W2 L1 - From retina to cortex

Ambiguity

  • there is inherent a,biguity in the visual signals our eyes recieve


Visual system

  • The Eye

  • pupil - where the light enters eye

  • iris - adjusts aperture, constricts in briglight to make pupil smaller

  • cornea and lens - focuses light on retina

  • accomodation - ciliary muscles change shape of lens to bring objects into focus at different distances


Retina

  • phtoreceptors - cells with light sensitive phoyopigments in outer segments

  • rods - contain rhodopsin, respond in dim light, none in fovea

  • cones - 3 types with photogipments sensitive to different wavebands - long, medium or short - daytime vision

  • last stage in retina processing:

- large parasol ganglion cells - extensive connections,ger area in retina

- small midget ganglio cells - small connection, cover smaller area of vision

- cells have ‘receptive fields’ - the part of the retina from which the genglion cells recieves input


Ganglion cell receptive fields

  • single cell recording from retinal ganglion cells

  • no light - baseline activity

  • light fills RF - baseline

  • light fills center - activity increases

  • light fills surround - activity decreases

  • poor at spotting gradual change

  • good at picking out sharp edges

  • filters the input for useful information


Lateral geniculate nucleous

  • deals with visual signals

  • koniocellular cells - in between layers, blue-yellow

  • mifget (parvocellular) - colour and detail

  • parasol (magnocellular) - movement and flicker


Visual Cortex

  • 50% dedicated to vision

  • primary cortex/ V1/ striate cortex


Retinotopy

  • parts of the cortex whose receptive field is a particular point in space

  • can be mapped - what part of the visual cortex is reponding to the visual field

  • v1 - encodes orientation


role of development / critical periods

  • relevance of neuron orientation selectiovity is shapped by environment experienced early on in life

  • cats experiment:

  • kittens raised in tubes - vertical and horizontal stripes

  • 5h per day in tube

  • 5m later - no response to orientation not in tube

  • recorded from cells in visual cortex - no neurons that respond to orientation absent in tube

  • critical period


cortex pathways

  • dorsal - movement

  • ventral - colour