Outline

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Outline

• Announcements
• Human Visual Information Processing
• Anatomy of visual system
• Major steps in human visual information
  processing
• Models of some visual processing stages

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Announcements
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Quantum Theory of Light

• Newton proposed that light is a stream of
  particles traveling in a straight line. Each
  particle is called a quantum and each quantum of
  light is a photon. Thus the intensity of light is
  measured in number of photons.
• the visible spectrum is from 380 nm (violet) to
  760 nm (red)
• refraction occurs when light enters a different
  medium causing the velocity of the light to
  change, this change bends the direction of the
  light
• Short wavelengths (violet) of light are refracted
  more than longer wavelengths (red). This is why a
  spectrum is formed from white light passing
  through a prism and it also causes the problem of
  chromatic aberration

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Spectrum
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Visual Pathway
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Visual Pathway cont.

• Vision is generated by photoreceptors in the
  retina, a layer of cells at the back of the eye.
• The information leaves the eye by way of the
  optic nerve, and there is a partial crossing of
  axons at the optic chiasm. After the chiasm, the
  axons are called the optic tract.
• The optic tract wraps around the midbrain to get
  to the lateral geniculate nucleus (LGN), where
  all the axons must synapse.
• From there, the LGN axons fan out through the
  deep white matter of the brain as the optic
  radiations, which will ultimately travel to
  primary visual cortex, at the back of the brain.

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Cross section of a human eye
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Retina
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Retina cont.

• transparent sheet of tissue and composed of 5
  cells types
• photoreceptors - rods and cones
• bipolar cells
• horizontal cells
• amacrine cells
• ganglion cells
• light passes through all the layers of the retina
  before reaching the photosensitive element of the
  photoreceptors
• the photoreceptors are apposed to the pigment
  epithelium which has a rich blood supply to
  provide oxygen for the retina

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Lateral Geniculate Nucleus
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LGN

• the majority of retinal axons terminate in the
  lgn
• the major subcortical relay station for
  processing of visual information
• nuclei in the thalamus, a left and a right lgn
• in primates each lgn has 6 layers
• 4 parvocellular layers and 2 magnocellular layers
  
• the inputs from the 2 eyes remain segregated into
  layers in the lgn
• each layer has an orderly topographic map of the
  visual field
• inputs to a lgn represent the opposite visual
  field

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Visual Cortex
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Primary Visual Cortex
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Brain Imaging
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How to understand the visual perception

• Neurophysiology
• Recording of cell responses
• Functional MRI
• Psychophysics
• Determination of the relationship between the
  magnitude of a sensation and the magnitude of the
  stimulus that gave rise to the that perceptual
  sensation

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Retinal Ganglion Cell Responses
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Retinal Ganglion Receptive Fields

• Has a circular center-surround organization
• Two major classes
• On-center
• Off-center
• How do they respond to a small spot of light?

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Simple Cells in the Visual Cortex
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Simple Cells

• rectangular shaped receptive fields
• segregated ON and OFF zones
• respond to a bright or dark bar
• represent a restricted region in the visual field
  
• respond best to a specific orientation
• non-optimally oriented stimuli will be
  ineffective in stimulating the neuron

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Complex Cells

• larger receptive field than simple cells
• orientation tuned
• ON and OFF zones are mixed in the receptive field
  
• respond well to a moving bar
• direction selective

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Hyper-complex Cells

• receptive field is selective for the length of
  the stimulus
• similar to complex cell receptive fields
  (orientation and direction selective)
• selective for features of shape such as length
  and width of the bar of light.

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Visual Perception

• Modern view is that visual transformation is a
  creative process
• Vision transforms light stimuli on the retina
  into mental constructs of a stable 3D world
• Visual perception is a 3D percept of the world
  that is invariant to a wide range of changes in
  illumination, size, shape, and brightness of the
  image

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Adaptation

• Adaptation
• Prior exposure affects the perception of
  brightness

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Webs Law

• The difference threshold is not constant
• The difference threshold changes as a function of
  the magnitude of the standard stimulus

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Contrast sensitivity function
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Single Channel or Multiple Channels
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Neural Spatial Frequency Channels

• Neural receptive fields are tuned to the spatial
  frequency of the stimulus
• There seems to be a range of neural spatial
  frequency channels, each tuned to a different
  spatial frequency
• A spatial frequency channel can be adapted

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Virtual Contours
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Reconstruction of Visual Perception
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Reconstruction of Visual Perception