Pulse-rate variability as the autonomic read-out
The interval between heartbeats is never perfectly even. It tends to shorten slightly on inhalation and lengthen on exhalation, and the size of those fluctuations reflects the balance between sympathetic and parasympathetic influence on the heart.
That balance is one of the clearest available windows onto physiological regulation, which is why the platform reads it alongside the neural record rather than on its own.
