 Heart rate variability analysis Linear analysis Sequences of 300 consecutives heart beats are selected inside each experimental phase. The mean and the variance of heart period are expressed in msec and msec2 respectively. Autoregressive spectral density is factorized into components each of them characterized by a central frequency. A spectral component is labeled as LF if its central frequency is between 0.04 and 0.15 Hz, while it is classified as HF if its central frequency is between 0.15 and 0.5 Hz. The HF power is considered to represent respiration-driven vagal modulation of heart rate. To rule out the effect of changes of total power spectrum densities on LF and HF components, spectral values are also expressed in normalized units NU. Normalization consisted in dividing the power of a given spectral component by the total power minus the power below 0.04 Hz Very Low Frequency VLF spectral component, and multiplying the ratio by 100. The ratio of the LF power to the HF LFHF is considered an indicator of the balance between sympathetic and vagal modulation directed to the heart. 