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  • 1. Outline
    • Announcements
    • Human Visual Information Processing
      • Anatomy of visual system
        • Major steps in human visual information processing
      • Models of some visual processing stages
  • 2. Announcements
  • 3. 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
  • 4. Spectrum
  • 5. Visual Pathway
  • 6. 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.
  • 7. Cross section of a human eye
  • 8. Retina
  • 9. 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
  • 10. Lateral Geniculate Nucleus
  • 11. 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
  • 12. Visual Cortex
  • 13. Primary Visual Cortex
  • 14. Brain Imaging
  • 15. 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
  • 16. Retinal Ganglion Cell Responses
  • 17. 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?
  • 18. Simple Cells in the Visual Cortex
  • 19. 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
  • 20. 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
  • 21. 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.
  • 22. 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
  • 23. Adaptation
    • Adaptation
      • Prior exposure affects the perception of brightness
  • 24. Web’s Law
    • The difference threshold is not constant
    • The difference threshold changes as a function of the magnitude of the standard stimulus
  • 25. Contrast sensitivity function
  • 26. Single Channel or Multiple Channels
  • 27. 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
  • 28. Virtual Contours
  • 29. Reconstruction of Visual Perception
  • 30. Reconstruction of Visual Perception