The manual haptic perception of orientations and the oblique effect in patients with left visuo-spatial neglect

This study addresses the limits of haptic orientation deficit observed in patients with left visuo-spatial neglect (VSN) in the fronto-parallel plane. We concentrated on two aspects of the haptic perception of vertical, horizontal and oblique orientations: first, the global level of performances compared with normal subjects and, second, the occurrence of the oblique effect (i.e. lower performances in oblique orientations than in vertical-horizontal orientations). Subjects were asked to position a rod, presented in the fronto-parallel plane, to one of four orientations: vertical, horizontal, left 45 degrees oblique and right 45 degrees oblique. First, we found a haptic orientation deficit in neglect patients: The precision was lower in the neglect patients than in the normal (young adults and seniors) subjects. Second, we observed in both neglect patients and control subjects the occurrence of a similar haptic oblique effect and there were no differences between the results in the left and right hemispaces. Taken together, this means that, in spite of the global haptic orientation production deficit observed in VSN patients, no specific pattern was observed in the haptic production of different orientations in these subjects as compared to the two other groups. The haptic orientation deficit of neglect patients seems to affect in the same way all values and spatial positions of orientations.