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Scotopic vision Vision at the low ambient light levels typical of nighttime, mediated by rods Photopic vision Vision at the high ambient light levels typical of daylight, mediated by cone photoreceptors Mesopic vision Vision at ambient light levels intermediate between photopic and scotopic levels, typical of dusk, mediated by both rods and cones Luminance grating A laboratory stimulus used to study spatial vision; it is a striped pattern containing alternating light and dark bars, commonly with a sine wave Spatial frequency A measure of the fineness of a grating's bars, in terms of the number of grating cycles (bright-dark bar pairs) per degree of visual angle Spatial contrast threshold The minimum contrast between the lightest and darkest parts of a pattern required for it to be reliably detected by an observer; lower values indicate better performance. Spatial contrast sensitivity The reciprocal of spatial contrast threshold (1/threshold); higher values indicate better performance Contrast sensitivity function A graph of spatial contrast sensitivity to luminance gratings, plotting sensitivity as a function of grating spatial frequency. Contrast ratio The ratio between the amount of contrast in an optical images, and the contrast in the original stimulus; values near unity indicate near-perfect transference. Optical transfer function A graph of an optical system's ability to transfer luminance gratings, plotting contrast ratio as a function of grating spatial frequency. Temporal contrast sensitivity The reciprocal of the amount of contrast between the brightest and darkest phases of a flickering stimulus required for a subject to detect the flicker Temporal frequency The alternation rate of a flickering stimulus, measuring in hertz, or the number of flicker cycles (bright-dark alternations) per second Temporal contrast sensitivity function A graph of temporal contrast sensitivity as a function of flicker temporal frequency Spatiotemporal contrast sensitivity Sensitivity to combinations of spatial modulation and temporal modulation, namely luminance gratings with flickering bars. Space-time plot A polt that represents luminance variations over both space and time, with space plotted horizontally and time plotted vertically. Spatiotemporal CSF A graph of spatiotemporal contrast sensitivity, plotted sensitivity as a function of grating spatial frequency and temporal frequency. Sustained channel A channel of processing in the visual system that is most sensitive at high spatial frequencies and low temporal frequencies Transient channel A channel of processing in the visual system that is most sensitive at low spatial frequencies and high temporal frequencies Threshold elevation The contrast threshold for a test stimulus is higher following exposure to an adapting stimulus than before adaptation Size after-effect A change in the apparent spatial frequency of a test grating following exposure to an adapting grating Tilt after-effect A change in the apparent tilt of a test stimulus following the exposure to an adapting stimulus Masking A rise in the contrast threshold for a test stimulus in the presence of a second, masking stimulus. Vernier acuity An observer's acuity for detecting the direction of small spatial offsets between collinear lines or edges. Hyperacuity Acuity performance in which the observer can detect changes in the spatial location that are smaller than the distance between adjacent retinal photoreceptors. Zero-crossing A point where the value of a function such as a receptive field response changes from positive to negative Peak A point where the value of a function such as a receptive field response reaches its maximum Primal sketch A relatively primitive representation of local spatial structure in an image, produced by the initial stages of visual processing Visual texture Repetitive variation in properties such as local luminance, contrast, size, and/'or orientation to create an extended pattern