Eye‐specific visual processing in the mouse suprachiasmatic nuclei

Key points Daily changes in global levels of illumination synchronise daily physiological rhythms via bilateral retinal projections to the suprachiasmatic nuclei. We aimed to determine how retinal signals are integrated within the suprachiasmatic nuclei. By monitoring electrophysiological responses to visual stimuli we show that most suprachiasmatic neurons receive input from just one eye. Our results establish that suprachiasmatic neurons measure local light intensity and that any assessment of global light levels occurs at the network level.Abstract Internal circadian clocks are important regulators of mammalian biology, acting to coordinate physiology and behaviour in line with daily changes in the environment. At present, synchronisation of the circadian system to the solar cycle is believed to rely on a quantitative assessment of total ambient illumination, provided by a bilateral projection from the retina to the suprachiasmatic nuclei (SCN). It is currently unclear, however, whether this photic integration occurs at the level of individual cells or within the SCN network. Here we use extracellular multielectrode recordings from the SCN of anaesthetised mice to show that most SCN neurons receive visual input from just one eye. While we find that binocular inputs to a subset of cells are important for rapid responses to changes in illumination, we find no evidence indicating that individual SCN cells are capable of reporting the average light intensity across the whole visual field. As a result of these local irradiance coding properties, our data establish that photic integration is primarily mediated at the level of the SCN network and suggest that accurate assessments of global light levels would be impaired by non‐uniform illumination of either eye.