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The visual system II Eye and retina

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The primary visual pathway From perret-optic.ch

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Techniques for understanding vision TMS Lesions and cooling Stimulation electrodes Genetic tools

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Receptive fields The population of stimulus properties that evokes neuronal firing

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Photoreceptor layer Bipolar cell layer Ganglion cell layer ~1 mm LIGHT

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Mller glial cells Franze et al. (2007)

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Photoreceptor layer Bipolar cell layer Ganglion cell layer Phototransduction ~1 mm

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Photoreceptors Contain pigment disks In the dark, receptors are depolarized and glutamate is released constantly. This inhibits some target cells and activates others During illumination, pigments deteriorate and receptors hyperpolarize Thus, glutamate flow decreases, and activation/inhibition of target cells reverses Rods Cones 20 : 1

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Photoreceptors

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Rods Grayscale Long integration time ( ~ 12 Hz) Detect 2% contrast change React to 1 photon Outside fovea Converge on bipolars Dim light Cones Colour Short integration time ( up to 55Hz) Detect 10% contrast change Require several photons Mainly in fovea Often 1 cone to 1 bipolar Precise shape and colour

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Photoreceptor layer Bipolar cell layer Ganglion cell layer 100 000 000 photoreceptors 1 000 000 retinal ganglion cells ~1 mm

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The bipolar layer Transforms and compresses input from photoreceptors to retinal ganglion cells Two layers of lateral interaction: Bipolar cells receive direct excitatory input from 1 cone or several rods and contact retinal ganglion cells Horizontal cells are activated by a large number of photoreceptors and inhibit bipolar cells Amacrine cells are activated by many bipolar cells and inhibit retinal ganglion cells/bipolars These lateral interactions produce e.g. fast gain control

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Lateral inhibition 0.1 5 mm!

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Mexican hat function Inhibition slower than excitation because of transduction through interneurons! How much slower depends on depth in the bipolar cell layer!

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The bipolar layer The bipolar layer has at least 10 sub-layers, 10 types of horizontal cells and over 30 types of amacrine cells Each cell type has different spatial extents (0.1 to several mm) and different temporal properties (transient to sustained) Deeper layers are usually faster As a result, bipolar cells have a graded, linear response with On/Off center and selectivity for stimuli of different spatial/temporal extent

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Photoreceptor layer Bipolar cell layer Ganglion cell layer ~1 mm Phototransduction Center-surround interactions, filtering for different spatial and temporal frequencies, gain control The emergence of the spike

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Retinal ganglion cells

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Retinal ganglion cells Magno- and parvocellular pathway Magno Large receptive fields Transient responses Grayscale Stimulus movement! Parvo Small receptive fields More sustained responses Colour selectivity

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The network of retinal ganglion cells- Synchrony Slow scale (40 100 ms): Shared input from photoreceptors Medium scale (2 40 ms): gap junctions from amacrine cells Fast scale (< 1 ms): gap junctions between ganglion cells Meister & Berry (1999)

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Summary II The retina is one of the most successful circuits in evolution is one of the best-understood examples of a neuronal network contains a large number of different cell types tuned to different spatial and temporal frequencies, including On/Off/On-Off, magnocellular and parvocellular as well as direction selective ganglion cells Contributes crucially to gain control and motion processing favours interdependence and synchrony of individual discharges