Arousal modulates retinal output

At various stages of the visual system, visual responses are modulated by arousal. Here we find that this modulation operates as early as in the first synapse from the retina, and even in retinal ganglion cells. To measure retinal activity in the awake brain, we imaged the synaptic boutons of retinal axons in the superior colliculus. Their activity depended not only on vision but also on running speed and pupil size, regardless of retinal illumination. Arousal typically reduced their visual responses and their selectivity for direction and orientation. Recordings from retinal axons in the optic tract revealed that arousal modulates the firing of retinal ganglion cells. Arousal had similar effects postsynaptically, in colliculus neurons, independent of activity in the other main source of visual inputs to colliculus, the primary visual cortex. These results indicate that arousal modulates activity at every stage of the visual system.