W3 - Receptive fields ✅

Question 1

What is the function of retinal ganglion cells?

Answer 1

Condense raw information from photoreceptors, extract important bits then send it to brain

Question 2

What is meant by receptive field and how did psychologists identify it?

Answer 2

Definition: the area on retina when stimulated by light, elicits a change in the firing rate of the cell (detect by using single cell recording)

2 regions: excitatory (increase) and inhibitory (decrease)
- ON centre - OFF surround
- OFF centre - ON surround
-> if both regions stimulated (baseline activity - effects canceled out)

=> Receptive fields of all ganglion cells together cover the whole visual field

Question 3

What is the importance of having receptive fields for ganglion cells?

Answer 3

• Ideal for detecting spots of light & edges but fail to detect orientation of bars (circular receptive field)
• No change in response to different intensity of contrast (as long as there is one)
• Reduce amount of information from retinal image by finding boundaries (contrast-sensitive)

Question 4

How to explain Hermann grid illusion using receptive fields theory?

Answer 4

1. Seeing gray circle in white intersection between dark squares:
   - When receptive fields at intersection, more light falls on OFF-region so receive more inhibition
   - Seen at less bright so perceive a dark spot in the middle
2. When focus on intersection gray circle disappear?
   - Neuro-convergence -> smaller receptive field
   - Full focus at fovea reduces ambiguity

Question 5

How to explain simultaneous contrast illusion using receptive field theory?

Answer 5

• With darker background: less light fall to OFF-surround so ON-centre perceives as brighter (increase response rate) -> lighter square
• With lighter background: more light fall to OFF-surround so ON-centre perceives as darker (decrease response rate) -> darker square

Question 6

What is lateral inhibition and how can it explain receptive field?

Answer 6

Ganglion cell receives converging signal from multiple photoreceptors

Lateral inhibition - the signal from retinal ganglion cells is inhibited when their neighbouring cells (amacrine and horizontal) are stimulated
-> these areas are seen as darker aka OFF-regions

Allow better perception of contrast and boundaries