Lecture 8 Study Guide

Question 1

the models of neural convergence and understand how these lead to different levels of acuity vs. sensitivity in circuits of cones and rods.

Answer 1

Rods are more sensitive to 
light than cones
– Rods take less light to 
respond.
– Rods have greater 
convergence which results 
in summation of the inputs 
of many rods onto ganglion 
cells, increasing the 
likelihood of response.
– Trade-off is that rods cannot 
distinguish detail.

All-cone vision in fovea results 
in high visual acuity.
– One-to-one wiring leads to 
ability to discriminate fine 
details.
– However, cones need more 
light to respond than rods. 

“see one light” “see two lights” -> Same distal stimulus –> Different perceptions

Question 2

lateral inhibition

Answer 2

Experiments with eye of Limulus
(Horseshoe crab)

– Operating at Marr’s 3rd level of analysis (implementation) we record signals from receptor A.

– When only receptor “A” is activated with light, the receptor fires rapidly.

– If light is also shown on “B” we see a decrease in activation recorded from“A”.

– Increasing the intensity at “B” reduces activity from “A” even further.

– Signals from neighboring receptors travel across the lateral plexus and laterally inhibit the receptor at “A”.

Question 3

explain phenomena like the Hermann grid, Mach Bands, and simultaneous contrast

Answer 3

The simple notion of lateral inhibition can be used to

explain some aspects of lightness perception.

Question 4

herman grid algorithm

Answer 4

1. Directly shining a light on a receptor sends 100 units of excitatory activity to a target bipolar cell.
2. When each bipolar cell is activated, it sends 10 units of inhibitory activity to neighboring bipolar cells.
3. The target bipolar cell sums this activity from receptor and neighboring cells. This becomes the output of the target bipolar cell.
   - - - - - - - - - - - - - - - - - - - - - - - - -
4. Five receptors are in the visual field. Each receptor synapses on a target bipolar cell.
5. Using our algorithm, we assume that the “A” receptor send 100 units of activity to it’s
   bipolar cell.
6. The 4 neighboring bipolar cells send 10 units of inhibition to “A’s” bipolar cell.
7. The summed output is 60, meaning a dim
   patch will be perceived at that spot.

Question 5

mach bands and lateral inhibition

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 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 contrast

Answer 6

An area that is of the same physical intensity appears:

• lighter when surrounded by a dark area.
• darker when surrounded by a light area.

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

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

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

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

Question 7

How does neural convergence affect processing further on in the brain?

The summation of excitatory and inhibitory connections in the
retina creates….

Answer 7

…. receptive fields for neurons in the visual system.

Question 8

neural convergence –> can lead to lateral inhibition –>

Answer 8

influence the receptive field and thus what we perceive