Enriched experience increases reciprocal synaptic connectivity and coding sparsity in higher-order cortex

The integration of new information during sleep reshapes cortical representations that support categorical knowledge. Autoassociative attractor network theories predict that reciprocal excitatory connections help form stable categorical attractors, but direct evidence is missing. We tested this using 10 weeks of enriched experience [environmental enrichment (ENR)] in mice as a model for knowledge accumulation and recorded single-unit activity across the hippocampus and neocortex. ENR induced significant remodeling in high-level but not low-level neocortex, with a major shift from unidirectional to bidirectional functional excitatory-excitatory connections, suggestive of increased "cell assemblies." This was accompanied by increased inhibitory-to-excitatory connections and sparser, more orthogonal population activity during awake rest and slow-wave sleep, particularly in deep layers. Thus, ENR reorganizes cortical circuits into a symmetric, inhibition-balanced network that improves coding efficiency, supporting long-standing attractor network predictions.