Herter Troy M, Guitton Daniel
Montreal Neurological Institute and McGill University, 3801 University Street, Montreal, QC, Canada H3A 2B4.
Exp Brain Res. 2007 Sep;182(1):11-25. doi: 10.1007/s00221-007-0969-1. Epub 2007 May 22.
In three patients who had one cortical hemisphere removed surgically (hemidecortication), we studied visually-triggered saccades directed contralateral to the intact cortical hemisphere (i.e., ipsilesional saccades). Both saccade reaction times (SRTs) and accuracy of these saccades have been reported as abnormal in hemidecorticate patients, but not monkeys. One explanation for this difference is that deficits in hemidecorticate patients may not have been directly caused by removal of cortical oculomotor structures themselves, but may have been a manifestation of compensatory strategies used to cope with contralesional hemianopia. We hypothesized that deficits in saccade performance to the ipsilesional (seeing) visual hemifield would be directly linked to how easily patients could localize targets in their blind hemifield with searching saccades. To test this hypothesis, we examined how deficits in our patients varied when targets were: (1) randomly presented to either the seeing or blind hemifield for long durations thereby permitting searching saccades in the blind hemifield; (2) presented as in Experiment 1, but briefly flashed thereby removing visual feedback prior to saccade onset thereby rendering searching saccades useless; (3) briefly flashed as in Experiment 2, but at random locations in only the seeing hemifield (blind hemifield irrelevant). Mean SRTs to the seeing hemifield were 165 ms longer than normal in Experiment 2, but only about 40 ms longer in Experiments 1 and 3. Saccade accuracy was characterized by task-dependent hypometria in all three experiments with a mean undershoot of about twice the amplitude variance. The largest undershoots were in Experiments 2 and 3. Our data suggest that deficits resulted from the direct effects of the lesions themselves coupled with context-dependent strategies used to cope with contralesional hemianopia.
在三名接受了一侧大脑半球手术切除（大脑半球切除术）的患者中，我们研究了朝向完整大脑半球对侧的视觉触发扫视（即同侧扫视）。据报道，大脑半球切除患者的扫视反应时间（SRT）和这些扫视的准确性均异常，但猴子没有。对此差异的一种解释是，大脑半球切除患者的缺陷可能并非直接由皮质动眼结构本身的切除所致，而是可能是用于应对对侧偏盲的补偿策略的一种表现。我们假设，向同侧（可见）视觉半视野的扫视表现缺陷将直接与患者使用搜索扫视在其盲半视野中定位目标的难易程度相关。为了验证这一假设，我们研究了在以下情况下患者的缺陷如何变化：（1）目标随机长时间呈现于可见或盲半视野，从而允许在盲半视野中进行搜索扫视；（2）如实验1那样呈现，但短暂闪烁，从而在扫视开始前消除视觉反馈，使搜索扫视变得无用；（3）如实验2那样短暂闪烁，但仅在可见半视野的随机位置呈现（盲半视野无关）。在实验2中，对可见半视野的平均SRT比正常时长165毫秒，但在实验1和3中仅长约40毫秒。在所有三个实验中，扫视准确性的特征是任务依赖性的运动幅度不足，平均下冲约为幅度方差的两倍。最大的下冲出现在实验2和3中。我们的数据表明，缺陷是由病变本身的直接影响以及用于应对对侧偏盲的情境依赖性策略共同导致的。
