Processing visual information - pathways Retina (vertical and horizontal pathways) Lateral geniculate nucleus of thalamus (Subcortical areas) Primary visual cortex Secondary visual cortex 1

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) 2

Retinal wiring 3

Retinal connections (Guyton & Hall) 4

LIGHT  Photoreceptor hyperpolarisation Visual transduction REMEMBER LIGHT  Photoreceptor hyperpolarisation 5

Vertical pathway in the fovea ON and OFF bipolar cells in the dark (Kolb) 6

Vertical pathway in the fovea ON and OFF bipolar cells in light (G protein coupled glutamate receptors can be either excitatory or inhibitory) 7

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) 8

Convergence in the vertical pathway Fovea: no convergence One cone  two ganglion cells Periphery: convergence Many photoreceptors  one ganglion cell 9

Function of horizontal cells Effects of light on cone connecting directly to an “on” bipolar cell: Cone is hyperpolarised Bipolar cell is depolarised Ganglion cell is depolarised – 10

Function of horizontal cells Effects of light on area surrounding an “on” bipolar cell: Edge cone is hyperpolarised Horizontal cell is hyperpolarised Centre cone is depolarised Bipolar cell is hyperpolarised Ganglion cell is hyperpolarised  – –  + Light on edge of receptive field antagonises light on centre Horizontal cells produce surround inhibition 11

Result: ganglion cell receptive fields (Kandel et al) 12

Result: ganglion cell receptive fields (Kandel et al) 13

Result: ganglion cell receptive fields Responds poorly to diffuse light (Kandel et al) 14

Central visual pathways 15

Outputs from the retina (Kandel et al) 16

Outputs from the retina Pretectum Pupil constriction/ dilation Superior colliculus Eye movement in pursuit of objects Lateral geniculate nucleus (LGN) Perceived visual image (relay station in cortical pathway) (Kandel et al) 17

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 18

Vertical specificity of pathways from retina to cortex (Kandel et al) 19

Effects of visual pathway damage (Kandel et al) 20

Primary visual cortex 21

The visual cortex (Guyton & Hall) 22

Medial surface of occipital cortex: large foveal representation Primary visual cortex Medial surface of occipital cortex: large foveal representation (Kandel et al) 23

Primary visual cortex Area of cortex devoted to fovea is huge... because High convergence in periphery - no convergence in fovea (Mountcastle) 24

Edge detection 25

Edge detection Edge detection: perhaps the most important aspect of image processing Begins with ganglion cells, continues in the cortex 26

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Edge detection begins in the retina Ganglion cell responds best when the edge just touches the central region of the receptive field (Kandel et al) 30

Edge detection in primary visual cortex Three types of cells continue the processing: Simple cells Complex cells End-stopped complex cells 31

How the recordings were made 32

Simple cell (Kandel et al) 33

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) 34

Varieties of simple cell receptive field (Kandel et al) 35

Constructing the simple cell receptive field LGN neurones Simple cell (Kandel et al) 36

Complex cell (Kandel et al) 37

Complex cell 38

Complex cell A complex cell: responds to a straight edge at a specific angle in any position within a large receptive field The edge stimulus can continue beyond the edge of the receptive field (Kandel et al) 39

End-stopped complex cell Like a complex cell... ...but inhibited if a straight edge stimulus continues beyond the receptive field Not inhibited by corners (Hubel) 40

Edge detection in primary visual cortex Simple cells Straight edges in a specific place Complex cells Straight edges within a defined region End-stopped complex cells Corners and curves 41

Colour vision 42

Colour opponent ganglion cells see http://psych.hanover.edu/JavaTest/Media/Chapter6/MedFig.EarlyColorVision.html 43

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) 44

Colour “blindness” Subject can’t distinguish colours in this frequency range 45

Testing colour vision Ishihara test (Kandel et al) 46

Testing colour vision Constructing the Ishihara test 3. Sum of 1 + 2 1. blue-yellow 2. red-green 3. Sum of 1 + 2 47

Simulated colour “blindness” Normal Protanope (no red cones) 48

Simulated colour “blindness” Normal Deuteranope (no green cones) 49

Simulated colour “blindness” Normal Tritanope (no blue cones) 50

Depth perception 51

Monocular depth cues (Kandel et al) 52

Monocular depth cues 53

Binocular disparity (Kandel et al) 54

Binocular disparity neurones (Kandel et al) 55

A pure binocular disparity stimulus (Kandel et al) 56