Descending Inputs and the Decoding of Temporally Encoded Sensory Information

感官信息往往是通过主动探索获得的。 世界的知识是通过用手探索复杂的表面或用眼睛探索视觉场景来获得的。 然而，关于神经元如何在感知行为的自然模式的背景下编码感觉刺激，或者大脑中的感觉处理区域如何区分外部世界的属性和动物自身行为的感觉结果，我们知道的相对较少。一个特别明显的主动感知的例子是在长吻电鱼中发现的。电鱼利用电感觉在黑暗中导航和寻找猎物，通过发出短暂的电器官放电（EOD）脉冲来探测环境。附近的物体会干扰鱼周围的电场，这些干扰会被鱼皮肤上的电感受器检测到。 每个受体编码局部场强的变化，作为EOD后个体动作电位精确计时的小偏移。因此，鱼获得了世界的一系列“快照”，其中关于周围物体的信息被编码在动作电位的时间中。在自然界中，这种快照序列的频率和规律性取决于行为背景，无论鱼是探测物体，觅食还是安静地休息。有趣的是，鱼选择的频率对每个快照中的电感受器动作电位的时间有明显的影响：较高的频率移动尖峰较晚，较低的频率移动尖峰较早。这些影响的大小与这些鱼赖以为生的小型无脊椎动物猎物的影响相当。当鱼自身的感觉行为对感觉输入有如此强烈的影响时，它是如何发现和捕获猎物的？这项研究提供了探索鱼的大脑的感觉处理区域如何解决模糊性的机会，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身的感觉行为。 神经元在电感觉处理的第一阶段将来自电感受器的输入与来自鱼类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。这样的运动指令信号可以，原则上，“撤销”EOD率对电感受器输入的影响。这项研究有望更好地理解动物如何利用其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。 在更多的细胞水平上，这些实验也有望更好地理解神经回路如何解码或解释动作电位精确计时中包含的信息。

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.