Vision

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Vision

• Without vision, the people perish.

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Visual processes

• The physics of light and eye Transmission
• The retina Transduction and organization
• Sensory receptors No axons
• Graded receptor potentials
• The visual pathways Projection
• Visual cortex Perception

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The physics of light

• Light energy characteristics
• Waves and particles
• Frequency or wavelength
• The visible spectrum 380nm to 760 nm
• Ultraviolet and infrared
• Hue
• Amplitude or intensity Brightness
• Purity Saturation

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Hue, brightness, and saturation
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The structure of the eye and the physics of light
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Physics of light and the visual system
Sensitivity and reliability Accommodation Binocula
r disparity
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The retina Transduction

• Cellular layout in the retina
• Blood vessels
• Ganglion cell layer
• Amacrine cells
• Bipolar cells
• Horizontal cells
• Photoreceptors Rods and cones
• Scotomas The optic disk

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Duplex theory of vision

• Rod Scotopic Dim lightAchromatic
• Cone Photopic Bright light Chromatic
• Physical distribution of rods and cones
• Fovea All cones
• Periphery Fewer cones, maximum rods at 20o
• Nasal vs. temporal hemiretina

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Spectral sensitivity phenomena

• Spectral sensitivity curves
• Scotopic curve peaks at 520 nm (green)
• Photopic curve peaks at 560nm (yellow)
• Purkinje shift

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Spectral sensitivity curves
Photopic (Cones)
Scotopic (Rods)
380 450 500 550 600 650 700 750
780
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Habituation

• Saccadic movement
• Stabilizing the retinal image

Mounting an LED or a miniature projector on a
contact lens produces a fixed retinal image.
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Transduction in rods Overview

• Photoabsorption by rhodopsin in lamellae
• Bleaching of rhodopsin
• 11-cis retinal rod opsin rhodopsin
• all-trans retinal releases rod opsin Bleaching
• Activates transducin and phosphodiesterase
• Cyclic GMP and sodium channels
• Inhibition of release of glutamate NT

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Bleaching of rhodopsin
11-cis retinal
all-trans retinal
Rod opsin
Rod opsin
Light
The bleaching of rhodopsin activates transducin,
a G-protein.
Transducin activates phosphodiesterase.
One phosphodiesterase eliminates over 2,000
molecules of cGMP.
With less cGMP, NA and Ca2 channels close,
hyperpolarizing the rod.
The hyperpolarized rod releases less glutamate
NT Inhibition.
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Transduction in rods Details

• Bleaching of rhodopsin
• 11-cis retinal rod opsin rhodopsin
• all-trans retinal releases rod opsin Bleaching
• Activates transducin, a G-protein
• The a sub-unit (GDP) is replaced by GTP,
  releasing the GDP.
• The freed GDP a sub-unit activates
  phosphodiesterase
• Phosphodiesterase hydrolizes cGMP, which has been
  holding Na channels open
• So, Na channels close.

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What happens next?

• The rod releases less glutamate, an inhibitory
  neurotransmitter.
• Since glutamate normally hyperpolarizes
  (inhibits) the bipolar cells, less glutamate
  relatively depolarizes the bipolar cells.
• Depolarized bipolar cells then release more
  excitatory neurotransmitter on the ganglion cell.
• Light increases firing in the optic nerve.

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Transduction in cones

• Transduction in cones is thought to follow a
  process similar to the bleaching of rhodopsin.
• In each cone, a cone opsin is associated with
  retinal.
• Cyanolabe maximum absorption at 419nm
• Chlorolabemaximum absorption at 531nm
• Erythrolabemaximum absorption at 559nm

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The hemidecussated visual pathways

• Retina-geniculate-striate pathway
• P or parvocellular pathways Retina to LGN
• Outer layers 3 - 6 of dorsal LGN 1,4, 6
  contralateral
• Respond to color and fine detail (Cone input)
• Detect stationary or slowly moving objects
• M or magnocellular pathways Retina to LGN
• Inner layers 1 - 2 of dorsal LGN
• Detect movement
• Mostly rod input
• K or koniocellular sublayers Retina to LGN
• Ventral to layers 1 - 6 of dorsal LGN
• Detect blue cone input.

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Retinotopic specificity and the retinal mosaic

• At the fovea, there is one ganglion cell for each
  cone, contributing to greater acuity.
• At greater distances from the fovea, there are
  more receptors than ganglion cells, leading to
  larger visual fields for each ganglion cell and
  lower acuity.
• Ganglion cells each carry information from a
  piece of the retinal mosaic.

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Visual pathways...

• From the LGN,optic radiations connect to the
  primary visual cortex around the calcarine
  fissure of the occipital lobe.
• The primary visual cortex has a prominent dark
  band of cells, so it is also called the striate
  cortex.
• Retinotopic representation persists at the
  cortex.

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Other visual pathways

• To the SCN of hypothalamus
• To accessory optic nuclei of brainstem and to
  cerebellum Synchronize eye and head movements
• To pretectum to control pupil diameter
• To superior colliculi, for control of visual
  attention
• To ventral LGN, as a relay

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Feature detection

• Edge enhancement Mach bands
• Contrast enhancement Lateral inhibition
• Receptive fields

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Mach bands
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Lateral inhibition

• Edges as contrast elements between lighter and
  darker areas may be enhanced by lateral
  inhibition.
• When one receptor is activated, it inhibits its
  neighbors.

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Lateral inhibition...

• Cells receiving more light on one side of an edge
  are less inhibited by their neighbors receiving
  less light on the other side of the edge.
• Thus, cells on the brighter side of an edge are
  less inhibited and those on the darker side are
  more inhibited, physiologically enhancing the
  contrast across the edge.

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Lateral inhibition...

• Receptors across
  an edge

show the greatest difference in response. The
receptor on the brighter side of an edge is
inhibited by fewer neighbors, while the receptor
on the darker side is inhibited by more.
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Receptive fields

• The piece in the retinal mosaic to which a given
  cell responds.
• Receptive fields for rods and cones are simple
  and round.
• Receptive fields for ganglion cells, LGN cells,
  and cortical cells are complex.

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Receptive fields...

• Complex receptive fields are often doughnut
  shaped, with stimuli in the center producing a
  response opposite to that in the surround.
• Some are center off - surround on.
• Others are center on - surround off.
• Cells with complex on - off fields enhance
  contrast from edges and from movement.