Learning shapes spatiotemporal brain patterns for flexible categorical decisions. 
Learning is thought to facilitate our ability to perform complex perceptual tasks and optimize brain circuits involved in decision making.  However, little is known about the experience-dependent mechanisms in the human brain that support our ability to make fine categorical judgments.  Previous work has focused on identifying spatial brain patterns (i.e., areas) that change with learning.  Here, we take advantage of the complementary high spatial and temporal resolution of simultaneous electroencephalography-functional magnetic resonance imaging (EEG-fMRI) to identify the spatiotemporal dynamics between cortical networks involved in flexible category learning.  Observers were trained to use different decision criteria (i.e., category boundaries) when making fine categorical judgments on morphed stimuli (i.e., radial vs. concentric patterns).  Our findings demonstrate that learning acts on a feedback-based circuit that supports fine categorical judgments.  Experience-dependent changes in the behavioral decision criterion were associated with changes in later perceptual processes engaging higher occipitotemporal and frontoparietal circuits.  In contrast, category learning did not modulate early processes in a medial frontotemporal network that are thought to support the coarse interpretation of visual scenes.  These findings provide evidence that learning flexible criteria for fine categorical judgments acts on distinct spatiotemporal brain circuits and shapes the readout of sensory signals that provide evidence for categorical decisions.