Following neuronal signals of multiple visual stimuli through cortical pathways to identify attentional gating mechanisms

自然场景的处理需要我们大脑的视觉系统同时处理大量独立的视觉刺激。在视网膜图像中，它们通常是紧密相连的。由于神经元连接沿着视觉处理通路的强烈发散和汇聚，神经元通常会接收来自不止一个视觉刺激的信号。然而，如果选择性地注意到其中一个刺激，神经元就能够选择性地处理这个刺激。他们能够做出反应，就好像只有被注意到的刺激才会出现，并抑制来自其他刺激的往往更多、更强的信号。虽然这种注意力依赖的选择性刺激加工已经被很好地记录下来，但潜在的神经元机制并没有得到很好的理解和有争议的讨论。我们实验室和其他实验室之前的结果表明，这种非凡的能力可以用注意力依赖的门控机制来解释。它们允许编码注意到的刺激的神经元输入信号的适当子集通过。相反，所有其他输入信号都被抑制。这种门控可以用两种主要的机制来解释。所谓的异步机制假设，特定的电路以注意力依赖的方式干扰信号传递，例如通过调制突触传递或取消具有匹配的抑制输入的兴奋性信号。另一方面，同步机制意味着特定的时空活动模式，调制信号接收神经元和传入输入之间的信号传输。与行为相关的输入在伽马频段(30-100赫兹)与信号接收神经元同步振荡，并具有特定的相位差，从而实现最佳信号传输。所有其他输入的活动模式避免了这种特定的阶段关系，因此相当受抑制。我们以前已经证明了相应的注意依赖的同步模式，但还不知道它们是负责信号选通还是更确切地说是附带现象。本项目的主要目标是调查注意依赖的选通是来自异步机制还是同步机制。使用我们之前开发的方法，我们将标记单个刺激的神经元信号。这将允许跟踪视觉皮质区域之间的信息流，并观察偶尔偏离最佳相位是否与来自所关注的刺激的信号传输的减少有关。如果信号传输确实严格依赖于相位关系，则结果将有力地支持同步选通机制。如果信号传输的波动最终证明是相位独立的，则必须摒弃同步机制的假设，转而支持异步机制。

Processing of natural scenes requires the visual system of our brain to handle a large number of independent visual stimuli at the same time. Often they are located close together in the retinal image. As a consequence of strong divergence and convergence of neuronal connections along the visual processing pathways, neurons typically receive signals originating from more than one visual stimulus. Nevertheless, neurons are capable to process selectively one of those stimuli if selective attention is directed to this stimulus. They are capable to respond as if only the attended stimulus would be present and suppress the often more numerous and stronger signals from other stimuli. While such attention-dependent selective stimulus processing is well documented, the underlying neuronal mechanisms are not well understood and discussed controversially.Previous results from our lab and others suggest, that this remarkable capability can be explained by attention-dependent gating mechanisms. They allow the appropriate subset of a neuron's input signals that encode the attended stimulus to pass. In contrast, all other input signals are suppressed. This gating can be explained by two major types of mechanisms. The so called asynchronous mechanisms assume, that specific circuitry interferes in an attention-dependent manner with signal delivery, e.g. by modulating synaptic transmission or canceling excitatory signals with matched inhibitory input. On the other hand, synchronous mechanisms imply specific spatio-temporal activity patterns modulating the transmission of signals, between the signal receiving neurons and the afferent input. The behaviorally relevant inputs oscillate synchronously with the signal receiving neurons in the gamma-band (30 - 100 Hz) and with a specific phase difference, allowing for optimal signal transmission. The activity patterns of all other inputs avoid this specific phase relation and are therefore rather suppressed. We have previously demonstrated corresponding patterns of attention dependent synchronization, but it is not known whether they are responsible for signal gating or rather epiphenomenal.Major goal of the project is to investigate, whether attention-dependent gating results from an asynchronous or a synchronous mechanism. Using a method we developed previously, we will tag the neuronal signals of individual stimuli. This will allow to follow the flow of information between visual cortical areas and to observe whether occasional deviations from the optimal phase are associated with reduction of transmission of signals from the attended stimulus. The results will strongly support the synchronous gating mechanism if signal transmission depends indeed strictly on phase relations. If fluctuations of signal transmission turn out to be phase-independent, the hypothesis of a synchronous mechanism has to be rejected in favor of an asynchronous mechanism.