Illusion processing in hemispatial neglect

Twelve patients with hemispatial neglect and two control groups were tested to examine the effects of the Muller-Lyer and Judd illusions on bisection behaviour. The studies were designed to investigate whether neglect patient s were indeed unaware of the left sides of the illusory figures. In Experim ent 1, participants were asked to describe the illusory figures prior to bi section, whereas in Experiment 2, they compared two illusions whose fins, i n the critical condition, differed on the left and then performed the bisec tion. It was found that the illusions worked equally well in all three grou ps. Interestingly, apart from one exception, almost all neglect patients ex plicitly reported the left-sided fins in Experiment 1. Only five patients f ailed to do so but only on an average of 16% of trials. In Experiment 2, si x patients made errors in the comparison task but four of these patients di d not neglect any left-sided fins in Experiment 1 (with the exception of th ree overall trials for LC and EdR). This finding seems a good indication th at the two tasks differ in their requirements. The comparison task may be p erceived as harder as it requires discrimination rather than detection and thus lead to more neglect type errors than the bisection task. In one negle ct patient, the illusions consistently failed to work. This patient present ed with an occipito-temporal and basal ganglia lesion and the mechanisms re sponsible for the processing of simple visual features might have possibly been impaired in her case. (C) 2001 Elsevier Science Ltd. All rights reserv ed.