Preferences and asymmetries in saccadic responses to delayed bihemifield stimuli.

Saccaadic response preferences and latencies were measured using dual targets presented at 5, 10 and 15 deg along the horizontal meridian in either one or both visual hemifields. In the unihemifield condition, subjects exhibited a strong preference (about 95%) in favor of the target presented closer to the original fixation point. Further, the presence of the second target did not increase the response latency to the preferred target. In the bihemifield condition using spatially symmetric targets, directional preference varied across subjects from 57 to 100% (mean = 83%). There was also a significant increase in response latency as compared with responses to either single targets or dual targets presented in one hemifield. This latency increase was observed for responses in both the preferred and nonpreferred directions and was taken as evidence of mutual interaction between the two hemispheres. The response preferences could be eliminated by delaying the onset of the preferred target by about 100 msec in the unihemifield condition and by about 50 msec in the bihemifield condition. Further, the interactions between the effects of the targets on response time in the bihemifield condition were found to be asymmetrical in that a target presented in the preferred hemifield has a greater effect than a target in the nonpreferred hemifield on responses to targets presented in the counterhemifield. The results suggest an asymmetry in hemispheric interaction which has not previously been demonstrated in the visualoculomotor system.