Matching velocity in central and peripheral vision.

The apparent velocity of peripheral, drifting sinusoidal gratings was measured as a function of eccentricity and viewing distance. Gratings appeared to move more slowly in the periphery. Apparent velocities in fovea and periphery could be matched by an appropriate spatial scaling of peripheral gratings. This scaling factor provides a psychophysical measure of the changing spatial grain of the visual system with eccentricity. Scaling factors were found to be the same for lower threshold of motion and for velocity matching with standard gratings of 2 and 6 Hz. The finding generalised over a range of standard temporal frequencies (less than 7 Hz) and spatial frequencies (1.2-9 c/deg). The psychophysically determined scaling factors were found to be proportional to the square root of macaque mean cortical receptive field area as a function of eccentricity. The data support a ratio strategy for encoding motion in which motion information is expressed relative to the changing spatial grain of the visual system. Locations for the apparent identity of physically identical grating motion fell along a straight line in space, prompting an explanation of these visual field effects in terms of the acquisition of environmental information from optic flow.