Distinguishing sensory and motor biases in parietal and frontal neglect

Left neglect after right-hemisphere damage may involve perceptual and/or motor impairments. Here we discuss the limitations of previous attempts to separate these components, and introduce a new method. Six neglect patients (three with right inferior parietal lesions and three with right inferior frontal lesions) moved their right hand to a target light, which appeared unpredictably on either the left or the right of central fixation. The target appeared alone or with a distractor light in the opposite hemifield. Any directional motoric bias was measured by comparing reaches from a central start position with those for the same visual displays, but starting from the left of both possible targets (thus requiring only rightward reaches) or from the right (requiring only leftward reaches). All patients were slower to initiate reaches to left than right targets from a central start, which could reflect perceptual and/or motor biases. Critically, in the parietal neglect group only, initiation speed for left targets improved when a rightward reach was required to these (from a left start) rather than a leftward reach. This suggests a deficit in programming leftward movements into left hemispace, in addition to any visual impairment, for parietal neglect. A control task confirmed that this effect of start position was due to the associated change in reach direction and not to afferent inputs from the hand as it rested at the start position. Frontal neglect patients were slow to execute reaches to left targets, regardless of movement direction. Right visual distractors slowed visual reaction times to left targets more than vice versa in frontal neglect patients, and likewise for reach execution times in parietal neglect patients, suggesting that visual distractors on the neglected side have less impact. Distractor effects were unaffected by start position in the frontal neglect group (suggesting a perceptual basis), but distractors slowed reach initiation in the parietal neglect group only from left and central starts. Taken together, these findings demonstrate a directional motor component to parietal but not frontal neglect, and suggest that in man the inferior parietal lobe plays a role not only in perception but also in the programming of selective reaches. These conclusions are related to recent single-unit data from the monkey parietal lobe.