RIGHTWARD ATTENTIONAL BIAS AND LEFT-HEMISPHERE DOMINANCE IN A CUE-TARGET LIGHT DETECTION TASK IN A CALLOSOTOMY PATIENT

Six normal subjects and a callosotomized man with a prefrontal lesion, mostly on the right side, were tested in a reaction time (RT) task in volving a key-pressing response to an extrafoveal light target precede d by an extrafoveal light cue. Cues and targets were presented along t he horizontal meridian at 4 degrees and 12 degrees on the right and le ft of fixation. Fixation was maintained throughout each trial. The cue signalled the occurrence of the target within a lime window extending from 200 to 4000 msec from the cue, but did not predict target locati on. Normal controls responded faster to medial than to lateral targets in both fields, but showed no between-held difference, and their RT w as not affected by cue location. Furthermore, they showed the so-calle d 'ipsilateral inhibition' or 'inhibition of return' effect, their RT being longer when cues and targets occurred in the same field than whe n they occurred in opposite fields. The RT of the callosotomized subje ct showed a left-right gradient for both cue location and target locat ion, being longest for the leftmost location and shortest for the righ t locations. In addition, he showed a significant advantage for the ri ght hand regardless of cue and target location, as well as a consisten t ipsilateral inhibition in the left field, whereas in the right field there was ipsilateral inhibition only at the two longest stimulus ons et asynchronies. These results suggest that, at least under these expe rimental conditions, there was a rightward orientational bias which re flected the taking over of the control of performance by the left hemi sphere. This attentional bias was reminiscent of that seen in patients with hemi-inattention from right hemisphere damage, although the call osotomized patient showed no sign of such hemi-inattention in routine clinical tests. On the basis of several considerations the rightward b ias could be attributed to the callosal interhemispheric disconnection rather than to the right prefrontal lesion. (C) 1997 Elsevier Science Ltd.