learning by enhancing the influence of feed-forward afferent inputs to the cortex carrying sensory information and decreasing excitatory feedback activity mediating retrieval (Hasselmo, 2006). ACh can also alter firing of neurons on a rapid time scale, as in fear-conditioning, when foot-shock results in direct cholinergic activation of interneurons in the auditory cortex that contribute to learning (Letzkus et al., 2011). All these models are consistent with a primary role of ACh as a neuromodulator that changes the state of an ensemble of neurons in response to changing environmental conditions.
