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. The competition among multiple objects can be biased by both bottom-up sensory-driven mechanisms (exogenous attention), such as stimulus salience, and top-down, goal-directed influences, such as selective (endogenous) 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. We previously reported that monkeys with lesions of prefrontal cortex (PFC) are impaired in their ability to switch top-down control. We then asked whether monkeys with lesions of posterior parietal cortex (PPC) would show similar or different behavioral effects. Our results showed that, unlike monkeys with PFC lesions, those with PPC lesions are not selectively impaired in their ability to switch top-down control. Rather, they have a selective impairment in spatially locating targets they are required to discriminate. Thus, the PFC plays a critical role in the ability to switch attentional control on the basis of changing task demands, whereas the PPC plays a critical role in allocating attentional resources to behaviorally relevant spatial locations. Another major goal has been 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 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. Previous research has suggested that the right middle frontal gyrus (rMFG) may serve as a node of interaction between neural networks for top-down goal-directed endogenous attention and bottom-up, stimulus-driven exogenous attention. We tested this hypothesis by comparing the performance on an orientation discrimination task of a patient with a rMFG resection (to remove a brain tumor) and healthy controls. On endogenous attention trials, a valid central cue predicted with 90% accuracy the location of a perithreshold Gabor patch. On the 10% invalid trials, the Gabor patch appeared in the opposite location to the cue. On exogenous attention trials, a cue appeared briefly at one of two peripheral locations, followed, after a variable interstimulus interval (ISI;range 0 to 700 ms), by a Gabor patch in either the same (valid) or opposite location (invalid). Analysis of behavioral data showed that for both patient and controls, valid cues facilitated faster reaction times compared to invalid cues, on endogenous and short ISI exogenous trials. However, at longer ISI exogenous trials, the patient was unable to withhold his responses, resulting in reduced performance compared to controls. This may be related to the patient's inability to reorient attention in a top-down fashion after the effect of the exogenous cue has dissipated, and suggests a putative role of the rMFG in switching between exogenous and endogenous modes of attention. We are continuing to test this hypothesis.
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