Functional Anatomy Of Perceptual and Attentional 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。多个对象之间的竞争可能会受到自下而上的感觉驱动机制(外源性注意)和自上而下的目标导向影响(如选择性(内源性)注意)的影响。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。 我们之前曾报道，前额叶皮质(PFC)受损的猴子切换自上而下控制的能力受损。然后，我们询问后顶叶皮质(PPC)受损的猴子是否会表现出相似或不同的行为效果。我们的结果表明，与患有PFC病变的猴子不同，那些患有PPC病变的猴子在切换自上而下控制的能力方面没有选择性地受损。相反，他们在空间定位他们需要区分的目标方面存在选择性损害。因此，PFC在根据不断变化的任务需求切换注意控制的能力中起着关键作用，而PPC在将注意资源分配到行为相关的空间位置方面起着关键作用。 另一个主要目标是通过测试以下假设来更好地描述ADHD患者注意力分散的性质：注意力分散是由增加的感觉驱动干扰引起的，还是由低效的自上而下控制引起的。我们采用了注意过滤范式，在该范式中，辨别难度和分心突显被参数控制。辨别难度的增加应该会增加自上而下过程的负担，而分心突出程度的增加应该会导致更强的感觉干扰。我们发现，辨别难度和分心显著度之间存在显著的交互作用：对于困难的辨别，ADHD儿童过滤分心项的效率与健康儿童和成人一样高，而且所有组对高显着性分心物的反应都慢于低显着性分心物。相比之下，在容易辨别的情况下，ADHD儿童比健康儿童和成年人慢得多，犯错误也更多。为了容易辨别，健康的儿童和成年人像过滤低显著干扰物一样容易过滤掉高显著分心物，但ADHD儿童在低显着分心物试验中的反应比在高显著分心物试验中的反应慢。ADHD儿童在高任务要求时表现出有效的注意过滤，但在低任务要求下表现出缺乏和非典型的分心过滤，这表明这些儿童的过滤机制保持不变，但激活注意的触发被选择性地受损。 以前的研究表明，右额中回(RMFG)可能是自上而下目标导向的内源性注意和自下而上的刺激驱动的外源性注意的神经网络之间相互作用的节点。我们通过比较接受rMFG切除(切除脑瘤)的患者和健康对照组在方位辨别任务中的表现来检验这一假设。在内源性注意试验中，有效的中央线索以90%的准确率预测了阈值周围Gabor贴片的位置。在10%的无效试验中，Gabor补丁出现在与线索相反的位置。在外源性注意试验中，线索在两个外围位置中的一个短暂出现，然后在可变的刺激间间隔(ISI；范围0到700ms)之后，在相同(有效)或相反的位置(无效)出现Gabor补丁。行为数据分析显示，在内源性和短ISI外源性试验中，与无效线索相比，对于患者和对照组来说，有效线索有助于更快的反应时间。然而，在更长的ISI外源性试验中，患者无法抑制他的反应，导致与对照组相比表现降低。这可能与患者在外源性线索的影响消失后无法以自上而下的方式重新定向注意有关，并提示rMFG在外源性和内源性注意模式之间的转换中可能起到了作用。我们正在继续检验这一假设。

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.