Mechanisms underlying spatial representation revealed through studies of hemispatial neglect

The representations that mediate the coding of spatial position were examined by comparing the behavior of patients with left hemispatial neglect with that of nonneurological control subjects. To determine the spatial coordinate system(s) used to define "left" and "right," eye movements were measured for targets that appeared at 5 degrees, 10 degrees, and 15 degrees to the relative left or right defined with respect to the midline of the eyes, head, or midsaggital plane of the trunk. In the baseline condition, in which the various egocentric midlines were all aligned with the environmental midline, patients were disproportionately slower at initiating saccades to left than right targets, relative to the controls. When either the trunk or the head was rotated and the midline aligned with the most peripheral position while the eyes remained aligned with the midline of the environment, the results did not differ from the baseline condition. However, when the eyes were rotated and the midline aligned with the peripheral position, saccadic reaction time (SRT) differed significantly from the baseline, especially when the eyes were rotated to the right. These findings suggest that target position is coded relative to the current position of gaze (oculocentrically) and that this eye-centered coding is modulated by orbital position (eye-in-head signal). The findings dovetail well with results from existing neurophysiological studies and shed further light on the spatial representations mediated by the human parietal cortex.