The visual system

Lecture 1:•Structure of the eye•Photoreceptors: transduction and adaptation

Lecture 2:•Retinal processing•Primary visual cortex: simple and complex cells, edge and feature detection•Colour vision, depth perception

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Visual processing

Processing visual information- pathways

•Retina (vertical pathway)

•Lateral geniculate nucleus of thalamus

•(Subcortical areas)

•Primary visual cortex

•Secondary visual cortex

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Processing visual information- processes

•Contrast and edge detection (retina)•Straight edges, curves and corners (primary cortex)•Colour (retina, secondary cortex)•Movement and complex form (secondary cortex)•Depth (primary and secondary cortex)

Important concept: parallel processing (different aspects of the same image are processed simultaneously by different cortical regions)

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Retinal wiring

Retinal connections

(Guyton & Hall)

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Vertical pathway in the foveaON and OFF bipolar cells in the dark

(Kolb) 5

Vertical pathway in the foveaON and OFF bipolar cells in light

(G protein coupled glutamate receptors can be either excitatory or inhibitory)

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Vertical pathway in the fovea

(Kandel et al)

Fovea: each cone connects to both on and off bipolar cells

+ –

+ +

Connections from bipolar to ganglion

cells are all excitatory

Ganglion cells produce the first

action potentials in the visual pathway

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Convergence in the vertical pathway

Fovea:no convergenceOne cone two

ganglion cells

Periphery: convergenceMany photoreceptors one ganglion cell

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Vertical and horizontal pathways

Vertical pathway:•Transmission of the image•Photoreceptorbipolar cell ganglion cell•Action potentials generated in ganglion cells•Axons of ganglion cells form the optic nerve

Horizontal pathway:•Horizontal cells responsible for lateral inhibition•This allows for contrast and edge detection

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Central visual pathways

Outputs from the retina

(Kandel et al) 10

Outputs from the retina

(Guyton & Hall)

http://psych.hanover.edu/JavaTest/Media/Chapter3/MedFig.RetinaLGN.html

LGN:•“Relay station”•Receptive fields similar to those of ganglion cells•Input from each eye into separate layers•Right worldleft LGNLeft worldright LGN

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Primary visual cortex

The visual cortex

(Guyton & Hall) 12

Primary visual cortex

(Kandel et al)

Medial surface of occipital cortex:

large foveal representation

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Edge detection

Edge detection

•Edge detection: perhaps the most important aspect of image processing•Begins with ganglion cells, continues in the cortex

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Edge detection in primary visual cortex

Various types of cells continue the processing:

we’ll look at one type, the simple cells

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How the recordings were made

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Simple cell

(Kandel et al) 20

Simple cell

(Kandel et al)

A simple cell:•responds to a straight edge•at a specific angle•in a specific position in the visual field

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Colour vision

Spectral sensitivity of rods and cones

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...depends on sequence differences in opsins

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Colour “blindness”

(Kandel et al)

•Genes on X chromosome for L (red) and M (green):recombination may make mutant forms or else cause gain/loss•Males, having only one X, may lack red or green cones

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Colour “blindness”

Subject can’t distinguish colours in this frequency range

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Testing colour vision

(Kandel et al)

Ishihara test

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Testing colour visionConstructing the Ishihara test

1. blue-yellow

2. red-green

3. Sum of 1 + 2

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Depth perception

Binocular disparity

(Kandel et al) 28

Binocular disparity neurones

(Kandel et al) 29

A pure binocular disparity stimulus

(Kandel et al) 30