Dissociated long lasting improvements of straight-ahead pointing and line bisection tasks in two hemineglect patients.

In this experiment, we evaluated over a longer time period the previously demonstrated effects of a short prism adaptation on hemispatial neglect. We followed two patients (PE and SA), during a period of 5 days (1 day before and 4 days after the prism adaptation procedure), repeatedly measuring their performances on a straight-ahead pointing task and a line bisection task. We also assessed the comparative temporal evolution of the rightward biased egocentric reference frame (as measured by the straight-ahead demonstration) and a classical neuropsychological symptom of neglect, namely the rightward bias observed on line bisection. Firstly, the results showed that prismatic effect could be maintained for 4 days, on the two tasks (separately straight-ahead for PE, line bisection for SA). This long-term effect implies a very profound action of prism adaptation based on active processes and opens large possibilities for clinical applications. Secondly, no correlation was found between the evolution of the performances on the two tasks, neither for patient PE, nor for patient SA (within-subject double-dissociation). Moreover, a double-dissociation between subjects was demonstrated on long-term effects. A new conception has thus to be found to explain the various symptoms manifested in neglect and more investigations have to be performed in order to establish to what extent they can be considered independent. It can be concluded that elucidating the mechanism through which prism adaptation affects neglect could lead to a better understanding of the neglect syndrome.