Inappropriate rightward saccades after right hemisphere damage: Oculomotor analysis and anatomical correlates

Patients with right hemisphere damage and visual neglect have severe problems to orient attention towards left-sided objects, often associated with the tendency to produce inappropriate rightward saccades. In its most severe form, this tendency can assume the compulsive character of a rightward deviation of gaze as soon as the visual scene deploys (so-called “magnetic attraction of gaze”). However, little is known about the exact nature of inappropriate rightward saccades, their relation with impaired conscious perception of left-sided stimuli, and their lesional correlates. To explore these issues, we studied three groups of patients with right brain damage: patients with signs of left visual neglect associated to left homonymous hemianopia, neglect patients without hemianopia, and patients without neglect or hemianopia. Participants searched for a gap missing within a target, presented among distractors. Manual responses for target detection were required, while participants were encouraged to move their eyes during search. Endogenous attention could be summoned to the target location by a central cue. All the three groups of patients produced inappropriate rightward saccades, which could not be completely overcome by the endogenous orienting of attention induced by the cues. Anatomical analysis indicated a specific implication of damage to the right frontal eye field and to a long-range white matter tract, the fronto-parietal superior longitudinal fasciculus. Fronto-parietal networks in the right hemisphere appear thus to be essential to integrate covert and overt orienting of attention, and to thoroughly explore space in order to become aware of the multiple competing objects around us.右侧半球损伤合并视觉忽视（visual neglect）的患者，在将注意力定向至左侧物体时存在严重功能障碍，此类障碍常伴随出现不当右侧扫视（saccade）行为。在最严重的情况下，该倾向可表现为一种强迫性的眼球右侧偏斜行为，一旦视觉场景展开便会出现，即所谓的‘眼球磁性吸引征（magnetic attraction of gaze）’。然而，目前学界对不当右侧扫视的确切本质、其与左侧刺激意识感知受损的关联，以及其病灶相关性仍知之甚少。为探究上述问题，我们将右侧脑损伤患者分为三组：伴左侧同向性偏盲（homonymous hemianopia）的左侧视觉忽视患者、无偏盲的视觉忽视患者，以及既无忽视也无偏盲的患者。受试者需在由干扰项构成的视觉场景中，搜寻目标内缺失的缺口。实验要求受试者通过手动操作完成目标检测任务，并鼓励其在搜索过程中移动眼球。中央提示可将内源性注意力引导至目标位置。三组患者均出现了不当右侧扫视行为，且无法通过提示引发的内源性注意力定向完全克服该现象。解剖学分析显示，右侧额叶眼区（frontal eye field）以及远距离白质束——额顶叶上纵束（fronto-parietal superior longitudinal fasciculus）的损伤与该现象存在特异性关联。由此可见，右侧半球的额顶叶网络对于整合内隐与外显的注意力定向、全面探索空间以感知周围众多相互竞争的物体而言，是必不可少的。