Eye movements were recorded with the scleral search coil method while striate cortex (area 17) was stimulated in alert cats with their heads fixed. Regardless of where stimulation was applied in the retinotopic map, eye position at the onset of stimulation strongly affected the amplitudes of evoked saccades, but had much less influence on their directions. Application of long stimulus trains evoked repeated saccades at all sites tested. Highly convergent or goal-directed saccades were not observed. Cortically evoked saccades appeared to habituate with repeated stimulation and had higher thresholds and longer latencies that those reported for saccades evoked from the superior colliculus. The directions of cortically evoked saccades generally agreed with those predicted from the retinotopic coordinates of the stimulus sites, but saccade amplitudes were usually lower than expected. It is suggested that these findings are consistent with certain characteristics of eye-head coordination in the cat's normal visual orienting behavior. The results are difficult to reconcile with the hypothesis that goal-directed saccades are a normal response to targets outside the cat's oculomotor range.

Positive- and negative-contrast stimuli yield the perceptions of brightness and darkness, respectively, and are processed separately by ON and OFF neural pathways. The properties of these morphologically and pharmacologically distinct subsystems were measured in humans by recording visual evoked potentials (VEPs). These electrical responses from the visual cortex were elicited by novel positive- and negative-contrast stimuli, designed to emphasize, selectively, contributions from ON and OFF pathways. Results revealed differential processing of the two types of contrast information, suggesting asymmetries in ON and OFF subsystems; OFF subsystems have finer spatial tuning and greater contrast gain than ON subsystems. These VEPs may be useful in diagnosing neurological disorders that involve primarily one subsystem.