Recovery of oculo-motor bias in neglect patients after prism adaptation

Patients with "neglect dyslexia" usually make errors in reading the left part of words and non-words. It has been shown that "neglect dyslexia" can improve following a short period of adaptation to wedge prisms [Neuropsychologia 40 (2002) 718], however the mechanisms underlying this amelioration are still unknown. The present study evaluated the effect of prism adaptation (PA) on ocular scanning behaviour of neglect dyslexia patients by investigating: (a) the first saccade landing position during reading a letter string and (b) the distribution of fixation time as a function of the side of oculo-motor exploration of words and non-words. This in order to assess whether possible changes (in reading performance) after the adaptation might be attributed to a resetting of the oculo-motor system. Eye movements' performances were recorded before and after a single prismatic exposure on right brain-damaged patients with left hemispatial neglect and "neglect dyslexia". The results obtained in Experimental neglect patients before and after PA were then compared to that of control neglect patients, who were wearing goggles with neutral lenses. Moreover, in order to provide normative data on ocular scanning behaviour during letter string reading, neurologically healthy subjects were also studied. Following a single session of prism adaptation, the results showed, in the Experimental neglect patients, an improvement of neglect dyslexia, an increased left-sided exploration of the letter string and an increased amplitude of the first left-sided saccade. In contrast, in the Control neglect patients, neglect dyslexia as well as the oculo-motor system behaviour, remained the same after the use of goggles with neutral lenses. These findings demonstrate that the beneficial effect induced by a single prismatic adaptation may be conceived as a complex interaction between sensory stimulation and a resetting of oculo-motor system.