Visual system 2

Visual system 2

1. What does the visual system do?

   • Localisation of an object or an event
   • Object recognition
   • Identifies the direction of motion and can predict where the object will be in the next moment of time
   • Colour processing
   • Adaptation

2. Adaptation

   • Prevents saturation
     • Visual neuron can be either saturated or not sensitive enough
     • Therefore, it should be plastic
       • Desensitise when contrast increases
       • Resensitise when contrast increases
     • One strategy: increase in gain i.e. overall sensitivity to the stimulus
     • Another strategy: hyperpolarisation
   • To temporal contrast (modulation of light intensity)
     • Recording from ganglion cells
     • Responses to increase or decrease in 
temporal contrast
       • Overall spiking rate gradually decreases after increase in contrast: adaptation. One of the main functions of adaptation is to prevent saturation of visual neurons
   • Two types
     • Do not confuse facilitation and recovery from depression. Recovery from depression occurs when the stimulus decreases in amplitude while facilitation occurs when the stimulus amplitude increases    
     • Depression
       • Decrease in sensitivity
       • Is a result of depletion of a readily releasable pool
         • I.e. depression has synaptic origin
     • Facilitation
       • Increase in sensitivity
       • Is a result of circuit mechanisms
         • Depression of an inhibitory cells leads to sensitisation in other cells

3. Visual scenes

   • Have different properties
     • Different patches of the visual scene differ in average light intensity
     • Different patches of the visual scene differ in modulation of light intensity (contrast)
     • Because eyes constantly move, the retina receives modulation in light intensity

4. Localisation of an object or event

   • E.g.
     • Orienting reflex (orientation of the head and eyes to focus salient stimulus on the fovea)
       • When novel stimulus appears animals tend to turn their heads and eyes in a position suitable for inspection of this stimulus
         • Ablation of optic tectum (superior colliculus in lower vertebrates) leads to disappearance of orienting reflex
     • Smooth pursuit (following moving object)
     • Prediction of motion during prey capture
     • Saccadic movements during object inspection
       • Superior colliculus
         • Receives input from ganglion cells, auditory system, somatosensory system
         • Integrates information from different sensory modalities
         • Main function = regulation of saccadic movements

5. Retinotopic maps

   • organisation whereby neighboring cells in the retina feed information to neighboring places in their target structures (LGN, SC, cortex)
   • Command neurons
     • Regulate eye saccades
     • Have retinotopic organisation
     • Deeper layers in Superior Colliculus contain neurons that spike before the saccadic movements
       • These neurons are organised in maps, similar to retinotopic maps and send projections to layers, regulating eye movements
         • Maps of both layers are aligned
           • “Foveation hypothesis” states that interaction between these maps initiates orienting reflex
             • Interaction between the maps seems to be indirect and foveation hypothesis, therefore, is not valid. Nevertheless, such interaction is a basis of object localisation

6. Motion detection and direction selectivity

   • Types of motion
     • Bar moves in certain direction
     • Approaching motion : looming spot
   • Direction selectivity is evident in the retina
     • Typical ON/OFF cells
       • Responds to movements in some directions but not others
       • Morphology of DS cells
         • Highly asymmetric
         • Prefered direction can be guessed from the morphology
     • DS cells receive excitation from bipolar cells and inhibition from amacrine cells
       • In preferred direction excitation is larger and Inhibition is smaller and delayed
         • In null direction, the excitation is smaller and delayed and inhibition is larger
   • Motion is analysed by the dorsal pathway in the brain
   • Response to moving vs stationary dots
   • Why do we need motion perception?
     • Patient LM who 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 and didn’t perceive fluid rising and often lets coffee spill.
       • “People were suddenly here or there, but I have not seen them moving... When I'm looking at the car first it seems far away, but then when I want to cross the road suddenly the car is very near”
       • Akinetopsia

7. Processing of colour

   • Why do we need colour vision?
     • Helps with object recognition
   • In retina
     • Three types of cones with different spectral sensitivities
       • These types activate different populations of bipolar cells
     • The output of the retina then projects to V1 cortex via LGN
       • Chromatic cells project to specific areas of the cortext called “blobs”
   • Colour opponent ganglion cells

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