A prominent idea about the organisation of the brain is that different brain areas perform specific, largely non-overlapping functions, such as sensation, decision-making, or motor control. An increasing number of studies shows however that responses from single neurons or brain areas exhibit a mixture of functionality, e.g. neurons in primary visual cortex integrate visual information with information about locomotion speed and the subject‘s level of arousal. This integration of sensory information with information about behaviour and internal state may improve the processing of sensory input by adapting the brain to specific demands and contexts.
Behavioural modulation in the visual system has mostly been studied in cortical areas. Here I show that it is evident at even earlier stages of visual processing: in the superior colliculus, which receives the majority of retinal projections, and even in the output of the retina. We have recorded responses of large populations of neurons in the superior colliculus and of retinal axons projecting to the superior colliculus in mice that were free to run on a treadmill. Using these methods, we found that tectal neurons and their retinal inputs are modulated by running and pupil-linked arousal. To test whether further behavioural variables affect neural activity in the superior colliculus, we have trained mice to perform a visual detection task, where they were rewarded for corrected responses. We found that reward increases neural responses to successive visual stimuli and that this reward effect is independent from effects of pupil-linked arousal.
These results show that several behavioural variables impact visual processing early on in the processing hierarchy. Questions we still need to answer are: what is the purpose of this modulation and how is it achieved?