Rapid associative spine enlargement is required for cognitive function and stable wakefulness

Awake cognition is usually attributed to arousal systems and ongoing electrical activity, yet these do not explain how cortical processing is coupled to prior context and memory formation. Associative synaptic plasticity provides a cellular mechanism for such coupling, but is viewed as memory-related rather than an online requirement for cognition. Here we developed SynC, an A/C-activated chemogenetic perturbation that selectively and reversibly blocks associative spine enlargement while sparing non-associative spine dynamics, excitability, synaptic transmission and NMDA receptor-mediated responses. In mice with broad frontoparietal SynC expression, acute A/C reversibly impaired open-field centre exploration, abolished laser-dot chasing, and delayed feeding initiation. During quiet-wake immobile epochs, cortical firing rates and {gamma}-band power were preserved, but pairwise spike correlations were reduced. Mice also intermittently showed sudden behavioural arrest with reduced {gamma} activity but without the robust {delta} elevation of slow-wave sleep. Critically, in vivo two-photon glutamate uncaging induced seconds-scale associative spine enlargement in [~]24% of neocortical spines, acutely blocked by SynC-A/C. These A/C-induced behavioural, circuit and spine-enlargement effects recovered within 1 h. Thus, wake-like firing and {gamma} activity are not sufficient to sustain awake cognitive function. Rapid associative spine enlargement is required to maintain functional cortical coupling and stable wakefulness.