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，并且最强烈地发生在接受野水平。多个物体之间的竞争可能会受到自下而上的感官驱动机制(如刺激突显)和自上而下的影响(如选择性注意)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。最近，我们报道了患有前额叶皮质(PFC)损伤的猴子在切换自上而下控制的能力方面选择性地受损。在过去的一年里，我们询问了后顶叶皮质(PPC)受损的猴子是否会表现出相似或不同的行为影响。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。这些调查结果正在准备出版。 在过去的一年里，我们还旨在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，还是来自低效的自上而下控制。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慢于在高显着分心物试验中的反应。ADHD儿童在高任务要求时表现出有效的注意力过滤，但在低任务要求下表现出缺乏和非典型的分心过滤，这一事实表明这些儿童的过滤机制保持完好，但激活注意力的触发器选择性地受损。 知觉学习是指由于对视觉刺激的重复体验而改善了对视觉刺激的检测和识别。关于注意力在知觉学习中所起的作用，文献中有相互矛盾的证据。为了进一步研究这个问题，我们独立地操纵了外源性(非自愿)和内源性(自愿)注意，并测量了不同视野象限呈现的定向刺激的知觉学习速度。通过这种方式，我们可以跟踪有人值守、有人值守和无人值守地点的学习情况。我们还测量了训练前和训练后刺激的对比阈值。我们的结果表明，对于外源性和内源性注意，在有人注意的地点进行定向辨别的准确性比在无人注意的地点有更大程度的提高。然而，重要的是，只有外源性注意力才能改善对比度阈值。这些发现表明，外源性和内源性注意都有助于知觉学习，但只有外源性注意才能提高对训练后的视觉刺激的敏感性。&#8232； 人类大脑的许多区域，特别是与内源性(自愿)注意相关的枕叶视皮层和额叶和顶叶区域，都有记录表明视网膜选择性是通过神经元活动模式来衡量的，这种模式与视觉刺激的位置一致。我们假设，视网膜选择性也存在于外源性(非自愿)注意的活跃区域。为了验证这一假设，我们获得了受试者在中心十字保持注视时的fMRI数据。以不可预测的时间间隔，由一系列快速扩展的圆圈组成的刺激出现在六个空间位置中的一个位置。在多个大脑区域，包括颞顶交界区(TPJ)，发现了对刺激呈现的阳性fMRI激活，这是一个先前与外源性注意有关的区域。TPJ的激活似乎没有被组织成横跨皮质表面的地图。然而，多体素模式识别分析成功地预测了15个刺激位置对中每一个的fMRI反应，证明了TPJ的活动模式取决于视网膜上的刺激位置。这是第一次证明空间位置出现在与外源性注意力相关的大脑区域。

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 reported that monkeys with lesions of prefrontal cortex (PFC) are selectively impaired in their ability to switch top-down control. In the past year, we 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. These findings are being prepared for publication.

 During the past year, we also 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 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.

 Perceptual learning refers to improved detection and discrmination of visual stimuli as a result of repeated experience with such stimuli. There is conflicting evidence in the literature regarding the role played by attention in perceptual learning. To further examine this issue, we independently manipulated exogenous (involuntary) and endogenous (voluntary) attention and measured the rate of perceptual learning of oriented stimuli presented in different quadrants of the visual field. In this way, we could track learning at attended, divided-attended, and unattended locations. We also measured contrast thresholds of the stimuli before and after training. Our results showed that, for both exogenous and endogenous attention, accuracy in performing the orientation discrimination improved to a greater extent at attended than at unattended locations. Importantly, however, only exogenous attention resulted in improved contrast thresholds. These findings suggest that both exogenous and endogenous attention facilitate perceptual learning, but that only exogenous attention enhances sensitivity to a trained visual stimulus. 
 Retinotopic selectivity, as measured by neuronal activity patterns that vary consistently with the location of visual stimuli, has been documented in many human brain regions, notably occipital visual cortex and frontal and parietal regions associated with endogenous (voluntary) attention. We hypothesized that retinotopic selectivity also exists in regions active during exogenous (involuntary) attention. To test this hypothesis, we acquired fMRI data while subjects maintained fixation on a central cross. At unpredictable time intervals, stimuli consisting of an array of rapidly expanding circles appeared at one of six spatial locations. Positive fMRI activations to the stimulus presentations were identified in multiple brain regions including the temporoparietal junction (TPJ), a region previously implicated in exogenous attention. The TPJ activations did not appear to be organized as a map across the cortical surface. However, multivoxel pattern recognition analysis successfully predicted fMRI responses to every one of the fifteen stimulus location pairs, demonstrating that patterns of activity in TPJ depend on the retinotopic stimulus location. This is the first demonstration that spatial locations are represented in a brain region associated with exogenous attention.