Visual Physiology. Perception and Attention. Graham Hole. Problems confronting the visual system: Solutions: The primary visual pathways: The eye:

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1 Problems confronting the visual system: Visual Physiology image contains a huge amount of information which must be processed quickly. image is dim, blurry and distorted. Light levels vary enormously. Perception and Attention Graham Hole Solutions: Enhance the important information (edges and contours). Process relative intensities, not absolute light levels. Use two systems, one for bright light and another for twilight (humans). The primary visual pathways: The right visual field is represented in the left hemisphere. The eye: 50% of light entering the eye is lost by absorption & scattering (diffraction). The rest strikes retinal photoreceptors. The left visual field is represented in the right hemisphere 1

2 Photoreceptors: Why are two kinds of photoreceptor needed? Rod Outer Segment Cone The retina contains two kinds of photoreceptors, rods and cones. Surface White paper in starlight White paper in moonlight White paper in artificial light Computer monitor, TV White paper in sunlight Luminance (cd/m 2 ) ,000 Scotopic Photopic Inner Segment Ambient light levels can vary hugely, by a factor of 10,000,000. Pupil diameter can vary retinal illumination by only a factor of 16. Rods respond at low (scotopic) light levels. Cones respond at high (photopic) light levels. Pedicle Number and distribution of receptors: Differences between rods and cones: Sensitivity Number distribution Visual Pigment There are about 120 million rods, and 8 million cones. Cones are concentrated in central vision, around the fovea. Rods cover the entire retina, with the exception of the fovea. Visual pigments in receptors: How are receptors connected to ganglion cells? Receptors Bipolar, horizontal, amacrine cells Ganglion cells Normalised Absorbance Optic nerve fibres A network of cells processes the photoreceptor responses Wavelength (nm) All rods contain the same pigment rhodopsin (" visual purple"). Each cone can contain one of three different pigments short, medium, or long. Output from the retina is via ganglion cell fibres that form the optic nerve. 2

3 The layered structure of the retina: 0.1 mm 0.2 mm Cone Rod Receptor layer LIGHT Outer nuclear layer Outer plexiform layer Horizontal cell Bipolar cell Inner nuclear layer Amacrine cell Inner plexiform layer Ganglion layer Ganglion cell Nerve fibres Bipolar cells connect vertically between receptors and ganglion cells. Horizontal and amacrine cells connect horizontally across the bipolar cell junctions. Photoreceptors are indirectly connected to ganglion cells: 128 million photoreceptors connect to 1 million ganglion cells (and hence 1 million optic nerve fibres). Each optic nerve fibre is connected to an average of 8 cones or 120 rods. Ratio increases with retinal eccentricity. 45 billion neurones in the cerebral hemispheres are directly involved in visual perception. Acuity and eccentricity: Concept of a "receptive field": Each ganglion cell is connected to many photoreceptors, and hence to a "patch" of retina its "receptive field". Adjacent ganglion cells have overlapping receptive fields. Receptive fields are small in central vision, and progressively larger in peripheral vision. Receptive fields consisting of rods are much larger than RF's consisting of cones. Oncentre/offsurround retinal ganglion cell receptive field: Offcentre/onsurround retinal ganglion cell receptive field: photoreceptors: Excitatory ganglion cell Optic nerve fibre photoreceptors: Excitatory ganglion cell Optic nerve fibre Inhibitory Inhibitory 3

4 Ganglion cell receptive fields: Oncentre ganglion cell Roughly circular, with centresurround organisation. Features of centresurround receptive fields: Some cells are excited by light in the centre and inhibited by light in the surround (oncentre/off surround). Emphasise edges, discard information from large unchanging (uninformative) areas. Offcentre ganglion cell Other cells are inhibited by light in the centre and excited by light in the surround (offcentre/onsurround). Relative light intensity is important for vision, not absolute intensity. Centresurround RFs signal relative intensity. Lateral Geniculate Nucleus (LGN): Inputs from the two eyes remain separate in layers of each LGN. topography preserved in LGN (i.e. spatially preserved maps). Receptive fields similar to those of retinal ganglion cells. From the LGN, fibres project to the primary visual cortex. Two functionally distinct pathways can be identified: Magnocellular (M) and Parvocellular (P). Left Visual Field Left Left Temporal Nasal Retina Retina Left LGN Layer 6 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1 Left Eye Right Eye Column Column 4Cαα 4Cαα Left Striate Cortex Right Visual Field Right Right Nasal Temporal Retina Retina Right LGN Layer 6 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1 Left Eye Right Eye Column Column 4Cαα 4Cαα Right Striate Cortex Organisation in the visual pathways Each LGN has six layers of cells, segregated by eye (L, R), and by cell type (magno and parvo). Magno LGN cells project to layer 4Cαα in striate cortex, and parvo cells project to layer in striate cortex. Cortical cells with receptive fields in one eye are segregated from cells with receptive fields in the other eye ( ocular dominance columns ). Primary visual cortex: The primary visual cortex: Known as Brodmann s Area 17, V1 or Striate Cortex. Located posteriorly in occipital cortex. First stage of cortical processing of the retinal signal. Six (subdivided) layers: LGN cells project to layer 4c. Contains cells with several different types of receptive field: e.g. "simple" cells and "complex" cells, some of which are "endstopped". 1 2 } 3 4A } 4B 4C 5 6 Small complex cell RFs (directionspecific, endstopped) Simple cells LGN projections CS and simple cells Large complex cell RFs (no length summation) Large complex cell RFs (length summation) 1 mm 4

5 Cortical simple cells: Cortical complex cells: + + Simple Cell Prefer oriented lines or edges. RF can be mapped into excitatory and inhibitory regions. Complex Cell Prefer oriented lines or edges. RF cannot be mapped into excitatory and inhibitory regions. + Most common type of cortical cell in primary visual cortex (75%) + Directionselective complex cells: Organisation in V1: 1020% of cells in upper cortical layers respond selectively to motion direction. On a large scale, RF positions are organised topographically. More cortical tissue is devoted to fovea than peripheral retina (cortical magnification factor). Autoradiograph of monkey visual cortex. Organisation in V1: On a smaller scale, cells are arranged in a highly ordered way, in terms of RF orientation and eye preference. In a vertical column through the cortex, all cells have the same preferred orientation and eye dominance. Higher cortical areas: Horizontally across the cortex, orientation preference and eye dominance change in an orderly way. Primary visual cortex is only the first stage of processing. A number of other visual areas have been identified. Processing seems to divide into streams specialised for the analysis of specific image properties: form (V2/V3), motion (MT), colour (V4). 5

6 Visual areas in the rhesus macaque: M & P pathways: LGN, 2 Magnocellular layers LGN, 4 Parvocellular layers V1, layer 4c V1, layer 4cb V1, layer 4b V1, cytochrome oxidase blobs V1, interblobs V2, V3, MT (V5) V2, thin stripes V2, pale stripes V4, TE Parietal lobe "Where" system: movement and depth Temporal lobe "What" system: colour and shape 6

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