Stimulus Localisation

Question 1

Remember you have ocular dominance columns, orientation columns and blobs

What is the four type of cell?

Answer 1

Direction columns

Question 2

Why do we study motion and finding stimulus location together?

Answer 2

Because they are interlinked

Question 3

What examples are there of objection localisation mechanisms in the visual system?

Answer 3

There is the orientation reflex - this reflex allows for the orientation of the head and the eyes to focus an important stimulus on the fovea

The smooth persuit mechanism - allows you to follow a moving object

Motion anticipation prediction mechanism - predicts movement

Saccadic movement mechanism - allows for object recognition via object inspection

Question 4

Saccadic movement mechanism - allows for object recognition via object inspection

How does this happen?

Answer 4

The fovea detects very specific parts of an object

It inspects and object and recognises this. It mainly does this by looking at borders

I.e. if you’re looking at an objects face you look at borders and regions such as the eyes.

Question 5

Orientation reflex ?

Answer 5

Just when a novel stimulus appears

Animals tends to turn head and eyes for suitable inspection of the organism

Question 6

What was the experiment on fishs brain which demonstrated the orientation reflexes origin?

Answer 6

Ablation of the optic tectum leads to disappearance of orientation reflex

This is called the superior colliculus in lower vertebrates

Question 7

What is the mechanism of motion capture?

Answer 7

This is because there is a phototransduction cascade

You see on photon absorbed by one opsin, this activates transducin molecules which activates a PDE enzyme

This causes cGMP to be converted to GMP

CGMP sensitive ion channels then close

This causes hyperpolarisation and decreases glutamate release

Remember glutamate release decrease causes signals to be sent from photoreceptors

Question 8

Why do we need motion capture anticipation?

Answer 8

This takes 60ms

So you actually see the recent past

This helps you to see things at really fast speeds.

Question 9

Areas important to stimulus location and motion processing?

Answer 9

The retina
Dorsal stream in the visual cortex
The superior and inferior colliculi

Question 10

Where does the superior colliculus receive innervation from?

Answer 10

From ganglion cells, auditory systems and somatosensory systems

It integrates information from different sensory modalities

Question 11

What is the main role of the superior colliculus?

Answer 11

Regulation of saccadic movements

So remember this is turning of the eyes to detect key features in the visual field

Lesions of the superior colliculus leads to the disappearance of the orientation reflex

Saccardia region - is responsible for when you turn head and eyes

Question 12

What is a retinotopic map?

Answer 12

This is when a set of cells in the retina supplies information to target structures in the brain

Knowing the retinotopic map for cells related to telling you location helps you to pinpoint where they are.

Question 13

What are command neurones?

Answer 13

These regulate eye saccades

These also have retinotopic like organisation

Cells in the retina send signals to the V1 cortex which help regulate eye movement.

Question 14

What happens when you stimulate the upper layers of the superior colliculi? What does the deeper layers of the superior colliculi do?

Answer 14

Stimulating the upper layers can also stimulate the lower layers of the superior colliculi

The deep layer of the superior colliculi contains neurones which spike before saccadic movements occur.

Stimulating certain motor neurones causes the eye to move.

Question 15

What are the superior colliculi maps? What is the foveation hypothesis?

Answer 15

There are two maps of the superior colliculi

The upper superior colliculi has sensory neurones

The deeper map collections have motor neurones.

These topographic maps overlap…

The foveation hypothesis suggests that interactions between these maps initiates the orientation reflex. This interaction leads to eye rotation

Question 16

Disproving the foveation hypothesis?

Answer 16

Interaction between the maps of the upper and deeper layers of the superior colliculi seem indirect

The interactions are said to be the basis of object localisation.

Question 17

The dorsal stream? And how do maps relate to this stream?

Answer 17

Remember the dorsal stream in ganglion light detections is as follows:

Magnocellular ganglion send info to the LGN, then V1,2,3 neurones. Then the parietal cortex.

Maps:

• the dorsal stream and ventral stream are two different maps
• they both need each other
• ventral stream tells us what we’re seeing - object recognition
• dorsal stream tells us where object is

Question 18

How do we discriminate objects moving in different directions?

Answer 18

Neurones V1 cortex and empty cortex are able to discriminate motion in different directions.

Question 19

Describe the experiment which shows direction selectivity and orientation:

Answer 19

A bar is moved in different directions

Neurones don’t respond when bar moves direction unless its in the right orientation.

But at some point by slowly rotating the object you get the right orientation.

If you move the bar in the opposite orientation then there is no response to the object

The direction that creates the most response is called the preferred direction. The other direction is called no direction.

Question 20

What can ganglion cells in the retina show?

Answer 20

Direction selectivity

Remember direction selectivity usually happens in the V1 and the LGN

Question 21

What in the retina helps with direction selectivity? And how can we experiment on this? And what important about morphology of direction selective cells?

Answer 21

On / off cells

In experiments to test direction selectivity of the retina we can take them out and flatten them

The retina has ganglion cells on top in this instance. Which is useful for patch clamp techniques

Morphology of direction selective cells note are highly asymmetric.

Question 22

What type of input do ganglion cells receive?

Answer 22

They receive excitatory input from bipolar cells. The inputs here a glutaminergic

They receive inhibitory input from amacrine cells

Question 23

How can excitatory and inhibitory inputs be measured from the same gangion cell?

Answer 23

Use patch clamp. Alter membrane potential so that it matches the reversal potential of either AMPA or Gaba receptors

You either record AMPA or gaba currents then to measure excitation and inhibition

Note to record GABA currents you need to clamp the membrane potential similar to the AMPA reversal potential.

Question 24

How do the ganglion cells which are direction selective measure direction?

Answer 24

Remember bipolar cells provide direction selective ganglion cells with excitation

When an object moves in the preferred direction the specific direction selective ganglion spike

When an object moves in the opposite direction in a different orientation the direction selective ganglion receive inhibition inputs from amacrine cells

This means the ganglion no longer spike to the stimulus of the moving object as it isnt in the preferred orientation and direction

Question 25

Summary of direction selective movement from ganglions?

Answer 25

Object moving in preferred direction means excitatory input is high and the inhibitor input is low

And remember bipolar cells provide summative innervation for ganglion cells

Question 26

Describe the flag illusion model?

Answer 26

This is when a circle, circles around a dot in the middle of the screen

There is a flash of light in the centre of the circle that covers the whole circle

However when you stare at the dot it appears that the circle isnt fully covered. Only parts closest to the dot in the centre in the screen

If you look at the circle and not the dot you see the flashing light covering the whole circle

This tells us that we dont predict motion anticipation with the fovea, but we do it with cells which surround the fovea.

Question 27

What is the difference in spiking rate or direction selective gangion, for stationary and moving objects?

Answer 27

Moving objections means ganglion spike earlier when an object moves compared to when its stationary.