THE human visual system detects discontinuities in the spatial or temporal pattern of light falling on the retina; it is relatively poor at detecting steady or slowly changing patterns in either domain. These insensitivities to low rates of change in space and time are not independent: a gentle gradient of illumination can be made more visible by moving it, or modulating its brightness in time. Patterns of alternating light and dark bars with a sinusoidal luminance profile across the bars - sinusoidal gratings - are commonly used to study spatial interactions in the visual system. Some of the earliest workers to use these stimuli noticed that human sensitivity to steady sinusoidal gratings of low spatial frequency is poor, but is markedly enhanced if the grating is moved or modulated in time1,2.
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