A biophysical mechanism for preferred direction enhancement in fly motion vision

Seeing the direction of motion is essential for survival of all sighted animals. Consequently, nerve cells that respond to visual stimuli moving in one but not in the opposite direction, so-called ‘direction-selective’ neurons, are found abundantly. In general, direction selectivity can arise by either signal amplification for stimuli moving in the cell’s preferred direction (‘preferred direction enhancement’), signal suppression for stimuli moving along the opposite direction (‘null direction suppression’), or a combination of both. While signal suppression can be readily implemented in biophysical terms by a hyperpolarization followed by a rectification corresponding to the nonlinear voltage-dependence of the Calcium channel, the biophysical mechanism for signal amplification has remained unclear so far. Taking inspiration from the fly, I analyze a neural circuit where a direction-selective ON-cell receives inhibitory input from an OFF cell on the preferred side of the dendrite, while excitatory ON-cells contact the dendrite centrally. This way, an ON edge moving along the cell’s preferred direction suppresses the inhibitory input, leading to a release from inhibition in the postsynaptic cell. The benefit of such a two-fold signal inversion lies in the resulting increase of the postsynaptic cell’s input resistance, amplifying its response to a subsequent excitatory input signal even with a passive dendrite, i.e. without voltage-gated ion channels. A motion detector implementing this mechanism together with null direction suppression shows a high degree of direction selectivity over a large range of temporal frequency, narrow directional tuning, and a large signal-to-noise ratio.

感知运动方向是所有具视觉动物生存的必要前提。因此，能够对朝单一方向而非相反方向运动的视觉刺激产生响应的神经细胞——即所谓的“方向选择性神经元（direction-selective neurons）”——广泛分布于生物体内。
一般而言，方向选择性可通过三种途径产生：对细胞首选方向运动的刺激实施信号放大（即“首选方向增强（preferred direction enhancement）”）、对沿相反方向运动的刺激实施信号抑制（即“零方向抑制（null direction suppression）”），或是二者结合。
尽管信号抑制可通过生物物理方式轻松实现：先产生超极化，再伴随与钙离子通道（Calcium channel）非线性电压依赖性相对应的整流作用，但信号放大的生物物理机制迄今仍未阐明。
本研究以果蝇为灵感来源，分析了一类神经环路：方向选择性ON型细胞（ON-cell）从其树突首选侧的OFF型细胞（OFF-cell）接收抑制性输入，而兴奋性ON型细胞则在树突中央与之形成突触连接。
通过该环路结构，沿细胞首选方向运动的ON边缘会抑制该抑制性输入，使突触后细胞（postsynaptic cell）获得解除抑制的效果。
这种双重信号反转机制的优势在于，它可提升突触后细胞的输入电阻，即便在仅具备被动树突（即不含电压门控离子通道（voltage-gated ion channels））的情况下，也能放大突触后细胞对后续兴奋性输入信号的响应。
将该机制与零方向抑制相结合的运动检测器，可在较宽的时间频率范围内展现出极高的方向选择性，同时具备狭窄的方向调谐特性与优异的信噪比（signal-to-noise ratio）。