W4 - Visual cortex ✅

Question 1

What is meant by optic nerve, optic chiasm AND optic tract?

Answer 1

Optic nerve: ganglion cells fibres form a bundle -> transmit information from from retina

Optic chiasm = fibres cross over point - left eye sends information to right brain hemisphere and vice versa

Optic tract: information from visual field transmitted in nerve fibres extended from the eye to the brain (LGN) beyond optic chiasm

Question 2

Why is the retinal image inverted and is processed from the opposite hemisphere to the eye?
(not tested, just explanation)

Answer 2

Light refracted because of the eyes’ lens and corneas -> inverted retinal image in optic tract

Optic chiasm -> information from the right visual field processes by the left brain and vice versa.

The brain then corrects and interprets as the correct orientation

Question 3

What is meant by Lateral Geniculate Nucleus? (LGN)

Answer 3

• Bilateral
• Each receives input from both eyes but keep the information separate (relay centre between eyes and V1)
• Similar receptive field as ganglion cells -> can detect spots & edges only

Question 4

What is meant by V1 (primary visual cortex/striate cortex)

Answer 4

• Ocipital lobe
• Receive stimulus from LGN l
• Responsible for processing basic features of the visual scene
• How to measure? Single cell recording

Question 5

What are 4 organisation styles of V1?

Answer 5

-> V1 cells prefer lines of a particular orientation

• Retinotopic mapping (neighboring cells can also detect)
• Cortical magnification (Fovea)
• Orientation columns (same orientations)
• Ocular dominance columns

Question 6

What is meant by retinotopic mapping?

Answer 6

Objects close together in the visual scene are analysed by V1’s neighbouring parts -> group things together

Question 7

What is meant by cortical magnification?

Answer 7

• Retinal image at fovea is processed by more amount of cortex in V1 -> central point
• Fovea accounts for 0.01% of retina but is represented by 8-10% of V1

Question 8

What is meant by orientation columns?

Answer 8

• Same column cells in V1 have same orientation preferences
• Investigated by recording of electrode penetrating the cortex either:
  1. perpendicular to the surface: all cells have same orientation preference
  2. at an angle to the surface: cells’ orientation preferences vary systematically

Question 9

What is meant by ocular dominance columns?

Answer 9

• Some V1 cells (although almost all are binocular) have preference for inputs from one eye = ocular dominance
• Cells with the same ocular dominance (same eye preference) are arranged in columns

Question 10

Receptive field organisation in V1?

Answer 10

3 different types of cell:
1. Simple cells
2. Complex cells
3. Hypercomplex cells

-> receptive fields increase in their complexity

Question 11

Simple cells’ receptive fields in V1?

Answer 11

Similar to ganglion’s cell receptive cells but instead of points, they are elongated

Excitatory responses:
- Perpendicular/perfectly align: big/rapid
- Tilted: some but weaker -> orientation tuning (also response to similar orientation to their preference)
- Horizontal: inhibitory (more stimulation in outer regions)

Sub-types of simple cells:
1. ON-centre vs OFF-centre receptive fields
2. Bar (3 regions) vs Edge detectors (2 regions)
-> But all have orientation preferences

Question 12

Complex cells’ receptive fields in V1?

Answer 12

• Respond to oriented lines but no discrete ON/OFF regions
• Phase insensitive (as long as light hits -> all gives same response regardless of regions)
• Response best to:
  1. Moving bars and edges AND
  2. Particular direction

Question 13

Hypercomplex cells’ receptive fields in V1?

Answer 13

• End-stopped cells
• Respond to bars of BEST fit:
  1. Particular orientation AND
  2. Moving in a particular direction AND
  3. Particular length

Question 14

Neural processing beyond V1?

Answer 14

• Over 30 visual areas
• Seemingly specialised (e.g. V3 - form, V4 - colour, V5 - motion)
• Interconnected areas
• Visual information being sent from V1 is interactive (flows backward and upward):
  1. Dorsal visual stream -> posterior parietal cortex
• determined location and how to act upon object
  2. Ventral visual stream -> to inferotemporal cortex - recognition and discriminating objects

Question 15

Evidence for “what” and “where” pathways?

Answer 15

1. Monkey lesions
   - monkeys that have damage to inferotemporal cortex failed to locate food based on recognition task (triangle blocks vs rectangle blocks)
   - still can perform landmark task (identify food based on location near cylinder)
   - opposite from monkeys with lesion to posterior parietal cortex
2. Neuropsychology
   - Visual form agnosia has damage to ventral pathway -> can’t identify objects despite knowing their features
   - Optic agnosia has damage to dorsal pathway -> can’t reach to grasp object but can identify and describe them