The ability to encode the direction of image motion is fundamental to our sense of vision. Direction selectivity along the four cardinal directions is thought to originate in direction-selective ganglion cells (DSGCs) because of directionally tuned GABAergic suppression by starburst cells. Here, by utilizing two-photon glutamate imaging to measure synaptic release, we reveal that direction selectivity along all four directions arises earlier than expected at bipolar cell outputs. Individual bipolar cells contained four distinct populations of axon terminal boutons with different preferred directions. We further show that this bouton-specific tuning relies on cholinergic excitation from starburst cells and GABAergic inhibition from wide-field amacrine cells. DSGCs received both tuned directionally aligned inputs and untuned inputs from among heterogeneously tuned glutamatergic bouton populations. Thus, directional tuning in the excitatory visual pathway is incrementally refined at the bipolar cell axon terminals and their recipient DSGC dendrites by two different neurotransmitters co-released from starburst cells.

对图像运动方向进行编码的能力是我们视觉的基础。沿着四个基本方向的方向选择性被认为起源于方向选择性神经节细胞 (DSGC)，因为星爆细胞对 GABA 能进行定向调节抑制。在这里，通过利用双光子谷氨酸成像来测量突触释放，我们揭示了沿所有四个方向的方向选择性在双极细胞输出中比预期更早出现。单个双极细胞包含四个不同的轴突末端纽扣群，具有不同的首选方向。我们进一步表明，这种特定于 bouton 的调节依赖于星爆细胞的胆碱能兴奋和广域无长突细胞的 GABA 能抑制。 DSGC 接收来自异质调整的谷氨酸能布顿群体的调整的定向排列输入和未调整的输入。因此，兴奋性视觉通路的定向调节在双极细胞轴突末端及其受体 DSGC 树突处通过星爆细胞共同释放的两种不同神经递质逐渐完善。