Functional Anatomy Of Perceptual And Memory Systems In The Primate Brain

一个典型的场景包含许多不同的对象，由于视觉系统的处理能力有限，这些对象竞争神经表示。在神经水平上，多个刺激之间的竞争表现为它们的视觉诱发反应的相互抑制，并且最强烈地发生在接受野水平。多个对象之间的竞争可能会受到自下而上的感官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向信号不仅在有视觉刺激的情况下，而且在没有视觉刺激的情况下，都可以调节视觉皮质的神经活动。虽然刺激之间对表征的竞争最终在视觉皮质内解决，但自上而下的偏向信号的来源可能来自额叶和顶叶皮质区域的分布网络。这种有偏见的注意力竞争模型表明，一旦注意力资源耗尽，就不可能进行进一步的处理。然而，现有的数据表明，情绪刺激“自动”激活大脑区域，很大程度上不受注意力控制。我们测试了另一种可能性，即对带有情绪内容的刺激的神经处理不是自动的，相反需要一定程度的注意。我们的结果显示，与流行的观点相反，所有对情绪面孔做出不同反应的大脑区域，包括杏仁核，只有在有足够的注意力资源来处理面孔时才会这样做。因此，与其他刺激类别的加工类似，面部表情的加工是自上而下控制的。 在我们过去一年完成的工作中，我们研究了前额叶皮质对自上而下控制视觉注意的具体贡献。我们准备了单侧前额叶皮质损伤的猴子，并切断了前脑连合。其结果是，视觉加工只能在一个半球受到前额叶反馈的调节。训练猴子注视一个中心点，并辨别在对照或受PFC损毁影响的半球的有色干扰物之间呈现的有色目标栅格的方向。刺激的位置是不同的，中央线索的颜色指定了每次试验中目标的颜色。这项任务的关键部分是，一次试验中的特定靶子格栅可能会分散下一次试验的注意力。在多次实验中，当线索保持不变时，猴子在使用自上而下的机制将注意力引导到目标位置方面几乎是正常的(根据定向阈值来衡量)。然而，当线索在试验中频繁切换时(一组试验中不超过5次)，猴子会受到严重损害。对错误的分析表明，在线索改变颜色的试验中，动物经常错误地应用前一次试验或几次试验中的线索信息。在不需要自上而下注意控制的改变目标的弹出任务中，猴子没有受到损害。这表明，当动物必须根据时刻变化的信息和任务需求灵活地重新分配注意力时，前额叶皮质起着关键作用。这一结果也可能有助于解释额叶损伤患者的顽固性。

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. In our work completed during the past year, we have examined the specific contributions made by prefrontal cortex to the top-down control of visual attention. We prepared monkeys with unilateral lesions of the prefrontal cortex in combination with transection of the forebrain commissures. As a result, visual processing could be modulated by prefrontal feedback in only one hemisphere. Monkeys were trained to fixate a central spot and discriminate the orientation of a colored target grating presented among colored distracters in either the control hemifield or the hemifield affected by the PFC lesion. The location of the stimuli was varied, and the color of a central cue specified the color of the target on each trial. The critical component of this task was that the specific target grating on one trial might be a distracter on the next. When the cue was held constant for many trials, the monkeys were nearly normal (as measured by orientation thresholds) in using top-down mechanisms to guide attention to the target location. However, the monkeys were severely impaired when the cue was switched frequently across trials (5 trials or less in a block). An analysis of errors indicated that on trials in which the cue changed color, animals frequently continued to incorrectly apply the cue information from the immediately preceding trial, or trials. The monkeys were not impaired in a pop-out task with changing targets that did not require top-down attentional control. This suggest that prefrontal cortex plays a critical role when animals must flexibly reallocate attention on the basis of changing information and task demands from one moment to the next. The results may also help explain perseveration seen in patients with frontal lobe damage.