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Visual Physiology

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Visual Physiology Visual Physiology Perception and Attention Graham Hole Problems confronting the visual system: Retinal image contains a huge amount of information - which must be processed quickly. Retinal image is dim, blurry and distorted. Light levels vary enormously. 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 left visual field is represented in the right hemisphere The eye: 50% of light entering the eye is lost by absorption & scattering (diffraction). The rest strikes retinal photoreceptors. Photoreceptors: Rod Outer Segment Cone The retina contains two kinds of photoreceptors, rods and cones. Inner Segment Pedicle Why are two kinds of photoreceptor needed? 2 S Surface Luminance (cd/m ) c o t o White paper in starlight 0.05 p i c White paper in moonlight 0.5 White paper in artificial light 250 P ho t Computer monitor, TV 100 o p i White paper in sunlight 30,000 c 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. Differences between rods and cones: Sensitivity Number Retinal distribution Visual Pigment Number and distribution of receptors: 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: 420 498 534 564 ce n a b r o s b A d se li a m r o N 400 500 600 Wavelength (nm) All rods contain the same pigment – rhodopsin (" visual purple"). Each cone can contain one of three different pigments – short, medium, or long. How are receptors connected to ganglion cells? Receptors Bipolar, horizontal, amacrine cells Ganglion cells Optic nerve fibres A network of cells processes the photoreceptor responses. Output from the retina is via ganglion cell fibres that form the optic nerve. The layered structure of the retina: 0.1 mm 0.2 mm Cone Bipolar cells Rod Receptor layer connect ‘vertically’ between LIGHT receptors and Outer nuclear layer ganglion cells. Outer plexiform layer Horizontal and Horizontal cell amacrine cells Bipolar cell Inner nuclear layer connect Amacrine cell ‘horizontally’ Inner plexiform layer across the bipolar cell junctions. Ganglion layer Ganglion cell Nerve fibres 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. 4-5 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. On-centre/off-surround retinal ganglion cell receptive field: Retinal Retinal Optic photoreceptors: ganglion cell nerve fibre Excitatory Inhibitory Off-centre/on-surround retinal ganglion cell receptive field: Retinal Retinal Optic photoreceptors: ganglion cell nerve fibre Excitatory Inhibitory Ganglion cell receptive fields: On-centre ganglion cell Roughly circular, with centre-surround organisation. Some cells are excited by light in the centre and inhibited by light in the Stimulus Cell Response surround (on-centre/off surround). Off-centre ganglion cell Other cells are inhibited by light in the centre and excited by light in the surround (off-centre/on- surround). Stimulus Cell Response Features of centre-surround receptive fields: Emphasise edges, discard information from large unchanging (uninformative) areas. Relative light intensity is important for vision, not absolute intensity. Centre- surround RFs signal relative intensity. Lateral Geniculate Nucleus (LGN): Inputs from the two eyes remain separate in layers of each LGN. Retinal 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 Right Visual Field Organisation in the visual pathways Left Left Right Right Temporal Nasal Nasal Temporal Retina Retina Retina Retina Left LGN Right LGN • Each LGN has six layers of Layer 6 Layer 6 cells, segregated by eye (L, Layer 5 Layer 5 R), and by cell type (magno Layer 4 Layer 4 and parvo). Layer 3 Layer 3 Layer 2 Layer 2 Layer 1 Layer 1 • Magno LGN cells project to layer 4Cα in striate cortex, and parvo cells project to Left Eye Right Eye Left Eye Right Eye Column Column β Column Column layer 4Cβ in striate cortex. Layer Layer Layer α Layer α α 4Cα 4Cα 4Cα 4Cα Layer Layer Layer β Layer • Cortical cells with receptive β β 4Cβ 4Cβ 4C 4C fields in one eye are Left Striate Cortex Right Striate Cortex segregated from cells with receptive fields in the other eye (‘ocular dominance columns’). 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 "end-stopped". The primary visual cortex: 1 2 Small complex cell RFs (direction-specific, 3` end-stopped) 4A Simple cells 4B` LGN projections 4C C-S and simple cells Large complex cell RFs 5 (no length summation) Large complex cell RFs 6 (length summation) 1 mm Cortical simple cells: Simple Cell Prefer oriented lines or edges. - + - RF can be mapped into excitatory and - inhibitory regions. + - - + - - + - Stimulus Cell Response Cortical complex cells: 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%) Stimulus Cell Response Direction-selective complex cells: 10-20% of cells in upper cortical layers respond selectively to motion direction. Stimulus Cell Response Organisation in V1: 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. Horizontally across the cortex, orientation preference and eye dominance change in an orderly way. Higher cortical areas: 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). Visual areas in the rhesus macaque: M & P pathways: LGN, LGN, 2 Magnocellular 4 Parvocellular layers layers V1, layer 4c V1, layer 4cb V1, V1, cytochrome interblobs V1, layer 4b oxidase blobs V2, V3, MT (V5) V2, thin stripes V2, pale stripes V4, TE Parietal lobe Temporal lobe "Where" system: "What" system: movement and depth colour and shape.
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