Top-down control of visual processing and awareness: Studies with transcranial magnetic stimulation and electroencephalography.

人类只知道周围环境的一小部分。神经系统能够对进入意识的感觉输入和没有进入意识的感觉输入进行分类。我们的假设是，大脑额叶中控制行为的区域，能够影响大脑后部对来自感官的输入的处理。更具体地说，我们想研究大脑中与眼睛运动有关的区域，即额叶视野，是如何与其他负责视觉的区域相互作用的。我们使用功能性磁共振成像来拍摄每个参与者的大脑，并识别当他/她移动眼睛或看视觉显示时活跃的区域。然后我们可以特别地操纵这些区域的活动，即使在没有运动或视觉刺激的情况下。这是通过经颅磁刺激（TMS）实现的。颅磁刺激是通过间接无痛地通过颅骨诱导电流，刺激大脑的一个有限区域（约1平方厘米），持续十分之一毫秒。感应电流干扰正在进行的神经元活动。如果经颅磁刺激脉冲导致行为或生理变量发生变化，那么我们可以得出结论，受刺激的大脑区域对调节这些变量至关重要。在第一项研究中，我们建议在前额视野上应用经颅磁刺激来评估它是否影响视觉运动的感知。事实上，当我们移动眼睛时，我们并没有意识到视网膜图像位移引起的模糊。这是因为眼球运动时对视觉运动的敏感性被抑制了。我们的方法将允许我们测试前额视野是否可能是这种抑制的起源。此外，我们将通过结合两个区域的TMS来测试前额视野是否与一个已知对视觉运动感知至关重要的区域V5/MT相互作用。这一点很重要，因为大多数视觉理论认为，意识是通过视觉区域之间的多重追溯相互作用来调节的。在第二组实验中，我们将在前额眼野上应用经颅磁刺激，同时用脑电图（EEG）测量其对其他视觉区域活动的影响。脑电图是一种允许研究人员记录头皮表面电流的技术。这些电流每毫秒都反映了潜在神经元群的活动。因此，我们将能够直接检查来自额叶的信号如何影响位于枕叶的远端神经元，枕叶对视觉刺激的反应是活跃的。这是一个重要的问题，因为许多心理物理学研究表明，涉及眼球运动的大脑区域在注意力中起着至关重要的作用。注意是负责选择感官特征进入意识的主要机制之一。我们提出的方法在方法上具有挑战性，因为它结合了几种复杂的技术。但与更传统的脑成像方法相比，它有几个优势。首先，它允许我们在空间和时间上精确地瞄准一个区域。其次，它不仅建立了大脑活动和行为变量之间的相关性，还使我们能够测试某个区域是否以及在什么时候对给定任务是必要的。第三，它使我们能够评估两个大脑区域之间相互作用的时间和功能意义。最后，它在概念上接近非人灵长类动物中使用的技术，这使我们能够将我们的结果与在动物中获得的大量数据进行比较。该提案与BBRSC动物科学委员会当前的优先事项“从神经元到行为”高度相关。获得的数据肯定会拓宽我们对人类视觉系统的理解。它们将有助于支持这样一种观点，即从它们进入大脑开始，感官信号就受到反馈控制的影响，这取决于我们的行为、期望和记忆。

Humans are aware of only a fraction of their environment. The nervous system is able to sort the sensory inputs that enter consciousness and those that do not. Our hypothesis is that brain regions that control action, in the frontal lobe, are able to influence the processing of input coming from the senses in the posterior part of the brain. More specifically we want to study how a brain region involved in moving the eyes, the frontal eye field, also interacts with other regions dedicated to vision. We use functional magnetic resonance imaging to take pictures of each participant brain and identify the regions active when he/she moves the eyes or looks at visual displays. Then we can manipulate specifically the activity of these regions, even in the absence of movement or visual stimuli. This is achieved with transcranial magnetic stimulation (TMS). TMS consists in stimulating a restricted area of the brain (about 1cm2) for a tenth of millisecond, by indirectly and painlessly inducing a current through the skull. The induced current interferes with the ongoing neuronal activity. If consequently to a TMS pulse a behavioural or physiological variable changes, then we can conclude that the stimulated brain region is crucial for regulating this variable. In a first study, we propose to apply TMS over the frontal eye field to assess whether it influences the perception of visual motion. Indeed, when we move the eyes, we are not aware of the blurring induced by the retinal image displacement. This is because sensitivity to visual motion is suppressed during eye movement. Our approach will allow us to test whether the frontal eye field could be at the origin of this suppression. Moreover, we will test whether the frontal eye field interacts with a region known to be crucial for visual motion perception, V5/MT, by combining TMS of the two regions. This is important since most theories of vision suggest that awareness is mediated through multiple retroactive interactions between visual areas. In a second set of experiments, we will apply TMS over the frontal eye field while we measure its effects on other visual areas activity with electroencephalography (EEG). EEG is a technique that allows researchers to record currents at the surface of the scalp. Those currents reflect, every millisecond, the activity of underlying neuronal populations. Therefore we will be able to examine directly how a signal originating from the frontal lobe influences distal neurons, located in the occipital lobe, which are active in response to a visual stimulation. This is an important question since many psychophysical studies indicate that the brain regions involved in eye movements play a crucial role in attention. Attention is one of the main mechanisms responsible for selecting sensory features to enter awareness. The approach we propose is challenging methodologically, since it combines several complex techniques. But it has several advantages over more conventional brain imaging methods. First, it allows us to target precisely one region in space and time. Second, more than establishing a correlation between brain activity and behavioural variables, it enables us to test whether and at what moment a region is necessary for a given task. Third, it allows us to assess the timing and functional significance of interactions between two brain regions. Finally, it is close conceptually to techniques employed in non-human primates, which enables us to compare our results with the large amount of data obtained in animals. This proposal is highly relevant to the current priority of the BBRSC Animal Sciences Committee ' From Neurons to Behaviour'. The data gained will certainly broaden our understanding of the human visual system. They will contribute to support the view that, from their entry into the brains, the sensory signals are shaped by feedback control, depending on our actions, our expectations, our memory.