No direction home: Extinction is affected by implicit motion

Following lesions to (usually) the right parietal lobe, patients may fail to report stimuli on their contralesional side if a stimulus is also presented ipsilesionally. The problem can be ameliorated if the stimuli form part of a common action (e.g., a bottle pouring into a glass), when the contralesional item may be brought to awareness. We examined whether this improved awareness depended on implied motion from one object to another. This was tested using pairs of stimuli in which one had implied motion towards or away from the other stimulus. The results showed that patients were more aware of the presence of two objects on trials when one object had implied motion towards the other, compared with when motion was directed away from the second object. This held when the implied motion was in the contralesional as well as when it was in the ipsilesional field. In a single case, this effect held even when the direction of motion could not be explicitly discriminated. The data suggest that motion was coded implicitly and that it helped to link objects together as a perceptual unit. Coding objects as a single perceptual unit reduces the spatial bias in selection that produces extinction.