Cardiac autonomic state and neural manifold geometry co-vary non-monotonically across working-memory task phases

Using multimodal EEG–ECG–PPG–pupillometry recordings from 62–65 healthy adults during a digit-span working-memory task, we show that cardiac autonomic state and neural manifold geometry are both systematically task-modulated, with a non-monotone, stage-shifted profile: the RMSSD-driven vagal proxy peaks during passive listening while neural trajectory efficiency peaks at the easiest active memory condition — a 24-fold rest-to-task difference. Direct autonomic–manifold coupling is modest and between-subject correlations are null, indicating co-modulation by task phase rather than causal autonomic gating.