Adaptation to reversing prisms: Pointing in patients with right-parietal damage

Abstract

Right parietal patients can show errors in pointing to targets in the contralesional (left) visual field (unilateral optic ataxia (OA)). It is unclear if these errors are due to deficits in visual or motor coordinates. To test this, we trained stroke patients and age-matched controls to point at remembered visual targets while looking through left-right optical reversing prisms. Targets appeared briefly to either the left or right of a central fixation point. Baseline accuracy was tested with 2 blocks of 20 pointing trials (10 to the left and 10 to the right). Subjects were then adapted with 11 blocks using the reversing prisms, followed by 2 blocks without the prisms (recovery phase).

If ‘OA’ errors remained fixed in visual coordinates, reversing vision would reverse the errors left to right. If the errors were fixed in motor coordinates, they should not be affected by the prism task. Eight patients with right parietal (and other) damage were tested, but only one, with damage localized to the right superior parietal lobule, showed baseline inaccurate pointing to the left (but accurate right pointing). This patient quickly learned the prism task and showed a pattern of pointing that was more accurate and symmetric over-all, but with slightly greater errors for right pointing. Surprisingly, when the prisms were removed, this adapted pattern then reversed (i.e., errors were once again greater on the left, but far less than the original baseline errors). Symptoms of a left visual field ‘neglect’ (i.e., failure to respond to the stimulus) also stayed fixed in visual coordinates in this subject as well as another. In general, these results support the idea that posterior parietal cortex encodes the visual goal of the movement (upstream from the vision-to-motor transformation) rather than the downstream motor command. Our results also suggest that learning this reversal promotes the development of alternative pathways that might be useful for rehabilitation.

Footnotes

 Funding support provided by OGSST and CIHR, Canada.