果蝇如何保持单一目标：果蝇 FC2 中的归一化而非选择
How the fly holds a single goal: normalization, not selection, in Drosophila FC2
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中文总结 AI 辅助
研究果蝇 FC2 神经元如何保持单一目标，通过追踪连接体发现抑制几乎全是全局的，FC2 对外部目标归一化而非选择，FB5A 可能是全局归一化器，明确未解决问题并提出实验检验解释。
中文摘要 AI 辅助
行走的果蝇会朝着目标方向转向，该目标方向以扇形体 FC2 神经元上的活动峰值形式存在。这些神经元还会在一定距离上相互抑制，距离越远抑制越强，这种反馈被认为能使果蝇保持单一目标。我们从连接体出发，探究是何种电路产生这种抑制，以及它是让 FC2 在竞争者中主动选择一个目标（胜者全得），还是仅仅将在其他地方设定的目标保持为一个清晰的峰值。通过追踪单个 FlyWire 大脑中的布线，我们发现这种抑制几乎完全是全局性的：四个 FB5A 细胞大致平等地抑制每个 FC2 神经元，hDelta 中间神经元有较小的、与距离相关的贡献，直接成分可忽略不计。环状吸引子胜者全得（指南针所用的那种）需要局部循环兴奋，而 FC2 布线缺乏这种兴奋，所以这种结构无法构建一个胜者全得机制；在一系列动态模型中，包括一个尖峰网络，在连接体规模的参考耦合下，没有一个电路版本能锁定胜者。因此，FC2 对外部设定的目标进行归一化而非选择，FB5A 可能充当全局归一化器，就像 APL 神经元在蘑菇体中所起的作用一样。我们明确指出两个未解决的问题：一种不同的机制，即两个竞争目标之间的相互抑制（由 hDelta 提供），原则上在非常强的耦合下可能进行选择，我们对其进行限制而非排除；FB5A 的抑制特性是连接体递质分类器的低置信度预测，尚未测量，且可能不是 GABA 能的。然后我们探究目标实际是在哪里设定的：连接体提名一个上游 hDelta 网络，并排除了主要的替代方案，其神经元提供的输入不到 FC2 的 0.2%。最后，我们提出一个直接实验，在对 FC2 成像时沉默 FB5A，以检验这一解释。
英文摘要
A walking fly steers toward a goal direction, held as a bump of activity across the FC2 neurons of the fan-shaped body. These neurons also inhibit one another over distance, more strongly the farther apart they are, a feedback proposed to keep the fly on a single goal. We asked, from the connectome, what circuit produces this inhibition, and whether it lets FC2 actively choose one goal among competitors (a winner-take-all) or simply keeps a goal set elsewhere as one clean bump. Tracing the wiring in a single FlyWire brain, we find the inhibition is almost entirely global: four FB5A cells inhibit every FC2 neuron roughly equally, with a smaller, distance-dependent contribution from hDelta interneurons and a negligible direct component. A ring-attractor winner-take-all (the kind the compass uses) requires local recurrent excitation that the FC2 wiring lacks, so this geometry cannot build one; and across a range of dynamical models, including a spiking network, no version of the circuit locks onto a winner at the connectome-scaled reference coupling. FC2 therefore normalizes an externally set goal rather than selecting it, with FB5A likely acting as the global normalizer, much as the APL neuron does in the mushroom body. We are explicit about two open points: a different mechanism, mutual inhibition between two competing goals (which hDelta supplies), could in principle select at very strong coupling, and we bound rather than exclude it; and FB5A's inhibitory identity is a low-confidence prediction of the connectome's transmitter classifier, not yet measured, and likely not GABAergic. We then ask where the goal is actually set: the connectome nominates an upstream hDelta network and rules out the leading proposed alternative, whose neurons supply under 0.2% of FC2's input. Finally, we propose a direct experiment, silencing FB5A while imaging FC2, that would test the account.
