Abnormal binocular rivalry in unilateral neglect: evidence for a non-spatial mechanism of extinction.

Unilateral spatial neglect (USN) is considered to be an attention deficit, which is primarily related to space. Recent evidence points to the relevance of non-spatially lateralized mechanisms, with impairments found in rapid stimulus presentation conditions. Here we used the phenomenon of binocular rivalry (BR) to explore a non-spatial deficit over long temporal intervals. Six right-hemisphere damaged (RHD) patients with contralesional neglect (USN+), five RHD patients without neglect (USN-) and six normal controls were tested on the basic properties of BR induced by dichoptic presentation of orthogonal gratings at fixation. USN+ patients had much slower perceptual alternations compared to the USN- and normal groups (factors 2.5 and 4, respectively), and were much more sensitive to inter-ocular changes in relative stimulus contrast, which, unlike normals, altered both the suppression and dominance phases. Most notably, a small advantage of one monocular stimulus caused a long-term extinction of the other stimulus in the USN+ group alone. We explain the results in terms of impaired habituation to dominant and attended stimuli, which normally prevents a winner-takes-all behavior and extinction of the weak. This impaired habituation may in turn contribute to inappropriate environmental monitoring and attenuated novelty-seeking behavior.