Is motor perseveration in unilateral neglect 'driven' by the presence of neglected left-sided stimuli?

Unilateral spatial neglect refers to a difficulty in detecting or acting on information in a particular region of space. When asked to cross out stimuli distributed across a page, patients with neglect may miss many targets on the left. In addition, they have a tendency to return to, and re-mark, right-sided targets that they have already cancelled. A recent retrospective study has shown this effect to be specific to unilateral neglect rather than a consequence of right hemisphere damage in general. Here, a consecutive group of seven right-hemisphere neglect patients performed five versions of a standard cancellation task, each version differing in the quantity of left-sided information presented. All of the participants showed perseveration on right-sided targets in the basic task. A highly significant and linear reduction in perseverative behaviour was observed as left-sided information was removed. In a second study, left-sided targets were again progressively removed but, in this case, were replaced with an additional distractor item, keeping the total quantity of stimuli presented in each condition constant. Again, a significant reduction in right-sided perseveration was observed, indicating a high degree of selectivity to the effect. The results show that a difficulty in perceiving existing cancellation marks, or a non-spatially specific motoric perseveration, are unlikely to fully account for this behaviour. As the patients omitted almost all targets on the left side of the basic cancellation task, the results suggest a striking influence on apparently intentional behaviour from un-cancelled information within the neglected field.