Chromatic signal processing starts at the level of photoreceptors where visual signals are separated into different input channels, based on the wavelength selectivity of the photoreceptors’ opsins. Unlike the temporal and spatial aspects of retinal signal integration, however, the properties of chromatic integration are largely unknown and direct assessments are lacking. Mechanistic investigations of spatial integration then elucidated the role of retinal bipolar cells in shaping signal transmission through the retina 15, 16, 17, 18 and helped characterize the suppressive receptive field surround 11, 19, 20, 21. Studies of spatial integration, in particular, refined the idea of receptive fields 10, 11 and later resulted in the distinction of linear and nonlinear spatial integration, as originally exemplified by the X and Y cells of the cat retina 12, 13, 14. Temporal and spatial signal integration have been intensively studied in the retina, where they have been linked to phenomena such as temporal filtering, adaptation, motion detection, and other specific visual functions 1, 2, 3, 4, 5, 6, 7, 8, 9. Downstream neurons, therefore, integrate visual information over time and space as well as over color channels, and the signal transformations inherent in these integration processes shape the computation and feature extraction associated with a given neuron.

In the visual system, the signal detection occurs in the photoreceptors, which are distributed over space on the retina and which belong to different chromatic channels, depending on their spectral sensitivity. Neurons in sensory systems encode different features of the environment by integrating signals that are detected in receptor cells. Furthermore, nonlinear chromatic integration occurs independently of nonlinear spatial integration, depends on contributions from the rod pathway and on surround inhibition, and may provide information about chromatic boundaries, such as the skyline in natural scenes. Unlike the latter two, nonlinear On cells display response suppression rather than activation under balanced chromatic stimulation. This reveals nonlinear chromatic integration in subsets of On, Off, and On–Off ganglion cells. Analogous to classical studies of spatial integration, we here study chromatic integration in mouse retina by identifying chromatic stimuli for which activation from the green or UV color channel is maximally balanced by deactivation through the other color channel. Unlike the former two, chromatic integration is largely unexplored. In the retina, ganglion cells integrate visual information over time, space, and chromatic channels. The computations performed by a neural circuit depend on how it integrates its input signals into an output of its own.

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