vision - from retina to cortex

Question 1

what do we do with visual information?

Answer 1

• See facial expressions
  
  • Looking for things
• Perceive information

Question 2

the problem of vision

Answer 2

• There is inherent ambiguity in the visual signals our eyes receive e.g., in shape
  
  • Cars are presented in different shapes depending on the angle you look at it.

Question 3

the eye

Answer 3

• Pupil - where light enters eye and then passes through the lens
  
  • Iris - adjustable aperture, constructs in bright light to make pupil smaller, allows how much light is let in
  • Cornea and lens - focuses light on retina
• Accommodation - ciliary muscles change shape of lens to bring objects into focus at different distances

Question 4

retina and photoreceptors

Answer 4

• Photoreceptors - cells with light sensitive photopigments in outer segments.
  
  • Types of photoreceptors: rods and cones
  • Rods - contain rhodopsin, respond in dim light, none in fovea, cant see colour (colour blind) in raw light
• Cones - three types with photopigments sensitive to different wavebands (long, medium, short) - daytime vision and colour vision

Question 5

retinal ganglion cells

Answer 5

• Sit downstream of the retina
  
  • Important ones are: large parasol and small midget
  • Last stage in retinal processing:
  • Large parasol ganglion cells - large receptor fields
  • Small midget ganglion cells - small receptor fields
  • Cells have ‘receptive fields’ - the part of the retina from which the ganglion cell receives input
• the midget cell goes with parvocellular
• parasol goes with magnocellular

Question 6

retinal ganglion processing

Answer 6

· Poor at spotting gradual change
· Good at picking out sharp edges
· Filters the input for useful information

Question 7

perceptual effects of retinal ganglion cells

Answer 7

· You get dots in your periphery
· There more light falling on the outside of the junction so there’s less inhibition
· Thought to be the explanation for many decades
· Hermann grid? (Baumgartner, 1960)

Question 8

but, is it really due to retinal ganglion cells?

Answer 8

· Wiggly version makes illusion go away (Geier, Bemath, Hudak, Sera, 2008)

Question 9

perceptual effects of retinal ganglion cells

Answer 9

· They can help you detect contrast
· E.g, the two small boxes look different colours, but they are the same
· This is due to one having more light around them
· It is not straight forward and has more going on
- Simultaneous contrast

Question 10

lateral geniculate nucleus (LGN)

Answer 10

· Magnocellular cells - movement and flicker
· Parvocellular cells - colour and detail
· Koniocellular cells - blue-yellow

Question 11

visual cortex

Answer 11

· >50% of cortex dedicated to vision (visual cortex and beyond)
· Primary visual cortex/ V1/ striate cortex

Question 12

role of development/critical periods

Answer 12

· Prevalence of neuron types shaped by environment experienced early on
· Kittens raised in striped tubes from birth, vertical or horizontal stripes
· 5 hours per day in tube
· 5 months later: no response to orientation not in tube
· Recorded from cells in visual cortex: no neurons that respond to orientation absent in tube
· Example of neural plasticity, but with a critical period - ‘use it or lose it’
· (Blakemore & Cooper, 1970)

Question 13

summary

Answer 13

1. Processing of some features (i.e., edges) begins at the retina, and gets more complex “higher” in the visual processing stream.
   
   2. Cells in different parts of the visual stream are selective to different features and take in information from a particular receptive field.
   3. Early levels of visual processing have retinotoptic maps where each part of the visual field is represented.