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. Visual Perception Modeling
Announcements
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Visual Perception Modeling

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
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Visual Perception Modeling

4. Visual Perception Modeling
Spectrum
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5. Visual Perception Modeling
Visual Pathway
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6. Visual Perception Modeling
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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7. Cross section of a human eye
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8. Visual Perception Modeling
Retina
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9. Visual Perception Modeling
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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10. Lateral Geniculate Nucleus
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11. Visual Perception Modeling
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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12. Visual Perception Modeling
Visual Cortex
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13. Visual Perception Modeling
Primary Visual Cortex
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14. Visual Perception Modeling
Brain Imaging
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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
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16. Retinal Ganglion Cell Responses
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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?
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18. Simple Cells in the Visual Cortex
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19. Visual Perception Modeling
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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20. Visual Perception Modeling
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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21. Visual Perception Modeling
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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22. Visual Perception Modeling
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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23. Visual Perception Modeling
Adaptation
Adaptation
Prior exposure affects the perception of brightness
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24. Visual Perception Modeling
Web’s 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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25. Contrast sensitivity function
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26. Single Channel or Multiple Channels
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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
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28. Visual Perception Modeling
Virtual Contours
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29. Reconstruction of Visual Perception
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30. Reconstruction of Visual Perception
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