Primary Visual Cortex & Beyond

Question 1

What is the Striate Cortex?

Answer 1

another name for the primary visual cortex (V1)

Question 2

What is the Extrastriate visual cortex?

Answer 2

visual areas of the cortex that aren’t primary visual cortex

Question 3

Which neurones would be required to resolve the highest spatial frequencies?

Answer 3

Parvocellular neurones

-have small receptive fields, and so help us see 30 cycles per degree (finest detail we can see)

Question 4

What are the Magnocellular and Parvocellular response properties?

Answer 4

M cells have higher contrast sensitivity, and respond to higher temporal frequencies

• very sensitive to small contrast
• generate fast and transient responses

P cells respond to higher spatial frequencies and to lower temporal frequencies
-see fine detail

Question 5

Describe the M and P pathways up to V1.

Answer 5

M and P neurones are separated in different layers within the LGN, and partially maintained in the primary visual cortex (V1).

The visual cortex is a general scheme. The inputs from sensory systems via the thalamus terminate in layer 4.

However, within the visual cortex, anatomical studies show that axons from the M layers of the LGN terminate in different subdivisions of layer 4 compared with the P layers.

This difference is maintained, to some extent, in onward projections.

Question 6

Describe the Onward projections of M and P neurones from V1.

Answer 6

P and M neurones target different sublayers within layer 4 in the middle of the visual cortex. Then, the onward projections are also partly separate.

M cells project to V2 (secondary visual cortex) and then to area MT. There are also direct M projections from V1 to MT.

P cells project to V2 and then V4. There are also direct P projections from V1 to V4.

Question 7

What is Area MT?

Answer 7

An area of the cortex, at the junction of the parietal and temporal lobes, that receives direct and indirect (via V2) magnocellular input from V1, and appears to be specialised for the processing of visual motion

Question 8

What are Parallel and separate processing of image attributes in different cortical areas?

Answer 8

· colour
· form
· motion
· depth
· orientation

Question 9

How are the visual pathways broadly divided in the monkey brain?

Answer 9

Dorsal pathway which projects onto the parietal lobe

Ventral pathway which projects towards the temporal lobe.

Question 10

How similar are the visual pathways in humans to monkeys?

Answer 10

Very similar

By direct homology with the monkey, you can directly find human V1 homologous to monkey V1, similar for V2, V3 and so on. Slightly trickier as you get to higher visual cortical areas, however there is no doubt that there is this broadly similar functional parcellation into a dorsal (parietal) stream and a ventral (temporal) stream.

Question 11

What did Ungerleider and Mishkin suggest about the visual pathways?

Answer 11

In 1982, Ungerleider and Mishkin suggested:

• dorsal pathways had a spatial function (WHERE)
• ventral pathways had roles in object recognition (WHAT).

This is based upon evidence discovered in lesion experiments (double dissociation between recognition and location)

Question 12

How can we test for functional specialisation?

Answer 12

· Lesion studies
· Neuropsychological assessment following focal brain damage in humans
· Electrophysiology
· Imaging

Question 13

What is the Complex visual response properties of neurones in the inferotemporal cortex (IT)?

Answer 13

· Large receptive fields, sometimes extending across midline, usually including fovea
· Response invariant: retinal size and precise position of stimulus within receptive field doesn’t affect response (different from V1 responses)
· Many cells are ‘face cells’ (neurones responding to faces), but other patterns can be effective, and may be modifiable with training, attention etc.

This is suggestive of the neurone doctrine

Question 14

What is the neurone doctrine?

Answer 14

This is the idea that as you ascend higher and higher in neural pathways, neural responses become more specific and increasingly selective, such that in the end, our actual perception of specific elements, would be caused by the activation of some specific small population of neurones that are absolutely selective for those particular elements

• termed ‘grandmother’ or cardinal cells
• implies population size must decrease as selectivity increases (sparse coding)

Question 15

Motion as a visual sensation: what are the functions of motion in the visual stimulus?

Answer 15

• Draws attention (capturing visual attention)
• Helps figure-ground segregation (movement can help you distinguish figure from ground)
• Location and navigation

Question 16

How do we detect motion?

Answer 16

Reichardt Model

-detects movement across the retina by discriminating between motions in two directions

Question 17

Explain the Reichardt Model.

Answer 17

It consists of two symmetrical subunits. Both subunits have a receptor that can be stimulated by light. In each subunit, when an input is received, a signal is sent to the other subunit. At the same time, the signal is delayed in time within the subunit, and after the temporal filter, is then multiplied by the signal received from the other subunit. Thus, within each subunit, the two brightness values, one received directly from its receptor with a time delay and the other received from the adjacent receptor, are multiplied. The multiplied values from the two subunits are then subtracted to produce an output. The direction of selectivity or preferred direction is determined by whether the difference is positive or negative. The direction which produces a positive outcome is the preferred direction.

Question 18

Which brain region is involved in motion processing?

Answer 18

Area MT (V5)

Question 19

Which pathway is area MT a part of?

Answer 19

V5 (Area MT) which is commonly considered part of the dorsal stream, receiving predominantly but not exclusively magnocellular inputs via specific pathways was one of the first extrasiate visual areas specifically identified with a specific visual function.

-these neurones are excited by motion signals (not orientation, wavelength etc.)

Question 20

Describe Area MT (V5) and motion processing.

Answer 20

Considered part of the dorsal stream:

• receives input from M pathway via V1 and V2
• retinotopically mapped
• cells sensitive to direction and speed of motion
• columnar organisation

Question 21

Many V1 cells respond to orientation of a particular bar, but many also respond to movement at that orientation. Therefore, if we have V1 cells that are sensitive to motion, why do we need area MT/V5 for motion perception?

Answer 21

Not fully known, but the V1 cells aren’t capably of unambiguously signalling complex motion.
-V1 cells cannot determine the actual direction of the real object

The V1 cell is excited in both scenarios (in pic), and the brain cannot figure out the ‘global’ direction of motion. Hence, we need a separate area to determine the direction of motion

Question 22

What is Plaid Gratings and Motion?

Answer 22

· Elemental motion components are the direction of each grating
· Superimposed, there is a global motion component
· Many cells in area MT respond to global motion; all in V1 respond to elemental motion