Slow binocular rivalry in hemispatial neglect

Hemispatial neglect is considered to be a disorder, which is primarily related to space. Inability to allocate attention properly to spatial-coded stimuli is of its essence. Recent evidence shows that hemispatial neglect might incorporate impairment in temporal attention as well as in spatial attention. For example, it was found that extreme elongation in time of the "attentional blink" affects the perception of visual stimuli in midsagittal position in neglect. Here we show that the temporal deficit previously demonstrated with rapid perceptual events, is also reflected in a slow perceptual switching in binocular rivalry, a phenomenon that does not involve any perceptual load and is considered to be "automatic". Three groups of observers - normal controls, patients with left-hemisphere damage without neglect, and patients with right-hemisphere damage with neglect, were tested on three aspects of binocular rivalry - alternation speed, contrast dependency and orientation preference. Observers monitored the perceptual alternations induced by two patches of orthogonal (horizontal and vertical) gratings presented to different eyes at varied contrast levels. Overall, neglect patients (right-hemisphere damage) had (1) much slower perceptual alternations (7-15 sec) as compared with normal (1-3 sec) or non-neglect patients (4-7 sec), (2) were more sensitive to contrast differences, with small changes in contrast altering both the suppression and dominance phases, and (3) had preference for horizontal gratings. We further found that the recovery from neglect correlated with shortening of the switching intervals. These results demonstrate a non-spatial fundamental disturbance in neglect: impairment in the temporal organization of the switching mechanisms, which seems to be crucial for proper environmental monitoring. At the same time, it demonstrates the critical role of attention in binocular rivalry.