Mind the Gap: Why Neural Memory Fails Under Semantic Density

Abstract: The brain solves a problem that current AI architectures struggle to manage: storing specific episodic facts without corrupting general semantic knowledge. Neuroscience explains this through Complementary Learning Systems theory - a fast hippocampal system for episodic storage using pattern-separated representations, and a slow neocortical system for extracting statistical regularities. Current AI systems lack this separation, attempting both functions through neural weights alone. We identify the 'Stability Gap' in online neural memory: fast-weight mechanisms that write facts into shared continuous parameters collapse to near-random accuracy within tens of semantically related facts. Through semantic density (rho), we show collapse occurs with as few as N=5 facts at high density (rho > 0.6) or N ~ 20-75 at moderate density - a phenomenon we formalise as the Orthogonality Constraint. This failure persists even with perfect attention and unlimited context, arising from write-time interference when storage and retrieval share the same substrate. We also identify schema drift and version ambiguity as primary failure modes in production systems, observing 40-70% schema consistency and 0-100% clean correction rates. Context-based memory incurs 30-300% cost premium over selective retrieval. We propose Knowledge Objects (KOs): discrete, typed memory units with controlled vocabularies and explicit version chains. Paired with neural weights, KOs enable a true complementary learning architecture, suggesting reliable AI memory may require this bicameral design.