Rees G, Wojciulik E, Clarke K, Husain M, Frith C, Driver J
Division of Biology 139-74, California Institute of Technology, Pasadena 91125, USA.
Brain. 2000 Aug;123 ( Pt 8):1624-33. doi: 10.1093/brain/123.8.1624.
Visual extinction is a sign classically associated with right parietal damage. The patient can see a single stimulus presented in the ipsilesional or contralesional visual field, but is characteristically unaware of the same contralesional stimulus during simultaneous stimulation of both fields. The ipsilesional stimulus is said to 'extinguish' the contralesional stimulus from awareness during bilateral stimulation, perhaps due to a pathological bias in attention towards the ipsilesional side. Recent psychophysical evidence suggests that, although extinguished stimuli are not consciously seen, they may undergo residual processing and exert implicit effects on performance. However, the neural structures mediating such residual processing for extinguished stimuli remain unknown. Here we studied the neural activity evoked by an extinguished visual stimulus, using event-related functional MRI (fMRI), in a patient with circumscribed right inferior parietal damage and profound left-sided extinction. Monochrome objects (faces or houses) were presented in the left or right field, either unilaterally or bilaterally on each trial, with the patient indicating by button press whether he saw an object on the left, the right or on both sides. He usually saw only the right object on bilateral trials, yet the fMRI data showed activation of visual cortex contralateral to the extinguished left stimulus on these trials (compared with right-only stimulation), in both striate and early extrastriate areas of the right hemisphere. This activity had a similar location and time-course to that resulting from a single stimulus in the left versus right visual field. Cortical pathways involved in the normal processing of a single seen stimulus can thus still be activated by an unseen, extinguished stimulus after right parietal damage. Comparison of fMRI responses for faces versus houses revealed some category-specific activation for extinguished stimuli in right fusiform regions, but only at low statistical threshold. These results are discussed in terms of theoretical accounts for parietal extinction and, more generally, for the neural substrates of visual awareness.
视觉消失是一种典型与右侧顶叶损伤相关的体征。患者能够看见呈现于患侧或对侧视野中的单个刺激，但在双侧视野同时受到刺激时，其特征性表现为对同一对侧刺激没有察觉。据说在双侧刺激期间，患侧刺激会使对侧刺激从意识中“消失”，这可能是由于注意力存在朝向患侧的病理性偏向。最近的心理物理学证据表明，尽管消失的刺激未被有意识地看见，但它们可能会经历残余加工并对行为产生隐性影响。然而，介导这种对消失刺激的残余加工的神经结构仍然未知。在此，我们使用事件相关功能磁共振成像（fMRI），对一名患有局限性右侧顶叶下部损伤且左侧存在严重视觉消失的患者，研究了由消失的视觉刺激诱发的神经活动。在每次试验中，单色物体（面部或房屋）呈现在左侧或右侧视野，或者单侧呈现，或者双侧呈现，患者通过按键指示他是否看到左侧、右侧或两侧的物体。在双侧试验中，他通常只能看到右侧物体，但fMRI数据显示，在这些试验中（与仅右侧刺激相比），右侧半球的纹状区和早期纹外区中，与消失的左侧刺激对侧的视觉皮层被激活。这种活动的位置和时间进程与左、右视野中单个刺激所产生的活动相似。因此，在右侧顶叶损伤后，参与正常处理单个可见刺激的皮层通路仍然可以被一个未被看见、消失的刺激激活。对fMRI对面部与房屋反应的比较显示，在右侧梭状回区域，消失刺激存在一些类别特异性激活，但仅在低统计阈值时出现。我们将根据关于顶叶视觉消失的理论解释，以及更广泛地根据视觉意识的神经基础来讨论这些结果。
