Motion and vision. IV. Isotropic and anisotropic spatial responses.

When the thresholds for periodic spatial patterns containing two or more differently oriented components (e.g., crossed gratings) are measured under normal, unstabilized conditions, each component seems to be detected almost independently of the others if their angular orientations are sufficiently different. This psychophysical behavior has been attributed to anisotropic or orientation-tuned units in the visual cortex. Here we report that when the image of such a multicomponent pattern is stabilized on the retina, the independent-detection behavior vanishes. Under stabilized-image conditions, the contrast sensitivity is governed by the maximum local contrast at the retina. The number and relative contrast of individual components, even orthogonal ones, behave almost additively in making up the threshold contrast. We confirmed this conclusion with a variety of patterns that give orientation-tuning effects in unstabilized viewing. Controlled image motion (resembling the effect of the natural drifts of the eye) restores the independent-detection behavior in every case, as do other forms of temporal modulation (e.g., flicker or flash presentations). We infer (1) that orientation-tuned units in man do not respond to unchanging stimuli--they cannot function unless the pattern on the retina is temporally modulated, and (2) in the absence of temporal modulation, spatial patterns are detected by isotropic units of relatively low sensitivity.