Week 1 Wednesday

Outline

• Announcements

• Human Visual Information Processing– Anatomy of visual system

• Major steps in human visual information processing

– Models of some visual processing stages

04/11/23 Visual Perception Modeling 2

Announcements


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


Spectrum


Visual Pathway


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.


Cross section of a human eye


Retina


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


Lateral Geniculate Nucleus


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


Visual Cortex


Primary Visual Cortex


Brain Imaging


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


Retinal Ganglion Cell Responses


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?


Simple Cells in the Visual Cortex


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


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


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.


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


Adaptation

• Adaptation– Prior exposure

affects the perception of brightness


Web’s Law

• The difference threshold is not constant

• The difference threshold changes as a function of the magnitude of the standard stimulus


Contrast sensitivity function


Single Channel or Multiple Channels


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


Virtual Contours


Reconstruction of Visual Perception

Week 1 Wednesday

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Transcript of Week 1 Wednesday