Week 1 Friday

Outline

• Human Visual Information Processing – cont.– Models of some visual processing stages

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Visual Pathway

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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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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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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

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Exercise #1

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Answers to Exercise #1

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Answers to Exercise #1 – cont.

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Answers to Exercise #1 – cont.

• Paolina Borgheseby Antonio Canova (1757-1822) - Galleria Borghese, Rome