• 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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• 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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– 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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• 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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– 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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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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• 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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• 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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• 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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• 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
– Prior exposure affects the perception of brightness
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• The difference threshold is not constant
• The difference threshold changes as a function of the magnitude of the standard stimulus
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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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Reconstruction of Visual Perception
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Reconstruction of Visual Perception
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