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，它们通常紧密地排列在一起。由于沿视觉处理通路的神经元连接的强烈发散和收敛，神经元通常接收来自多个视觉刺激的信号。然而，如果选择性注意被引导到这些刺激上，神经元能够选择性地处理其中一个刺激。他们有能力做出反应，就好像只有被关注的刺激才会出现，并抑制来自其他刺激的更多、更强的信号。虽然这种注意力依赖的选择性刺激处理有很好的记录，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。我们实验室和其他人之前的研究结果表明，这种非凡的能力可以用注意力依赖门控机制来解释。它们允许神经元输入信号的适当子集通过，这些信号编码了被关注的刺激。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类型的机制来解释。所谓的异步机制假设，特定的电路以注意依赖的方式干扰信号传递，例如通过调节突触传递或通过匹配的抑制性输入取消兴奋性信号。另一方面，同步机制意味着在信号接收神经元和传入输入神经元之间调节信号传输的特定时空活动模式。行为相关的输入与信号接收神经元在伽马波段（30 - 100 Hz）同步振荡，并具有特定的相位差，从而实现最佳的信号传输。所有其他输入的活动模式都避免了这种特定的相位关系，因此受到相当大的抑制。我们之前已经证明了相应的注意依赖同步模式，但尚不清楚它们是否负责信号门控或更确切地说是副现象。该项目的主要目标是调查，注意力依赖门控是由异步还是同步机制产生的。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层区域之间的信息流，并观察偶尔偏离最佳相位是否与来自参与刺激的信号传输减少有关。如果信号传输确实严格依赖于相位关系，结果将有力地支持同步门控机制。如果信号传输的波动被证明是与相位无关的，那么同步机制的假设必须被拒绝，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.