Can task specific perceptual bias be distinguished from unilateral neglect?

The present study examined visuoperceptual bias in 12 right hemisphere damaged patients, eight of whom showed left unilateral neglect on standard clinical tests, and in 30 normal controls. In the chimeric faces task, subjects were required to judge which of a pair of faces appeared happier. Stimuli comprising each pair were mirror images, with the smiling half on the left of one face and on the right of the other. In the grey scales task, subjects were required to indicate which of two shaded rectangles appeared to be darker overall. Again, stimuli were mirror images, with the darker end appearing either on the left or on the right. Patients exhibited a significant rightward bias on both experimental tasks, in contrast to the significant leftward bias exhibited by controls. There was no significant correlation between patients' performances on standard clinical tests and the extent of bias on the two experimental tasks, suggesting that such patients exhibit distinct impairments of spatial cognition which are differentially indexed by the two types of task. Moreover, for both patients and controls, scores obtained on the two perceptual bias tasks were unrelated, suggesting that they may engage stimulus-specific processes which have different underlying patterns of asymmetrical processing. These data provide further support for models which propose that the heterogeneity of disorders of spatial cognition arise from disruption of distinct neural mechanisms.