Baroreceptor-sensitive fluctuations of heart rate and pupil diameter

It is generally known that the pupil is under the control of the autonomic nervous system. Recently, those rhythms characterizing the autonomic fluctuations of heart period and arterial blood pressure have been detected in spontaneous pupil diameter (PD) fluctuations. The physiological mechanisms underlying such variability have not been widely investigated. The aim of this study was to investigate the origin of the pupil fluctuations in humans, using a non-invasive modulation of carotid baroreceptors by neck suction (NS). To this purpose, we simultaneously recorded ECG, respiration activity, NS pressure and PD fluctuations from 10 normal subjects. The equipment for the PD measurement and the NS stimulation was developed in our laboratory. The response of the pupil to the NS was studied at stimulation frequencies of 0.10 and 0.20 Hz, by using parametric spectral and cross-spectral estimation. In all subjects, the NS rhythms were clearly detectable in heart rate variability series in both stimulation frequencies and also in the PD spectra with significant coherences (>0.5). These findings suggest that blood pressure fluctuations propagate to the pupil via carotid baroreceptor afferent pathways. However a central contribution can not be excluded.