Eye-movements and bisection behavior in spatial neglect syndrome. Representational biases induced by the segment length and spatial dislocation of the stimulus

Systematic spatial biases in the visually guided actions were observed for patients with right hemisphere damage. Neglect patients generally show an inability to take into account information coming from the left side of space. Typical symptoms of neglect are rightward errors in line bisection and left-side deficits in visual search task. The present study explored behavioral and eye-movement measures in spatial unilateral neglect in response to an online bisection task. Bisection stimuli were horizontal gaps represented by two red spheres, one to either side of the midline, that were presented, after a fixation point, on a white background. The experimental subjects (patients N = 10; control N = 10) could give their response pointing the perceived midpoint starting from stimuli onset. Eye-movement (total number of fixations, fixations length and direction of the first fixations) and bisection responses were considered. Consistent spatial biases were found for patients in comparison with controls (mixed repeated measure ANOVA) for both bisection position and fixations as a function of segment length (from shorter to longer) and segment spatial dislocation (from right to left spatial dislocation). The eccentric left-position induced a greater rightward bias in patients, with increasing more rightside bisection and visual right-directed fixation. Contrarily, segment length produced significant differences between-groups only for eye movement behavior, with increased fixation count and duration rightward oriented in response to longer segment. Nevertheless, the left-to-right and longer-to-shorter “continuous-gradient effect” was not totally supported by our results, whereas an “extreme left-gradient effect” was suggested and discussed.