The relationship of displacement thresholds for oscillating gratings to cortical magnification, spatiotemporal frequency and contrast.

The threshold amplitude for detecting square-wave oscillatory motion of a sinusoidal grating (contrast 0.3) was approximately 14 sec arc for foveal viewing, and was independent of spatial frequency in the range 2-16 c/deg. Threshold displacement amplitude (TD) was greater for eccentric stimuli; but when these were scaled inversely with the cortical magnification factor (M) it was found that TD was equivalent to a constant displacement in cortically-scaled units (0.03 mm). As grating contrast was reduced, invariance of TD over spatial frequency disappeared and consequently for a contrast of 0.03 TD was minimum at 6-8 c/deg. TD was systematically elevated at low contrast and low spatial frequency. Sensitivity to sinusoidal displacements was maximum at 2-10 Hz; for temporal frequencies below 1 Hz, TD was proportional to velocity. The results provide evidence that oscillatory displacement thresholds in the hyperacuity range depend on motion-detecting mechanisms whose spatial dimensions are correlated with M-1.