Estimating the cortex and autonomic nervous activity during a mental arithmetic task
Highlights
► The heart rate variability (HRV) and directed transfer function (DTF) methods are used. ► The direction of information flow between brain and heart are investigated during a mental arithmetic (MA) task.
Introduction
Brain activity such as emotional events and mental stress can precipitate cardiac arrhythmia and sudden death [1] and different cortex and subcortex regions were concerned during the process [2], [3]. Stimulations to the cortex could arouse autonomic nerve system and then result in the changes of heart rate and blood pressure [3]. Autonomic regulation and network of brain regions have complex interacting mechanisms [4]. The autonomic-related regions of the cerebral cortex were located at forebrain or post-central areas according to extensive different researches conducted on human and animals. For example, functional neuroimaging and near infrared spectroscopy studies have showed that the anterior cingulated cortex and prefrontal cortex were correlated with stress-induced changes in the sympathetic activity [5], [6]. During lower body negative pressure experiments and the decision making task, the sympathetic skin response was associated with significant activity in bilateral extrastriate visual cortices and parietal lobe, suggesting the involvement of post-central regions in the control of peripheral autonomic function [3], [7]. However, the coupling patterns of brain and heart associated with sympathetic and parasympathetic activity have not yet been identified.
A number of approaches have been proposed to estimate the functional connectivity with electroencephalographic (EEG) signals, such as the cross-correlation, spectral coherence and synchronization [8], [9], [10]. These techniques likelihood have some limitation when dealing with bivariate time series because they do not make use of the whole covariance structure for multivariate data. In addition, these techniques cannot reflect the direction of the information flow within the functional coupling of EEG rhythms and were difficultly to analyze the connectivity of brain and heart. The directed transfer function (DTF) method can overcome this limitation by multivariate autoregressive (MVAR) process [11]. Heart rate variability (HRV) and RR intervals were the effective indices of autonomic activity affecting the heart performance [12]. Considering that EEG alpha activity was synchronized to the cardiac cycle and the fluctuation of HRV [13], it is possible that the spectral power of EEG and the index of HRV as RR intervals can be used to describe the functional relevance between brain and heart. In the study, we applied the DTF to investigate the directional features of EEG-RR intervals and the cortico-cortical functional coupling under mental stress on conscious human.
Mental arithmetic (MA) tasks are routinely used in autonomic function testing and commonly provoke changes in cardiac function after increasing sympathetic arousal [1], [5]. Gray et al. have used a serial seven subtraction mental stress task to enhance sympathetic cardiovascular arousal among cardiac patients. They found heartbeat-evoked potential amplitude at left temporal and lateral frontal electrode locations correlated with stress-induced changes in cardiac output [1]. Critchley et al. asked the subjects to perform serial subtractions of numbers and found that the mental stress task was associated with increased heart rate (HR). Significant common activations were observed in cerebellar vermis, brainstem and right anterior cingulate [3]. Similar to these studies, the MA task was also used in the study.
Section snippets
Subjects
43 healthy male subjects (22–27 years old) voluntarily participated in the study and the subjects were all accustomed to operate computer keyboard. The experiment was carried out with the approval of the Xi’an Jiaotong University Ethics Committee, and informed written consent was obtained from each subject after the experimental protocol had been explained.
Experimental protocol
Experiments were performed between 7 p.m. and 10 p.m. The participants were instructed to avoid alcohol, tea, coffee and strenuous exercise
Information flow among brain regions
During the MA stress task, the DTFdiff values from C4 to P3, P4 and Pz and the values from FCz and Cz to P3, P4, Pz, O1, O2 and Oz were all positive. Compared with the control, these DTFdiff values significantly increased at alpha and beta bands (only the significant values were showed). The results indicated that the information flow from central areas to the parietal and occipital regions in the alpha and beta frequency range was increased and the cortico-cortical functional coupling became
Discussion
Many diseases are related to arrhythmia and some of antiarrhythmic medicines can themselves paradoxically lead to life-threatening rhythm disorders and increase the mortality. There has been, therefore, an increasingly shift towards non-pharmacological therapies for cardiac arrhythmias [24]. The autonomic nervous system has a significantly effect on heart and some non-pharmacological methods related to ANS were used to improve arrhythmia. In the study, the characteristic of information flow
Conclusions
Clinical studies have revealed that autonomic responses could be disrupted in patients with cerebral injury and it meant that there is a close relationship between cortical functions and cardiovascular health. The integration of information in cerebral cortex might depend on the formation and dissolution of synchronized neuron assemblies characterized by various frequency bands and different directions of information flow. DTF difference might be a sensitive measure of cortico-cortical and
Acknowledgements
This work is partly supported by National Natural Scientific Foundation of China (No. 10972177) and the schoolmaster fund of Xi’an Jiaotong University.
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