Neural Circuits of Stimulus Localisation

Question 1

What are examples of object localisation in the visual system? (4)

Answer 1

• Orienting reflex
• Smooth pursuit
• Prediction of motion during prey capture
• Saccadic movements during object inspection

Question 2

What is the orienting reflex?

Answer 2

Orientation of the head and eyes to focus a stimulus on the fovea

Question 3

What is smooth pursuit?

Answer 3

Following a moving object

Question 4

What does ablation of the optic tectum/superior colliculus result in?

Answer 4

Loss of the orienting reflex

Question 5

Is the orienting reflex present at birth?

Answer 5

No, develops as babies get a bit older

Question 6

Why do we need motion anticipation?

Answer 6

Phototransduction takes a relatively long time (in terms of being hunted by predators)

Question 7

Which areas of the brain are involved in stimulus localisation and motion processing? (3)

Answer 7

• Retina (orientation selective ganglion cells, Magnocellular cells)
• Dorsal stream in the cortex
• Superior and inferior colliculus

Question 8

What are the inputs into the superior colliculus? (3)

Answer 8

• Ganglion cells
• (Auditory system)
• Somatosensory system

Question 9

What is the function of the superior colliculus? (2)

Answer 9

• Integrates information from different sensory modalities
• Regulates saccadic movements

Question 10

What is the structure of the superior colliculus? (4)

Answer 10

• Retinotopic map
• Organised into layers based on where the areas receive input from and where they send output to
• Superficial layers receive input from the retina
• Inputs from the cortex (V1, somatosensory) do processing

Question 11

What is a retinotopic map?

Answer 11

Organisation whereby neighbouring cells in the retina feed information to neighbouring places in their target structures

Question 12

Which areas of the brain have retinotopic maps? (3)

Answer 12

• Visual cortex
• LGN
• Superior colliculus

Question 13

Which brain areas are involved in orientation during hunting in zebrafish? (5)

Answer 13

• Retina
• Optic tectum
• Pretectum
• Hindbrain
• Reticulospinal system signalling causes movement towards the stimulus

Question 14

Which pathway is responsible for processing motion in mammals?

Answer 14

Dorsal stream

Question 15

What is the dorsal stream pathway for processing motion? (7)

Answer 15

• Magnocellular ganglion cells
• LGN
• V1 cortex
• V2 cortex
• V3 cortex
• Medial temporal cortex
• Parietal cortex

Question 16

What does a stroke in the medial temporal (MT) cortex cause? (2)

Answer 16

• Difficulty perceiving motion
• Objection recognition and colour perception intact (ventral stream)

Question 17

Where does direction selectivity occur?

Answer 17

On-off direction selective (DS) cells in the retina only respond to movements in specific directions

Question 18

How does the morphology of DS cells link to their function?

Answer 18

They have highly asymmetric dendritic trees so motion in the same direction as the dendritic tree will stimulate the cell

Question 19

What inputs do retinal ganglion cells receive? (4)

Answer 19

• Excitatory input from the bipolar cells
• Inhibitory input from the amacrine cells
• Interaction occurs in the inner plexiform layer (IPL)
• Bipolar cells activate amacrine cells, amacrine cells inhibit bipolar cells

Question 20

Where does direction selectivity arise in the retina?

Answer 20

Inner plexiform layer (IPL)

Question 21

How does direction selectivity arise in the IPL? (4)

Answer 21

• Direction selective (DS) ganglion cells receive excitatory input from bipolar cells and inhibitory input from amacrine cells
• In the DS preferred direction excitation is large, inhibition is small and delayed
• In the null direction excitation is small and delayed, inhibition is large
• Summation causes greater depolarisation in the preferred direction and less in the null direction