Larsson Jonas, Amunts Katrin, Gulyás Balázs, Malikovic Aleksandar, Zilles Karl, Roland Per E
Division of Human Brain Research, Department of Neuroscience, Karolinska Institute, 17177 Stockholm, Sweden.
Exp Brain Res. 2002 Mar;143(1):1-10. doi: 10.1007/s00221-001-0895-6. Epub 2002 Jan 16.
Objects in natural scenes are rarely seen in isolation, but are usually overlapping or partially occluding other objects. To recognize individual objects, the visual system must be able to segregate overlapping objects from one another. Evidence from lesions in humans and monkeys suggest that perceptual segregation of occluded or overlapping objects involves extrastriate visual cortex. In monkeys, area V4 has been shown to play an important role in recognizing occluded or poorly salient shapes. In humans, a retinotopic homologue of ventral V4 (V4v) has been described, but it is not known whether this area is also functionally homologous to area V4 in monkeys. In this study, we tried to localize the visual cortical regions involved in perceptual segregation of overlapping shapes using positron emission tomography (PET). Regional cerebral blood flow (rCBF) was measured in seven subjects while they discriminated the relative areas of simultaneously presented rectangular shapes. In the control condition, the shapes were displayed without overlaps; in a second condition, the shapes overlapped each other partially. In a third condition, the shapes did not overlap but had been reduced in salience by adding random noise to the stimuli. Contrasting the overlapping shape condition with the control condition identified a single region in the left posterior lateral occipital cortex. The rCBF in this region also increased, though more weakly, during discrimination of shapes embedded in noise, relative to the control condition. The region activated by segregation of overlapping shapes was located in the posterior occipital cortex close to the anterior border of area V2, near the average location of human V4v as determined by retinotopic mapping studies. The activation of this region of extrastriate visual cortex by a task that involved segregation of overlapping shapes is consistent with monkey V4 and human V4v being functionally homologous. We conclude that discrimination of overlapping shapes involves in particular a region of extrastriate visual cortex located in the left lateral occipital cortex and that this region may correspond to human V4v.
自然场景中的物体很少孤立出现，而是通常相互重叠或部分遮挡其他物体。为了识别单个物体，视觉系统必须能够将重叠的物体彼此分离。来自人类和猴子损伤的证据表明，对被遮挡或重叠物体的感知分离涉及纹外视觉皮层。在猴子中，V4区已被证明在识别被遮挡或显著性较差的形状方面发挥重要作用。在人类中，已描述了腹侧V4区（V4v）的视网膜拓扑对应物，但尚不清楚该区域在功能上是否也与猴子的V4区同源。在本研究中，我们试图使用正电子发射断层扫描（PET）定位参与重叠形状感知分离的视觉皮层区域。在七名受试者辨别同时呈现的矩形形状的相对面积时，测量了局部脑血流量（rCBF）。在对照条件下，形状无重叠显示；在第二种条件下，形状部分相互重叠。在第三种条件下，形状不重叠，但通过向刺激中添加随机噪声降低了显著性。将重叠形状条件与对照条件进行对比，在左侧枕叶后外侧皮质中识别出一个单一区域。相对于对照条件，在辨别嵌入噪声中的形状时，该区域中的rCBF也增加了，尽管增加幅度较小。由重叠形状分离激活的区域位于枕叶后皮质，靠近V2区的前边界，接近通过视网膜拓扑映射研究确定的人类V4v的平均位置。通过涉及重叠形状分离的任务对该纹外视觉皮层区域的激活与猴子V4区和人类V4v在功能上同源是一致的。我们得出结论，重叠形状的辨别特别涉及位于左侧枕叶皮质的一个纹外视觉皮层区域，并且该区域可能对应于人类V4v。
