The neural basis of color vision: (I) photoreceptor interactions and origin of color opponent processing.

色觉是一种重要的感官能力，使许多动物能够区分独立于强度的光谱内容。它为动物提供了额外的力量来检测物体和与环境的互动。我们对其神经基础的大部分知识来自脊椎动物的研究，特别是对视网膜的研究，其中许多潜在的神经回路、细胞类型、突触相互作用和处理机制已经被揭示。我们知道，具有不同光谱灵敏度的光感受器信号通过神经系统拮抗地结合，采用类似减法的机制。例如，人类和其他三色灵长类结合短(S)，中(M)和长(L)波长敏感的锥状光感受器来创建L - M和S - (L+M)颜色对抗通路。这种颜色对抗处理克服了光感受器固有的混淆波长和强度的缺点，使光谱对比度的检测和增强成为可能。我们对非脊椎动物色彩视觉的神经基础知之甚少。结合行为和计算研究表明，颜色对抗处理是在果蝇的神经系统中实现的（可能“所有”具有色觉的动物）。然而，迄今为止，从果蝇的颜色检测系统的细胞中进行生理记录还不可行。在我们最近的研究中，我们克服了这一主要障碍，我们记录了大约40种不同基因型的约3500个光感受器的信号。与该领域的普遍观点相反，我们的记录表明，颜色对立已经在髓质内光感受器R7/R8的末端实现。我们揭示了潜在的突触苍白和黄色特定电路机制的详细见解，我们推导了一个简化的电路模型，现在可以让我们预测“缺失”的处理步骤。基于这些预测和我们的技术成果，我在这里提出了一个全面的研究计划，以揭示直接的下一个加工机制，特别是R7p/y和R8p/y的光谱反馈抑制的神经基础。我们将通过应用代表该领域最先进技术的遗传、解剖和生理方法来实现这一目标。我们将描述识别神经元的感受野特性，重点是它们的颜色和时空组织。最后，我们将利用获得的信息来扩展我们的模型，并识别果蝇颜色视觉系统的新的候选神经元。通过这项工作，我们将有助于更好地了解生态上重要的昆虫群体的色觉，我们将为不同分类群的色觉电路实现的可变性提供见解。

Color vision is an important sensory capability that enables many animals to differentiate spectral content independent of intensity. It provides animals with additional power for the detection of objects and interactions with the environment. Most of our knowledge on its neural basis comes from vertebrate research, in particular on the retina, where many of the underlying neuronal circuits, cell types, synaptic interactions, and processing mechanisms have been revealed. We know that signals of photoreceptors with different spectral sensitivities are combined antagonistically by the nervous system, employing a mechanism akin subtraction. Humans and other trichromatic primates for instance combine short (S), middle (M) and long (L) wavelength sensitive cone photoreceptors to create L–M and S–(L+M) color opponent pathways. This color-opponent processing overcomes the shortcoming of photoreceptors that inherently confound wavelength and intensity and enables the detection and enhancement of spectral contrast. Little is known about the neural basis of color vision in none-vertebrates. Combined behavioral and computational studies suggest that color-opponent processing is implemented in the nervous system of Drosophila (and likely ‘all’ animals with color vision). However, physiological recordings from cells of the color detection system in Drosophila have not been feasible so far. ln our recent study we overcame this major roadblock and we recorded the signals of ~3500 photoreceptors in about 40 different genotypes. Contradicting the common thought in the field, our recordings showed that color-opponency is already implemented in the terminals of inner photoreceptors R7/R8 in the medulla. We revealed detailed insights into the underlying synaptic pale- and yellow-specific circuit mechanisms and we deduced a simplified circuit model that now allows us to predicting ‘missing’ processing steps. Based on these predictions and our technical achievements I am here presenting a comprehensive research plan to reveal the immediate next processing mechanisms, in particular the neural basis of spectral feedback inhibition of R7p/y and R8p/y, respectively. We will reach this goal by applying a combined genetic, anatomical, and physiological approach that represents the state-of-the-art in the field. We will characterize the receptive field properties of identified neurons with an emphasis on their chromatic and spatiotemporal organization. Finally, we will use the obtained information to extend our model and to identify novel candidate neurons of the color vision system in Drosophila. By this work we will contribute to a better understanding of color vision in the ecologically important group of insects and we will provide insights into the variability of the circuit implementation of color vision across taxa.