Functional Anatomy Of Perceptual And Memory Systems

一个典型的场景包含许多不同的对象，由于视觉系统有限的处理能力，这些对象竞争神经表征。在神经层面上，多种刺激之间的竞争表现为视觉诱发反应的相互抑制，并在感受野层面上发生得最为强烈。多个对象之间的竞争可能受到自下而上的感觉驱动机制和自上而下的影响（如选择性注意）的影响。脑功能成像研究表明，选择性注意引起的偏置信号不仅可以在有视觉刺激的情况下，也可以在没有视觉刺激的情况下调节视觉皮层的神经活动。虽然刺激之间的表征竞争最终在视觉皮层内得到解决，但自上而下的偏倚信号的来源可能来自额叶和顶叶皮层区域的分布式网络。这种注意力的偏见竞争模型表明，一旦注意力资源耗尽，就不可能再进行进一步的处理。然而，现有数据表明，情绪刺激会“自动”激活大脑区域，在很大程度上不受注意力控制的影响。我们测试了另一种可能性，即，神经处理带有情绪内容的刺激不是自动的，而是需要一定程度的注意力。我们的研究结果表明，与主流观点相反，只有当有足够的注意力资源来处理这些面孔时，包括杏仁核在内的所有大脑区域才会对情绪激动的面孔做出不同的反应。因此，与其他刺激类别的加工类似，面部表情的加工也受到自上而下的控制。

A typical scene contains many different objects that compete for neural representation due to the limited processing capacity of the visual system. At the neural level, competition among multiple stimuli is evidenced by the mutual suppression of their visually evoked responses and occurs most strongly at the level of the receptive field. The competition among multiple objects can be biased by both bottom-up sensory-driven mechanisms and top-down influences, such as selective attention. Functional brain imaging studies reveal that biasing signals due to selective attention can modulate neural activity in visual cortex not only in the presence but also in the absence of visual stimulation. Although the competition among stimuli for representation is ultimately resolved within visual cortex, the source of top-down biasing signals likely derives from a distributed network of areas in frontal and parietal cortex. This biased competition model of attention suggests that once attentional resources are depleted, no further processing is possible. Yet, existing data suggest that emotional stimuli activate brain regions "automatically," largely immune from attentional control. We tested the alternative possibility, namely, that the neural processing of stimuli with emotional content is not automatic and instead requires some degree of attention. Our results revealed that, contrary to the prevailing view, all brain regions responding differentially to emotional faces, including the amygdala, did so only when sufficient attentional resources were available to process the faces. Thus, similar to the processing of other stimulus categories, the processing of facial expression is under top-down control.