Distinguishing sensory and motor biases in parietal and frontal neglect

Left neglect after right-hemisphere damage may involve perceptual and/or mo tor impairments. Here we discuss the limitations of previous attempts to se parate these components, and introduce a new method, Six neglect patients ( three with right inferior parietal lesions and three with right inferior fr ontal lesions) moved their right hand to a target light, which appeared unp redictably on either the left or the right of central fixation, The target appeared alone or with a distracter light in the opposite hemifield, Any di rectional motoric bias was measured by comparing reaches from a central sta rt position with those for the same visual displays, but starting from the left of both possible targets (thus requiring only rightward reaches) or fr om the right (requiring only leftward reaches). All patients were slower to initiate reaches to left than right targets from a central start, which co uld reflect perceptual and/or motor biases. Critically, in the parietal neg lect group only, initiation speed for left targets improved when a rightwar d reach was required to these (from a left start) rather than a leftward re ach. This suggests a deficit in programming leftward movements into left he mispace, in addition to any visual impairment, for parietal neglect. A cont rol task confirmed that this effect of start position was due to the associ ated 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 exec ute reaches to left targets, regardless of movement direction. Right visual distracters slowed visual reaction times to left targets more than vice ve rsa in frontal neglect patients, and likewise for reach execution times in parietal neglect patients, suggesting that visual distracters on the neglec ted side have less impact. Distracter effects were unaffected by start posi tion in the frontal neglect group (suggesting a perceptual basis), but dist racters slowed reach initiation in the parietal neglect group only from lef t and central starts. Taken together, these findings demonstrate a directio nal motor component to parietal but not frontal neglect, and suggest that i n man the inferior parietal lobe plays a role not only in perception but al so in the programming of selective reaches. These conclusions are related t o recent single-unit data from the monkey parietal lobe.