Department of Biomedical Engineering, Northwestern University, Evanston, IL, USA.
Department of Biological Structure, Washington National Primate Research Center, University of Washington, Seattle, WA, USA.
Vis Neurosci. 2023 May 23;40:E003. doi: 10.1017/S0952523823000019.
In a recent study, visual signals were recorded for the first time in starburst amacrine cells of the macaque retina, and, as for mouse and rabbit, a directional bias observed in calcium signals was recorded from near the dendritic tips. Stimulus motion from the soma toward the tip generated a larger calcium signal than motion from the tip toward the soma. Two mechanisms affecting the spatiotemporal summation of excitatory postsynaptic currents have been proposed to contribute to directional signaling at the dendritic tips of starbursts: (1) a "morphological" mechanism in which electrotonic propagation of excitatory synaptic currents along a dendrite sums bipolar cell inputs at the dendritic tip preferentially for stimulus motion in the centrifugal direction; (2) a "space-time" mechanism that relies on differences in the time-courses of proximal and distal bipolar cell inputs to favor centrifugal stimulus motion. To explore the contributions of these two mechanisms in the primate, we developed a realistic computational model based on connectomic reconstruction of a macaque starburst cell and the distribution of its synaptic inputs from sustained and transient bipolar cell types. Our model suggests that both mechanisms can initiate direction selectivity in starburst dendrites, but their contributions differ depending on the spatiotemporal properties of the stimulus. Specifically, the morphological mechanism dominates when small visual objects are moving at high velocities, and the space-time mechanism contributes most for large visual objects moving at low velocities.
在最近的一项研究中，首次记录了猕猴视网膜星爆型无长突细胞中的视觉信号，并且与小鼠和兔子一样，从树突末梢附近记录到钙信号中观察到的方向偏置。与从末梢向体细胞的运动相比，来自体细胞的刺激运动产生了更大的钙信号。有两种机制被提出影响兴奋性突触后电流的时空总和，以有助于星爆型树突末梢的方向信号传递：（1）一种“形态”机制，其中兴奋性突触电流的电紧张传播沿着树突优先总和双极细胞输入，以促进向心刺激运动；（2）一种“时空”机制，依赖于近端和远端双极细胞输入的时间进程差异，以有利于向心刺激运动。为了探索这两种机制在灵长类动物中的作用，我们基于猕猴星爆细胞的连接重建和其来自持续和瞬态双极细胞类型的突触输入分布，开发了一个现实的计算模型。我们的模型表明，这两种机制都可以在星爆树突中引发方向选择性，但它们的贡献因刺激的时空特性而异。具体来说，当小的视觉物体以高速运动时，形态机制占主导地位，而当大的视觉物体以低速运动时，时空机制的贡献最大。
