Stimulus localisation and processing of motion Flashcards
How are processing of motion and stimulus localisation interlinked?
By identifying the direction of motion - can predict where the stimulus will be in the next moment of time
What are 4 examples of object localisation in the visual system? What are these?
1) Orienting reflex (Orientation of head so that the eyes can focus important stimulus on the fovea)
2) Smooth pursuit (Following a moving object)
3) Prediction of motion during prey capture (motion anticipation)
4) Saccadic movement of the eye (eyes flicker)
What does the orienting reflex happen independently of?
The stimulus
What happens when a stimulus appears in the visual field?
Why?
Orienting reflex:
Move head and eyes so this part of the visual field is focused on the fovea - highest visual acuity
When motion anticipation needed?
Why?
When something is moving fast:
- Speed at which the brain processes information doesn’t allow to follow the stimulus at any moment in time
- Phototransduction cascade takes time
Describe saccadic eye movements during object inspection?
- They are not random
- Some parts are inspected more than others
What does ablation of the optic tectum/superior colliculus result in?
Disappearance of the orienting reflex
What is the optic tectum/superior colliculus?
Optic tectum is the lower vertebrate homologue of the superior colliculus (mammals)
What are the important areas in the brain involved in motion processing?
- Retina - M-type ganglion cells
- Dorsal stream in the visual cortex
- Superior and inferior colliculus
Where does the superior colliculus receive inputs from?
What is the role of the superior colliculus?
ALL different sensory modalities (smell, taste, hear, see, touch)
Role: to INTEGRATE the information from different sensory modalities, in order to make decisions on where to move, issue motor commands
What is the MAIN function of the superior colliculus? (regulate…)
Regulates saccadic eye movements
Orienting reflex
What do lesions in the superior colliculus result in?
Disappearance of the orienting reflex
How are different brain areas organised?
Describe this organisation
Retinotopically:
- Neighbouring cells in the retina feed information to neighbouring places in the target structure
What changes the activity in the brain and why?
Position of the stimulus - activity of the brain is different
Firing in the TECTUM is not random
Where are retinotopic maps present in the brain?
- Retina
- Superior colliculus
- LGN
- Early visual cortex
What do ‘command’ neurons regulate?
Eye saccades
What do command neurons do?
Spike before saccadic movements, in order to regulate them
Where are command neurons present?
In the deeper layers of the superior colliculus
How are the command neurons organised?
What is this similar to?
In maps
Similar to retinotopic maps
Where do command neurons send their projections to?
What does this result in?
Different layers, to stimulate SPECIFIC neurons
Stimulation of a specific neuron leads to the eye shifting to a specific angle
How can processing in the superior colliculus happen, where motor commands are issued?
Describe the processing
Retinotopic map (visual map) is aligned with a deeper motor map:
- Stimulus in the visual field - stimulation of specific visual neurons in the superior colliculus
- If the activation of this neuron is large enough - leads to activation of the lower neurons (in the motor map) which are responsible for moving the eye in that direction
- This activation is via COMMAND NEURONS (send projections from visual map to motor map)
Along the dorsal stream, what neurons become present?
Neurons that respond to moving objects (motion)
Along the dorsal stream, what does movement of a stimulus in different directions cause?
Different responses of the specific neurons
What is the direction of movement that causes the maximum response in a specific neuron?
Preferred direction
What is the direction of movement that causes the minimum response in a specific neuron?
What is this opposite to?
Null direction
Opposite to the preferred direction (90 degree angle)
Where is direction selectivity of neurons first apparent?
In the retina
When do neurons in the retina start spiking/end spiking during a motion?
Start spiking when motion starts
Finish spiking when motion ends
Describe the morphology of direction selective neurons
What can be determined from this morphology?
Highly asymmetric dendrites
Preferred direction of the neurons can be guessed from with direction the dendrites are in
What inputs do retinal ganglion cells receive?
Both excitatory and inhibitory
Where do the excitatory input to ganglion cells come from?
What mediates this?
From bipolar cells
Mediated by glutamate
Where do the inhibitory input to ganglion cells come from?
What mediates this?
From amacrine cells
Mediated by GABA
What inputs do direction selective ganglion cells receive?
Excitatory input from BIPOLAR cells
Inhibitory input from AMACRINE cells
What happens to the activity in the ganglion cell in the stimulus is moving in the PREFERRED direction?
What does this cause?
- Excitation is larger
- Inhibition is smaller and delayed
Causes depolarisation of the membrane to THRESHOLD
Neuron spikes
What happens to the activity in the ganglion cell in the stimulus is moving in the NULL direction?
What does this cause?
- Excitation is smaller and delayed
- Inhibition is larger
Still causes depolarisation but DOESN’T REACH threshold
How are static stimuli and moving stimuli processed in the visual system?
Differently
What cells in the retina predict the location of moving objects?
How?
How is this different for a static object?
Ganglion cells
They begin spiking BEFORE the object approaches the receptive field of the cell
Different for a static object - response is delayed
