human vision

Question 1

the eye

Answer 1

• Photoreceptors are at the back, therefore the optic nerve creates a blind spot
• Three layers of neurons in the eye.
• Visual processing starts in the eye not the brain

Question 2

colour processing - 3 cone types

Answer 2

• There are 4 photoreceptor types
• Rod system is more sensitive in low light levels
• Colour vision is mediated by RGB cones, such
  colour perception starts with mixed activity of
  the 3 channels.
• Like an inverse TV

Question 3

adaptation

Answer 3

• Photoreceptors adapt to the dark
• Rods become more sensitive
  than cones in the dark
• Peripheral vision can be better
  than central in darkness
• Photo-tranduction ‘bleaches’ the photon sensitive molecules
• Photoreceptors ‘fatigue’ and
  respond less to the same stimuli
  Cone responses

Question 4

retinal ganglion cells

Answer 4

• The axons of the RGCs forms the optic nerve to the brain
• This is a bottleneck, with a
  bandwidth of only ~10Mbps
• There are 10+ types of RGC
• TGC take inputs from
  photoreceptors via bipolar cells.
• But they don’t just add up the
  response of the photoreceptors

Question 5

lateral inhibitation

Answer 5

• Many RGCs show on-off receptive fields
• These are formed by lateral
  inhibition between photoreceptors
• The ‘preferred’ stimuli would be a bright spot
• The optic nerve is transmitting
  signals about visual contrast, not
  simple brightness

Question 6

visual pathway to the cortex

Answer 6

• Half of each optic nerve cross to the contralateral side
• Left visual field is processed on the right and vice versa
• Information goes to Primary visual cortex via the Lateral Geniculate Nucleus (LGN)
  of the thalamus
• Visual information also goes to Superior Colliculus
• SC has a role in attention, eye
  movements and multi-modal integration

Question 7

visual pathways

Answer 7

• In V1 cells respond to contrast
  edges of a particular orientation at a particular region in space
• So called ‘simple’ and ‘complex’
  cells
• Dorsal stream.
  – local/global motion, optic flow,
  heading, self-motion
• Ventral stream.
  – shapes, colour, objects, body parts

Question 8

specialised object regions in the ventral pathway

Answer 8

• Fusiform face area in the fusiform gyrus responds to faces more than other objects
• It responds to the invariant aspects of faces (identity)
• Other face areas extract the variable properties (gaze, emotion etc.)

Question 9

define pareidolia

Answer 9

a psychological phenomenon where people perceive familiar patterns, such as faces or objects, in random or ambiguous stimuli, like clouds or tree trunks

Question 10

visual receptive fields

Answer 10

• Photoreceptors
• Retinal ganglion cells
• Primary visual cortex
• FFA
• Memory centres