Department of Psychology and Key Laboratory of Machine Perception (Ministry of Education), Peking University, Beijing 100871, People's Republic of China.
J Neurosci. 2010 Dec 8;30(49):16692-8. doi: 10.1523/JNEUROSCI.3578-10.2010.
In natural images, visual objects are typically occluded by other objects. A remarkable ability of our visual system is to complete occluded objects effortlessly and see whole, uninterrupted objects. How object completion is implemented in the visual system is still largely unknown. In this study, using a backward masking paradigm, we combined psychophysics and functional magnetic resonance imaging to investigate the temporal evolvement of face completion at different levels of the visual processing hierarchy. Human subjects were presented with two kinds of stimuli that were designed to elicit or not elicit the percept of a completed face, although they were physically very similar. By contrasting subjects' behavioral and blood oxygenation level-dependent (BOLD) responses to completed and noncompleted faces, we measured the psychophysical time course of the face completion and its underlying cortical dynamics. We found that face completion manifested its effect between 50 and 250 ms after stimulus onset. Relative to noncompleted faces, completed faces induced weaker BOLD response at early processing phases in retinotopic visual areas V1 and V2 and stronger BOLD response at late processing phases in occipital face area and fusiform face area. Attending away from the stimuli largely abolished these effects. These findings suggest that face completion consists of two synergetic phases: early suppression in lower visual areas and late enhancement in higher visual areas; moreover, attention is necessary to these neural events.
在自然图像中,视觉对象通常被其他对象遮挡。我们的视觉系统具有一项非凡的能力,即能够毫不费力地完成对被遮挡物体的补全,并看到完整、未被打断的物体。然而,物体补全是如何在视觉系统中实现的,目前仍知之甚少。在这项研究中,我们采用了后向掩蔽范式,结合心理物理学和功能磁共振成像,研究了面部补全在视觉处理层次的不同水平上的时间演变。研究中向人类被试呈现了两种刺激,这两种刺激的设计旨在引起或不引起完整面部的感知,尽管它们在物理上非常相似。通过对比被试对完整和不完整面部的行为和血氧水平依赖(BOLD)反应,我们测量了面部补全的心理物理学时间进程及其潜在的皮层动力学。研究发现,面部补全在刺激开始后 50 到 250 毫秒之间表现出其效果。与不完整的面部相比,完成的面部在枕叶面孔区和梭状回面孔区等高级视觉区域的早期处理阶段引起较弱的 BOLD 反应,而在后期处理阶段引起较强的 BOLD 反应。将注意力从刺激上移开在很大程度上消除了这些效应。这些发现表明,面部补全由两个协同阶段组成:在较低的视觉区域中进行早期抑制,在较高的视觉区域中进行晚期增强;此外,注意力对于这些神经事件是必要的。