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, such as stimulus salience, and top-down influences, such as selective attention. 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. Recently, we found that monkeys with lesions of prefrontal cortex (PFC) are selectively impaired in their ability to switch top-down control. In humans, we dound that attentive switching results in enhanced activation of several fronto-parietal and posterior visual areas. Taken together, our findings indicate that both frontal and parietal cortices are involved in generating top-down control signals for attentive switching, which may then be fed back to visual processing areas. The PFC in particular plays a critical role in the ability to switch attentional control on the basis of changing task demands. The 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 most recent work, we aimed to better characterize the nature of distractibility in ADHD by testing hypotheses about whether distractibility arises from increased sensory-driven interference or from inefficient top-down control. We employed an attentional filtering paradigm in which discrimination difficulty and distractor salience were parametrically manipulated. Increased discrimination difficulty should add to the load of top-down processes, whereas increased distractor salience should result in stronger sensory interference. We found a striking interaction of discrimination difficulty and distractor salience: For difficult discriminations, ADHD children filtered distractors as efficiently as healthy children and adults, and all groups were slower to respond with high vs. low salience distractors. In contrast, for easy discriminations, ADHD children were much slower and made more errors than healthy children and adults. For easy discriminations, healthy children and adults filtered out high salience distractors as easily as low salience distractors, but ADHD children were slower to respond on trials with low salience distractors than they did on trials with high salience distractors. The fact that ADHD children exhibit efficient attentional filtering when task demands are high, but show deficient and atypical distractor filtering under low task demands suggests that filtering mechanisms remain intact in these children but the trigger for activating attention is selectively impaired.
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