Left hand movements and right hemisphere activation in unilateral spatial neglect: a test of the interhemispheric imbalance hypothesis

The aim of the present study was to check one of the main assumptions of the interhemispheric imbalance hypothesis, namely, the prediction that the severity of neglect should be reduced by conditions activating the right hemisphere. To achieve this, a group of neglect patients was studied using a slightly modified version of the limb activation technique. The (verbal or visuo-spatial) nature of the stimuli to be processed by the patient and the (left or right) side of space where the left hand moved were considered as the critical variables to check the interhemispheric imbalance hypothesis. Three traditional and one new methods were used to measure changes induced in the severity of neglect by the material to be processed or by the side of space where the left hand moved. The traditional methods, all based on counting omissions, consisted of measuring: (a) the overall number of omissions; (b) the number of omissions made on the left half sheet; or (c) the difference between the omissions made on the left and right sides of the sheet. The new index, based on the notion of the 'attentional field' and defined as the spatial distribution of stimuli detected by the patient, was operationally measured by computing the distance between each stimulus crossed out by the patient and the right margin of the sheet. The study was conducted by rating the severity of neglect in 42 cancellation sheets which had used, respectively letters (N=21) and small geometric figures (N=21) as targets. The two sets of cancellation sheets were obtained from seven neglect patients during a limb activation task requiring the cancellation of a given target in three different conditions: (a) baseline; (b) active movements of the left hand in the left half space; (c) active movements of the left hand in the right half space. Results were at variance with the predictions based on Kinsbourne's model, since the verbal or visual spatial nature of the material to be processed did not influence the severity of unilateral spatial neglect (USN) and since left hand movements produced a significant reduction in the severity of neglect only when these movements were made on the left side of space.