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，由于视觉系统有限的处理能力，这些对象竞争神经表征。在神经层面上，多种刺激之间的竞争表现为视觉诱发反应的相互抑制，并在感受野层面上发生得最为强烈。多个对象之间的竞争可能存在外因（由自下而上的感觉驱动机制，如刺激显著性）和内因（由自上而下的影响，如选择性注意）。虽然刺激之间的表征竞争最终在视觉皮层内得到解决，但自上而下的偏倚信号的来源可能来自额叶和顶叶皮层区域的分布式网络。最近，我们报道了前额皮质（PFC）损伤的猴子选择性地削弱了他们切换自上而下控制的能力。我们接下来问是否猴子损伤后顶叶皮层（PPC）会表现出相似或不同的行为影响。我们的研究结果表明，与患有PFC病变的猴子不同，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受损。更确切地说，他们在空间定位目标方面有选择性障碍，他们需要区分目标。因此，PFC在根据任务需求的变化转换注意力控制的能力中起着关键作用，而PPC在将注意力资源分配到与行为相关的空间位置方面起着关键作用。这些发现正在准备发表。
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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 both exogenously, by bottom-up sensory-driven mechanisms, such as stimulus salience, and endogenously, by 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. We next 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.

 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 these two types of attention may be mediated by different neural mechanisms. A paper reporting these findings was published this past year. Retinotopic selectivity, as measured by fMRI 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 passive 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. The data collection for this study is now complete. Threatening stimuli evoke exogenous (involuntary) attention and enhanced neural processing, but this can vary considerably even among healthy individuals. By manipulating expectation of threat in an event-related fMRI study of fearful vs. neutral face categorization, we aimed to better understand the spectrum of threat processing sensitivity. We altered expectation explicitly by presenting face images within runs containing three different proportions of fearful(F):neutral (N)faces: 80F:20N, 20F:80N, and 50F:50N, and subjects were instructed to report as fast and as accurately as possible whether the face was fearful (i.e. signaled threat) or not. A neutral cue preceded each face by 4 seconds (s). Overall subjects responded faster to the face type that was expected. However, across the subjects there was a spectrum of response times. By taking the difference in reaction time between responses to fearful and neutral faces, we quantified a fear reaction time bias (faster to fearful than neutral faces) for all subjects. This bias positively correlated with late trial fMRI activation during unexpected fearful face trials in the ventromedial prefrontal cortex bilaterally, the left subgenual cingulate cortex, and the right caudate nucleus and negatively correlated with early trial fMRI activation during expected neutral faces trials in the dorsal striatum bilaterally and in right ventral striatum. These results underscore that a large amount of variability exists in the processing of stimuli that signal threat in healthy adults, which is reflected not only in behavior but also in the magnitude of activation in brain regions implicated in value-based decision making. This study is being prepared for publication. It is well known that people take advantage of prior knowledge to bias decisions, that is, things they have experienced and attended in the past. To investigate this phenomenon behaviorally and in the brain, we acquired fMRI data while human subjects viewed ambiguous abstract shapes and decided whether a shape was of Category A (smoother) or B (bumpier). The decision was made in the context of one of two prior knowledge cues, 80/20 and 50/50. The 80/20 cue indicated that upcoming shapes had an 80% probability of being of one category, e.g., B, and a 20% probability of being of the other. The 50/50 cue indicated that upcoming shapes had an equal probability of being of either category. The ideal observer would bias decisions in favor of the indicated alternative at 80/20 and show zero bias at 50/50. We found that subjects did bias their decisions in the predicted direction at 80/20 but did not show zero bias at 50/50. Instead, at 50/50 the subjects retained biases of the same sign as their 80/20 biases, though of diminished magnitude. The signature of a persistent though diminished bias at 50/50 was also evident in fMRI data from frontal and parietal regions previously implicated in decision-making. Behavioral and fMRI data from nave subjects reflected decision biases closer to those of the ideal observer. The results indicate that practice making decisions in the context of non-equal prior probabilities biases decisions made later when prior probabilities are equal. This finding may be related to the "anchoring and adjustment" strategy described in the psychology, economics, and marketing literatures, in which subjects adjust a first approximation response - the "anchor" - based on additional information, typically applying insufficient adjustment relative to the ideal observer. A paper describing these findings was published in the past year.