used their
acceptance lectures to defend contradictory
positions. In spite of Cajal’s evidence, which had
persuaded almost everyone else, Golgi clung to the
theory that all nerve cells merge directly into one
22. THE STRUCTURES OF
AN ANIMAL CELL
• Neurons have much in common with the rest of the
body’s cells. The surface of a cell is its membrane
(or plasma membrane), a structure that separates
the inside of the cell from the outside environment.
• Except for mammalian red blood cells, all animal cells
have a nucleus, the structure that contains the
chromosomes.
• A mitochondrion (plural: mitochondria) is the
structure that performs metabolic activities,
providing the energy that the cell uses for all
activities.
• Ribosomes are the sites within a cell that synthesize
new protein molecules. Proteins provide building
materials for the cell and facilitate chemical
reactions.
• Some ribosomes float freely within the cell, but
others are attached to the endoplasmic
reticulum, a network of thin tubes that transport
newly synthesized proteins to other locations.
23. THE STRUCTURE OF
A NEURON
• Unlike most other body cells, neurons have
long branching extensions.
• All neurons include a soma (cell body), and
most also have dendrites, an axon, and
presynaptic terminals. The tiniest neurons
lack axons and some lack well-defined
dendrites.
24. • A motor neuron, with its soma in the spinal
cord, receives excitation through its
dendrites and conducts impulses along its
axon to a muscle.
• A sensory neuron is specialized at one
end to be highly sensitive to a particular
type of stimulation, such as light, sound,
or touch.
THE STRUCTURE OF
A NEURON
25. • Dendrites are branching fibers that get
narrower near their ends. (The term dendrite
comes from a Greek root word meaning
“tree.” A dendrite branches like a tree.)
• The cell body, or soma (Greek for “body”;
plural: somata), contains the nucleus,
ribosomes, and mitochondria.
• The axon is a thin fiber of constant diameter.
(The term axon comes from a Greek word
meaning “axis.”) The axon con?veys an
impulse toward other neurons, an organ, or a
muscle.
• The end of each branch has a swelling,
called a presynaptic terminal, also known as
an end bulb or bouton (French for “button”).
At that point the axon releases chemicals that
cross through the junction between that
neuron and another cell.
26. • Other terms associated with neurons are
afferent, efferent, and intrinsic.
• An afferent axon brings information into
a structure;
• an efferent axon carries information
away from a structure.
• If a cell’s dendrites and axon are entirely
contained within a single structure, the
cell is an interneuron or intrinsic neuron
of that structure.
• For example, an intrinsic neuron of the
thalamus has its axon and all its dendrites
within the thalamus
27. Neurons vary enormously in size, shape, and
function.
The shape of a neuron determines its
connections with other cells and thereby
determines its function (see Figure 1.8).
VARIATIONS AMONG
NEURONS
28. • Glia (or neuroglia), the other components of the nervous
system, perform many functions
(see Figure 1.9).
• The term glia, derived from a Greek word meaning “glue,”
reflects early investigators’ idea that glia were like glue that
held the neurons together.
The brain has several types of glia.
• Astrocytes help synchronize the activity of the axon by
wrapping around the presynaptic terminal and taking up
chemicals released by the axon.
• Astrocytes dilate the blood vessels to bring more nutrients
into brain areas that have heightened activities.
• Tiny cells called microglia act as part of the immune system,
removing viruses and fungi from the brain. And also
removes dead dying or damaged neurons.
• Oligodendrocytes (OL-i-go-DEN-druh-sites) in the brain
and spinal cord and Schwann cells in the periphery of the
body build the myelin sheaths that surround and insulate
certain vertebrate axons.
• Radial glia guide the migration of neurons and
• their axons and dendrites during embryonic development.
GLIA
29.
30. • Although the brain, like any other organ,
needs to receive nutrients from the blood,
many chemicals cannot cross from
• the blood to the brain (Hagenbuch, Gao,
& Meier, 2002).
• The mechanism that excludes most
chemicals from the vertebratebrain is
known as the blood–brain barrier. Before
we examine how it works, let’s consider
why we need it.
THE BLOOD BRAIN
BARRIER
31. HOW THE BLOOD–BRAIN
BARRIER WORKS
For certain other chemicals, the brain uses
active transport, a protein-mediated process
that expends energy to pump chemicals from
the blood into the brain. Chemicals that are
actively transported into the brain include
glucose (the brain’s main fuel), amino acids
(the building blocks of proteins), purines,
choline, a few vitamins, and iron
32. NOURISHMENT OF
VERTEBRATE
NEURONS
NEURONS RELY HEAVILY ON GLUCOSE,
THE ONLY NUTRIENT THAT CROSSES THE
BLOOD–BRAIN BARRIER IN LARGE
QUANTITIES. THEY NEED THIAMINE
(VITAMIN B1) TO USE GLUCOSE.
34. THIS IS A NEURON
are nerve cells, which are the
fundamental unit of the brain and
the nervous system.
THIS IS THE NEURON
MEMBRANE
Embedded in the membrane are
protein channels that permit ertain ions
to cross through the membrane at a
controlled rate.
35. METHODS FOR
RECORDING ACTIVITY
OF A NEURON
• the electrode used is a fine glass tube filled with a salt
solution tapering to a tip diameter of 0.0005 mm or less.
• the neuron'sinterior has a negative potential relative to its
exterior.
• Magnitude varies but typically its level is -70 millivolts
36. FORCES ACTING ON SODIUM AND POTASSIUM IONS
• Not all chemicals can pass through as
freely. Ions go through gates that are
always open but for several important ions
like sodium and potassium go through
membranes channels (gates) that are
sometimes closed.
• when the membrane is at rest, then these
channels are also closed limiting the flow
of potassium to almost none and no flow of
sodium.
• the sodium-potassium pump is an active
trnasport that requires energy, repeatedly
transporting three sodium ions out of the
cell while drawing two potassium ions into it.
• making the concentration of sodium ions 10x
more concentrated outside the membrane
than inside and potassium ions more
concentrated inside than ou
SELECTIVE PERMEABILITY SODIUM-POTASSIUM PUMP
37. ION CHANNELS IN THE
MEMBRANE OF A NEURON
• When a channel
opens, it permits
some type of ion to
cross the membrane.
When it closes, it
prevents passage of
that ion.
38. THE SODIUM AND POTASSIUM GRADIENTS FOR A
RESTING MEMBRANE
Sodium ions are more concentrated
outside the neuron and potassium ions
are more concentrated inside. Protein
and chloride ions (not shown) bear
negative charges inside the cell. At rest,
almost no sodium ions cross the
membrane except by the sodium–
potassium pump. Potassium tends to
flow into the cell because of an electrical
gradient but tends to flow out because of
the concentration gradient. However,
potassium gates retard the flow of
potassium when the membrane is at
rest.
39. WHY A RESTING POTENTIAL?
• it takes a lot of energy to operate the sodium-potassium pump which
if not stimulated remains at a resting potential.
• The resting potential prepares the neuron to respond rapidly.
THE ACTION POTENTIAL
• Messages sent by axons are called action potentials
• When an axon’s membrane is at rest, the recordings show a
negative potential inside the axon.
40. Hyperpolarization: an increased
polarization caused by increasing the
negative charge inside a neuron.
THE ACTION POTENTIAL
. If we apply a small depolarizing
current, we get a result like this:
With a slightly stronger depolarizing
current, the potential rises slightly higher
but again returns to the resting level as
soon as the stimulation ceases:
41. Stimulation beyond the threshold of
excitation produces a massive
depolarization of the membrane.
THE ACTION POTENTIAL
Any subthreshold stimulation produces a
small response that quickly decays. Any
stimulation beyond the threshold,
regard?less of how far beyond, produces
a big response like the one shown, known
as the action potential.
The peak of the action potential is +30mV
in this illustration, varies from one axon to
another
42. THE ALL OR NONE LAW
• the intensity of the stimu?lus cannot cause a neuron to
produce a bigger or smaller action potential, or a faster
or slower one. (Slight variations can occur at random,
but not because of the stimulus.)
• the all-or-none law is that the am?plitude and velocity of
an action potential are independent of the intensity of
the stimulus that initiated it, provided that the stimulus
reaches the threshold.
43. THE MOLECULAR BASIS OF THE ACTION
POTENTIAL
The chemical events behind the action potential may seem complex, but
they make sense if you remember three principles:
1. At the start, sodium ions are mostly outside the neuron, and potassium
ions are mostly inside.
2. When the membrane is depolarized, sodium and potas?sium channels in
the membrane open.
3. At the peak of the action potential, the sodium channels close
44. • The axon channels regulating sodium and potassium.
• their permeability depends on the voltage difference across the membrane.
VOLTAGE-GATED CHANNELS
• When the depolarization reaches the threshold
of the membrane, the sodium channels open
wide enough for sodium to flow freely. Driven
by both the concentration gradient and the
electrical gradient, the sodium ions enter the
cell rapidly, until the elec?trical potential across
the membrane passes beyond zero to a
reversed polarity, as shown in the following
diagram
45. THE MOVEMENT OF SODIUM AND
POTASSIUM IONS DURING AN ACTION
POTENTIAL
Sodium ions cross during the peak of the action potential,
and potassium ions cross later in the opposite direction,
returning the membrane to its
original polarization.
46. The term propagation of the
action potential describes
the transmission of an action
potential down an axon.
In a sense, the action potential
gives birth to a new action
potential at each point along
the axon.
The propagation of an animal
species is the production of
offspring.
PROPAGATION OF THE ACTION
POTENTIAL
47. As an action potential occurs at one
point on the axon, enough sodium
enters to depolarize the next point to
its threshold, producing an action
potential
at that point. In this manner the action
potential flows along the axon,
remaining at equal strength
throughout. Behind each area of
sodium entry,
potassium ions exit, restoring the
resting potential.
48. LET'S REVIEW
THE ACTION
POTENTIAL
• When an area of the axon membrane
reaches its threshold of excitation, sodium
channels and potassium channels open.
• At first, the opening of potassium channels
produces little effect.
• Opening sodium channels lets sodium ions
rush into the axon.
• Positive charge flows down the axon and
opens voltagegated sodium channels at the
next point.
49. LET'S REVIEW
THE ACTION
POTENTIAL
• At the peak of the action potential, the sodium
gates snap shut. They remain closed for the
next millisecond or so, despite the
depolarization of the membrane.
• Because voltage-gated potassium channels
remain open, potassium ions flow out of the
axon, returning the membrane toward its
original depolarization.
• A few milliseconds later, the voltage dependent
potas-sium channels close.
51. THE MYELIN SHEATH
and Saltatory Conduction
• In the thinnest axons, action
potentials travel at a velocity of
less than 1 meter/second.
Increasing the diameter brings
induction velocity up to about 10
m/s. An impulse along an axon
between a giraffe's spinal cord
and its foot takes about half a
second.
Imagine a job in which it is possible
to take written messages over a
long distance without using any
mechanical device. The best
solution would be to station people
at moderate distances along the
route and throw the message-
bearing ball from person to per-son
until it reaches its destination.
52. CUTAWAY VIEW OF AXON
WRAPPED IN MYELIN
An axon surrounded by a myelin sheath and
interrupted by nodes of Ranvier
The inset shows a cross section through both
the axon and the myelin
sheath. The anatomy is distorted here to show
several nodes; in fact, the
distance between nodes is generally at least 100
times as long as a node.
53. SALTATORY CONDUCTION IN A
MYELINATED AXON
An action potential at the node triggers flow of
current to the next node,
where the membrane regenerates the action
potential. In reality, a myelin
sheath is much longer than shown here, relative
to the size of the nodes of
Ranvier and to the diameter of the axon.
54. SALTATORY CONDUCTION IN A
MYELINATED AXON
This flow of charge moves considerably faster than the regeneration of an action potential at
each point along the axon. The jumping of action potentials from node to node is referred to
as saltatory conduction, from the Latin word saltare, meaning "To Jump"
In multiple sclerosis, the immune system attacks myelinsheaths. An axon that never had a
myelin sheath conducts impulses slowly but steadily. People with multiple sclerosis suffer a
variety of impairments, ranging from visual impairments to poor muscle coordination.
55. THE REFRACTORY
PERIOD
THE ELECTRICAL POTENTIAL ACROSS THE MEM-BRANE IS STILL
ABOVE THE THRESHOLD. WHY DOESN'T THE CELL PRODUCE
ANOTHER ACTION POTENTIAL DURING THIS PERIOD? (IF IT
DID, OF COURSE, IT WOULD ENDLESSLY REPEAT ONE ACTION
POTENTIAL AFTER ANOTHER). THE REFRACTORY PERIOD
DEPENDS ON TWO FACTS: THE SODIUM CHANNELS ARE
CLOSED AND POTASSIUM IS FLOWING OUT FASTER THAN
USUAL.
56. LOCAL NEURONS
• Neurons produce action potentials, but many
small neu-rons have no axon. Neurons without an
axon exchange information with only their closest
neighbors. These neurons are called local neurons
because they do not follow the all-or-none law.
• "They say we use only 10 percent of our brain,"
has been quoted as saying since the early 1900s.
Other origins have also been suggested for this
belief. Does it mean that you could lose 90
percent of your brain and still behave normally?
Good luck with that one.
57. GLOSSARY
• Absolute refractory method - the first part of the refractory period in which the membrane cannot
produce more action potentials despite stimulus.
• Action Potential - occurs when a neuron sends information down an axon, away from the cell body.
Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The action
potential is an explosion of electrical activity that is created by a depolarizing current.
• Active transport - a protein-mediated process that expends energy to pump chemicals from the blood
into the brain.
• Afferent axon - brings information into a structure. "Admit".
• All-or-None Law - is a principle that states that the strength of a response of a nerve cell or muscle fiber
is not dependent upon the strength of the stimulus.
• Astrocytes - The star-shaped wrap around the synapses of functionally related axons. Astrocytes dilate
the blood vessels to bring more nutrients into brain areas that have heightened activity (Filosa et al.,
2006; Takano et al., 2006).
• Axon - is a thin fiber of constant diameter. (The term axon comes from a Greek word meaning “axis.”)
The axon conveys an impulse toward other neurons, an organ, or a muscle
• Blood-brain barrier - The mechanism that excludes most chemicals from the vertebrate brain.
58. GLOSSARY
• Concentration Gradient results from the unequal distribution of particles (e.g. ions) between two solutions, i.e.
the intracellular fluid (the solution inside the cell) and the extracellular fluid (the solution outside the cell). The
particles may move along or against their concentration gradient.
• Dendrites - are branching fibers that get narrower near their ends. (The term dendrite comes from a Greek root
word meaning “tree.” A dendrite branches like a tree.)
• Depolarize - is a decrease in the absolute value of a cell's membrane potential. Thus, changes in membrane
voltage in which the membrane potential becomes less positive or less negative are both depolarizations.
• Efferent axon - carries information away from a structure. "Exit".
• Endoplasmic reticulum - isolation, modication, transport of proteins, and other substances.
• Glia (or neuroglia) - The term glia, derived from a Greek word meaning “glue,” reflects early investigators’ idea
that glia were like glue that held the neurons together.
• Graded Potential - what a local neuron has instead of an "action potential" since it doesn't have an axon, the
intensity of a stimulus decays as it travels.
• Hyperpolarization - is a change in a cell's membrane potential that makes it more negative. It is the opposite of
depolarization.
• Interneuron or intrinsic neuron - If a cell’s dendrites and axon are entirely contained within a single structure.
59. GLOSSARY
• Local Anesthetic - they attach to sodium channels which prevents sodium ions from entering which then
prevents you from feeling pain.
• Local Neuron - Neurons with no axons, they don't follow all or none law,
• Microelectrode - is a very small size terminal used in electrophysiology for either recording of neural
signals or electrical stimulation of nervous tissue.
• Microglia - act as part of the immune system, removing viruses and fungi from the brain. They proliferate
after brain damage, removing dead or damaged neurons.
• Mitochondrion (plural: mitochondria) - is the structure that performs metabolic activities, providing the
energy that the cell uses for all activities.
• Motor neuron - with its soma in the spinal cord, receives excitation through its dendrites and conducts
impulses along its axon to a muscle.
• Myelin sheath - insulating material.
• Myelinated Axons - can only be found in vertebrates and are covered in fats and proteins.
• Neurons - receive information and transmit it to other cells.
• Nucleus - the structure that contains the chromosomes.
60. GLOSSARY
• Oligodendrocytes - in the brain and spinal cord and Schwann cells in the periphery of the body build the
myelin sheaths that surround and insulate certain vertebrate axons.
• Polarization - pertains to the act or process of producing a positive electrical charge and a negative
electrical charge such that between a nerve cell internal electrical charge, which is negative, and the
surrounding environment of a nerve cell, which is positive.
• Presynaptic terminal - also known as an end bulb or bouton (French for “button”).
• Propagation of the action potential - describes the transmission of an action potential down an axon
• Refractory Period - a state of a cell when it's trying to resist the production of more action potentials.
• Relative Refractory Period - the second part of the refractory period wherein a stronger than usual
stimuli is needed to initiate an action potential.
• Resting Potential - the imbalance of electrical charge that exists between the interior of electrically
excitable neurons (nerve cells) and their surroundings.
• Saltatory Conduction - from the latin word "Saltare" meaning "to jump" The jumping of the action
potentials from node to node.
61. GLOSSARY
• Selective Permeability - is a property of cellular membranes that only allows certain molecules to enter or
exit the cell. This is important for the cell to maintain its internal order irrespective of the changes to the
environment.
• Sodium–Potassium Pump - also known as the Na+/K+ pump or Na+/K+-ATPase, this is a protein pump
found in the cell membrane of neurons (and other animal cells). It acts to transport sodium and potassium
ions across the cell membrane in a ratio of 3 sodium ions out for every 2 potassium ions brought in.
• Threshold - is the lowest point at which a particular stimulus will cause a response in an organism.
• Voltage-Gated Channel - are multi-sub-unit protein complexes that respond to changes in membrane
potential with conformational changes that lead to gating, or opening and closing, of an ion-selective
transmembrane pore.
62. REFERENCE:
KALAT, J. W. (2018). BIOLOGICAL PSYCHOLOGY (13TH
ED.) [E-BOOK]. CENGAGE LEARNING.
64. TOPIC OUTLINE
• PROPERTIES OF SYNAPSES
• RELATIONSHIP AMONG EPSP,
IPSP, AND ACTION POTENTIALS
MODULE 2.1:
THE CONCEPT OF THE SYNAPSE
MODULE 2.2:
CHEMICAL EVENTS AT THE SYNAPSE
• THE DISCOVERY OF CHEMICAL
TRANSMISSION AT SYNAPSE
• THE SEQUENCE OF CHEMICAL
EVENTS AT THE SYNAPSE
• HORMONES
65. WHAT IS SYNAPSE?
SMALL OR
MICROSCOPIC GAP
AT THE END OF A
NEURON THAT
ALLOWS A SIGNAL TO
PASS FROM ONE
NEURON TO THE
NEXT.
REFERRED TO AS THE SYNAPTIC CLEFT OR SYNAPTIC
GAP.
66. WHAT IS SYNAPSE?
WORD SYNAPSE WAS FIRST USED IN A BOOK CALLED A TEXTBOOK OF
PHYSIOLOGY, THE CENTRAL NERVOUS SYSTEM, BY MICHAEL FOSTER AND
ASSISTED BY CHARLES S. SHERRINGTON, IN 1897.
THERE ARE 100 TRILLION TO 1,000 TRILLION SYNAPSES
DERIVED FROM GREEK WORDS:
"SYN" - TOGETHER
"HAPTEIN" - TO CLASP
67. PARTS OF THE SYNAPSE
PRESYNAPTIC ENDING
• CONTAINS NEUROTRANSMITTERS THAT
SENDS INFORMATION
( TRANSMITS CHEMICAL MESSAGES)
SYNAPTIC CLEFT
• FOUND BETWEEN THE TWO NERVE CELLS
POSTSYNAPTIC ENDING
• CONTAINS RECEPTOR SITES THAT RECEIVES
INFORMATION
(RECEIVES CHEMICAL MESSAGES)
68. Ramón y Cajal
• regarded as the great pioneer of modern neuroscience.
THE CONCEPT OF THE SYNAPSE
CAJAL ANATOMICALLY DEMONSTRATED A
NARROW GAP SEPARATING ONE NEURON
FROM ANOTHER IN LATE 1800S.
69. Charles Scott Sherrington
• regarded as the great pioneer of modern neuroscience.
THE CONCEPT OF THE SYNAPSE
SHERRINGTON PHYSIOLOGICALLY
DEMONSTRATED THAT COMMUNICATION
BETWEEN ONE NEURON AND THE NEXT
DIFFERS FROM COMMUNICATION ALONG A
SINGLE AXON IN 1906
70. PROPERTIES OF SYNAPSES
REFLEXES
AUTOMATIC MUSCULAR RESPONSES TO STIMULI
REFLEX ARC
THE CIRCUIT FROM SENSORY NEURON TO MUSCLE RESPONSE.
• IF ONE NEURON IS SEPARATE FROM ANOTHER, A REFLEX MUST
REQUIRE COMMUNICATION BETWEEN NEURONS.
72. • SPEED OF A REFLEX AND DELAYED TRANSMISSION AT THE SYNAPSE
THE CONCEPT OF THE SYNAPSE
• SHERRINGTON MEASURED THE TOTAL DISTANCE THAT THE
IMPULSE TRAVELED FROM SKIN RECEPTOR TO SPINAL CORD TO
MUSCLE AND CALCULATED THE SPEED AT WHICH THE IMPULSE
TRAVELED TO PRODUCE THE RESPONSE.
• HE FOUND THAT THE SPEED OF CONDUCTION THROUGH THE
REFLEX ARC VARIED BUT WAS NEVER MORE THAN ABOUT 15
METERS PER SECOND (M/S).
73. • SPEED OF A REFLEX AND DELAYED TRANSMISSION AT THE SYNAPSE
THE CONCEPT OF THE SYNAPSE
74. • TEMPORAL SUMMATION (SUMMATION OVER TIME)
THE CONCEPT OF THE SYNAPSE
• SHERRINGTON PROPOSED THAT ALTHOUGH THE SUBTHRESHOLD
EXCITATION IN THE POSTSYNAPTIC NEURON DECAYS OVER TIME, IT
CAN COMBINE WITH A SECOND EXCITATION THAT FOLLOWS IT
QUICKLY.
• GRADED DEPOLARIZATION KNOWN AS AN EXCITATORY
POSTSYNAPTIC POTENTIAL (EPSP). IT RESULTS FROM A FLOW OF
SODIUM IONS INTO THE NEURON. IF AN EPSP DOES NOT CAUSE
THE CELL TO REACH ITS THRESHOLD, THE DEPOLARIZATION
DECAYS QUICKLY.
75. • SPATIAL SUMMATION (SUMMATION OVER SPACE)
THE CONCEPT OF THE SYNAPSE
• SHERRINGTON CONCLUDED THAT PINCHING TWO POINTS
ACTIVATED SEPARATE SENSORY NEURONS, WHOSE AXONS
CONVERGED ONTO ONE NEURON IN THE SPINAL CORD.
EXCITATION FROM EITHER SENSORY AXON EXCITED THAT SPINAL
NEURON, BUT NOT ENOUGH TO REACH THE THRESHOLD
77. • INHIBITORY SYNAPSES
THE CONCEPT OF THE SYNAPSE
• PLAYS A CRUCIAL ROLE IN REGULATING THE FLOW OF SENSORY
INFORMATION THROUGH THE SPINAL CORD.
• INHIBITORY POSTSYNAPTIC POTENTIAL (IPSP)—RESEMBLES AN
EXCITATORY POSTSYNAPTIC POTENTIAL (EPSP).
80. A set of nerves called the sympathetic nervous system accelerates the
heartbeat, relaxes muscle, dilates the pupils of the eyes, and reguate
other organs.
81. T.R Elliott, suggested that
the sympathetic nerves
stimulates muscles by
releasing adrenaline or other
similar chemical.
83. 1. Neuron synthesizes chemicals that serves as
neurotransmitters.
2. Action potentials travel down the axon.
3. The released molecules diffuse across the narrow cleft,
attach to receptors, and alter the activity of the postsynaptic
neuron.
5. The neurotransmitter molecules may be taken back into the
presynaptic neuron.
6. Some postsynaptic cells send reverse messages to control
the further release of neurotransmitter.
85. It is the oddest transmitter. It dilates nearby blood vessels by
increasing blood flow to the brain area
Nitric Oxide
86. Synthesis of
Transmitters
The amino acid tryptophan, the precusor to
serotonin, crosses the blood-brain barrier by a
special transport system thay shares with other large
amino acids
87. Storage of
Transmitters
Most neurotransmityers are synthesized in the
presynaptic terminal, near the point of release. The
presynaptic terminal stores high concentrations of
neurotransmitter molecules in vesicles.
88. Release and Diffuse of
Transmitters
At the end of an axon, an action potential itself does not release the
neurotransmitter. Exocytosis a bursts of release of transmitter from
presynaptic neuron
Glutamate is the most
abundant neurotransmitter in
the nervous system.
89. Activating Receptors of the Postsynaptic
Cell
The effect of a neurotransmitters depends on its
receptor on the postsynaptic cell.
90. Metabotropic Effects and Second Messenger System
Metabotropic effects emerge 30ms or more after the
release of the transmitter.
91. Neuropeptides
This also refers as neuromodulators because they have
properties that set them apart from other transmitters.
92.
93. Variation in
Receptors
A given receptor can have a different effects for different
people or in different parts of one person's brain.
94. Drugs That Act by Binding to
Receptors
Drug that chemically resembles a neurotransmitter can
bind to its receptor.
95. 1. Amphetamine- Block reuptake of dopamine and several other
transmitters.
2. Opiate Drugs-Stimulates endorphin receptors
3. Nicotine- Stimulates nicotinic-type acetylcholine receptors that
increases dopamine release.
4. Cocaine- Block reuptake of dopamine and several other
transmitters.
5. Methylphenidate- It also block reuptake dopamine but gradually.
6. Ecstasy- Releases dopamine and serotonin.
7. Cannaboids- Excites negative-feedback receptors on presynaptic
cells
8. Hallucinogens- Stimulates serotonin type 2A receptors
96. Inactivation and Reuptake of
Neurotransmitters
A neurotransmitter does not linger at the postsynaptic membrane, where
it might continue exciting or inhibiting the receptor. Various
neurotransmitters are inactivated in different ways
97. Serotonin and the catecholamines (dopamine, norepinephrine,
and epinephrine) do not break down into inactive fragments at
the postsynaptic membrane. They simply
detach from the receptor. At that point, the next step varies.
The presynaptic neuron takes up much or most of the released
neurotransmitter molecules intact and reuses them.
This process, called reuptake, occurs through special membrane
proteins called transporters.
98. Electrical Synapse
At the electrical synapse, the
membrane of one neuron
comes into direct contact
with the membrane of
another this is called gap
junction
electrical transmission is faster than even the fastest chemical
transmission.
99. Hormones
It is a chemical secreted by cells in one part of the body and
conveyed by the blood to influence other cells.
102. Pituitary Glands
is attached to hypothalamus, it
has two parts namely: anterior
pituitary and posterior
pitutuitary, which releases
different sets of hormones.
103.
104. Glossary
Action Potential- a sudden, fast, transitory, and propagating change
of the resting membrane potential.
Axon- portion of a nerve cell (neuron) that carries nerve impulses
away from the cell body
Cocaine- This is a stimulant type of Drugs. Most commonly used
recreational drug.
Excitatory Postsynaptic Potential (EPSP)- synaptic inputs that
depolarize the postsynaptic cell, bringing the membrane potential
closer to threshold and closer to firing an action potential.
Inhibitory Postsynaptic Potential (IPSP)- a temporary
hyperpolarization of postsynaptic membrane caused by the flow of
negatively charged ions into the postsynaptic cell.
Neuron- the basic working unit of the brain, a specialized cell
designed to transmit information to other nerve cells, muscle, or
gland cells.
Reflex- an involuntary and nearly instantaneous movement in
response to a stimulus
Reflex Arc- the nerve pathway involved in a reflex action, including at
its simplest a sensory nerve and a motor nerve with a synapse
between.
Postsynaptic Ending- contains receptor sites that receives
information.
Presynaptic Ending- contains neurotransmitters that send
information.
Serotonin- A chemical that regulates happiness and mood.
Spatial Summation- refers to the sensory summation that involves
stimulation of several spatially separated neurons at the same time.
Synapse- a small gap at the end of a neuron that allows a signal to
pass from one neuron to the next.
Temporal Summation- refers to the sensory summation that involves
the addition of single stimuli over a short period of time.
Transmitter- A chemical which a neuron uses to influence the activity
of an anatomically adjacent cell body.
105.
106.
107.
108.
109.
110.
111.
112.
113.
114.
115.
116.
117.
118.
119.
120.
121.
122.
123.
124.
125.
126.
127.
128.
129.
130.
131.
132.
133.
134.
135.
136.
137.
138.
139.
140.
141.
142. Chapter Outline
Module 1.1
The Mind–Brain Relationship
Biological Explanations of Behavior
The Brain and Conscious Experience
Research Approaches
Career Opportunities
In Closing: Your Brain and Your Experience
Summary
Answers to Stop & Check Questions
Thought Questions
Author’s Answer About Machine Consciousness
Module 1.2
The Genetics of Behavior
Mendelian Genetics
Heredity and Environment
The Evolution of Behavior
In Closing: Genes and Behavior
Summary
Answers to Stop & Check Questions
Thought Questions
Module 1.3
The Use of Animals in Research
Reasons for Animal Research
The Ethical Debate
In Closing: Humans and Animals
Summary
Answers to Stop & Check Questions
Terms
Suggestions for Further Reading
Websites to Explore
Exploring Biological Psychology CD
ThomsonNow
Main Ideas
1. Biological explanations of behavior fall into sev-
eral categories, including physiology, development,
evolution, and function.
2. Nearly all current philosophers and neuroscien-
tists reject the idea that the mind exists indepen-
dently of the physical brain. Still, the question re-
mains as to how and why brain activity is connected
to consciousness.
3. The expression of a given gene depends on the en-
vironment and on interactions with other genes.
4. Research with nonhuman animals can produce im-
portant information, but it sometimes inflicts dis-
tress or pain on the animals. Whether to proceed
with a given experiment can be a difficult ethical
issue.
It is often said that Man is unique among animals.
It is worth looking at this term “unique” before
we discuss our subject proper. The word may in
this context have two slightly different meanings.
It may mean: Man is strikingly different—he is
not identical with any animal. This is of course
true. It is true also of all other animals: Each spe-
cies, even each individual is unique in this sense.
But the term is also often used in a more absolute
sense: Man is so different, so “essentially differ-
ent” (whatever that means) that the gap between
him and animals cannot possibly be bridged—he
is something altogether new. Used in this absolute
sense the term is scientifically meaningless. Its
use also reveals and may reinforce conceit, and
it leads to complacency and defeatism because it
assumes that it will be futile even to search for
animal roots. It is prejudging the issue.
Niko Tinbergen (1973, p. 161)
Biological psychologists study the “animal roots”
of behavior, relating actions and experiences to
genetics and physiology. In this chapter, we consider
three major issues and themes: the relationship be-
tween mind and brain, the roles of nature and nurture,
and the ethics of research. We also briefly consider
prospects for further study.
1
1
The Major Issues
Opposite: It is tempting to try to “get inside the mind” of
people and other animals, to imagine what they are thinking
or feeling. In contrast, biological psychologists try to explain
behavior in terms of its physiology, development, evolution,
and function.
Source: George D. Lepp/CORBIS
143. Biological psychology is the study of the physio-
logical, evolutionary, and developmental mecha-
nisms of behavior and experience. It is approximately
synonymous with the terms biopsychology, psycho-
biology, physiological psychology, and behavioral
neuroscience. The term biological psychology empha-
sizes that the goal is to relate the biology to issues of
psychology. Neuroscience as a field certainly includes
much that is relevant to behavior, but it also includes
more detail about anatomy and chemistry.
Much of biological psychology is devoted to study-
ing brain functioning. Figure 1.1 offers a view of the
human brain from the top (what anatomists call a dor-
sal view) and from the bottom (a ventral view). The
labels point to a few important areas that will become
more familiar as you proceed through this text. An in-
spection of brain areas reveals distinct subareas. At the
microscopic level, we find two kinds of cells: the neu-
rons (Figure 1.2) and the glia. Neurons, which convey
messages to one another and to muscles and glands,
vary enormously in size, shape, and functions. The glia,
generally smaller than neurons, have many functions
but do not convey information over great distances.
The activities of neurons and glia somehow produce
an enormous wealth of behavior and experience. This
book is about researchers’ attempts to elaborate on that
word “somehow.”
Biological psychology is the most interesting topic
in the world. No doubt every professor or textbook au-
thor feels that way about his or her field. But the oth-
ers are wrong. Biological psychology really is the most
interesting topic.
When I make this statement to a group of students,
I always get a laugh. But when I say it to a group of bio-
logical psychologists or neuroscientists, they nod their
heads in agreement, and I do in fact mean it seriously.
I do not mean that memorizing the names and func-
tions of brain parts and chemicals is unusually inter-
esting. I mean that biological psychology addresses
some fascinating issues that should excite anyone who
is curious about nature.
Actually, I shall back off a bit and say that biolog-
ical psychology is about tied with cosmology as the
most interesting topic. Cosmologists ask why the uni-
Module 1.1
The Mind–Brain Relationship
Figure 1.1 A dorsal view (from above) and a ventral view (from below) of the
human brain
The brain has an enormous number of divisions and subareas; the labels point to a few
of the main ones on the surface of the brain.
Anterior
Posterior
Frontal lobe
Precentral gyrus
Postcentral gyrus
Parietal lobe
Occipital lobe
Central sulcus
Longitudinal
fissure
Olfactory
bulbs
Optic
nerves
Spinal cord
Frontal
lobe of
cerebral
cortex
Temporal
lobe of
cerebral
cortex
Medulla
Cerebellum
Dr.
Dana
Copeland
2 Chapter 1 The Major Issues
146. one; hence, it is larger in breeding males than in fe-
males or immature birds. That brain area enables a
mature male to sing.
Ontogenetic explanation: In many species, a young
male bird learns its song by listening to adult males.
Development of the song requires both the genes
that prepare him to learn the song and the opportu-
nity to hear the appropriate song during a sensitive
period early in life.
Evolutionary explanation: In certain cases, one species’
song closely resembles that of another species. For
example, dunlins and Baird’s sandpipers, two shore-
bird species, give their calls in distinct pulses, un-
like other shorebirds. This similarity suggests that
the two evolved from a single ancestor.
Functional explanation: In most bird species, only the
male sings, and he sings only during the reproduc-
tive season and only in his territory. The functions
of the song are to attract females and warn away
other males. As a rule, a bird sings loudly enough
to be heard only in the territory he can defend. In
short, birds have evolved tendencies to sing in ways
that improve their chances for mating.
We improve our understanding of behavior when
we can combine as many of these approaches as pos-
sible. That is, ideally, we should understand the body
mechanisms that produce the behavior, how it devel-
ops within the individual, how it evolved, and what
function it serves.
STOP & CHECK
1. How does an evolutionary explanation differ from a
functional explanation?
Check your answer on page 10.
The Brain and
Conscious Experience
Explaining birdsong in terms of hormones,
brain activity, and evolutionary selection
probably does not trouble you. But how
would you feel about a physical explana-
tion of your own actions and experiences?
Suppose you say, “I became frightened be-
cause I saw a man with a gun,” and a neu-
roscientist says, “You became frightened
because of increased electrochemical ac-
tivity in the central amygdala of your
brain.” Is one explanation right and the
other wrong? Or if they are both right, what is the con-
nection between them?
Biological explanations of behavior raise the mind–
body or mind–brain problem: What is the relationship
between the mind and the brain? The most widespread
view among nonscientists is, no doubt, dualism, the
belief that mind and body are different kinds of sub-
stance—mental substance and physical substance—
that exist independently. The French philosopher René
Descartes defended dualism but recognized the vexing
issue of how a mind that is not made of material could
influence a physical brain. He proposed that mind and
brain interact at a single point in space, which he sug-
gested was the pineal gland, the smallest unpaired
structure he could find in the brain (Figure 1.5).
Although we credit Descartes with the first explicit
defense of dualism, he hardly originated the idea. Men-
tal experience seems so different from the physical ac-
tions of the brain that most people take it for granted
that mind and brain are different. However, nearly all
current philosophers and neuroscientists reject dual-
ism. The decisive objection is that dualism conflicts
with a consistently documented observation in phys-
ics, known as the law of the conservation of matter and
energy: So far as we can tell, the total amount of matter
and energy in the universe is fixed. Matter can trans-
form into energy or energy into matter, but neither one
appears out of nothing or disappears into nothing.
Because any movement of matter requires energy, a
mind that is not composed of matter or energy would
seem unable to make anything happen, even a muscle
movement.
The alternative to dualism is monism, the belief
that the universe consists of only one kind of sub-
stance. Various forms of monism are possible, grouped
into the following categories:
• materialism: the view that everything that exists is
material, or physical. According to one version of
Figure 1.5 René Descartes’s conception of brain and mind
Descartes understood how light from an object reached the retinas at
the back of the eyes. From there, he assumed the information was all
channeled back to the pineal gland, a small unpaired organ in the brain.
(Source: From Descartes’ Treaties on Man)
50
5
3
1
1
5
3
A
B
C
6
6
4
4
2
2
a
b
c
H.
B
1.1 The Mind–Brain Relationship 5
147. this view (“eliminative materialism”), mental events
don’t exist at all, and the common folk psychology
based on minds and mental activity is fundamen-
tally mistaken. However, most of us find it difficult
to believe that our mind is a figment of our imagi-
nation! A more plausible version is that we will
eventually find a way to explain all psychological
experiences in purely physical terms.
• mentalism: the view that only the mind really ex-
ists and that the physical world could not exist un-
less some mind were aware of it. It is not easy to test
this idea—go ahead and try!—but few philosophers
or scientists take it seriously.
• identity position: the view that mental processes are
the same thing as certain kinds of brain processes
but are described in different terms. In other words,
the universe has only one kind of substance, but it
includes both material and mental aspects. By anal-
ogy, one could describe the Mona Lisa as an extraor-
dinary painting of a woman with a subtle smile, or
one could list the exact color and brightness of each
point on the painting. Although the two descriptions
appear very different, they refer to the same object.
According to the identity position, every mental ex-
perience is a brain activity, even though descrip-
tions of thoughts sound very different from descrip-
tions of brain activities. For example, the fright you
feel when someone threatens you is the same thing
as a certain pattern of activity in your brain.
Note how the definition of the identity position is
worded. It does not say that the mind is the brain. Mind
is brain activity. In the same sense, fire is not really a
“thing.” Fire is what is happening to something. Sim-
ilarly, mental activity is what is happening in the brain.
Can we be sure that monism is correct? No. How-
ever, we adopt it as the most reasonable working hy-
pothesis. That is, we conduct research on the assump-
tion of monism and see how far we can go. As you will
find throughout this text, experiences and brain activ-
ities appear inseparable. Stimulation of any brain area
provokes an experience, and any experience evokes
brain activity. You can still use terms like mind or men-
tal activity if you make it clear that you regard these
terms as describing an aspect of brain activity. However,
if you lapse into using mind to mean a ghostlike some-
thing that is neither matter nor energy, don’t under-
estimate the scientific and philosophical arguments
that can be marshaled against you (Dennett, 1991).
(Does a belief in monism mean that we are lower-
ing our evaluation of minds? Maybe not. Maybe we
are elevating our concept of the material world.)
Even if we accept the monist position, however,
we have done little more than restate the mind–brain
problem. The questions remain: Why is consciousness
a property of brain activity? Is it important or just an
accident, like the noises a machine makes? What kind
of brain activity produces consciousness? How does it
produce consciousness? And what is consciousness,
anyway? (You may have noted the lack of a definition.
A firm, clear definition of consciousness is elusive. The
same is true for many other terms that we feel comfort-
able using. For example, you know what time means,
but can you define it?)
The function (if any) of consciousness is far from
obvious. Several psychologists have argued that many
nonhuman species also have consciousness because
their behavior is so complex and adaptive that we can-
not explain it without assuming consciousness (e.g.,
Griffin, 2001). Others have argued that even if other
animals are conscious, their consciousness explains
nothing. Consciousness may not be a useful scientific
concept (Wynne, 2004). Indeed, because we cannot ob-
serve consciousness, none of us knows for sure even
that other people are conscious, much less other spe-
cies. According to the position known as solipsism
(SOL-ip-sizm, based on the Latin words solus and
ipse, meaning “alone” and “self”), I alone exist, or I
alone am conscious. Other people are either like ro-
bots or like the characters in a dream. (Solipsists don’t
form organizations because each is convinced that all
other solipsists are wrong!) Although few people take
solipsism seriously, it is hard to imagine evidence to
refute it. The difficulty of knowing whether other peo-
ple (or animals) have conscious experiences is known
as the problem of other minds.
David Chalmers (1995) has proposed that in dis-
cussions of consciousness we distinguish between
what he calls the easy problems and the hard problem.
The easy problems pertain to many phenomena that
we call consciousness, such as the difference between
wakefulness and sleep and the mechanisms that en-
able us to focus our attention. These issues pose all
the usual difficulties of any scientific question but no
philosophical problems. In contrast, the hard prob-
lem concerns why and how any kind of brain activity
is associated with consciousness. As Chalmers (1995)
put it, “Why doesn’t all this information-processing
go on ‘in the dark,’ free of any inner feel?” (p. 203).
That is, why does brain activity feel like anything at
all? Many scientists (Crick & Koch, 2004) and philos-
ophers (Chalmers, 2004) agree that we have no way to
answer that question, at least at present. We don’t even
have a clear hypothesis to test. The best we can do is
determine what brain activity is necessary or sufficient
for consciousness. After we do so, maybe we will see
a way to explain why that brain activity is associated
with consciousness, or maybe we won’t.1
1Note the phrasing “is associated with consciousness,” instead of
“leads to consciousness” or “causes consciousness.” According
to the identity position, brain activity does not cause or lead to
consciousness any more than consciousness leads to brain activ-
ity. Each is the same as the other.
6 Chapter 1 The Major Issues
148. Why are most of us not solipsists? That is, why do
you (I assume) believe that other people have minds?
We reason by analogy: “Other people look and act
much like me, so they probably have internal experi-
ences much like mine.” How far do we extend this
analogy? Chimpanzees look and act somewhat like
humans. Most of us, but not all, are willing to assume
that chimpanzees are conscious. If chimpanzees are
conscious, how about dogs? Rats? Fish? Insects? Trees?
Rocks? Most people draw the line at some point in this
sequence, but not all at the same point. A similar prob-
lem arises in human development: At what point be-
tween the fertilized egg and early childhood does some-
one become conscious? At what point in dying does
someone finally lose consciousness? And how could
we possibly know?
Speculating on these issues leads most people to
conclude that consciousness cannot be a yes-or-no
question. We can draw no sharp dividing line between
those having consciousness and those lacking it. Con-
sciousness must have evolved gradually and presum-
ably develops gradually within an individual (Edelman,
2001).
What about computers and robots? Every year,
they get more sophisticated and complicated. What if
someone builds a robot that can walk, talk, carry on an
intelligent conversation, laugh at jokes, and so forth?
At what point, if any, would we decide that the robot
is conscious?
You might respond, “Never. A robot is just a ma-
chine that is programmed to do what it does.” True,
but the human brain is also a machine. (A machine is
anything that converts one kind of energy into an-
other.) And we, too, are programmed—by our genes
and our past experiences. (We did not create ourselves.)
Perhaps no robot ever can be conscious, if conscious-
ness is a property of carbon chemistry (Searle, 1992).
Can you imagine any conceivable evidence that would
persuade you that a robot is conscious? If you are cu-
rious about my answer, check page 11. But think about
your own answer first.
STOP & CHECK
2. What are the three major versions of monism?
3. What is meant by the “hard problem”?
Check your answers on page 10.
Research Approaches
Even if the “hard problem” is unanswerable at pres-
ent, it might be possible to determine which kinds of
brain activity are associated with consciousness (Crick
& Koch, 2004). For the most part, researchers have as-
sumed that even though you might be conscious of
something and unable to report it in words (e.g., as in-
fants are), if you can describe something you saw or
heard, then you must have been conscious of it. Based
on that operational definition of consciousness,2 it is
possible to do research on the brain activities related
to consciousness. Let’s consider two examples.
One clever study used this approach: Suppose we
could present a visual stimulus that people consciously
perceived on some occasions but not others. We could
then determine which brain activities differed be-
tween the occasions with and without consciousness.
The researchers flashed a word on a screen for
29 milliseconds (ms). In some cases, it was preceded
and followed by a blank screen:
In these cases, people identified the word almost 90%
of the time. In other cases, however, the researchers
flashed a word for the same 29 ms but preceded and
followed it with a masking pattern:
Under these conditions, people almost never iden-
tify the word and usually say they didn’t see any word
at all. Although the physical stimulus was the same
in both cases—a word flashed for 29 ms—it reached
consciousness in the first case but not the second.
Using a brain scan technique that we shall examine in
Chapter 4, the researchers found that the conscious
stimulus activated the same brain areas as the uncon-
scious stimulus, but more strongly. Also, the conscious
stimuli activated a broader range of areas, presumably
because strong activation in the initial areas sent exci-
tation to other areas (Dehaene et al., 2001).
These data imply that consciousness of a stimu-
lus depends on the amount of brain activity. At any
moment, a variety of stimuli act on your brain; in ef-
fect, they compete for control (Dehaene & Changeux,
2004). Right now, for example, you have the visual
sensations from this page, as well as auditory, touch,
and other sensations. You cannot be simultaneously
conscious of all of them. You might, however, direct
SALTY
GROVE
2An operational definition tells how to measure something or
how to determine whether it is present or absent.
1.1 The Mind–Brain Relationship 7
149. your attention to one stimulus or another. For exam-
ple, right now what is your conscious experience of
your left foot? Until you read that question, you prob-
ably had no awareness of that foot, but now you do.
Because you directed your attention to it, activity has
increased in the brain area that receives sensation
from the left foot (Lambie & Marcel, 2002). Becoming
conscious of something means letting its information
take over more of your brain’s activity.
STOP & CHECK
4. In the experiment by Dehaene et al., how were the
conscious and unconscious stimuli similar? How were
they different?
5. In this experiment, how did the brain’s responses
differ to the conscious and unconscious stimuli?
Check your answers on page 10.
Here is a second kind of research. Look at Fig-
ure 1.6, but hold it so close to your eyes that your nose
touches the page, right between the two circles. Better
yet, look at the two parts through a pair of tubes, such
as the tubes inside rolls of paper towels or toilet paper.
You will see red and black vertical lines with your left
eye and green and black horizontal lines with your
right eye. (Close one eye and then the other to make
sure you see completely different patterns with the
two eyes.) Seeing something is closely related to see-
ing where it is, and the red vertical lines cannot be in
the same place as the green horizontal lines. Because
your brain cannot perceive both patterns in the same
location, your perception alternates. For a while, you
see the red and black lines, and then gradually the
green and black invade your consciousness. Then your
perception shifts back to the red and black. Some-
times you will see red lines in part of the visual field
and green lines in the other. These shifts, known as
binocular rivalry, are slow and gradual, sweeping
from one side to another. The stimulus seen by each
eye evokes a particular pattern of brain response,
which researchers can measure with the brain scan-
ning devices described in Chapter 4. As that first per-
ception fades and the stimulus seen by the other eye
replaces it, the first pattern of brain activity fades also,
and a different pattern of activity replaces it. Each
shift in perception is accompanied by a shift in the
pattern of activity over a large portion of the brain
(Cosmelli et al., 2004; Lee, Blake, & Heeger, 2005). (A
detail of procedure: One way to mark a pattern of brain
activity is to use a stimulus that oscillates. For exam-
ple, someone might watch a stationary pattern with
one eye and something flashing with the
other. When the person perceives the flash-
ing stimulus, brain activity has a rhythm
that matches the rate of flash.)
By monitoring brain activity, a researcher can lit-
erally “read your mind” in this limited way—know-
ing which of two views you perceive at a given mo-
ment. What this result says about consciousness is that
not every physical stimulus reaches consciousness. To
become conscious, it has to control the activity over a
significant area of the brain.
The overall point is that research on the biologi-
cal basis of consciousness may be possible after all.
Technological advances enable us to do research that
would have been impossible in the past; future meth-
ods may facilitate still more possibilities.
Career Opportunities
If you want to consider a career related to biological
psychology, you have a range of options. The relevant
careers fall into two categories—research and therapy.
Table 1.1 describes some of the major fields.
A research position ordinarily requires a PhD in
psychology, biology, neuroscience, or other related
field. People with a master’s or bachelor’s degree might
work in a research laboratory but would not direct it.
Many people with a PhD hold college or university
positions in which they perform some combination of
teaching and research. Depending on the institution and
the individual, the balance can range from almost all
teaching to almost all research. Other individuals have
pure research positions in laboratories sponsored by
the government, drug companies, or other industries.
Fields of therapy include clinical psychology, coun-
seling psychology, school psychology, several special-
izations of medicine, and allied medical practice such
try it
yourself
Figure 1.6 Binocular rivalry
If possible, look at the two parts through tubes, such
as those from the inside of rolls of toilet paper or paper
towels. Otherwise, touch your nose to the paper between
the two parts so that your left eye sees one pattern while
your right eye sees the other. The two views will compete
for your consciousness, and your perception will alternate
between them.
8 Chapter 1 The Major Issues
150. as physical therapy. These various fields of practice
range from neurologists (who deal exclusively with
brain disorders) to social workers and clinical psy-
chologists (who need to distinguish between adjust-
ment problems and possible signs of brain disorder).
Anyone who pursues a career in research needs to
stay up to date on new developments by attending con-
ventions, consulting with colleagues, and reading the
primary research journals, such as Journal of Neuro-
science, Neurology, Behavioral Neuroscience, Brain
Table 1.1 Fields of Specialization
Specialization Description
Research Fields Research positions ordinarily require a PhD. Researchers are employed by universities,
hospitals, pharmaceutical firms, and research institutes.
Neuroscientist Studies the anatomy, biochemistry, or physiology of the nervous system. (This broad term
includes any of the next five, as well as other specialties not listed.)
Behavioral neuroscientist Investigates how functioning of the brain and other organs influences behavior.
(almost synonyms: psychobiologist,
biopsychologist, or physiological
psychologist).
Cognitive neuroscientist Uses brain research, such as scans of brain anatomy or activity, to analyze and explore
people’s knowledge, thinking, and problem solving.
Neuropsychologist Conducts behavioral tests to determine the abilities and disabilities of people with various
kinds of brain damage and changes in their condition over time. Most neuropsychologists
have a mixture of psychological and medical training; they work in hospitals and clinics.
Psychophysiologist Measures heart rate, breathing rate, brain waves, and other body processes and how they
vary from one person to another or one situation to another.
Neurochemist Investigates the chemical reactions in the brain.
Comparative psychologist Compares the behaviors of different species and tries to relate them to their habitats and
(almost synonyms: ethologist, ways of life.
animal behaviorist)
Evolutionary psychologist Relates behaviors, especially social behaviors, including those of humans, to the functions
(almost synonym: sociobiologist) they have served and, therefore, the presumed selective pressures that caused them to evolve.
Practitioner Fields of Psychology In most cases, their work is not directly related to neuroscience. However, practitioners
often need to understand it enough to communicate with a client’s physician.
Clinical psychologist Requires PhD or PsyD. Employed by hospital, clinic, private practice, or college. Helps
people with emotional problems.
Counseling psychologist Requires PhD or PsyD. Employed by hospital, clinic, private practice, or college. Helps
people make educational, vocational, and other decisions.
School psychologist Requires master’s degree or PhD. Most are employed by a school system. Identifies educa-
tional needs of schoolchildren, devises a plan to meet the needs, and then helps teachers
implement it.
Medical Fields Practicing medicine requires an MD plus about 4 years of additional study and practice
in a specialization. Physicians are employed by hospitals, clinics, medical schools and in
private practice. Some conduct research in addition to seeing patients.
Neurologist Treats people with brain damage or diseases of the brain.
Neurosurgeon Performs brain surgery.
Psychiatrist Helps people with emotional distress or troublesome behaviors, sometimes using drugs
or other medical procedures.
Allied Medical Field These fields ordinarily require a master’s degree or more. Practitioners are employed
by hospitals, clinics, private practice, and medical schools.
Physical therapist Provides exercise and other treatments to help people with muscle or nerve problems, pain,
or anything else that impairs movement.
Occupational therapist Helps people improve their ability to perform functions of daily life, for example, after a
stroke.
Social worker Helps people deal with personal and family problems. The activities of a clinical social
worker overlap those of a clinical psychologist.
1.1 The Mind–Brain Relationship 9
151. Research, Nature Neuroscience, and Archives of Gen-
eral Psychiatry. However, what if you are entering a
field on the outskirts of neuroscience, such as clinical
psychology, school psychology, social work, or physical
therapy? In that case, you probably don’t want to wade
through technical journal articles, but you do want to
stay current on major developments, at least enough
to converse intelligently with medical colleagues. I
recommend the journal Cerebrum, published by the
Dana Press, 745 Fifth Avenue, Suite 700, New York,
NY 10151. Their website is http://www.dana.org and
their e-mail address is danainfo@dana.org. Cerebrum
provides well-written, thought-provoking articles re-
lated to neuroscience or biological psychology, acces-
sible to nonspecialists. In many ways, it is like Scien-
tific American but limited to the topic of brain and
behavior.
Module 1.1
In Closing: Your Brain and Your Experience
In many ways, I have been “cheating” in this module,
like giving you dessert first and saving your vegetables
for later. The mind–brain issue is an exciting and chal-
lenging question, but we cannot go far with it until we
back up and discuss the elements of how the nervous
system works. The goals in this module have been to
preview the kinds of questions researchers hope to an-
swer and to motivate the disciplined study you will
need in the next few chapters.
Biological psychologists are ambitious, hoping to
explain as much as possible of psychology in terms of
brain processes, genes, and the like. The guiding as-
sumption is that the pattern of activity that occurs in
your brain when you see a rabbit is your perception of
a rabbit; the pattern that occurs when you feel fear is
your fear. This is not to say that “your brain physiology
controls you” any more than one should say that “you
control your brain.” Rather, your brain is you! The rest
of this book explores how far we can go with this guid-
ing assumption.
Summary
1. Biological psychologists try to answer four types of
questions about any given behavior: How does it
relate to the physiology of the brain and other or-
gans? How does it develop within the individual?
How did the capacity for the behavior evolve? And
why did the capacity for this behavior evolve? (That
is, what function does it serve?) (p. 3)
2. Biological explanations of behavior do not neces-
sarily assume that the individual understands the
purpose or function of the behavior. (p. 3)
3. Philosophers and scientists continue to address the
mind–brain or mind–body relationship. Dualism,
the popular view that the mind exists separately
from the brain, is opposed by the principle that only
matter and energy can affect other matter and energy.
(p. 5)
4. Nearly all philosophers and scientists who have
addressed the mind–brain problem favor some ver-
sion of monism, the belief that the universe consists
of only one kind of substance. (p. 6)
5. No one has found a way to answer the “hard ques-
tion” of why brain activity is related to mental ex-
perience at all. However, new research techniques
facilitate studies on what types of brain activity are
necessary for consciousness. For example, a stimu-
lus that becomes conscious activates the relevant
brain areas more strongly than a similar stimulus
that does not reach consciousness. (p. 6)
Answers to
STOP & CHECK
Questions
1. An evolutionary explanation states what evolved
from what. For example, humans evolved from ear-
lier primates and therefore have certain features
that we inherited from those ancestors, even if the
features are not useful to us today. A functional ex-
planation states why something was advantageous
and therefore evolutionarily selected. (p. 5)
2. The three major versions of monism are material-
ism (everything can be explained in physical terms),
mentalism (only minds exist), and identity (the
mind and the brain are the same thing). (p. 7)
3. The “hard problem” is why minds exist at all in a
physical world, why there is such a thing as con-
sciousness, and how it relates to brain activity.
(p. 7)
4. The conscious and unconscious stimuli were phys-
ically the same (a word flashed on the screen for
29 ms). The difference was that a stimulus did not
become conscious if it was preceded and followed
by an interfering pattern. (p. 8)
5. If a stimulus became conscious, it activated the
same brain areas as an unconscious stimulus, but
more strongly. (p. 8)
10 Chapter 1 The Major Issues
152. Thought Questions3
1. What would you say or do to try to convince a solip-
sist that you are conscious?
2. Now suppose a robot just said and did the same
things you did in question 1. Will you be convinced
that it is conscious?
Author’s Answer About
Machine Consciousness (p. 7)
Here is a possibility similar to a proposal by J. R. Searle
(1992): Suppose someone suffers damage to part of the
visual cortex of the brain and becomes blind to part of
the visual field. Now, engineers design artificial brain
circuits to replace the damaged cells. Impulses from
the eyes are routed to this device, which processes the
information and sends electrical impulses to healthy
portions of the brain that ordinarily get input from the
damaged brain area. After this device is installed, the
person sees the field that used to be blind, remarking,
“Ah! Now I can see it again! I see shapes, colors, move-
ment—the whole thing, just as I used to!” Evidently,
the machine has enabled conscious perception of vi-
sion. Then, the person suffers still more brain damage,
and engineers replace even more of the visual cortex
with artificial circuits. Once again, the person assures
us that everything looks the same as before. Next, engi-
neers install a machine to replace a damaged auditory
cortex, and the person reports normal hearing. One by
one, additional brain areas are damaged and replaced
by machines; in each case, the behavior returns to nor-
mal and the person reports having normal experiences,
just as before the damage. Piece by piece, the entire
brain is replaced. At that point, I would say that the ma-
chine itself is conscious.
Note that all this discussion assumes that these ar-
tificial brain circuits and transplants are possible. No
one knows whether they will be. The point is merely
to show what kind of evidence might persuade us that
a machine is conscious.
1.1 The Mind–Brain Relationship 11
3Thought Questions are intended to spark thought and discus-
sion. The text does not directly answer any of them, although it
may imply or suggest an answer in some cases. In other cases,
there may be several possible answers.
153. Everything you do depends on both your genes
and your environment. Without your genes or
without an adequate environment, you would not exist.
So far, no problem. The controversies arise when we
discuss how strongly genes and environment affect
various differences among people. For example, do
differences in human intelligence depend mostly on
genetic differences, mostly on environmental influ-
ences, or on both about equally? Similar issues arise
for sexual orientation, alcoholism, psychological dis-
orders, weight gain, and so forth. This module certainly
does not resolve the controversies, but it should help
you understand them as they arise later in this text or
in other texts.
We begin with a review of elementary genetics.
Readers already familiar with the concepts may skim
over the first three pages.
Mendelian Genetics
Prior to the work of Gregor Mendel, a late-19th-century
monk, scientists thought that inheritance was a blend-
ing process in which the properties of the sperm and
the egg simply mixed, much as one might mix two col-
ors of paint.
Mendel demonstrated that inheritance occurs
through genes, units of heredity that maintain their
structural identity from one generation to another. As a
rule, genes come in pairs because they are aligned along
chromosomes (strands of genes), which also come in
pairs. (As an exception to this rule, a male has unpaired
X and Y chromosomes, with different genes.) A gene
is a portion of a chromosome, which is composed of
the double-stranded molecule deoxyribonucleic acid
(DNA). A strand of DNA serves as a template (model)
for the synthesis of ribonucleic acid (RNA) molecules.
RNA is a single-strand chemical; one type of RNA mol-
ecule serves as a template for the synthesis of protein
molecules. Figure 1.7 summarizes the main steps in
translating information from DNA through RNA into
proteins, which then determine the development of
the organism. Some proteins form part of the structure
of the body; others serve as enzymes, biological cata-
lysts that regulate chemical reactions in the body.
Anyone with an identical pair of genes on the two
chromosomes is homozygous for that gene. An individ-
ual with an unmatched pair of genes is heterozygous
Module 1.2
The Genetics of Behavior
Figure 1.7
How DNA controls
development of
the organism
The sequence of bases
along a strand of DNA
determines the order
of bases along a strand
of RNA; RNA in turn
controls the sequence of
amino acids in a protein
molecule.
DNA
Self-replicating
molecule
Each base determines one base of the RNA.
A triplet of bases determines
one amino acid.
RNA
Copy of one strand
of the DNA
Protein
Some proteins become
part of the body’s structure.
Others are enzymes that
control the rate of chemical
reactions.
...
12 Chapter 1 The Major Issues
154. for that gene. For example, you might have a gene for
blue eyes on one chromosome and a gene for brown
eyes on the other.
Certain genes are dominant or recessive. A domi-
nant gene shows a strong effect in either the homozy-
gous or heterozygous condition; a recessive gene shows
its effects only in the homozygous condition. For ex-
ample, someone with a gene for brown eyes (domi-
nant) and one for blue eyes (recessive) has brown eyes
but is a “carrier” for the blue-eye gene and can trans-
mit it to a child. For a behavioral example, the gene for
ability to taste moderate concentrations of phenylthio-
carbamide (PTC) is dominant; the gene for low sensi-
tivity is recessive. Only someone with two recessive
genes has trouble tasting it (Wooding et al., 2004). Fig-
ure 1.8 illustrates the possible results of a mating be-
tween people who are both heterozygous for the PTC-
tasting gene. Because each of them has one high-taste
sensitivity (T) gene,4 each can taste PTC. However, each
parent transmits either a taster gene (T) or a nontaster
gene (t) to a given child. Therefore, a child in this fam-
ily has a 25% chance of being a homozygous (TT) taster,
a 50% chance of being a heterozygous (Tt) taster, and a
25% chance of being a homozygous (tt) nontaster.
Chromosomes and Crossing Over
Each chromosome participates in reproduction inde-
pendently of the others, and each species has a certain
number of chromosomes—for example, 23 pairs in
humans, 4 pairs in fruit flies. If you have a BbCc geno-
type, and the B and C genes are on different chromo-
somes, your contribution of a B or b gene is indepen-
dent of whether you contribute C or c. But suppose B
and C are on the same chromosome. If one chromosome
has the BC combination and the other has bc, then if
you contribute a B, you probably also contribute C.
The exception comes about as a result of crossing
over: A pair of chromosomes may break apart during
reproduction and reconnect such that part of one chro-
mosome attaches to the other part of the second chro-
mosome. If one chromosome has the BC combination
and the other chromosome has the bc combination,
crossing over between the B locus (location) and the C
locus leaves new chromosomes with the combinations
Bc and bC. The closer the B locus is to the C locus, the
less often crossing over occurs between them.
Sex-Linked and Sex-Limited Genes
The genes located on the sex chromosomes are known
as sex-linked genes. All other chromosomes are auto-
somal chromosomes, and their genes are known as
autosomal genes.
In mammals, the two sex chromosomes are desig-
nated X and Y: A female mammal has two X chromo-
somes; a male has an X and a Y. (Unlike the arbitrary
symbols B and C that I introduced to illustrate gene
pairs, X and Y are standard symbols used by all ge-
neticists.) During reproduction, the female necessar-
ily contributes an X chromosome, and the male con-
tributes either an X or a Y. If he contributes an X, the
offspring is female; if he contributes a Y, the offspring
is male.
The Y chromosome is small. The human Y chro-
mosome has genes for only 27 proteins, far fewer than
other chromosomes. The X chromosome, by contrast,
has genes for about 1,500 proteins (Arnold, 2004).
Thus, when biologists speak of sex-linked genes, they
usually mean X-linked genes.
An example of a human sex-linked gene is the re-
cessive gene for red-green color vision deficiency. Any
man with this gene on his X chromosome has red-green
color deficiency because he has no other X chromo-
some. A woman, however, is color deficient only if she
has that recessive gene on both of her X chromosomes.
So, for example, if 8% of human X chromosomes con-
tain the gene for color vision deficiency, then 8% of
all men will be color-deficient, but fewer than 1% of
women will be (.08 .08).
1.2 The Genetics of Behavior 13
Figure 1.8 Four equally likely outcomes of a
mating between parents who are heterozygous
for a given gene (Tt)
A child in this family has a 25% chance of being homozygous
for the dominant gene (TT), a 25% chance of being
homozygous for the recessive gene (tt), and a 50% chance
of being heterozygous (Tt).
4Among geneticists, it is customary to use a capital letter to indi-
cate the dominant gene and a lowercase letter to indicate the re-
cessive gene.
Father
Genes Tt
Heterozygous
Taster
T
T
t T t
T t
t
Mother
Child 1
Genes TT
Homozygous
Taster
Child 2
Genes Tt
Heterozygous
Taster
Child 3
Genes Tt
Heterozygous
Taster
Child 4
Genes tt
Homozygous
Nontaster
Genes Tt
Heterozygous
Taster
155. Distinct from sex-linked genes are the sex-limited
genes, which are present in both sexes but have effects
mainly or exclusively for one sex. For instance, genes
control the amount of chest hair in men, breast size in
women, the amount of crowing in roosters, and the
rate of egg production in hens. Both sexes have those
genes, but sex hormones activate them, so their effects
depend on male or female hormones.
STOP CHECK
1. Suppose you can taste PTC. If your mother can also
taste it, what (if anything) can you predict about your
father’s ability to taste it? If your mother cannot taste
it, what (if anything) can you predict about your father’s
ability to taste it?
2. How does a sex-linked gene differ from a sex-limited
gene?
Check your answers on page 21.
Sources of Variation
If reproduction always produced offspring that were
exact copies of the parents, evolution would not occur.
One source of variation is recombination, a new com-
bination of genes, some from one parent and some
from the other, that yields characteristics not found in
either parent. For example, a mother with curly blonde
hair and a father with straight black hair could have a
child with curly black hair or straight blonde hair.
A more powerful source of variation is a mutation,
or change in a single gene. For instance, a gene for
brown eyes might mutate into a gene for blue eyes. Mu-
tation of a given gene is a rare, random event, inde-
pendent of the needs of the organism. A mutation is
analogous to having an untrained person add, remove,
or distort something on the blueprints for your new
house. A mutation leading to an altered protein is al-
most always disadvantageous. A mutation that modi-
fies the amount or timing of protein production is
closer to neutral and sometimes advantageous. Many
of the differences among individuals and even among
species depend on quantitative variations in the ex-
pression of genes.
Heredity and Environment
Unlike PTC sensitivity and color vision deficiency, most
variations in behavior depend on the combined influ-
ence of many genes and environmental influences.
You may occasionally hear someone ask about a behav-
ior, “Which is more important, heredity or environ-
ment?” That question as stated is meaningless. Every
behavior requires both heredity and environment.
However, we can rephrase it meaningfully: Do the
observed differences among individuals depend more
on differences in heredity or differences in environ-
ment? For example, if you sing better than I do, the rea-
son could be that you have different genes, that you had
better training, or both.
To determine the contributions of heredity and
environment, researchers rely mainly on two kinds of
evidence. First, they compare monozygotic (“from one
egg,” i.e., identical) twins and dizygotic (“from two
eggs,” i.e., fraternal) twins. A stronger resemblance
between monozygotic than dizygotic twins suggests
a genetic contribution. Second, researchers examine
adopted children. Any tendency for adopted children
to resemble their biological parents suggests a heredi-
tary influence. If the variations in some characteristic
depend largely on hereditary influences, the charac-
teristic has high heritability.
Based on these kinds of evidence, researchers have
found evidence for a significant heritability of almost
every behavior they have tested (Bouchard McGue,
2003). A few examples are loneliness (McGuire Clif-
ford, 2000), neuroticism (Lake, Eaves, Maes, Heath,
Martin, 2000), television watching (Plomin, Corley,
DeFries, Fulker, 1990), and social attitudes (S. F. Pos-
ner, Baker, Heath, Martin, 1996). About the only be-
havior anyone has tested that has not shown a signifi-
cant heritability is religious affiliation—such as Jewish,
Protestant, Catholic, or Buddhist (Eaves, Martin,
Heath, 1990).
Possible Complications
Humans are difficult research animals. Investigators
cannot control people’s heredity or environment, and
even their best methods of estimating hereditary influ-
ences are subject to error (Bouchard McGue, 2003;
Rutter, Pickles, Murray, Eaves, 2001).
For example, it is sometimes difficult to distin-
guish between hereditary and prenatal influences. Con-
sider the studies showing that biological children of
parents with low IQs, criminal records, or mental ill-
ness are likely to have similar problems themselves,
even if adopted by excellent parents. The parents with
low IQs, criminal records, or mental illness gave the
children their genes, but they also gave them their pre-
natal environment. In many cases, those mothers had
poor diets and poor medical care during pregnancy.
Many of them smoked cigarettes, drank alcohol, and
used other drugs that affect a fetus’s brain develop-
ment. Therefore, what looks like a genetic effect could
reflect influences of the prenatal environment.
14 Chapter 1 The Major Issues
156. Another complication: Certain environmental fac-
tors can inactivate a gene by attaching a methyl group
(CH3) to it. In some cases, an early experience such as
malnutrition or severe stress inactivates a gene, and
then the individual passes on the inactivated gene to
the next generation. Experiments have occasionally
shown behavioral changes in rats based on experi-
ences that happened to their mothers or grandmothers
(Harper, 2005). Such results blur the distinction be-
tween hereditary and environmental.
Genes can also influence your behavior indirectly
by changing your environment. For example, suppose
your genes lead you to frequent temper tantrums. Other
people—including your parents—will react harshly,
giving you still further reason to feel hostile. Dickens
and Flynn (2001) call this tendency a multiplier ef-
fect: If genetic or prenatal influences produce even a
small increase in some activity, the early tendency will
change the environment in a way that magnifies that
tendency.
For a sports example, imagine a child born with
genes promoting greater than average height, running
speed, and coordination. The child shows early suc-
cess at basketball, so parents and friends encourage the
child to play basketball more and more. The increased
practice improves skill, the skill leads to more success,
and the success leads to more practice and coach-
ing. What started as a small advantage becomes larger
and larger. The same process could apply to school-
work or any other endeavor. The outcome started with
a genetic basis, but environmental reactions magni-
fied it.
Environmental Modification
Even a trait with a strong hereditary influence can be
modified by environmental interventions. For exam-
ple, different genetic strains of mice behave differently
in the elevated plus maze (Figure 1.9). Some stay al-
most entirely in the walled arms, like the mouse shown
in the figure; others (less nervous?) venture onto the
open arms. But even when different laboratories use the
same genetic strains and nearly the same procedures,
strains that are adventuresome in one laboratory are
less active in another (Crabbe, Wahlsten, Dudek,
1999). Evidently, the effects of the genes depend on
subtle differences in procedure, such as how the inves-
tigators handle the mice or maybe even the investiga-
tors’ odors. (Most behaviors do not show this much
variability; the elevated plus maze appears to be an
extreme example.)
Genes or prenatal influences Increase of some tendency
Environment that facilitates
For a human example, phenylketonuria (FEE-nil-
KEET-uhn-YOOR-ee-uh), or PKU, is a genetic inability
to metabolize the amino acid phenylalanine. If PKU is
not treated, the phenylalanine accumulates to toxic
levels, impairing brain development and leaving chil-
dren mentally retarded, restless, and irritable. Approx-
imately 1% of Europeans carry a recessive gene for
PKU; fewer Asians and almost no Africans have the
gene (T. Wang et al., 1989).
Although PKU is a hereditary condition, environ-
mental interventions can modify it. Physicians in many
countries routinely measure the level of phenylalanine
or its metabolites in babies’ blood or urine. If a baby
has high levels, indicating PKU, physicians advise the
parents to put the baby on a strict low-phenylalanine
diet to minimize brain damage (Waisbren, Brown, de
Sonneville, Levy, 1994). Our ability to prevent PKU
provides particularly strong evidence that heritable
does not mean unmodifiable.
A couple of notes about PKU: The required diet is
difficult. People have to avoid meats, eggs, dairy prod-
ucts, grains, and especially aspartame (NutraSweet),
which is 50% phenylalanine. Instead, they eat an ex-
pensive formula containing all the other amino acids.
Physicians long believed that children with PKU could
quit the diet after a few years. Later experience has
shown that high phenylalanine levels damage teenage
and adult brains, too. A woman with PKU should be
especially careful during pregnancy and when nurs-
ing. Even a genetically normal baby cannot handle the
enormous amounts of phenylalanine that an affected
mother might pass through the placenta.
1.2 The Genetics of Behavior 15
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