Lecture 2 - The brain

Question 1

Ganglion cells?

Answer 1

• each ganglion cell receives input from 126 photo receptors - input is not simply added but either excites or inhibits a ganglion cell
• this depends on where in a ganglion cell’s receptive field the stimulus is located

Question 2

What is the receptive field of a neuron?

Answer 2

the part of the visual field in which a stimulus can modify the neurons firing rate

Question 3

What is the excitatory area (+) of a ganglion cell?

Answer 3

• ‘on’ area
• increases the firing rate of a neuron when stimulated

Question 4

What is the inhibitory area (-) of a ganglion cell?

Answer 4

• ‘off’ area
• decreases the firing rate of a neuron when stimulated

Question 5

What are the 3 main types of ganglion cells?

Answer 5

1. magnocellular (M) cells
   - mostly input from rods
   - not colour specific
2. parvocellular (P) cells
   - input from single M or L cones
   - colour specific (green or red on/off)
3. koniocellular (K) cells
   - excitatory input from s cones
   - inhibitory input from M and L cones (blue on)

Question 6

Visual cortex?

Answer 6

• primary visual cortex = V1
• extra striate visual areas = V2, V3, V4, V5
• as visual information travels through the cortex there is ongoing neural convergence
• the neurons receptive fields increase
• visual information gets more complex with ongoing integration

Question 7

V1?

Answer 7

• striate cortex (stripy appearance)
• some V1 neurons are orientation selective =
  -> they respond when a stimulus in their receptive field matches their preferred orientation
  -> they have elongated receptive fields to capture edges in a particular orientation
• the less the stimulus matches the preferred orientation the lower the neurons firing
• other neurons are motion direction selective = they respond when a stimulus in their receptive field matches their preferred motion direction
• other V1 neurons are selective for colour and brightness

Question 8

V2?

Answer 8

• V2 receptive fields are about twice as large as V1 receptive fields
• V2 neurons respond to basic stimulus features such as orientation, motion direction and brightness
• they also respond to more complex features such as length, angles, arcs etc.

Question 9

Evidence for 2 separate streams?

Answer 9

• the parietal (dorsal) stream processes object locations (where)
• the temporal (ventral) stream processes object identities (what)
• moneys with parietal (where) lesions could distinguish a cube from a triangle (object discrimination) while monkeys with temporal (what) lesions could not
• monkeys with temporal (what) lesions could learn the positions of an object (location discrimination) while monkeys with parietal (where) lesions could not

Question 10

V3?

Answer 10

• is in the parietal/ dorsal (where) stream
• receptive fields are 5x larger in V3 than V1
• integrated information over a large area of the retina
• perfect for motion perception (where is an object)

Question 11

V4?

Answer 11

• is in the temporal/ ventral (what) stream
• receptive fields are 5x larger in v4 than in v1
• V4 neurons respond to object-defining features (what is an object) such as colour, orientation, complex shapes, texture