Gradients of detection in neglect: comparison of peripersonal and extrapersonal space.

Previous studies of target-cancellation performance in visuospatial neglect patients have reported lateral (left-right) and radial (near-far) gradients of attentional ability. The purpose of the present study was to replicate the reported attentional gradients in peripersonal space (within arms reach) and to examine whether lateral gradients of detection also appear in extrapersonal space (beyond arms reach), using equivalent tasks with no manual requirement. The relationship between radial gradients in peripersonal space and neglect severity (degree of lateral gradient) in extrapersonal space was also of interest. Right-hemisphere stroke subjects, with and without neglect, and healthy control subjects named visual targets on scanning sheets placed in peripersonal and extrapersonal space. The neglect group showed lateral gradients of increasing target detection from left to right in both peripersonal and extrapersonal space, which were not evident in the performance of either of the control groups. Double dissociations of neglect severity in peripersonal and extrapersonal space were also found in analyses of individual performance. Lesion analyses showed that peripersonal neglect was related to dorsal stream damage and extrapersonal neglect was related to ventral stream damage. Group analyses showed no significant radial gradients in peripersonal space in the three groups. In addition, while analyses of some individuals found significant near-far and far-near radial gradients, there was no correlation between radial gradients in peripersonal space and neglect severity in extrapersonal space. These results are discussed in terms of theorised hemispheric mechanisms of spatial attention and the relationship of neglect in the two co-ordinate spaces to the extent and location of damaged neurons in the right hemisphere.