Vision

• Light

  • wavelength of light determines colour
  • visible spectrum is only a small range
• The eyes

  • Light enters through cornea, pupil, lens
  • lens focuses light
  • inverts the image
  • why don't we see world upside down? bc brain re-inverts the image
  • this can be learned and unlearned
  • light is focused onto retina
• The retina

  • our retina is inverted
  • rods; very sensitive to light
  • cones; less light sensitive, specific colour for each eye
  • horizontal, bipolar, amacrine cells; collect input from rods and cones, pass to ganglion cells
  • ganglion cells; send message into brain
• The inverted vertebrate retina

  • light sensitive cells at back
  • causes blind spot
  • reduces amount of light
  • why?
  • evolutionary baggage
  • cells help limit damage to rods and cones
  • rods and cones require massive amounts of oxygen for signal amplification
  • blood vessels need to be close
  • blood absorbs light
• The big lie

  • rods and cones actually firing most of the time
  • being hit by light reduces their firing
  • most or all of their synapses onto bipolar and amacrine cells inhibitory
  • process:
  • light hits rod and cone cell
  • cell reduces firing
  • inhibition on bipolar cell decreases
  • bipolar cell fires more
  • the two negatives cancel out
• Neurons?

  • Rods and cones and many other sensory cells are weird:
  • no axons
  • no action potential
  • do release transmitter
  • all the time
  • except when inhibited by light
  • for cones:
  • diff frequencies inhibit more or less
• Perceiving colour

  • perception of most colours depends on the relative firing rates of all the diff cones
  • bipolar and amacrine cells combine diff stimulation to do initial processing of colour (shape, movement)
• Double vision

  • need detailed vision
  • discriminate tiger from a cat
  • need sensitive vision
  • see things fast (processing details takes time)
  • see in dark (details require light)
  • we have 2 kinds of vision:
  • at cellular level
  • rods; sensitive to low levels of light
  • cones; see colour but need light
  • at retina level
  • fovea; detailed, high acuity, lots of cells
  • periphery; mostly rods, low acuity, high sensitivity
  • at brain level
  • 2 diff pathways for processing; fast and detailed
• Visual pathways I

  • visual stimulus in the world
  • enter eyeball, impinges on retina
  • excites rods and cones
  • send signals to bipolar, horizontal cells
  • some processing (edge detection)
  • signals collected in ganglion cells
  • axons exit through blind spot
  • crossover at optic chiasm
  • info from each visceral field goes to contralateral visual cortex
• Visual pathway II

  • after optic chiasm
  • lateral geniculate nucleus (of thalamus)
  • some more processing
  • messages sent on to cortex
  • also received from cortex, for attention focusing
  • primary visual cortex (V1)
• Receptive fields

  • every cell responds to some stimuli
  • rods; very small region of space
  • cones; small region of space and one colour
  • ganglion cells: slightly larger region
  • V1 cells; areas of space, orientations of lines
  • often visuotopically arranged; nearby cells respond to similar orientations
  • this can be changed by experience
  • V2 and V3 cells; movement
  • V4 cells' colour and other things
  • we get representation of the important features of any stimulus
• Complex receptive fields

  • some cells in inferior temporal cortex respond selectively to a particular object
  • independent of orientation, size, lighting
  • cells can respond to:
  • movement, size, colour, shape, orientation
  • specific shapes only
  • parts of object
  • how is this determined?
  • specific sensitivity can be altered by learning
  • evolution has prepared us to pay attention to certain features
• Biological motion

  • objects that move like living things are perceived as the implied object
• Recognizing faces

  • humans are very good at seeing faces from a very young age
  • lots of activity in the fusiform gyrus
  • evolutionary advantage;
  • need to recognize members of our group
  • need to do it fast
  • must correct for ageing, injuries, hair loss
• What makes a face?

  • Damage to the fusiform gyrus causes prosopagnosia
  • inability to recognize faces
  • inability sometimes to tell faces from other objects
  • faces are recognized by the fusiform by configuration
  • upside down faces don't activate the fusiform
  • harder, slower to identify
• Other expertise

  • fusiform also responds to any visual stimulus people are experts at
  • car enthusiasts: can identify faster than other people at identifying cars
  • we need to create the expertise
  • greebles
  • train subjects to identify individuals and families
  • test with inverted or new greebles
  • greeble experts fusiform reacts to greebles
• Ventral and dorsal streams

  • diff aspects of visual stimuli are processed in parallel (at the same time)
  • what the object is; ventral stream
  • where the object is; dorsal stream
  • damaging to different areas leads to diff deficits
  • similar dual pathways exist for other sensory modalities
• Theories of colour vision

  • Trichromatic theory
  • we compare the activation of three cone types
  • does not explain colour afterimages well
  • opponent-process theory
  • has limitations
  • retinex theory
  • its all too complex for the retina, it must be the cortex
• Opponent-process theory

  • colour is detected along 3 axes: blue/yellow, red/green, white/black
  • bipolar cells react to changes in the rate of firing of cells
  • detection of blueness:
  • blue cone synapses onto bipolar cell
  • red/green cones inhibit bipolar cell
  • increase in blue cone firing leads to perception of blue
  • decrease in blue leads to yellow
  • afterimage:
  • blue cells gets tired form lots of firing
  • decreases level of firing, u see yellow
• Inhibition in the retina

  • Horizontal cells are weird
  • no axon, no action potentials
  • functioning:
  • whenever they are excited, they produce inhibition
  • locally; more inhibition closer to where it was excited
  • E.g; light hits rod #8
  • rod 8 excited, fires
  • excites bipolar (B) #8
  • excites horizontal (H)
  • H inhibits nearby B
  • B8 inhibited a lot but also excited
  • B6, B7, B8, B10 inhibited
• Edge detection

  • Light hits rod 6-10
  • B6-10 excited
  • H also excited; releases inhibitions
  • inhibits B6-10 lots
  • B7, B8,B9 inhibited from both sides
  • B6,B10 inhibited form one side only
  • inhibits B4,B5,B11,B12 some
  • final activity
  • B5,B11- least (inhibition only)
  • B7,B8,B9- weak (strong inhibition + excitation)
  • B6, B10- most (medium inhibitions+ strong excitation)