Neural Processing and Perception
Neural processing—the interaction of the signals in many neurons.
Lateral Inhibition and Perception
Lateral inhibition —inhibition that is transmitted across the retina
Lateral Inhibition in the Limulus
They chose the Limulus because the structure of its eye makes it possible to
stimulate individual receptors. The Limulus eye is made up of 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.
Nerve fiber of receptorA, they found that illumination of that receptor
caused a large response
But when they added illumination to the three nearby receptors at B, the
response of receptorAdecreased
They also found that further increasing the illumination of B decreasedA’s
response even more
Thus, illumination of the neighboring receptors at B inhibited the firing
caused by stimulation of receptorA. This decrease in the firing of receptor
Ais caused by lateral inhibition that is transmitted from B toAacross the
Limulus’s eye by the fibers of the lateral plexus
Lateral Inhibition and Lightness Perception
Lightness —the perception of shades ranging from white to gray to black.
The Hermann Grid: Seeing Spots at Intersections
You can prove that this grayness is not physically present by noticing that it
is reduced or vanishes when you look directly at an intersection or, better
yet, when you cover two rows of black squares with white paper.
Each bipolar cell has an initial response of 100. Bipolar cells B, C, D, and E
each send 10 units of inhibition to bipolar cell A, as indicated by the red
arrows. Because the total inhibition is 40, the final response of bipolarAis
The final response of bipolarAis determined by starting with its initial
response and subtracting any decrease caused by lateral inhibition, which is
indicated by the red arrows.
Although the initial responses of bipolarsAand D are the same, their final
responses are different, because D receives less lateral inhibition than A.
Lateral inhibition, therefore, explains the dark images at the intersection.
Mach Bands: Seeing Borders More Sharply
Mach bands, illusory light and dark bands near a light-dark border. The lightness is high as we begin moving to the right across the lighter
stripe, but then, near the border at B, the lightness becomes even higher.
The upward bump at B represents this slight increase in lightness we see
just to the left of the border. Once across the border, we encounter the dark
Mach band, indicated by the downward bump at C that represents the slight
decrease in lightness we see just to the right of the border
Each of the six receptors in this circuit sends signals to bipolar cells, and
each bipolar cell sends lateral inhibition to its neighbors on both sides
An example of simultaneous contrast: The receptors under the two small
squares receive the same illumination. However, the light area surrounding
the square on the left causes receptors under that area to respond rapidly
and to send large amounts of inhibition to the neurons below the center
square (large arrows). The dark area surrounding the square on the right
causes the receptors under that area to fire less rapidly, so they send less
inhibition to the neurons under the right square (small arrows). Because the
cells under the left square receive more inhibition than the cells under the
right square, their response is decreased more. This smaller response
compared to the response of the neurons under the right square causes the
left square to appear darker.
A Display That Can’t Be Explained by Lateral Inhibition
White’s illusion: RectangleA, on the left, which appears to be resting on
the white area under the black bars, looks much darker than rectangle B, on
the right, which appears to be located on the black bars. However,
rectanglesAand B reflect the same amount of light.
Our perception of lightness in influenced by a principle called
belongingness , which states that an area’s appearance is influenced by the
part of the surroundings to which the area appears to belong.
Processing From Retina to Visual Cortex and Beyond
Responding of Single Fibers in the Optic Nerve
The optic nerve, which leaves the back of the eye, contains about one
million optic nerve fibers in the human.
While recording from this teased-out fiber, Hartline illuminated different
areas of the retina and found that the fiber he was recording from responded
only when a small area of the retina was illuminated. He called the area that
caused the neuron to fire the nerve fiber’s receptive field, which he defined
as follows: The region of the retina that must receive illumination in order
to obtain a response in any given fiber
The cat receptive fields, it turns out, are arranged in a center-surround
organization , in which the area in the “center” of the receptive field
responds differently to light than the area in the “surround” of the receptive
field For the receptive field in Figure 3.21a, presenting a spot of light to the
center increases firing, so it is called the excitatory area of the receptive
field. In contrast, stimulation of the surround causes a decrease in firing, so
it is called the inhibitory area of the receptive field. This receptive field is
called an excitatory-center, inhibitory-surround receptive field . The
receptive field in Figure 3.21b, which responds with inhibition when the
center is stimulated and excitation when the surround is stimulated, is an
inhibitory-center, excitatory-surround receptive field .
The discovery that receptive fields can have oppositely responding areas
made it necessary to modify Hartline’s definition of receptive field to the
retinal region over which a cell in the visual system can be influenced
(excited or inhibited) by light
Center-surround antagonism , illustrated in Figure 3.22.Asmall spot of
light presented to the excitatory center of the receptive field causes a small
increase in the rate of nerve firing (a), and increasing the light’s size so that
it covers the entire center of the receptive field increases the cell’s response,
as shown in (b). Increasing stimulus size further causes a decrease in firing
due to center-surround antagonism.
Hubel and Wiesel’s Rationale for Studying Receptive Fields
Recording electrical signals from a fiber in the optic nerve of an
anesthetized cat. Each point on the screen corresponds to a point on the
The receptive field is always on the retina, because that is where the stimuli
Shows how 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 2–4 mm thick covering of the brain that plays a
central role in determining perception and cognition
The occipital lobe is the visual receiving area —the place where signals
from the retina and LGN first reach the cortex. Viewing the underside of
the brain shows the pathway from eye to cortex, plus the superior
colliculus , which receives some signals from the eye. This structure plays
an important role in controlling movements of the eyes.
The visual receiving area is also called the striate cortex , because it has a
striped appearance when viewed in cross section, or area V1 to indicate
that it is the first visual area in the cortex
One proposal of LGN function is based on the observation that the signal
sent from the LGN to the cortex is smaller than the input the LGN receives
from the retina