Application of the Kelvin-Voigt viscoelastic model to hippus reveals major insights into the autonomic nervous system activity.

This study characterized the autonomic nervous system impulses underlying the iris spontaneous cyclic activity, which is characterized by hippus occurrence, by taking into account also its viscoelastic properties using the Kelvin-Voigt model. Hippus recordings were carried out in young active individuals, in the supine and standing position to test the model sensitivity, simultaneously with cardiac activity. This was followed by the recording of a photomotor reflex sequence. A clustering analysis allowed the selection of relatively low-noise hippus, where the model showed a high degree of fit (mean error = 3.5 ± 1.1%). Despite the modest reproducibility of autonomic nervous system impulses, ranging from 0.35 ± 0.33 to 0.88 ± 30 in each participant, hippus showed strong similarities, suggesting the existence of an autonomic activity signature. Parasympathetic impulses were significantly (p = 3.4 × 10− 3) higher in the supine than in the standing position. The amount of expended energy was 7.5 times lower (p = 1.7 × 10− 7) during hippus than the photomotor reflex, indicating a less costly activity. The application of the model to hippus and the photomotor reflex provided information on the autonomic nervous system basal state and adaptive reserve, respectively. These analyses allow better understanding of the autonomic nervous system activity in both clinical and athletic contexts.