A cholinergic mechanism orchestrating task-dependent computation across the cortex

In an ever-changing environment, animals often need to switch between performing different tasks involving distinct sets of cognitive processes. Many such tasks involve neural activity distributed across the cortex, with dynamics that depend on both task demands and behavioral strategy. A fundamental but unanswered question is what circuit mechanisms orchestrate these task-dependent dynamics. Here, we hypothesized that acetylcholine release in the cortex plays a key role. The cortex's only long-range source of this neuromodulator is the basal forebrain cholinergic system, which targets the entire cortical sheet and can individually modulate single regions on sub-second timescales. To test our hypothesis, we first imaged cholinergic axons innervating the cortex while mice switched frequently between two navigational decision-making tasks in virtual reality (VR), only one of which required gradual accumulation of sensory evidence. We found that cholinergic input to the cortex is spatiotemporally heterogeneous and multiplexes sensory, motor, arousal, and cognitive signals in a task- and strategy-dependent fashion, with overall higher activity during evidence accumulation. Crucially, beyond contextual variables, cholinergic activity directly tracked task computations themselves, encoding an evidence-dependent decision variable only in the accumulation task. To test if acetylcholine release is causal to the performance of each task, we optogenetically silenced cholinergic terminals in the cortex while simultaneously imaging excitatory cortical activity. We found that this input is selectively required for evidence accumulation, and for large-scale cortical coding of evidence and choice during the accumulation task. Thus, we have identified a new cholinergic mechanism that orchestrates cortex-wide activity in a task-dependent manner and serves as a key node in the distributed brain network underlying the accumulation of sensory evidence.