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Chapter 3
Saturday, February 13, 2016 1:37 PM
Neural Processing and Perception
• The pathway for a neuron through the brain has stops and unexpected turns and sometimes
goes backwards
• Neural processing: the interaction of the signals in many neurons
Lateral Inhibition and Perception
• This section answers the question of what will happen when both convergence and inhibition
are present?
• Lateral inhibition: inhibition that is transmitted across the retina
○ Inhibition can cause perceptual effects
Lateral inhibition in the Limulus
○ The Limulus eye is made of up hundreds of tiny structures called ommatidia, and each
ommatidium has a small lens on the eye's surface that is located directly over a single
receptor.
○ When researchers used illumination on receptor A, it caused a large response. However,
when they added illumination to the three nearby receptors at B, the response of
receptor A decreased
○ This decrease in the firing of receptor A is caused by lateral inhibition that is transmitted
from B to A across the Limulu's eye by the fibers of the lateral plexus
Lateral Inhibition and Lightness Perception
○ Each of the perceptual phenomena that have been explained by lateral inhibition have
involved the perception of lightness (the perception of shades ranging from white to grey
to black)
The Hermann Grid: seeing spots at the intersections
▪ Know what a Hermann grid is
▪ Lateral inhibition is given by a percentage of how much one receptor affects the
receptor next to it.
▪ A point between two black squares will receive less lateral inhibition than a point at
the intersection of 4 black square. This means that the former will be lighter.
Mach Bands: Seeing Borders More Sharply
▪ Works the same way as Hermann grid, just with different shades next to each other
in a row
Lateral Inhibition and Simultaneous Contrast
▪ Simultaneous contrast occurs when our perception of the brightness oor color of
one area is affected by the presence of an adjacent or surrounding area
▪ This works by lateral inhibition, which should be self-explanatory once lateral
inhibition is understood
A display that can't be explained by lateral inhibition
○ White's illusion
○ Lateral inhibition combines with belongingness (an area's appearance is influenced by the
part of the surroundings to which the area appears to belong) to get this effect
▪ Belongingness isn't proven, only theory
Processing From Retina To Visual Cortex and Beyond
Responding of single fibers in the optic nerve
○ The optic nerve is made up of a bunch of nerve fibers just like a muscle. Each one
responds separately
○ A fiber's receptive field covers a much greater area than a single rod or cone receptor.
○ Fiber is receiving converging signals from all of these hundreds or thousands of receptors
○ Cats have a center-surrounded organization of receptive fields
▪ This means the area in the "center" of the receptive field responds differently to
light than the area in the "surround" of the receptive field.
▪ A spot of light to the center increases firing, so it is called the excitatory area of the
receptive field.
▪ In contrast, stimulation fo the surround causes a decrease in firing, so it is called the
inhibitory area
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▪ This receptive field is called excitatory-center, inhibitory- surround receptive field
○ Receptive field: the retinal region over which a cell in the visual system can be influenced
(excited or inhibited) by light
○ Center-surround receptive fields showed that neural processing could result in neurons
that respond best to specific patterns of illumination
○ Center-surround antagonism: increasing a light's size so that it covers the entire center of
the receptive field increases the cell's response.
○ Stimulation of the inhibitory surround counteracts the center's excitatory response
causing a decrease in the neuron's firing rate
○ Because of center-surround antagonism, this neuron responds best to a spot of light htat
is the size of the excitatory center of the receptive field
Hubel and Wiesel's rationale for studying receptive fields
○ Neurons at higher levels of the visual system become tuned to respond best to more and
more specific kinds of stimuli
○ Signals leaving the eye in the optic nerve travel to the lateral geniculate nucleus (LGN) and
then from the LGN to the occipital lobe of the cerebral cortex
○ The superior colliculus receives some signals from the eye and plays an important role in
controlling movements of the eye
○ The visual receiving area is also called the striate cortex or area V1, to indicate that it's the
first visual area in the cortex
○ Little change occurred in receptive fields when moving from the optic nerve fibers to
neurons in the LGN, which made researchers wonder about the function of the LGN.
▪ One suggestion is that the LGN is used to regulate neural information as it flows
from the retina to the cortex
▪ The LGN receives more signals from the cortex than from the retina
Receptive fields of neurons in the visual cortex
○ Cells with side-by-side receptive fields are called simple cortical cells
▪ This is the reason for more activity when viewing vertical bars
○ The relationship between orientation and firing is indicated by a neuron's orientation
tuning curve
○ As with simple cells, a particular neuron has a preferred orientation
○ Complex cells also respond best to bars of a particular orientation, however, unlike simple
cells, which respond to small spots of light or to stationary stimuli, most complex cells
respond only when a correctly oriented bar of light moves across the entire receptive
field
○ End-stopped cells fire to moving lines of a specific length or to moving corners or angles
○ Because simple cells, complex cells, and end-stopped cells only respond to certain things
they are sometimes called feature detectors
Do Feature Detectors Play a Role in Perception?
Selective Adaptation
○ Selective adaptation is that this firing causes neurons to eventually become fatigued, or
adapt. This adaptation causes two physiological effects: 1) the neuron's firing rate
decreases, and 2) the neuron fires less when that stimulus is immediately presented
again
○ Our psychophysical curve shows that adaptation selectively affects only some
orientations, just as neurons selectively respond to only some orientations
○ This evidence that feature detectors have something to do with perception means that
when you look at a complex scene, such as a city street or a crowded shopping mall,
feature detectors that are firing to the orientations in the scene are helping to construct
our perceptions of the scene
Selective Rearing
○ Selective rearing is that if an animal is reared in an environment that contains only certain
types of stimuli, than neurons that respond to these stimuli will become more prevalent
○ This follows from a phenomenon called neural plasticity or experience-dependent
plasticity: the idea that the response properties of neurons can be shaped by perceptual
experience
○ "use it or lose it" you can become blind to other orientations if you only see only for years
Higher-Level Neurons
○ Neurons in the IT cortex respond to complex stimuli
○ Some respond best to faces, some to hands
○ Researchers discovered an area on the underside of the temporal lobe of the human
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cortex that was named th fusiform face area because it responded strongly to faces
Sensory Code
• Sensory coding: how the firing of neurons represent various characteristics of the environment
• The idea that the firing of single neurons is the key to understanding sensory coding is called
specificity coding
Specificity coding: representation by the firing of a specialized neuron
○ Specificity coding proposes that a particular object is represented by the firing of a neuron
that responds only to that object and to no other objects
○ A grandmother cell is a neuron that responds only to a specific stimulus. This stimulus
could be a specific image, such as a picture, such as a picture of your grandmother, or a
concept, such as the idea of grandmothers in general
Distributed coding: representation by the firing of large groups of neurons
○ This is the representation of a particular object by the pattern of firing of a large number
of neurons
Sparse coding: representation by the firing of a small number of neurons
○ Occurs when a particular object is represented by a pattern of firing of only a small group
of neurons, with the particular neuron can respond to more than one stimulus.
▪ Each neuron responds a certain amount of strongness to a particular stimuli
The mind-body problem
• How do physical processes such as nerve impulses or sodium and potassium molecules flowing
across membranes become transformed into the richness or perceptual experience
• Difference between the easy problem and the hard problem of consciousness
