Beat-to-beat heart timing (the RR interval, in ms) is driven by two real autonomic rhythms superposed on a workload-dependent mean:
RR(t) = RR0(W) − A_LF(W)·sin(2π·0.10·t)
− A_HF(W)·sin(2π·f_resp·t) + noise
RR0(W) = 923 − 323·W (ms, mean interval)
A_LF(W) = 15 + 35·W (ms, ~0.1 Hz Mayer/baroreflex wave)
A_HF(W) = 5 + 70·(1−W) (ms, respiratory sinus arrhythmia)
W ∈ [0,1] is workload. The 0.1 Hz term is the classic Mayer wave (low-frequency, LF band, baroreflex/sympathetic); the respiration-locked term is the high-frequency (HF) band, mediated by the vagus nerve. Under cognitive load, sympathetic drive raises mean heart rate and LF amplitude while vagal (HF) modulation is suppressed — the textbook signature of stress-induced sympathovagal shift used in aerospace human-factors research.
Each new beat is plotted as a point on a Poincaré plot (RRₙ vs RRₙ₊₁) — a standard nonlinear HRV technique. Its scatter shape is summarised by two numbers computed from the live buffer:
SDSD = std(RRₙ₊₁ − RRₙ)
SD1 = √(0.5 · SDSD²) short-term / vagal variability
SD2 = √(2·SD(RR)² − 0.5·SDSD²) long-term / total variability
RMSSD = √mean((RRₙ₊₁ − RRₙ)²) time-domain vagal-tone index
- Workload slider — simulates task/instrument demand; raises heart rate, shrinks the Poincaré cloud along its short axis (falling SD1/RMSSD), and stretches it along the long axis (rising SD2) — reduced, more sympathetically-dominated variability.
- Respiration slider — sets the HF oscillation frequency; faster breathing compresses the scatter's cyclic structure.
- Startle event — an acute sympathetic surge (workload spikes, decays over ~6 s), the model used to represent an unexpected alarm or upset during simulator training.
This is the same class of model referenced in neuroergonomic flight-training research, where continuous HRV/EEG monitoring is used to infer a pilot's real-time cognitive workload and stress state.