ON–OFF CODING IS LATENT IN VERTEBRATE VISUAL CIRCUITS

Neural circuits across sensory systems represent stimulus changes of opposite sign as On and Off signals. In vertebrate vision, this polarity split is classically attributed to the photoreceptor–bipolar cell synapse via a fixed molecular dichotomy: Off bipolar cells use sign-preserving ionotropic glutamate receptors, whereas On bipolar cells use the sign-inverting receptor mGluR6. This textbook model implies that downstream circuits inherit pre-segregated polarity channels. However, many non-mammalian vertebrates exhibit predominantly mixed On–Off responses at the retinal output, challenging this view.
Here we combine comparative transcriptomics, pharmacology, genetics, and in vivo imaging in the diurnal zebrafish retina to test whether polarity splitting is actively maintained. We find that bipolar cells frequently co-express receptors of opposing polarity and many are intrinsically On–Off. Here, Off signalling depends on kainate receptors, whereas On signalling is mediated primarily by the glutamate transporter EAAT5b. Blocking either pathway or removing inhibition unmasks strong latent responses of the opposite polarity. Strikingly, additional sign inversions re-emerge downstream via Group III mGluR, and removing inhibition at the retinal output and in central visual circuits reveals widespread mixed-polarity signalling.
Together, our results show that On–Off coding is latent, whereas clean On or Off responses emerge through active suppression of one polarity. We suggest that the On/Off segregation prominent in mammals represents a derived specialization shaped by a nocturnal bottleneck where low-light constraints favoured early, robust polarity splitting. Polarity segregation is therefore repeatedly reconstructed across processing stages, challenging the canonical model of a single early pathway split.