Sensation is what our senses do to make us
aware of a stimulus, such as a tree or
someone calling our name or cupcakes baking
in the oven. Perception is what our brain does
with the information that allows us to form a
concept of the stimulus: the sight of the tree,
the sound of a friend’s voice calling or name,
or the sweet smell of the baking cupcakes.
However, the sights, sounds, smells, tastes,
and touches we experience are not a property
of the stimulus itself, but a reaction of neurons
in your brain to these particular stimuli.
Each of the receptors for the various senses in
our brain (for vision, hearing, taste, smell, and
touch) is specialized to use one kind of energy
and convert that energy into an electrochemical
pattern in our brain. The activity of your brain in
response to the stimuli does not duplicate the
object that your senses respond to, but the
representation of that object that the brain has
built because of our experiences using our various
senses in our world. A certain pattern of activity in
our neurons allows us to experience “green tree”
or “familiar voice” or “baking cupcakes.”
Light comes into our eye through an opening in the
center of the iris called the pupil. The lens of the eye
is adjustable and focuses the light and the cornea
(which is not adjustable) also helps focus the light and
it is then projected onto the retina at the rear surface
of the eye. The rear surface of the eye is lined with
visual receptors. The highest concentration of
receptors specialized for particularly detailed vision is
known as the fovea. The receptors send action
potentials to the brain along the optic nerve. The point
at which the optic nerve leaves the eye has no
receptor cells and is called the blind spot. We cannot
see anything with the part of the eye where the blind
spot is. The message about vision is carried to the
primary visual cortex in the occipital lobe of the brain.
The human eye with major parts labeled
The receptors that are located in the back of the eye in
the retina include the bipolar cells which send their
messages to ganglion cells, located closer to the
center of the eye than the bipolar cells. The ganglion
cells bind together and form the optic nerve. Another
set of receptors are amacrine cells, and these send
their messages to bipolar cells, other amacrine cells
and to ganglion cells. Horizontal cells are also receptor
cells, but they are cells that inhibit rather than excite.
In addition to the bipolar, ganglion, amacrine, and
horizontal cells, the retina also is made up receptor
cells called rods and cones. There are more rods at the
edges of the retina, and they respond to faint light and
seeing in the dark. There are more rods than cones in
the brain, but they are smaller. Cones are clustered in
and around the fovea and are more useful in bright
light, and they are critical for color vision. Cones have a
more direct route to the brain than rods do.
Rod (left) and cone (right)
Both rods and cones contain a chemical that releases energy when hit by
light called photopigments. Photopigments have a sensitivity to different
wavelengths of light. The different lengths of light waves are what allow us
to perceive distinct colors.
For humans, the shortest light waves are seen as violet and light waves
that get longer and longer are seen as blue, then green, then yellow, then
orange, and the longest are seen as red.
There are three major theories about how it is
that we see color and also shades of color
because a neuron in the visual system can only
vary the frequency of action potentials. A single
action potential can’t respond to both brightness
and color, for example. And we don’t have
separate receptors for different colors.
The first theory is called the trichomatic theory of color
vision or the Young-Helmholtz theory (after the people
who developed the theory). This theory suggests that
we have three kinds of cones: cones that respond best
to the short wavelength blue part of the visual color
spectrum, cones that respond best to the medium
wavelength green part of the visual color spectrum,
and cones that respond best to the long wavelength
red part of the visual color spectrum. We see different
shades of color depending on how active cones
sensitive to each particular color are because they are
firing at the same time.
The second theory is called the opponent-process theory.
This theory is based on the fact that if you stare at an
image of one of the colors a specific cone is sensitive to
for a minute or so and then look at a white surface or white
piece of paper, the color should be replaced by its
opposite. For example, in the picture below, if you stare at
the red square for a time and look away, you should see a
blue afterimage. This theory proposes that we see colors
based on paired opposites: red versus green, yellow
versus blue, and white versus black. This theory proposes
that shades of color occur both because of the active firing
of some cones but also a decrease in firing of cones
sensitive to a different color.
The third theory is called the retinex theory. This theory
explains the idea of color constancy, that is the ability to
recognize the color of an object even when light changes.
For example, if someone wearing a bright shirt walks into a
tunnel where it is pretty dark, the shirt will look more gray
than yellow, but we know the color of the shirt has not
actually changed. Also, we see how bright an object is by
comparing it with other objects. This theory suggests that
the visual cortex compares information from many parts of
the retina to decide the brightness and color of objects. This
theory suggests that vision requires thinking about what we
are seeing and having some experience with vision to be
able to make judgments about objects, including what the
object is, what color it is, and how bright it is.
Some people don’t see all of the colors in the light
spectrum because they have a condition known as
color vision deficiency or color blindness. Color vision
deficiency is caused by differences in our genes. Some
people are missing one or two kinds of cones, and
some have all three kinds of cones, but one kind of
cone has some abnormalities.
The most common type of color blindness occurs
because a person has the same kind of photopigments
in their long wavelength and medium wavelength cones
instead of different ones. This causes them to have
trouble telling the difference between red and green.
The optic nerve leaves the eye and forms the optic
tract to the brain. At a place called the optic chiasm, the
optic nerve sends part of its signal to the same side of
the brain and part of its signal to the opposite side of
the brain. Before the signal from the optic nerve gets to
the brain, the thalamus and some other brain areas
below the cerebral cortex help route the information to
the primary visual cortex in the occipital lobe of the
The primary visual cortex manages the first stage of
visual processing and is responsible for most of the
visual information of which we are consciously aware.
Some other areas of the cortex process visual
information about shape, other areas about movement,
and still other areas about brightness and color. Areas
in the temporal cortex of the brain process information
about what something is and where something is that
we are looking at. All of these areas of the brain work
together so that we see a complete object when we are
looking at it, even if different information about the
object is processed in different places in our brains.
Vision and the Brain
Optic chiasm and route of visual information to the brain
When we hear, we are processing sound waves that
are made up of compressions of air or water. We
hear vibrations in the air as they strike a part of the
ear called the eardrum and make it vibrate. These
vibrations are sent through other parts of the ear and
finally sent as action potentials to the brain.
Sound waves have both amplitude and frequency.
Amplitude is a sound’s intensity, and loudness is the
perception of that intensity. Frequency of a sound is
the number of compressions per second. Pitch is
closely related to frequency.
The part of the hearing system that we see on
the outside of the head is called the pinna (the
ear). It is designed to capture sound. When a
sound reaches the ear, it passes through the
tube called the external auditory canal until it
reaches the tympanic membrane or eardrum.
The eardrum vibrates at the same frequency as the
sound waves that hit it. Attached to the eardrum
are three very small bones (the smallest bones in
the body!) that also vibrate to the frequency of the
sound. These bones are known as the hammer,
anvil, and stirrup because of their shapes.
Together, they are known as the ossicles. The
three bones are attached to the oval window. The
oval window is the beginning of the inner ear.
The inner ear has the cochlea, a snail-shaped fluid-
filled structure. Vibrations from sound in the fluid in
the cochlea displace hair cells that are the neuron
receptors for sound at the bottom of the cochlea in
the basilar membrane. The tectorial membrane
covers the hair cells and protects them. The hair
cells send signals to the auditory nerve, which
sends a signal about sound to the temporal lobe of
Visualization of the Ear
There are three theories about hearing.
The first theory is known as the frequency theory.
This theory says that the basilar membrane that
holds the hair cells vibrates at the same
frequency as sound. This causes the auditory
nerve axons to produce action potentials at the
same frequency. However, the maximum firing
rate of a neuron is short of the highest
frequencies we can hear.
The second theory is known as the place theory.
This theory suggests that the basilar membrane is
similar to the strings of a piano and that each area
along the membrane is tuned to a specific
frequency and vibrates to that frequency. The
nervous system would have to decide among the
frequencies based on which neurons are active.
However, the problem with this theory is that some
parts of the basilar membrane are bound together
too tightly for any part to vibrate like a piano string.
The final theory is known as the volley principle. This
theory suggests we use methods that combine
aspects of the frequency theory and the place theory.
The basilar membrane is stiff at its base where the
stirrup connects with the cochlea and floppy at the
other end of the cochlea. Hair cells along the basilar
membrane would act differently depending on their
location. When we hear sounds at a very high
frequency, we use something like the place theory.
When we hear lower pitched sounds, we use
something like the frequency theory. So combining
parts of the first two theories explains how we hear
better than using either one of the first two theories
Information about hearing, just like information
about vision, is first routed through the thalamus
and other brain areas below the cortex before
reaching the primary auditory cortex, located in the
temporal lobe of the cerebral cortex. Different
areas of the auditory cortex, just like is true in the
visual cortex, process information in different ways,
including about the location of a sound and the
motion of sound. And just like vision, hearing
requires a certain amount of experience with
sounds for our hearing to be fully developed.
Areas of the brain associated with vision and
When you move your head, the organs of the
vestibular system adjust the movement and allow
your eyes to adjust as well. For example, when
your head moves left, your eyes move right and
when your head moves right your eyes move left.
This allows you to keep your eyes focused on
what you want to see.
The vestibular organ looks like it is part of the
structures used for hearing and is attached to the
hearing structures. It is composed of three
semicircular canals. Calcium carbonate particles
called otoliths lie next to the hair cells (similar to
the hair cells used in hearing) that are the
receptor cells for vestibular sensations. When the
head tilts in different directions, the otoliths push
against sets of hair cells and cause them to be
excited and produce action potentials.
The semicircular canals are on three different
planes, and they are filled with a jellylike
substance and lined with the hair cells. During
head movement, the jellylike substance also
pushes against the hair cells. This causes action
potentials to be produced, and they travel to the
brainstem and cerebellum. Two baglike
structures, known as saccule and utricle, are also
part of the vestibular system.