Descending Inputs and the Decoding of Temporally Encoded Sensory Information

感官信息通常是通过主动探索获得的。对世界的认识是通过用手探索一个复杂的表面或用眼睛探索一个视觉场景来获得的。然而，对于神经元如何在感知行为的自然模式下对感觉刺激进行编码，或者大脑中的感觉处理区域如何区分外部世界的属性与动物自身行为的感觉后果，人们所知相对较少。主动感应的一个特别明显的例子是电鱼。电鱼通过发出简短的电器官放电（EOD）脉冲来探测环境，利用电感在黑暗中导航和寻找猎物。附近的物体会扰乱鱼周围的电场，这些扰动会被鱼皮肤上的电感受器检测到。每个受体将局部场强的变化编码为排爆后个体动作电位精确时间的微小变化。因此，鱼获得了一系列关于世界的“快照”，其中关于周围物体的信息被编码为动作电位的时间。在自然界中，这种快照序列的频率和规律取决于行为环境，鱼是在探测物体、觅食还是安静地休息。有趣的是，鱼所选择的频率对每次快照中电感受器动作电位的时间有明显的影响：高频率的移峰较晚，低频率的移峰较早。这些影响的大小与这些鱼类赖以为生的小型无脊椎猎物的影响相当。当鱼自身的感知行为对感官输入有如此强烈的影响时，它是如何探测和捕获猎物的呢？这项研究为探索鱼大脑的感觉处理区域如何解决这种模糊性提供了机会，以及来自电感受器的输入的变化是由于外部刺激还是动物自己的感知行为。在电感觉处理的第一阶段，神经元将来自电感受器的输入与来自鱼的大脑其他区域的信号整合在一起，这些区域与唤起EOD的运动命令有关。原则上，这样的运动指令信号可以“撤销”排爆率对电感受器输入的影响。这项研究有望使人们更好地理解动物是如何利用自身行为的内部知识来区分外部世界的特性和自身行为的感官后果的。在细胞层面上，这些实验也有望更好地理解包含在动作电位精确计时中的信息是如何被神经回路解码或解释的。

Sensory information is often acquired through active exploration. Knowledge of the world is gained by exploring a complex surface with hands or a visual scene with eyes. Yet relatively little is known about how neurons encode sensory stimuli in the context of natural patterns of sensing behavior, or about how sensory processing regions in the brain distinguish properties of the external world from the sensory consequences of the animal's own behavior. A particularly clear example of active sensing is found in mormyrid electric fish. Electric fish use an electrical sense to navigate and find prey in the dark by probing the environment by emitting brief electric organ discharge (EOD) pulses. Nearby objects perturb the electric field around the fish, and these perturbations are detected by electroreceptors in the fish's skin. Each receptor encodes changes in local field strength as small shifts in the precise timing of individual action potentials following the EOD. The fish thus obtains a sequence of "snapshots" of the world, in which information about surrounding objects is encoded in the timing of action potentials. In nature, the frequency and regularity of this sequence of snapshots varies depending on the behavioral context, whether the fish is probing objects, foraging, or quietly resting. Interestingly, the frequency chosen by the fish has a clear effect on the timing of electroreceptor action potentials within each snapshot: higher rates shift spikes later, and lower rates shift spikes earlier. The size of these effects is comparable to the effects of small invertebrate prey on which these fish feed. How does the fish detect and capture prey when its own sensing behavior has such a strong effect on the sensory input? This study provides opportunity to explore how sensory processing regions of the fish's brain resolves the ambiguity, and whether a change in the input from electroreceptors is due to an external stimulus or to the animal's own sensing behavior. Neurons at the first stage of electrosensory processing integrate input from electroreceptors with signals from other areas of the fish's brain linked to the motor command that evokes the EOD. Such motor command signals could, in principal, "undo" the effects of EOD rate on electroreceptor input. The research is expected to lead to a better understanding of how animals use internal knowledge of their actions to distinguish properties of the external world from the sensory consequences of their own behavior. At a more cellular level, the experiements are also expected to lead to a better understanding of how information contained in the precise timing of action potentials is decoded or interpreted by neural circuits.