Psychophysiology of eye-movements and “attentional gradient effect” in unilateral spatial neglect

Neglect patients generally fail to respond appropriately toward stimuli located in the contralesional space and may ignore stimuli or reduce the extent eye and hand movement to objects occurring within this space. Thus, systematic spatial biases in the visually guided actions were observed for patients with right-hemisphere damage. As it was found in previous studies, in addition to the behavioral response, it is important to monitor the eye behavior during a visual search task to analyze the difficulties related to the exploration of the visual space by neglect patients. The present study analyzed behavioral and eye-movement measures in unilateral neglect patients in response to online bisection task (unfilled gap lines). Two different tasks were used to test the bisection performance, a pointing and a grasping strategy. Indeed, we hypothesized that neglect patients may be more directly impaired in task execution (online bisection) in case of pointing than of grasping strategy. Therefore, it was explored whether these different strategies may influence subjects’ behavioral and eye-movement measures. Secondly, segment length (from shorter to longer) and its spatial position (from right to left spatial location) were monitored, to verify the consistency of rightward bias increasing as a function of left-side more than right-side dislocation and of longer more than shorter segments. Eleven neglect patients and ten control subjects were included into the study. The subjects could give their response pointing the perceived midpoint starting from stimuli onset (pointing condition). In the grasping condition, subjects were required to imagine they had to grasp an object localized on the midpoint of the segment, instead of only perceptually visualize and point the midline of the segment itself. Eye movements were recorded using an infrared-based video tracking (Tobii X120), including the total number of fixations, fixations length, and direction of the first fixations during bisection task execution. Consistent spatial biases were found for both bisection responses, fixation count, and duration, as well as for the first fixation count in case of pointing task. The spatial gradient effect was found only in response to specific ‘‘leftward’’ positions. In addition, bisection task was affected by the cognitive strategy of patients, with a more consistent rightward response bias in case of a representation finalized to pointing more than grasping. Eye-movement behavior supported and confirmed this behavioral trend. An ‘‘extreme-left’’ gradient effect was suggested and discussed, with patients’ behavioral and eye measures more impaired. On the contrary, the patients’ performance was similar to the controls’ one in case the grasping task. The direct link of visual pointing and grasping strategy with two distinct cortical pathways was adduced to explain these results.