Neglect rehabilitation by prism adaptation: different procedures have different impacts.

Several studies point to prism adaptation as an effective tool for the rehabilitation of hemispatial neglect. However, some recent reports failed to show a significant amelioration of neglect after prism adaptation as compared to control treatments. This apparent contradiction might reflect important differences in the procedures used for treatment. Here we compare the effects of two treatments (performed for 10 sessions, over 2 weeks) in two groups of patients, based either on a Terminal (TPA) or a Concurrent (CPA) prism adaptation procedure. During TPA only the final part of the pointing movement is visible and prism adaptation relies most strongly on a strategic recalibration of visuomotor eye-hand coordinates. In contrast, during CPA the second half of the pointing movement is visible, and thus adaptation mainly consists of a realignment of proprioceptive coordinates. The present results show that both TPA and CPA treatments induced a greater improvement of neglect as compared to a control treatment of pointing without prisms. However, neglect amelioration was higher for patients treated with TPA than for those treated with CPA. At the same time, the TPA treatment induced a stronger deviation of eye movements toward the left, neglected, field as compared to the CPA treatment. Interestingly, in TPA patients the visuomotor and oculomotor effects of the treatment were directly related to the patients' ability to compensate for the optical deviation induced by prism during pointing (i.e., Error reduction effect). In summary, prism adaptation seems particularly effective for the recovery of visuo-spatial neglect when conducted with a procedure stressing a correction of visuomotor eye-hand coordinates, i.e., with a TPA procedure. The present observations may help to better understand the mechanisms underlying prism-induced recovery from neglect and the procedural basis for some of the contradictory results obtained when using this rehabilitative strategy.