G. Dennis Rains,
Kutztown University of Pennsylvania
| achromatopsia | Central (brain-based) color blindness.
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| akinetopsia | Central (brain-based) impairment in the perception of motion.
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| amacrine cells | Cells forming the inner plexiform layer of the retina, between the inner nuclear layer (bipolar cells) and the ganglion cell layer.
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| B cones | Cones most sensitive to 419 nanometer (nm) wavelength light.
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| binding problem | The problem of how the varied aspects of sensory (e.g., visual) input processed in different cortical areas are integrated to yield the coherent percepts and representations that we experience as the external world.
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| binocular disparity | The difference between the images of an object on the two retinas due to the slightly different location of the two eyes rela-tive to the viewed object. Information regarding binocular disparity is critical for stereoscopic depth perception.
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| binocular zone | The area of the visual field that projects to the retinas of both eyes.
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| bipolar cells | Neurons having long processes at both ends, with the cell body at a midpoint. Bipolar cells in the retina connect photoreceptors with ganglion cells, forming the inner nuclear layer of the retina.
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| blind spot | The area within the visual field where stimuli cannot be detected due to the presence of the optic disc.
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| blobs | Areas in V1 that stain darkly for the dye for cytochrome oxidase.
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| coextensive single-opponent cells | P ganglion cells that have an undifferentiated receptive field (no center-surround organization) in which the action of B cones is opposed to the combined action of G and R cones.
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| color constancy | The tendency of a surface to maintain its color across varying conditions of illumination.
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| complex cells | Neurons in the cortex with receptive fields consisting of an edge of illumination in a particular orientation in any portion of the visual field to which the cell is responsive. They have been hypothesized to receive input from a number of simple cells.
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| concentric broad-band cells | Ganglion cells in which R and G cones act together in each area of the receptive field but have opposite action in the two areas. Neurons with these characteristics are also found at higher levels of the visual system.
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| concentric double-opponent cells | Neurons in which R and G cones act together in each area of the receptive field but have opposite action in the two areas. This makes these neurons highly sensitive to the presence of contrasting colors.
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| cones | Photoreceptors in the retina. There are three different types of cones, each responsive to a different range of wavelengths of light. Together, these three cone types form the first stage of a system that makes color vision possible. Cones are found in the center (fovea) of the retina, the area of highest visual acuity in the visual field.
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| contrast enhancement | The phenomenon in which a darker area that is next to a lighter area appears to have an even darker band along the common border, whereas the lighter area appears to have an even lighter band along the common border. Lateral inhibition underlies this effect.
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| cortical module | A unit (or chunk) of cortex that is necessary and sufficient to process information impinging on one discrete point on a sensory surface.
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| cyclic guanosine monophosphate (cGMP) | A molecule synthesized in rods (by the enzyme guanylyl cyclase) in darkness (when the cis form of retinal is bound snugly to a site on the opsin component of rhodopsin).
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| cytochrome oxidase | An enzyme that, when subjected to a specific stain, can serve as a marker of enhanced metabolic activity in neural tissue.
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| dark current | The opening of Na+ channels, with a resulting influx of Na+, in rods in darkness mediated by high levels of cGMP.
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| dorsal hemiretina | The dorsal (upper) half of each retina.
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| dynamic form | Form in motion.
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| extrastriate cortex (prestriate) | Areas of cortex outside V1 that are involved in vision. Also called prestriate cortex.
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| fovea | The area of the retina that has the highest concentration of cones, resulting in the highest visual acuity. In this area the axons of retinal ganglion cells project away from the other cell layers at an acute angle, producing a kind of indentation in the retina and allowing more light to reach the photoreceptors.
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| ganglion cell layer | The layer of ganglion cells in the retina.
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| G cones | Cones most sensitive to 531 nm wavelength light.
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| grandmother cell | A hypothetical type of neuron that has been proposed to be a component in the neural mechanism underlying visual object recognition. According to this view, such cells have highly specific receptive fields (that register the presence of specific objects) because they are the end point in a converging hierarchy of neurons with progressively more specific receptive fields.
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| guanylyl cyclase | The enzyme that mediates the synthesis of cyclic guanosine monophosphate in rods in darkness.
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| homonymous hemianopia | Blindness in the same half (left or right) of the visual field in both eyes.
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| horizontal cells | Cells in the retina that form the outer plexiform layer, between the outer nuclear layer (photoreceptors) and the inner nuclear layer (bipolar cells).
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| human V4 | An area in humans hypothesized to be analogous to V4 in other species.
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| human V5 | An area in humans hypothesized to be analogous to V5 in other species.
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| inner nuclear layer | The layer of the retina composed of bipolar cells.
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| inner plexiform layer | The layer of cells in the retina between the inner nuclear layer (bipolar cells) and the ganglion cell layer, composed of amacrine cells.
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| interblobs | Areas of V1 that stain lightly for the dye for cytochrome oxidase. These areas surround the more darkly staining blobs in V1.
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| interstripes | Areas in V2 that stain lightly for cytochrome oxidase, lying between the thin stripes and the thick stripes. They receive input from the interblobs of V1, and neurons in these areas project to V4.
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| lateral inhibition | The process in which adjacent neural elements hold each other in check. This emphasizes borders between areas of different luminance, thereby accentuating contours.
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| lateral pathway | In the retina, the indirect pathway from photoreceptor to bipolar cell, via horizontal cells.
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| lightness constancy | The perception of lightness and darkness on the basis of differences in the reflectance of surfaces across varying conditions of light intensity (luminance).
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| lightness record | The hypothetical comparison by the visual system of the relative reflectance of different objects for short-, middle-, and long-wavelength light.
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| lumens | Units used to measure luminance (the magnitude of light intensity).
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| luminance | The magnitude of light intensity (measured in units called lumens).
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| Mach bands | Illusory bands that appear along the common border between areas of differing lightness and darkness. When a darker area is next to a lighter area, an even darker band appears in the dark area along the common border, whereas the lighter area has an even lighter band along the common border. These bands are mediated by lateral inhibition.
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| M ganglion cells | One of two types of ganglion cells in the retina. They have large receptive fields, are sensitive to low contrast, are not sensitive to wavelength, and respond in transient bursts. They project to neurons in the magnocellular layers of the lateral geniculate.
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| monocular zones | The two peripheral areas of the visual field, each of which, because of the position of the nose, projects to only one retina.
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| nasal hemiretina | The half of each retina closest to the nose.
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| ocular dominance column | A perpendicular column of neurons in V1 with similar ocular dominance characteristics.
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| ocular dominance hypercolumn | A section of visual cortex containing a set of ocular dominance columns that together represent the full range of ocular dominance characteristics.
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| opsin | The non-light-absorbing component of rhodopsin.
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| optic disc | The area of the retina where ganglion cell axons leave the retina. There are no photoreceptors in this area, so stimuli falling on this area cannot be seen. This results in the blind spot.
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| optic nerve | The nerve composed of axons of retinal ganglion cells (which convey information from the retina). These axons project to the lateral geniculate nucleus of the thalamus. By convention, after the optic chiasm, these axons are termed the optic tract.
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| optic tract | The term applied to the optic fibers (axons of retinal ganglion cells) posterior to the optic chiasm.
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| orientation columns | Columns of neurons in the visual cortex, perpendicular to the cortical surface, each unit of which responds to a bar of illumination of the same orientation.
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| orientation hypercolumn | A section of visual cortex containing a set of orientation columns that together represents the full range of stimulus orientations.
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| orientation-selective | A term referring to the finding that certain cells in the visual cortex have a stimulus at a particular orientation as their receptive field.
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| outer nuclear layer | The layer of cells in the retina composed of photoreceptors.
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| outer plexiform layer | The layer of cells in the retina (between the outer and inner nuclear layers) composed of horizontal cells.
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| parvocellular-blob channel | The pathway from P retinal ganglion cells to the parvocellular layers of the LGN to layer IVCß of V1 to the blobs of V1 to the thin stripes of V2 to V4. Processing in this channel is critical for the construction (perception) of color.
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| parvocellular-interblob channel | The pathway from P retinal ganglion cells to the parvocellular layers of the LGN to layer IVCß of V1 to the interblobs of V1 to the interstripes of V2 to V4. Processing in this channel is important for the perception of form.
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| P ganglion cells | One of two types of ganglion cells in the retina. They have small receptive fields, respond with sustained discharge, and are sensitive to wavelength. They project to neurons in the parvocellular layers of the lateral geniculate nucleus.
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| photoreceptors | Cells in the retina sensitive to light.
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| phototransduction | The process whereby the presence of light is coded (represented) in the form of changes in photoreceptor activity.
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| pigmented epithelium | The layer of cells in the back of the eye in which the photoreceptors are embedded.
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| primary visual cortex | Area in the occipital lobes that receives the primary projection from the lateral geniculate nucleus of the thalamus. Also called V1.
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| R cones | Cones most sensitive to 559 nm wavelength light.
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| receptive field | The stimulus characteristics that cause an individual neuron to fire maximally or minimally.
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| reentrant connections | Return connections (connections from one structure back to a structure from which it has received input).
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| reflectance | A measure of the fraction of the light illuminating a surface that the surface reflects.
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| retinal | The light-absorbing component of rhodopsin.
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| rhodopsin | The visual pigment in rods.
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| rods | Photoreceptors in the retina sensitive to dim light but not to color. They are found in the periphery of the retina.
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| temporal hemiretina | The half of each retina closest to the temple.
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| thick stripes | Thickly striped areas in V2 defined by the cytochrome oxidase staining technique. Thick stripes receive input from layer IVB of V1 and project to V3 and V5.
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| thin stripes | Thinly striped areas in V2 defined by the cytochrome oxidase staining technique. Thin stripes receive input from the blobs of V1 and project to V4.
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| ventral hemiretina | The ventral (lower) half of each retina.
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| vertical pathway | In the retina, the direct input from photoreceptor to bipolar cell.
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| visible spectrum | The band of wavelengths of electromagnetic radiation that humans are able to detect (400--700 nm).
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| visual agnosia | Impairment in the recognition of aspects of the visual world not due to an impairment in elementary components of vision, such as visual acuity.
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| visual association cortex | Areas of cortex outside V1 that are involved in vision. These areas are now more frequently referred to a extrastriate cortex or prestriate cortex.
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| visual hemifield | Half (left or right) of the visual field. Typically the terms right visual field and left visual field are used to mean right and left visual hemifield, respectively.
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| visual pigment | A molecule in a photoreceptor that changes its conformation when it absorbs (is activated by) a specific range of wavelengths of light. This is the only light-dependent step in vision. The three types of cones and the rods each have visual pigment with different absorption characteristics.
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| void condition | The condition in which a surface is viewed in isolation (without view of its surround).
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| Young-Helmholtz trichromatic theory of color vision | The theory that color vision is mediated by the three types of cones.
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2002 McGraw-Hill Higher Education