Conscious understanding of sound is necessary for communication and development of cognitive abilities [1] and our cognitive system provides the most important distinguishing characteristic for humans and other mammals [2].

Sound processing in the brain may be analyzed through cortical auditory evoked potentials (CAEP). This is a well-recognized technique for evaluation of central auditory mechanisms related to auditory processing [3]. It affords information regarding automatic perception, discrimination, passive, and sound recognition and is associated with the alert response during the early allocation of attention [4,5].

CAEP includes the P100 (P1), N100 (N1), P200 (P2), N200 (N2), and P300 (P3) waves. The P1 wave reflects synaptic transmission in the thalamus-primary cortical level [6]. The N1 and P2 waves correspond to secondary auditory cortex pathways from the thalamus and different cortical areas [7]. The N2 component is associated with passive and sound recognition [4]. Finally, P3 is linked to the alert response during early allocation of attention. It is elicited by a distractor stimulus [5] and has an association with prefrontal cortex activity [8,9].

Interaction between the autonomic nervous system (ANS) and auditory processing in the central nervous system has previously been reported for rats [8, 10]. Both the parasympathetic [9] and sympathetic [10] divisions of the ANS in rats have been found to be involved in autonomic and heart rate (HR) responses induced by auditory stimulation. It has been demonstrated that the auditory cortex has a pivotal role in autonomic responses elicited by sound.

Under these circumstances, heart rate variability (HRV) is a simple, inexpensive, reliable, and noninvasive method for analysis of autonomic HR regulation [11,12] previously validated in pharmacological studies [13,14]. HRV describes the fluctuations of the intervals between two consecutive heart beats (RR-interval of the electrocardiographic signature) and indicates the capacity of the heart to respond to external and physiological stimuli. HRV is analyzed by mathematical procedures based on the RR-interval. Here, these include standard deviation of normal-to-normal RR-intervals (SDNN), percentage of adjacent RR-intervals with a difference of duration greater than 50 milliseconds (pNN50) and root-mean square of differences between adjacent normal RR-intervals in a time interval (RMSSD) - Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology [15]. Time and frequency domains are the most commonly applied indices for linear HRV investigation [16].

Significant correlation of N2, P2 and P3 waves with resting HR in the left ear (right cortical hemisphere) has been reported [17]. This study suggested the hypothesis that HRV may be associated with sound discrimination and facilitation of attention and memory mechanisms during stimulus processing.

Although, as previously noted, there is an interaction between auditory mechanisms and the ANS [18], it is unclear whether auditory processes related to attentional mechanisms are associated with the ANS. Furthermore, mechanisms regarding auditory attention processing and parasympathetic modulation could provide additional evidence for the role of ANS in social interactions and engagement. Thus, here it was aimed to evaluate the association between cortical auditory processes and resting autonomic HR regulation.

Data collection for all subjects was undertaken in the same soundproofed room. Temperature was between 21 $ ^\circ $C and 25 $ ^\circ $C, and the relative humidity was 50% to 60%. Subjects were instructed not to consume alcohol, caffeine or other ANS stimulants for 24 hours prior to the evaluation. Data were individually collected, always between 13:00 and 17:00 to standardize circadian influences [25]. All procedures required for data collection were separately explained to each subject, and the subjects were instructed to remain rested and avoid conversation during data collection. Before auditory examination subjects remained seated for 10 minutes to record their HRV under spontaneous breathing with no auditory or visual stimulation. The auditory studies were then conducted.

The sample size calculation began with a pilot test. Online software from www.lee.dante.br was used to view the RMSSD index. The magnitude of statistically significant difference was assumed to be 7 ms, standard deviation 10 ms, with a 1% alpha and 80% beta risk, sample size 22 subjects. The Shapiro-Wilk normality test was used to evaluate distributions.

To evaluate the correlation between HRV indices at rest and CAEP components during the examination, the Pearson correlation coefficient for parametric distributions and the Spearman correlation coefficient for nonparametric distributions were employed to evaluate any correlation between either HRV indices or CAEP components, respectively. Strong correlation was defined as a r > 0.75; moderate correlation for r between 0.75 and 0.5, and weak correlation when r > 0.25 [26]. Statistical significance was accepted when the probability of a Type I error was less than 5% ( p < 0.05).

Following determination of significant correlations ( p < 0.05)in the selection of independent variables, simple linear regression models were applied to model HRV indices as outcome variables. Predictors included continuous variables representing the CAEP components. Due to the non-normality of pNN50 and LF/HF indices,prior to analysis these data were transformed by taking the square root and logarithm, respectively, so as to fit the regression model.

To compare the obtained CAEP waves (frequency protocol in right ear vs. duration protocol in right ear vs. frequency protocol in left ear vs. duration protocol in left ear), a one-way analysis of variance was applied to parametric distributions, followed by a Bonferroni post-test.

Effect size was calculated using "Cohen's d" to quantify the magnitude of difference between protocols.Values $ > $ 0.9 were considered to indicate large effect size, values between 0.25 and 0.5 were considered medium, and values $ < $ 0.25 were assumed to be small [27].

Raw data are available in the Supplementary Materials.

Baseline diastolic (DAP) and systolic arterial pressure (SAP), weight, height, and body mass index (BMI) of subjects are presented in Table 1.

Correlation of HRV time domain indices with N1, P2, N2 and P3 latency are illustrated in Table 2. No significant correlations were present.

With regard to N1, P2, N2 and P3 latency, significant positive correlation was found between N1 and pNN50 for the duration protocol, between N2 and pNN50 in the frequency protocol, and between N2 and SDNN for the left protocol in the left ear (see Table 3).

Table 4 shows data regarding latency of CAEP waves and spectral analysis of HRV. There was significant correlation in the left ear - a positive correlation of LF (n.u.) with P2 and N2 waves in the duration protocol, negative correlation of HF (n.u.) with P2 and N2 waves in the duration protocol, and positive correlation of LF/HF with P2 and N2 waves in the duration protocol. In the frequency protocol, there was a positive correlation of LF (n.u.) with the N2 wave, negative correlation of HF (n.u.) with the N2 wave, and positive correlation of LF/HF with the N2 wave.

Correlation between amplitude of CAEP waves and spectral analysis of HRV isindicated in Table 5. There was significant correlation for only the left ear - positive correlation of HF (ms2) with the N1 wave for the duration protocol and positive correlation of HF (ms2) for the N2 wave.

Simple linear regression analysis provided additional details of the association between resting HRV and CAEP. Amplitude of the N2 wave in the left ear was significantly influenced by SDNN (17.7%) and pNN50 (25.3%) indices for the frequency protocol (see Table 6).

Furthermore, for the duration protocol in the left ear, the latency of the P2 wave was significantly influenced by LF (n.u.) (20.8%), HF (n.u.) (21%), and LF/HF (22.4%). Latency of the N2 wave was significantly influenced by LF (n.u.) (25.8%), HF (n.u.) (25.9%), and LF/HF (28.8%) (see Table 6).

So as to control the false discovery rate when enforcing multiple statistical tests, CAEP waves were compared during each protocol.No significant differences were found with regard to N1 latency (p = 0.35, F = 1.096), P2 latency (p = 0.63, F = 0.574), N2 latency (p = 0.97, F = 0.063), P3 latency (p = 0.98, F = 0.04), N1 amplitude (p = 0.94, F = 0.12), P2 amplitude (p = 0.76, F = 0.38), N2 amplitude (p = 0.95, F = 0.108), P3 amplitude (p = 0.37, F = 1.03) or N2-P3 amplitude (p = 0.95, F = 0.11). Cohen's d calculation only indicated a small effect size for all comparisons ($ < $ 0.25).

To provide details regarding the relationship between central auditory processing and the ANS, this study investigated the association between CAEP components and resting time and frequency domain indices of HRV. The following outcomes are highlighted: (1) There was significant association of the vagal component of HR control and sympathovagal balance at rest with the N2 and P2 waves; (2) This association only occurred in the left ear, indicating involvement of the right cortical hemisphere; and (3) The hypothesis is proposed that the autonomic component of heart rhythm interacts with cortical sound processing.

According to the results reported here, the SDNN was significantly and positively correlated with the N2 amplitude, where the greater the N2 amplitude the larger the HRV. Also, linear regression indicated that SDNN significantly influenced the N2 amplitude (17.7%), thus if SDNN increases 1 ms the N2 amplitude increases 8.7 ms. Nevertheless, SDNN analysis alone was unable to confirm which component of the ANS is associated with CAEP. This is attributed to the mathematical computation of the SDNN index incorporating both sympathetic and parasympathetic regulation of HR. Thus, it does not allow identification of which component was responsible [28].

PNN50 was positively correlated with N2 amplitude. The pNN50 index was found to influence the N2 amplitude by 25.3% (1% change in pNN50 increases 1.136 ms in N2 amplitude), indicating its association with vagal tone.

Latency of the N2 wave was also found to be related to the sympathetic control of HR. Statistical analysis indicated the N2 latency was significantly influenced by LF (25.8%) and the LF/HF ratio (28.8%). It was observed that if LF (n.u.) increased one unit the N2 latency increased 0.322 ms and if the LF/HF ratio increased one unit, the N2 latency increased by 0.016 ms. This result suggests that increasing sympathetic tone slows the electrical activity of cortical auditory processing.

The HF band was observed to significantly influence N2 latency (25.9%). If HF increased one unit the N2 latency decreased by 0.322 ms, indicating that vagal HR modulation accelerates action potential velocity.

It is essential to highlight that the N2 component is involved in pre-attention mechanisms related to communication [29]. The N2 component is also related to obligatory (exogenous) cortical processes. Amplitude and latency of obligatory CAEP are contingent on the acoustic parameters of stimuli [30]. This wave is associated with the superior temporal cortex [31] and attentional orientation toward a visual target stimulus surrounded by several distracters [32]. The N2 wave represents the quality of sound perception, classification, and recognition [33].

Another related result is the association of P2 with HRV. Similarly with N2, P2 latency was significantly influenced by LF (20.8%) and LF/HF (22.4%). Linear regression indicated that if LF increases one unit the P2 latency increases 0.519 ms and if the LF/HF ratio similarly increases, the P2 latency increases 0.026 ms. Vagal HR control was likewise observed to significantly influence the P2 wave,if HF increased one unit the P2 latency was reduced by 0.519 ms.

The P2 wave is required for the ability to process sounds based on its phonetic and acoustically-related properties since it provides information regarding the arrival of an auditory stimulus at the cortex and onset of cortical processing, thus indicating whether a sound signal is properly acknowledged in the cortex [30,34].

Taken together, it is proposed that the parasympathetic control of HR is associated with sound reception in the cortex and sound processing control, while sympathetic tone degrades this. The superior temporal cortex is involved in this mechanism, reinforcing evidence that supports the relationship between the ANS and social functioning [35,36]. This observation is supported by Nakamura et al. [9,10], who reported the role of the auditory cortex in the sympathetic responses induced by auditory stimulation in rats.

The influence of the ANS on cortical auditory processing may be explained by previous animal studies. Acetylcholine, the main parasympathetic neurotransmitter, was found to play an important role in the auditory cortex [37]. It has been reported that acetylcholinergic synaptic mechanisms may mediate the effects of acetylcholine on receptive fields in auditory cortex. Also, sound processing may favor sensory information relayed through the thalamus to cortical activity in response to increased acetylcholine release [38]. This leads to a theory that parasympathetic activation inducing increased acetylcholine release may positively influence sound reception in the auditory cortex.

Consequently, an adrenergic mechanism has also been recognized to be involved in central auditory processing [39]. However, the exact role of adrenergic neurotransmission in auditory evoked responses remains unclear [40].

According to these data, only the left ear showed association with HRV, indicating that the right cortical hemisphere related to auditory processing is associated with HR autonomic regulation. Conversely, it does not explain which hemisphere plays the main role in the ANS. The left-sided forebrain areas were observed to have a primary function in modulating vagal activity and it has been suggested as the major cortical hemisphere related to parasympathetic nervous system activity [41,42,43]. Yet, there are studies in humans using neuroimaging with affective and cognitive tasks that indicate the right hemisphere as responsible for vagal activity [44,45].

Here only males were investigated to avoid gender-dependent effects on HRV. This was reinforced by a recent study that evaluated the role of gender regarding short-term HRV analysis [46]. Those authors detected significant gender effects that involved association between HRV and stress and indicated that gender also presented an important influence on short-term HRV analysis.

The findings reported here provide important information for comprehension of cognition, as evidence is presented for a role for the ANS in cortical auditory processing, which is relevant for communication and social behavior [1,2]. In this study, mechanisms of cortical auditory processing were revealed to be associated with the parasympathetic control of HR and sympathovagal balance. This signifies that the ANS may have a significant impact on specific cognitive processes. Support for this assumption requires further research, including pharmacological techniques of parasympathetic and sympathetic blockade.

There is significant association between resting HR autonomic control and right cortical auditory processing. Here, it is proposed that the parasympathetic regulation of HR and the sympathovagal balance at rest are associated with thalamo-cortical pathways and cortical-cortical circuits involved in auditory processing.

This study received financial support from FAPESP (Process 2012/01366-6).

All authors declare no conflicts of interest.