Department of Circuits-Computation-Models, Max Planck Institute of Neurobiology, Am Klopferspitz 18, 82152 Martinsried, Germany.
Research Institute of Molecular Pathology, Dr. Bohr-Gasse 7, 1030 Vienna, Austria.
Curr Biol. 2014 Feb 17;24(4):385-92. doi: 10.1016/j.cub.2014.01.006. Epub 2014 Feb 6.
Detecting the direction of visual motion is an essential task of the early visual system. The Reichardt detector has been proven to be a faithful description of the underlying computation in insects. A series of recent studies addressed the neural implementation of the Reichardt detector in Drosophila revealing the overall layout in parallel ON and OFF channels, its input neurons from the lamina (L1→ON, and L2→OFF), and the respective output neurons to the lobula plate (ON→T4, and OFF→T5). While anatomical studies showed that T4 cells receive input from L1 via Mi1 and Tm3 cells, the neurons connecting L2 to T5 cells have not been identified so far. It is, however, known that L2 contacts, among others, two neurons, called Tm2 and L4, which show a pronounced directionality in their wiring.
We characterized the visual response properties of both Tm2 and L4 neurons via Ca(2+) imaging. We found that Tm2 and L4 cells respond with an increase in activity to moving OFF edges in a direction-unselective manner. To investigate their participation in motion vision, we blocked their output while recording from downstream tangential cells in the lobula plate. Silencing of Tm2 and L4 completely abolishes the response to moving OFF edges.
Our results demonstrate that both cell types are essential components of the Drosophila OFF motion vision pathway, prior to the computation of directionality in the dendrites of T5 cells.
检测视觉运动的方向是早期视觉系统的一项基本任务。里夏德探测器已被证明是昆虫中基本计算的忠实描述。最近的一系列研究解决了果蝇中里夏德探测器的神经实现问题，揭示了在平行的 ON 和 OFF 通道中的总体布局，其来自光感受器层的输入神经元（L1→ON 和 L2→OFF），以及分别到小叶板的输出神经元（ON→T4 和 OFF→T5）。虽然解剖学研究表明 T4 细胞通过 Mi1 和 Tm3 细胞接收来自 L1 的输入，但迄今为止尚未鉴定出将 L2 连接到 T5 细胞的神经元。然而，已知 L2 与包括 Tm2 和 L4 在内的两个神经元接触，这两个神经元在其连接中表现出明显的方向性。
我们通过 Ca(2+)成像对 Tm2 和 L4 神经元的视觉反应特性进行了表征。我们发现 Tm2 和 L4 细胞以非选择性的方式对移动的 OFF 边缘表现出活性增加的反应。为了研究它们在运动视觉中的参与，我们在记录小叶板中的下游切线细胞时阻断了它们的输出。沉默 Tm2 和 L4 完全消除了对移动 OFF 边缘的反应。
我们的结果表明，这两种细胞类型都是果蝇 OFF 运动视觉通路的重要组成部分，在 T5 细胞的树突中进行方向计算之前。
