Behavioral-independence features of complex heartbeat dynamics

We test whether the complexity of the cardiac interbeat interval time serie s is simply a consequence of the wide range of scales characterizing human behavior, especially physical activity, by analyzing data taken from health y adult subjects under three conditions with controls: (i) a "constant rout ine" protocol where physical activity and postural changes are kept to a mi nimum, (ii) sympathetic blockade, and (iii) parasympathetic blockade. We fi nd that when fluctuations in physical activity and other behavioral modifie rs are minimized, a remarkable level of complexity of heartbeat dynamics re mains, while for neuroautonomic blockade the multifractal complexity decrea ses.