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 1

Visual processing

Processing visual information - pathways Retina (vertical pathway) Lateral geniculate nucleus of thalamus (Subcortical areas) Primary visual cortex Secondary visual cortex 2

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) 3

Retinal wiring

Retinal connections (Guyton & Hall) 4

Vertical pathway in the fovea ON and OFF bipolar cells in the dark (Kolb) 5

Vertical pathway in the fovea ON and OFF bipolar cells in light (G protein coupled glutamate receptors can be either excitatory or inhibitory) 6

Vertical pathway in the fovea 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 (Kandel et al) 7

Convergence in the vertical pathway Fovea: no convergence One cone  two ganglion cells Periphery: convergence Many photoreceptors  one ganglion cell 8

Vertical and horizontal pathways Vertical pathway: Transmission of the image Photoreceptorbipolar 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 9

Central visual pathways

Outputs from the retina (Kandel et al) 10

Outputs from the retina LGN: “Relay station” Receptive fields similar to those of ganglion cells Input from each eye into separate layers Right worldleft LGN Left worldright LGN (Guyton & Hall) http://psych.hanover.edu/JavaTest/Media/Chapter3/MedFig.RetinaLGN.html 11

Primary visual cortex

The visual cortex (Guyton & Hall) 12

Medial surface of occipital cortex: large foveal representation Primary visual cortex Medial surface of occipital cortex: large foveal representation (Kandel et al) 13

Edge detection

Edge detection Edge detection: perhaps the most important aspect of image processing Begins with ganglion cells, continues in the cortex 14

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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 18

How the recordings were made 19

Simple cell (Kandel et al) 20

Simple cell A simple cell: responds to a straight edge at a specific angle in a specific position in the visual field (Kandel et al) 21

Colour vision

Spectral sensitivity of rods and cones 22

...depends on sequence differences in opsins 23

Colour “blindness” 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 (Kandel et al) 24

Colour “blindness” Subject can’t distinguish colours in this frequency range 25

Testing colour vision Ishihara test (Kandel et al) 26

Testing colour vision Constructing the Ishihara test 3. Sum of 1 + 2 1. blue-yellow 2. red-green 3. Sum of 1 + 2 27

Depth perception

Binocular disparity (Kandel et al) 28

Binocular disparity neurones (Kandel et al) 29

A pure binocular disparity stimulus (Kandel et al) 30