Westwood David A, Danckert James, Servos Philip, Goodale Melvyn A
Department of Psychology, The University of Western Ontario, London, Ontario N6A 5C2, Canada.
Exp Brain Res. 2002 May;144(2):262-7. doi: 10.1007/s00221-002-1068-y. Epub 2002 Apr 10.
Visually guided prehension is controlled by a specialized visuomotor system in the posterior parietal cortex. It is not clear how this system responds to visual stimuli that lack three-dimensional (3D) structure, such as two-dimensional (2D) images of objects. We asked a neurological patient with visual-form agnosia (patient D.F.) to grasp 3D objects and 2D images of the same objects and to estimate their sizes manually. D.F.'s grip aperture was scaled to the sizes of the 2D and 3D target stimuli, but her manual estimates were poorly correlated with object size. Control participants demonstrated appropriate size-scaling in both the grasping and manual size-estimation tasks, but tended to use a smaller peak aperture when reaching to grasp 2D images. We conclude that: (1) the dorsal stream grasping system does not discriminate in a fundamental way between 2D and 3D objects, and (2) neurologically normal participants might adopt a different visuomotor strategy for target objects that are recognized to be ungraspable. These findings are consistent with the view that the dorsal grasping system accesses a pragmatic, spatial representation of the target object, whereas the ventral system accesses a more comprehensive, volumetric description of the object.
视觉引导的抓握动作由后顶叶皮质中的一个专门的视觉运动系统控制。目前尚不清楚该系统如何对缺乏三维（3D）结构的视觉刺激做出反应，例如物体的二维（2D）图像。我们让一位患有视觉形式失认症的神经学患者（患者D.F.）抓握3D物体以及相同物体的2D图像，并手动估计它们的大小。D.F.的抓握孔径与2D和3D目标刺激的大小成比例，但她的手动估计与物体大小的相关性很差。对照组参与者在抓握和手动大小估计任务中均表现出适当的大小缩放，但在伸手抓握2D图像时倾向于使用较小的峰值孔径。我们得出以下结论：（1）背侧流抓握系统在2D和3D物体之间不存在根本的区分，（2）神经功能正常的参与者可能会针对被认为无法抓握的目标物体采用不同的视觉运动策略。这些发现与以下观点一致，即背侧抓握系统获取目标物体的实用空间表征，而腹侧系统获取物体更全面的体积描述。
