Chaotic Dynamics and Effects of Cognitive State in Human Single Neurons

How the brain balances flexibility and stability during memory-guided behavior remains unclear. While large-scale brain activity exhibits signatures of chaos, it is unknown whether individual neurons also operate in a chaotic regime, and whether such dynamics are modulated by task engagement. Here, we analyzed over 1800 medial temporal lobe (MTL) neurons recorded intracranially during a declarative memory task. Using sample entropy, correlation dimension, and the largest Lyapunov exponent, we found robust signatures of deterministic chaos in a substantial proportion of individual neurons. Crucially, these signatures were not static: during memory task engagement, neurons exhibited greater entropy and dimensionality, and lower divergence, compared to task-irrelevant periods. Chaos was widespread across MTL subregions and independent of age or gender. These results suggest that chaos is an intrinsic, dynamic feature of single-neuron activity which is selectively amplified during cognitive engagement, and may support flexible temporal coding in the human memory system.