Heart rate variability is encoded in the spontaneous discharge of thalamic somatosensory neurones in cat

1. We studied the spontaneous discharge variability of thalamocortical somatosensory neurones in the awake cat in order to disclose its possible information content. The presence of slow (0.09-1.39 Hz) regular fluctuations in the discharge rate of these cells during the waking state has been previously reported. Oscillations in a similar frequency range are known to characterize the activity of central and peripheral neurones pertaining to the autonomic nervous system and the variability of heart period (RR interval variability). 2. A surrogate data test, performed on our database, confirmed the presence of slow (0.05-1 Hz) non-random fluctuations in firing rate. 3. Linear regression detected the presence of an inverse relationship between the values of RR interval and the concurrent levels of neural discharge. 4. Frequency domain analysis indicated that a significant coupling between the two variability signals preferentially occurred in two frequency bands: in the frequency of the respiratory sinus arrhythmia and in correspondence with a slower rhythm (0.07-0.3 Hz), the two signals being in phase opposition in most of the cases. 5. Coherent fluctuations could also be observed when epochs of evoked activity were analysed, while coupling between the two variability signals appeared to be disrupted after sleep onset. 6. We conclude that RR interval variability, an internally generated dynamic related to basic visceral regulation, is encoded in the discharge of single somatosensory thalamocortical neurones during wakefulness. A possible interaction with the transmission of somatosensory information has to be evaluated.