BMS248 Lecture 7 - Stimulus Localisation (Dorsal Stream)

Question 1

What is the orienting reflex? What is the brain area responsible?

Answer 1

The reflexive redirection of head/eyes toward the unexpected stimulus.
Originates in the superior colliculus (optic tectum)

Question 2

What are the areas involved in stimulus localisation and motion processing? (3)

Answer 2

1. Retina - M ganglion cells
2. Dorsal stream in the cortex
3. Superior + Inferior colliculus

Question 3

What is the main function of the superior colliculus?

Answer 3

Regulation of saccadic movements - rapid eye movements

Question 4

What areas/systems does the superior colliculus receive input from? (3)

Answer 4

1. Ganglion cells (retina)
2. Auditory system
3. Somatosensory system
   - Integrates information from different sensory modalities

Question 5

What are 3 features of the superior colliculus

Answer 5

1. Layering: different layers receive input from different brain areas
2. Temporal-Nasal organisation of inputs
3. Retinotopic organisation

Question 6

What is a retinotopic map

Answer 6

Organisation whereby neighbouring cells in the retina feed information to neighbouring places in their target structures e.g. the superior colliculus
Also present in the LGN and Cortex
Mirror image as left visual field goes to right side of brain and vice versa

Question 7

What is the dorsal stream involved in?

Answer 7

Speed, direction, location

Question 8

What is the organisation of the dorsal stream?

Answer 8

1. M-Ganglion cells
2. Magnocellular LGN
3. V1
4. V2
5. V3
6. MT cortex/Middle temporal visual area (V5) - movement
7. Parietal lobe - “where”

Question 9

Which area in the dorsal stream is responsible for the perception of motion?

Answer 9

MT cortex/Middle temporal visual area (V5)

Question 10

What area is responsible for direction selectivity?

Answer 10

V1 cortex
ALSO some direction selective ON/OFF Ganglion cells in the retina
- Respond to movements in some directions but not others

Question 11

What are the dendritic trees like in direction selective ganglion cells in the RETINA?

Answer 11

Highly asymmetric - faces direction of movement it responds to

Question 12

What inputs do direction selective retinal ganglion cells receive?

Answer 12

Both excitatory and inhibitory - inputs can be measured from the same cell

Question 13

What cells do retinal ganglion cells receive EXCITATORY inputs from, in the retina?

Answer 13

Excitatory - Bipolar cells

Question 14

What cells do retinal ganglion cells receive INHIBITORY inputs from, in the retina?

Answer 14

Inhibitory - Amacrine

Question 15

What defines direction selectivity in retinal ganglion cells?

Answer 15

The ratio between excitatory inputs from bipolar cells and inhibitory inputs from amacrine cells

Question 16

When the stimulus moves in the PREFERRED direction, what is the balance between the excitatory and inhibitory inputs?

Answer 16

Preferred direction:
- Excitation is larger
- Inhibition is smaller and delayed

Question 17

When the stimulus moves in the NULL direction, what is the balance between the excitatory and inhibitory inputs?

Answer 17

Null direction:
- Excitation is smaller and delayed
- Inhibition is larger