Neural Processing and Perception
• The pathway from receptors to brain is not a non-stop turnpike, every signal leaving a
receptor travels through a complex of interconnected neurons, often meeting, and being
affected by, other signals along the way.
• Neural processing: Operations that transform electrical signals within a network of
neurons or that transform the response of individual neurons.
Lateral Inhibition and Perception
• Lateral inhibition: Inhibition that is transmitted laterally across a nerve circuit. In the
retina, lateral inhibition is transmitted by the horizontal and amacrine cells.
• Primary work on lateral inhibition carried out on the horseshoe crab, or Limulus.
Lateral Inhibition in the Limulus
• Experiment done by Keffer Hartline, Henry Wagner, and Floyd Ratliff.
• The horseshoe crab’s eye structure makes it possible to stimulate individual receptors.
• Ommatidia: A structure in the eye of Limulus that contains a small lens, located directly
over a visual receptor. The Limulus eye is made up of hundreds of these ommatidia. The
limulus eye has been used for research on lateral inhibition because its receptors are
large enough so that stimulation can be applied to individual receptors.
• The eye on the crab has hundreds of ommatidia, each with a small lens on the eye’s
surface, located directly over a single receptor.
• Found that when they recorded receptor response for receptorA, it was large. When they
stimulated receptor B at the same time, the response inAdiminished.Also found that the
intensity of B was related to how muchAdecreased in response.
• This decrease in the firing of receptorAis caused by lateral inhibition that is transmitted
from B toAacross the Limulus’s eye by the fibres of the lateral plexus.
• The lateral plexus is analogous to horizontal and amacrine cells in humans.
Lateral Inhibition and Lightness Perception
• Lightness: The perception of shades ranging from white to grey to black.
The Hermann Grid: Seeing Spots at Intersections
• Hermann grid: A display that results in the illusion of dark areas at the intersection of 2
white “corridors.” This perception can be explained by lateral inhibition.
• Picture description: Grid. ReceptorAin centre where the corridors meet. BesideAin each
white corridor is a receptor labelled B through E.Assume each is receiving a signal of
100 from the white. Due to lateral inhibition, B through E are each sending an inhibitory
signal of -10 to A. This leaves Awith a total signal of 60, making it appear darker than
the other areas.
• Picture description: D receptor located in the middle of 2 white intersections. F and H are
in the black squares,Aand G are in the white corridor.A, D, and G are receiving white
light and will have a response of 100. F and H are receiving dark light, so let’s say their
response is 20 each. G andAwill inhibit D by 10 each, but F and H will only inhibit by 2
each. This leaves D with a perceived lightness of 76. • Comparing these two scenarios, we can see that in the intersection will appear darker
than the surrounding corridors.
Mach Bands: Seeing Borders More Sharply
• Mach bands illusion is perceived light and dark bands near a light-dark border.
• The perceived lightness graph (for Mach bands going from light to dark) is a straight line,
then up a bit, then down a lot, then up a bit, then straight until the next dark-light border.
• Picture explanation: 6 receptors, 3 low intensity stimuli, 3 high intensity stimuli. Each
high intensity has a signal of 100 while each low intensity has a signal of 20. The final
output for the 6 in a line after taking account for lateral inhibition is 80, 80, 88, 8, 16, 16.
As you can see, it follows that graph of perceived lightness.
Lateral Inhibition and Simultaneous Contrast
• Simultaneous contrast: The effect that occurs when surrounding one colour with another
changes the appearance of the surrounding colour. Occurs for chromatic and achromatic
• Picture description: 2 large squares, one dark, one light.Amini square in each that is the
same in-between shade. Lateral inhibition causes the box in the light square to appear
darker. The lateral inhibition from the dark square causes the box to appear lighter.
Therefore comparing the 2 boxes will make it seem as though they are different colours.
ADisplay That Can’t Be Explained by Lateral Inhibition
• White’s illusion: A display in which 2 rectangles are perceived as differing in lightness
even though they both reflect the same amount of light and even though the rectangle that
is perceived as lighter receives more lateral inhibition than the one perceived as darker.
• RectangleAis first followed by rectangle B.
• Looking at lateral inhibition for picture: RectangleAis receiving less lateral inhibition
than rectangle B.An explanation based on lateral inhibition would say that B should be
darker than A.
• Belongingness: The hypothesis that an area’s appearance is influenced by the part of the
surroundings that the area appears to belong to. This principle has been used to explain
the perception of lightness in the Benary cross and White’s illusion.
• Picture explanation from belongingness: Our perception of rectangleAwould be affected
by the white background because it appears to be resting on the white background. Our
perception of rectangle B would be affected by the black areas because it appears to be
resting on them. Thus, the principle of belongingness proposes that the light area makes
areaA appear darker and the dark area makes area B appear lighter.
Processing From the Retina to Visual Cortex and Beyond
• Now looking at how processing affects the responses of single neurons at higher levels of
the visual system.
Responding of Single Fibres in the Optic Nerve • Nerve fibres leaving the eye are the axons of the ganglion cells.
• Pre horseshoe crab research, Hartline used the opened eye cup of a frog. He isolated a
single nerve fibre. When illuminating areas of the eye, he found that the nerve was only
showing a response to a certain area, the receptive field.
• Receptive field: A neuron’s receptive field is the area on the receptor surface (the retina
for vision; the skin for touch) that, when stimulated, affects the firing of that neuron.
• Hartline emphasized that a fibre’s receptive field covered a much greater area than a
single rod or cone receptor. This means that a fibre is receiving converging signals from
receptors.Also noted that receptive fields overlap.
• Centre-surround organization: Arrangement of a neuron’s receptive fields in which one
area is surrounded by another area, like the hole of a doughnut (corresponding to the
centre) and the doughnut (the surround). Stimulation of the centre and surround causes
• Excitatory area: Area of a receptive field that is associated with excitation. Stimulation of
this area causes an increase in the rate of nerve firing.
• Inhibitory area: Area of a receptive field that is associated with inhibition. Stimulation of
this area causes a decrease in the rate of nerve firing.
• Excitatory centre, inhibitory-surround receptive field: A centre-surround receptive field
in which stimulation of the centre area causes an excitatory response, and stimulation of
the surround causes an inhibitory response.
• Inhibitory-centre, excitatory-surround receptive field: A centre-surround receptive field
in which stimulation of the centre causes an inhibitory response and stimulation of the
surround causes an excitatory response.
• Centre-surround receptive fields: A receptive field that has a centre-surround
• Centre-surround antagonism: The competition between the centre and surround regions
of a centre-surround receptive field, caused by the fact that one is excitatory and the
other is inhibitory. Stimulating centre and surround areas simultaneously decrease
responding of the neuron, compared to stimulating the excitatory area alone.
• Discovery of centre-surround receptive fields was important because it showed that
neural processing could result in neurons that respond best to specific patterns of
• Due to centre-surround antagonism, a neuron responds best to a spot of light that is the
size of its excitatory centre of the receptive field.
• Centre-surround receptive fields are created by the interplay between excitation and
Hubel and Wiesel’s Rationale for Studying Receptive Fields
• Instead of shining light directly into an animal’s eye as Hartline did, Hubel and Wiesel
had animals look at a screen on which they projected stimuli.
• The experiment had a cat or monkey sedated, and their eyes focused with a lens so that
any image on the screen would fall on the back of the retina.
• The eyes of the animal remain stationary, so a spot on the screen corresponds to a point
on the cat’s retina.
• The receptive field is always on the retina. • Lateral geniculate nucleus (LGN): The nucleus in the thalamus that receives inputs from
the optic nerve and, in turn, communicates with the cortical receiving area for vision.
• Cerebral cortex: The 2-mm-thick layer that covers the surface of the brain and contains
the machinery for creating perception, as well as for other functions, such as language,
memory and thinking.
• Occipital lobe: A lobe at the back of the cortex that is the site of the cortical receiving
area for vision.
• Visual receiving area: The area of the occipital lobe where signals from the retina and
LGN first reach the cortex.
• Superior colliculus: An area in the brain that is involved in controlling eye movements
and other visual behaviours. This area receives about 10% of the ganglion cell fibres that
leave the eye in the optic nerve.
• Eye → Receptors → Optic nerve → Optic chiasm → superior colliculus
→ LGN → Occipital lobe
• Striate cortex: The visual receiving