A causal investigation of cortical communication

Distributed regions in the brain continuously interact - forming dynamic pathways of information flow which underlie the diversity of behaviour. Understanding how areas in the brain communicate is contingent on understanding the principles by which neuronal activity in one area causally influences activity in another. Achieving this goal has been challenging, due to the ambiguities arising from inferring cause-effect interactions from statistical relations of neuronal activity. Therefore, it is still unclear how cortical communication causally affects ongoing population activity patterns and if these inter-areal interactions can change over behaviourally relevant timescales and in different behavioural states. Here I introduce a causal approach to measure feedforward and feedback communication between primary and secondary visual cortex in mice performing a visual discrimination task. I assessed the instantaneous effect of optogenetically silencing one area on its target’s population activity and found the effect one cortical area had on another was surprisingly variable over time. Both feedforward and feedback pathways reliably affected different subpopulations of target neurons at different moments during processing of a visual stimulus, resulting in dynamically changing communication dimensions between the two cortical areas. The influence of feedback on V1 became even more dynamic when visual stimuli were behaviourally relevant and associated with a reward, impacting different subsets of V1 neurons within tens of milliseconds. Consequently, the patterns of correlated variability in V1 population activity - important for stimulus coding - dynamically restructured on rapid time-scales in a context-dependent manner. Thus, while cortical areas are connected by static anatomical pathways, they interact through highly flexible communication channels while evaluating sensory information