Stimulus Localisation Flashcards
Remember you have ocular dominance columns, orientation columns and blobs
What is the four type of cell?
Direction columns
Why do we study motion and finding stimulus location together?
Because they are interlinked
What examples are there of objection localisation mechanisms in the visual system?
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
Saccadic movement mechanism - allows for object recognition via object inspection
How does this happen?
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.
Orientation reflex ?
Just when a novel stimulus appears
Animals tends to turn head and eyes for suitable inspection of the organism
What was the experiment on fishs brain which demonstrated the orientation reflexes origin?
Ablation of the optic tectum leads to disappearance of orientation reflex
This is called the superior colliculus in lower vertebrates
What is the mechanism of motion capture?
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
Why do we need motion capture anticipation?
This takes 60ms
So you actually see the recent past
This helps you to see things at really fast speeds.
Areas important to stimulus location and motion processing?
The retina
Dorsal stream in the visual cortex
The superior and inferior colliculi
Where does the superior colliculus receive innervation from?
From ganglion cells, auditory systems and somatosensory systems
It integrates information from different sensory modalities
What is the main role of the superior colliculus?
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
What is a retinotopic map?
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.
What are command neurones?
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.
What happens when you stimulate the upper layers of the superior colliculi? What does the deeper layers of the superior colliculi do?
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.
What are the superior colliculi maps? What is the foveation hypothesis?
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
Disproving the foveation hypothesis?
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.
The dorsal stream? And how do maps relate to this stream?
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
How do we discriminate objects moving in different directions?
Neurones V1 cortex and empty cortex are able to discriminate motion in different directions.
Describe the experiment which shows direction selectivity and orientation:
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.
What can ganglion cells in the retina show?
Direction selectivity
Remember direction selectivity usually happens in the V1 and the LGN
What in the retina helps with direction selectivity? And how can we experiment on this? And what important about morphology of direction selective cells?
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.
What type of input do ganglion cells receive?
They receive excitatory input from bipolar cells. The inputs here a glutaminergic
They receive inhibitory input from amacrine cells
How can excitatory and inhibitory inputs be measured from the same gangion cell?
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.
How do the ganglion cells which are direction selective measure direction?
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
Summary of direction selective movement from ganglions?
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
Describe the flag illusion model?
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.
What is the difference in spiking rate or direction selective gangion, for stationary and moving objects?
Moving objections means ganglion spike earlier when an object moves compared to when its stationary.
