Hue selectivity from recurrent circuitry in Drosophila

Circuit Mechanisms of Hue Selectivity in the Fruit Fly Visual System

Color perception is a key aspect of visual experience, playing an important role in the interaction between organisms and their environment. In primates and other animals with three types of photoreceptor cells, neurons in the visual cortex have been found to selectively respond to specific hues (such as blue, blue-green, and orange) as well as non-spectral colors (such as purple and magenta). However, the neural circuit basis of this hue selectivity has remained unclear.

In a recent study published in Nature Neuroscience, the lab of Rudy Behnia at Columbia University utilized the fruit fly, a genetically tractable model organism, to discover hue-selective neurons in its visual system and unravel the neural circuit mechanisms underlying this hue selectivity. Their findings provide new insights into the neural basis of color perception.

The researchers first used two-photon calcium imaging to measure the responses of several key neuron types in the fruit fly visual system to various spectral and non-spectral color stimuli. They found that the transmedullary neurons (TM neurons) tm5a, tm5b, and tm5c, which relay color information, exhibited highly selective responses to specific hues, similar to the hue-selective neurons in the primate visual cortex.

To elucidate the neural circuit mechanisms underlying this hue selectivity, the researchers traced the connectivity patterns of these TM neurons using the fruit fly whole-brain electron microscopy reconstruction data. Based on the reconstructed connectivity, they built a neural circuit model constrained by the anatomical connections, which could accurately reproduce the observed response patterns of the TM neurons.

Surprisingly, despite the TM neurons being only one or two synapses away from the photoreceptor input, the model predicted extensive recurrent connections between them. By disrupting these recurrent connections in both the model and experiments, the researchers found that the recurrent connections between neurons were crucial for generating hue selectivity, whereas nonlinear integration within individual neurons was not necessary.

This study provides new insights into the neural mechanisms of color information processing in visual perception, revealing a mechanism by which biological visual systems can implement nonlinear computations through recurrent connections. Although the fruit fly, as an invertebrate, has a visual system significantly different from that of primates, this finding suggests that the computation of hue selectivity may share some evolutionary conservation across different animal brains.

This research not only contributes to our understanding of the neural basis of visual perception but also offers new insights into general principles of brain computation. Future work could further explore the links between these hue-selective neurons and fruit fly behavior, as well as the existence and roles of similar neural mechanisms in other species.