Sabira K. Mannan

A total of 29 patients with homonymous visual field defects without neglect practised visual search in 20 daily sessions, over a period of 4 weeks. Patients searched for a single randomly positioned target amongst distractors displayed for 3 s. After training patients demonstrated significantly shorter reaction times for search stimuli (Pambakian et al. in J Neurol Neurosurg Psychiatry 75:1443–1448, 2004). In this study, patients achieved improved search efficiency after training by altering their oculomotor behaviour in the following ways: (1) patients directed a higher proportion of fixations into the hemispace containing the target, (2) patients were quicker to saccade into the hemifield containing the target if the initial saccade had been made into the opposite hemifield, (3) patients made fewer transitions from one hemifield to another before locating the target, (4) patients made a larger initial saccade, although the direction of the initial saccade did not change as a result of training, (5) patients acquired a larger visual lobe in their blind hemifield after training. Patients also required fewer saccades to locate the target after training reflecting improved search efficiency. All these changes were confined to the training period and maintained at follow-up. Taken together these results suggest that visual training facilitates the development of specific compensatory eye movement strategies in patients with homonymous visual field defects.

Searching a cluttered visual scene for a specific item of interest can take several seconds to perform if the target item is difficult to discriminate from surrounding items. Whether working memory processes are utilized to guide the path of attentional selection during such searches remains under debate. Previous studies have found evidence to support a role for spatial working memory in inefficient search, but the role of nonspatial working memory remains unclear. Here, we directly compared the role of spatial and nonspatial working memory for both an efficient and inefficient search task. In Experiment 1, we used a dual-task paradigm to investigate the effect of performing visual search within the retention interval of a spatial working memory task. Importantly, by incorporating two working memory loads (low and high) we were able to make comparisons between dual-task conditions, rather than between dual-task and single-task conditions. This design allows any interference effects observed to be attributed to changes in memory load, rather than to nonspecific effects related to “dual-task” performance. We found that the efficiency of the inefficient search task declined as spatial memory load increased, but that the efficient search task remained efficient. These results suggest that spatial memory plays an important role in inefficient but not efficient search. In Experiment 2, participants performed the same visual search tasks within the retention interval of visually matched spatial and verbal working memory tasks. Critically, we found comparable dual-task interference between inefficient search and both the spatial and nonspatial working memory tasks, indicating that inefficient search recruits working memory processes common to both domains.

We are very interested in the letter from Karnath and colleagues and welcome the opportunity to continue this stimulating debate about the anatomy of neglect. Although the arguments advanced by Karnath and his colleagues are interesting, we disagree with them. We hope they will reconsider their interpretation of our findings (Mort et al., 2003), as well as those of Binder et al. (1992), in the light of our responses. In addition, we draw attention to the fact that the limited new data provided in their letter does not actually support well the controversial claim that neglect is associated with superior temporal gyrus (STG) damage (Karnath et al., 2001).