We reported previously (Douglas et al, SFN, 2003) that the spatial vision of freely moving rodents can be measured rapidly with head tracking in a virtual optomotor system. In this task, animals usually begin to track soon after the grating starts to move, and the tracking reverses when the direction of rotation is reversed. In most animals, the psychophysical threshold (the point at which tracking stops) recorded for each direction of motion differed slightly. We investigated the nature of this asymmetry and the role of the cortex in the visuomotor behavior in rats and mice first tested as adults. Since occluders are not tolerated well by unrestrained animals, we temporarily sutured each eye closed and tested the acuity and contrast sensitivity of the open eye using both directions of motion. Under these monocular viewing conditions, the animals tracked only when the gratings moved in the temporal-to-nasal direction. The thresholds for temporal-to-nasal motion were identical to those measured with the sam...