The central autonomic network (CAN) has been described in animal models but has been difficult to elucidate in humans. Potential confounds include physiological noise artifacts affecting brainstem neuroimaging data, and difficulty in deriving non-invasive continuous assessments of autonomic modulation. We have developed and implemented a new method which relates cardiac-gated fMRI timeseries with continuous-time heart rate variability (HRV) to estimate central autonomic processing. As many autonomic structures of interest are in brain regions strongly affected by cardiogenic pulsatility, we chose to cardiac-gate our fMRI acquisition to increase sensitivity. Cardiac-gating introduces T1-variability, which was corrected by transforming fMRI data to a fixed TR using a previously published method [Guimaraes, A.R., Melcher, J.R., et al., 1998. Imaging subcortical auditory activity in humans. Hum. Brain Mapp. 6(1), 33-41]. The electrocardiogram was analyzed with a novel point process adaptive-filter algorithm for computation of the high-frequency (HF) index, reflecting the time-varying dynamics of efferent cardiovagal modulation. Central command of cardiovagal outflow was inferred by using the resample HF timeseries as a regressor to the fMRI data. A grip task was used to perturb the autonomic nervous system. Our combined HRV-fMRI approach demonstrated HF correlation with fMRI activity in the hypothalamus, cerebellum, parabrachial nucleus/locus ceruleus, periaqueductal gray, amygdala, hippocampus, thalamus, and dorsomedial/dorsolateral prefrontal, posterior insular, and middle temporal cortices. While some regions consistent with central cardiovagal control in animal models gave corroborative evidence for our methodology, other mostly higher cortical or limbic-related brain regions may be unique to humans. Our approach should be optimized and applied to study the human brain correlates of autonomic modulation for various stimuli in both physiological and pathological states.

Neuroimage. 2008 Apr 30. Napadow V, Dhond R, Conti G, Makris N, Brown EN, Barbieri R. MGH/MIT/HMS Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA; Department of Radiology, Logan College of Chiropractic, Chesterfield, MO, USA.

Subjects with high (Highs) and low (Lows) susceptibility to hypnosis show differences in the sensory-motor integration for postural control and in the cardiovascular response to stress and experimental pain. Aim of the experiment was to assess whether the cardiac response to gravity-related stimulation depending on changes in the body position were different in the two groups. Thus, heart rate (HR) and heart rate variability (HRV) were evaluated in sitting and upright position in Highs and Lows. Position-related HRV changes were studied in the time (statistical indexes, Poincaré Plot) and frequency (spectral analysis) domain. Results indicated that upright stance was associated with similar changes in heart rate and different modulation of HRV in the two groups. The association of time and frequency domain analyses allowed hypothesizing different control mechanisms as responsible for the cardiac response to upright stance in Highs and Lows, likely due to a different role of the Very Low Frequency (VLF) spectral component of HRV in the two groups. The results are in line with previous findings indicating a natural protection of Highs against cardiovascular events and suggest that the Highs' cardiac function might be less impaired by microgravity than the Lows' one.

Brain Res Bull. 2008 Mar 28;75(5):692-7. Epub 2007 Dec 18.Related Articles, Links Santarcangelo EL, Balocchi R, Scattina E, Manzoni D, Bruschini L, Ghelarducci B, Varanini M.

Department of Human Physiology, University of Pisa, Via San Zeno 31, Pisa, Italy. enricals@dfb.unipi.it