Primary auditory cortex is necessary for the acquisition and expression of categorical behavior.

The primary auditory cortex (ACtx) extracts behaviorally relevant information from complex sounds and supports the categorization of continuous acoustic features into discrete perceptual or behavioral classes. Although ACtx is known to be necessary for certain sound-guided behaviors, how its contributions evolve during learning remains unclear. This evolving role may be shaped by the diversity of its excitatory output pathways, which include multiple classes of projection neurons that transmit sensory information to downstream targets. To address this question, we trained head-fixed mice to categorize the temporal rate of sinusoidally amplitude-modulated (sAM) noise and used cell-type-specific optogenetic inhibition to assess ACtx function across learning. Silencing the ACtx impaired performance throughout learning and altered decision strategies by inducing a fluctuating choice bias and increasing reliance on prior choices, without affecting motor output. Inhibiting intratelencephalic (IT) or extratelencephalic (ET) projection neurons on just 20% of trials significantly impaired learning but had only a modest effect on expert performance. These findings demonstrate that the ACtx supports both the acquisition and expression of categorical behavior, with IT and ET projection pathways playing important roles in learning-related plasticity.