In referring to visuospatial attention, Doty mentions possible relationships between hemispheric interactions and the mechanisms for the focusing of attention. I would like to discuss some evidence that a section of the corpus callosum upsets the interhemispheric balance in the control of visuospatial attention in some simple tasks of light detection. It may be argued that the detection of a simple light stimulus in a visual field region must be preceded by a shift of the focus of attention toward that region. Proverbio and colleagues (1994) found that normal observers were equally fast in detecting light stimuli in the right and left visual fields, whereas in the same task a callosotomy patient exhibited a striking superiority of the right field over the left. Concurrent electro-physiological findings of positive correlations between the P300 component of the stimulus or event-related potentials in the two hemispheres and the reaction time (RT) data suggested that the advantage for RT to right-field stimuli and the disadvantage for RT to left-field stimuli could be accounted for by a monopolization of attentional control by the left hemisphere. A significant overall advantage for the right visual field in simple RT was also found in three commissurotomy patients out of four by Clarke and Zaidel (1989). A recent study from my laboratory (Berlucchi et al., 1997a) has reported a strong rightward attentional bias in another callosotomy patient who was tested in a different RT task, involving a manual keypress to the second of two identical light stimuli presented at random in the right and left fields. The first stimulus (the cue) did not predict the location of the second stimulus (the target) but forewarned the subject that the target would be presented within 0.24 s. Contrary to normal controls, who showed no inter-field differences, RT of the callosotomy patient was longest when both cue and target went to the left field and the right hemisphere, intermediate when cue and target occurred in opposite fields, and shortest when both cue and target went to the right field and the left hemisphere. It was as if the patient's focus of attention was consistently shifted toward the right visual field during the presentation of both cue and target, even though we made sure that his direction of gaze was firmly held at the center on each trial. In yet another RT task, involving a covert orienting toward a visual location indicated by an arrow, Zaidel (1995, Fig. 17.12) found a massive advantage for RT to right-field targets compared to RT to left-field targets in one of two com-missurotomy patients, regardless of whether the targets appeared at the cued location or elsewhere. All these findings of a rightward attentional bias, as well as other converging results obtained in more complex tasks (Corballis, 1995; Kingstone et al., 1995), seem to indicate that after interhemispheric disconnection the control of visuospatial attention is preferentially lateralized to the left hemisphere, at least when such control is voluntary rather than automatic (Gazzaniga, 1995; Berlucchi et al., 1997b). Since the rightward bias is not present in all tasks, and certainly is not apparent in normal daily life activities, we still need to arrive at an exact and exhaustive definition of the conditions in which attentional control is monopolized by the left hemisphere.
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