L8 - Stimulus Location and Processing of Motion Flashcards
What are the 4 examples of object localisation in the visual system?
Orienting reflex
Smooth pursuit
Prediction of motion during prey capture
Saccadic movements during object inspection
What is the orienting reflex?
Orientation of the head and eyes to focus salient stimulus on the fovea
When a novel stimulus appears animals turn their heads and eyes to allow inspection of stimulus
What is smooth pursuit?
Following moving object
What is prediction of motion during prey capture?
Motion anticipation
What is saccadic movement during object inspection?
Eyes move all the time
Some parts of the object are observed more than others
Ablation of what leads to disappearance of orienting reflex?
Optic tectum
Superior colliculus in lower vertebrates
Why do we need motion anticipation?
- 1 photon absorbed by 1 opsin
- 800 transducin molecules
- 800 PDE enzymes
- 4800 cGMP converted to GMP
- 200 cGMP sensitive ion channels close
- Causes hyperpolarisation and decrease in glutamate release
This process takes 60 ms
What areas are involved in stimulus localisation and motion processing?
Retina - orientation selective ganglion cells and motion anticipation
Dorsal stream in the cortex
Superior and inferior colliculus
What does the superior colliculus receive input from?
Ganglion cells
Auditory system
Somatosensory system
Where does the superior colliculus integrate information from?
Different sensory modalities
What is the function of the superior colliculus?
Regulation of saccadic movements
What do lesions of the superior colliculus lead to?
Disappearance of orienting reflex
What are retinotopic maps in the retina?
Organisation where neighbouring neurons in the retina feed information to neighbouring places in target structures
- Lateral geniculate nucleus, superior colliculus and visual cortex
How are eye saccades regulated?
Via command neurons
Where are command neurons found?
Deeper layers in superior colliculus contain neurons that spike before the saccadic movements
How are the deep layers of superior colliculus neurons organised?
Organised in maps (similar to retinotopic maps)
Send projections to layers regulating eye movements
Maps of the retina and superior colliculus are aligned
What is the foveation hypothesis?
Interaction between maps initiates orienting reflex
Whys is the foveation hypothesis wrong?
Interaction between maps seems to be indirect
Along what pathway does processing of motion in higher areas occur?
Dorsal stream
Dorsal stream pathway
- Magnocellular ganglion cell
- LGN Magn
- V1 cortical area
- V2 cortical area
- V3 cortical area
- Middle temporal visual area
- Parietal cortical area - parietal pathway (where)
Along the dorsal stream pathway the neurons are?
Motion sensitive
Have retinotopic organisation
Why we need motion perception example
A patient following a stroke had difficulty perceiving certain forms of motion
Object recognition and colour perception were intact
She couldn’t see coffee flowing into the cup - didn’t perceive fluid rising
- Coffee often spilt
Motion detection and direction selectivity
Movement of a bar in a particular direction causes a much bigger response than a bar moving in the other direction
What is preference direction?
Direction that produces the biggest response
What is null direction?
Direction that produces the smallest response
Where is direction selectivity in the retina?
Typical On/OFF cells
Responds to movements in some directions but not others
What is the morphology of direction selectivity cells?
Highly asymmetric – only have neurons in one direction
Preferred direction can be guessed from the morphology
Retinal ganglion cell inhibitory input?
Amacrine cells
Retinal ganglion cell excitatory input?
Bipolar cells
Direction selective cells receive input from?
Excitation from bipolar cells
Inhibition from amacrine cells
Direction selective On-OFF cell movement in preferred direction?
Excitation is larger
Inhibition is smaller and delayed
Direction selective On-OFF cell movement in null direction?
Excitation is smaller and delayed
Inhibition is larger
What is the flash lag illusion?
If yellow light is flashed on a blue cell while the cell is moving to the right we perceive it as only the left half of the cell turning yellow
The whole cell actually turns yellow
How does the retina predict the location of moving objects?
In a flashed object - Spike rate reaches greater Hz - Response is delayed In moving object - Spike rate reaches lower Hz - Response is not delayed
