Lecture 8 - Lateral Inhibition

Question 1

The Hermann Grid

Answer 1

Seeing spots at an intersection

Question 2

How does the Hermann grid illusion work?

Answer 2

the neighboring receptors inhibit the target cell, so the sum of the output is less and thus perception is dimmer

dimming of neural activity based on perceptual activity

Question 3

In the Herman grid why does the dim spot fade when you look directly at the
intersection?

Answer 3

The convergence is less in the fovea.

the fovea only has cones and cones have a special one to one relationship with the retinal ganglion cells = high acuity = no lateral inhibition

the cone fires and the cone sends the signal

Question 4

Mach Band

Answer 4

illusory light and dark bands near a border

getting lateral inhibition by the structure of the cells in the retina

Question 5

how Mach Band works

Answer 5

The perception of enhanced 
lightness and darkness at borders is a construction of the visual system.
– Actual physical intensities (distal 
stimulus) indicate that this is not in 
the distal stimulus itself.
– Receptors responding to low 
intensity (dark) area have smallest 
output (and least lateral inhibition).
– Receptors responding to high 
intensity (light) area have largest 
output (and most lateral inhibition).
– All receptors are 
receiving lateral 
inhibition from neighbors
– In purely low and high 
intensity areas, the 
amounts of inhibition at a 
receptor is the same as 
its neighbors. 
– Receptors on the border, 
however, receive 
differing levels of 
inhibition.

Question 6

Simultaneous Constrast

Answer 6

an illusion of changed brightness or color due to effect of adjacent area.
– An area that is of the same physical intensity appears:
• lighter when surrounded by a dark area.
• darker when surrounded by a light area.

effect of inhibition created by the context

Question 7

Receptors stimulated by bright
surrounding area send a large
amount of inhibition to cells in center.

Answer 7

– Receptor cells in the middle are
inhibited, and the resulting
perception is of a darker area than when this stimulus is viewed
alone

Question 8

Receptors stimulated by dark
surrounding area send a small
amount of inhibition to cells in center.

Answer 8

Resulting perception is of a lighter area than when this stimulus viewed alone.

Question 9

Lateral inhibition fail: White’s illusion

Answer 9

Possible explanation is that the bars are perceived more strongly as “belonging to” (rectangular) objects.
– Effect probably occurs in cortex rather than retina.
– Exact physiological mechanism is unknown
– Would likely take research done at Marr’s 3rd level of
analysis to get more raw data, then build a new model for
2nd level algorithm.

Question 10

receptive field

Answer 10

inputs that tell it how much it should be firing

Question 11

The summation of excitatory and inhibitory connections in the
retina creates….for neurons in the visual system.

Answer 11

receptive fields

Question 12

by activating different parts of the receptive field

Answer 12

you can get different firing profiles that might start to correlate to perception

Question 13

receptive field of a particular neuron

Answer 13

is going to be affected by the firing rate of some part of the receptors in the retina

and that field corresponds to some place in the real world