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Joe Lappin, Duje Tadin, Sonal S. Patel, Elizabeth A. Killingsworth; Psychophysical receptive fields for motion discrimination depend on contrast. Journal of Vision 2003;3(12):47. doi: 10.1167/3.12.47.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: to determine whether the size of the psychophysical receptive field for discriminating motion direction depends on contrast.
Background: The visibility of motion usually is expected to increase with stimulus contrast, spatial area, and temporal duration — which determine the contrast energy and statistical information for motion. Indeed, contrast thresholds for direction discrimination decrease as spatial area increases, and this quantitative relationship has been used to estimate receptive fields for psychophysically defined motion mechanisms (Anderson & Burr, 1991; Watson & Turano, 1995). Recently, however, we found that these intuitions about visual motion mechanisms are incorrect: Increasing the size and contrast of a moving pattern often makes its direction less visible (Tadin et al., in press). Thresholds for direction discrimination exhibit center-surround antagonism, resembling receptive fields of many neurons in area MT.
Method: Observers discriminated the motion directions (left/right) of brief moving random-pixel arrays displayed in stationary 2D spatial Gaussian windows. Contrast (9 – 92%) and spatial area (2s width = 15 − 360 arcmin) were independently varied. Temporal duration thresholds for direction-discrimination were estimated by adaptive adjustment (Quest) of the width of a temporal Gaussian window. (Light and dark pixels were 3x3 arcmin. The array was shifted by one pixel every 8.3 ms, with a 50% inter-frame correlation between successive arrays.)
Results: For a given contrast, temporal thresholds generally varied as a U-shaped function of size. The principal result was that the optimum size, for minimum threshold, increased as contrast decreased. Increasing contrast also yielded lower minimum duration thresholds and a more rapid increase in thresholds for sizes greater than the optimum.
Conclusion: Sizes of the psychophysical receptive fields for motion discrimination vary inversely with stimulus contrast. Spatial integration and differentiation by elementary motion mechanisms adjust adaptively to changes in contrast.
AndersonS. J.BurrD. C.(1991). Spatial summation properties of directionally sensitive mechanisms in human vision. Journal of the Optical Society of America A, 8, 1330–1339.
TadinD.LappinJ. S.GilroyL. A.BlakeR.(in press). Perceptual consequences of centre-surround antagonism in visual motion processing. Nature.
WatsonA. B.TuranoK.(1995). The optimal motion stimulus. Vision Research, 35, 325–336.
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