Abstract
Our emotional experiences depend on our interoceptive ability to perceive and interpret changes in our autonomous nervous system. An inaccurate perception and interpretation of autonomic changes impairs our ability to understand and regulate our emotional reactions. Impairments in emotion understanding and emotion regulation increase our risk for mental disorders, indicating that interoceptive deficits play an important role in the etiology and pathogenesis of mental disorders. We, thus, need measures to identify those of us whose interoceptive deficits impair their emotion understanding and emotion regulation. Here, we used cardiac measures to investigate how our ability to engage prefrontal and (para-)limbic brain region regions affects our ability to perceive and interpret cardiac changes. We administered a heartbeat detection task to a sample of healthy individuals (n = 113) whose prefrontal-(para-) limbic engagement had been determined on basis of a heart rate variability recording. We found a positive association between heartbeat detection and heart rate variability, implying that individuals with higher heart rate variability were more accurate in heartbeat detection than individuals with lower heart rate variability. These findings suggest that our interoceptive accuracy depends on our prefrontal-(para-)limbic engagement during the perception and interpretation of cardiac changes. Our findings also show that cardiac measures may be useful to investigate the association between interoceptive accuracy and prefrontal-(para-)limbic engagement in a time- and cost-efficient manner.
Introduction
Interoception refers to our ability to perceive and interpret changes in our autonomous nervous system (). Cardiac and respiratory activity are examples of autonomic processes that frequently change during emotional events (). The perception and interpretation of these autonomic changes forms the basis of our emotional experiences and helps us to understand and to regulate our emotional reactions (). For instance, those of us who are sensitive to cardiac changes have less difficulties in understanding and responding to emotional events than those of us who are insensitive to cardiac changes (; ; , ). Given that emotion understanding and emotion regulation is central for our mental health (), it is not surprising that alterations in the perception and interpretation of autonomic changes increase our risk for mental disorders (). For instance, those of us whose cardiac sensitivity is in the abnormal range have more mental health problems than those of us whose cardiac sensitivity is in the normal range (, ; ; ). The way we perceive and interpret autonomic changes, thus, appears to be of utmost importance for our mental health.
To understand how we perceive and interpret autonomic changes, we need measures that differentiate between different aspects of interoception (). Fortunately, these measures have already been developed by researchers. Depending on the research question, these measures assess interoceptive accuracy (i.e., objective accounts of interoception), interoceptive sensibility (i.e., subjective accounts of interoception) or interoceptive awareness (i.e., correspondence between objective and subjective accounts of interoception). The most popular measure is the heartbeat detection task (), an interceptive accuracy task that requires the tracking of heartbeats within different time intervals. By employing the heartbeat detection task, researchers were able to identify a network of brain regions that is relevant for the perception and interpretation of cardiac changes (). This network comprises several brain regions but prefrontal and (para-)limbic brain regions appear to be the most important ones. Prefrontal and (para-)limbic show the most pronounced activity changes during the perception and interpretation of cardiac changes (; ; ), implying a close association between prefrontal-(para-)limbic activity and interoceptive accuracy. It may, thus, be possible that our ability to perceive and interpret cardiac changes depends on our ability to engage prefrontal and (para-)limbic brain regions for this matter.
To test this possibility, we administered measures of interoceptive accuracy and prefrontal-(para-)limbic engagement to a sample of young adults. Interoceptive accuracy was measured with the heartbeat detection task and prefrontal-(para-)limbic engagement was measured with a heart rate recording. The heart rate recording was used for the determination of parasympathically induced heart rate changes, a measure of vagally mediated heart rate variability (). Parasympathetically induced heart rate changes are closely associated with activity changes in prefrontal and (para-)limbic brain regions (; ), indicating that vagally mediated heart rate variability reflects prefrontal-(para-)limibic engagement (; ). The heart rate recording, thus, allowed us to investigate the association between interoceptive accuracy and prefrontal-(para-)limbic engagement in an unobtrusive manner. In light of previous findings showing that activity changes in prefrontal and (para-)limbic brain regions are positively associated with the perception and interpretation of cardiac changes (; ; ), we expected to find a similar association between vagally mediated heart rate variability and heartbeat detection. Preliminary findings suggest that vagally mediated heart rate variability may be positively associated with heartbeat detection (). We, thus, expected to find a positive rather than negative association between vagally mediated heartrate variability and heartbeat detection.
Materials and Methods
Participants
We based our participant recruitment on an a priori power analysis with G∗Power 3.1.9.2 (). The power analysis suggested that 82 participants would provide sufficient data to detect meaningful associations between vagally mediated heartrate variability and heartbeat detection in our analyses [correlation analysis (two-tailed): α = 0.05, 1-ß = 0.80, r = 0.30; regression analyses (total number of predictors: 8, number of tested predictors: 1): α = 0.05, 1-ß = 0.80, f2 = 0.15]. Following this suggestion, we recruited 113 participants for the study (see Table 1). In order to be included in the study, the participants had to be native speakers and to be aged between 18 and 35 years. Participants who were currently in psychotherapeutic treatment were excluded from the study. Inclusion and exclusion of participants was determined on basis of an in-house questionnaire assessing sociodemographic (age and sex), anthropometric (height and weight) and medical (physical activity in terms of aerobic fitness, smoking status, medication status, treatment status) information (). All participants that were included in the study provided written-informed consent to the study protocol. The study protocol, which had been approved by the local ethics committee, was carried out in accordance with the Declaration of Helsinki.
TABLE 1
| M (SD) | 95% CI | |
| Sex (f/m, n) | 40/69 | |
| Age | 26.30 (3.91) | [25.55, 27.04] |
| Tobacco use (n) | 28 | |
| Medication use (n) | 10 | |
| Anti-allergic medication | 3 | |
| Endocrine medication | 5 | |
| Psychotropic medication | 2 | |
| Contraceptive use (n) | 22 | |
| Unspecified contraceptives | 19 | |
| Androgenic contraceptives | 1 | |
| Anti-androgenic contraceptives | 2 | |
| Body mass index (kg/m2) | 22.90 (2.83) | [22.40, 23.42] |
| Physical activity (h/week) | 6.19 (3.53) | [5.57, 6.86] |
| Respiratory activity (log pHF, Hz) | −0.71 (0.10) | [−0.72, −0.69] |
| Heart rate (bpm) | 74.70 (12.20) | [72.25, 76.88] |
| Heart rate variability | ||
| RMSSD (ms) | 43.54 (26.19) | [38.51, 48.79] |
| log RMSSD (ms) | 1.57 (0.25) | [1.52, 1.62] |
| pNN50 (%) | 20.72 (17.66) | [17.35, 24.20] |
| log pNN50 (%) | 1.14 (0.48) | [1.04, 1.23] |
| Heartbeat detection | ||
| HBDSC | 0.69 (0.18) | [0.66, 0.72] |
| HBDGA | 0.60 (0.26) | [0.56, 0.65] |
Participant characteristics.
pHF, peak of high frequency band (); RMSSD, root mean square of successive differences between consecutive heartbeats (); pNN50, number of successive heartbeat interval pairs that differ more than 50 ms divided by the total number of all heartbeat intervals (); HBDSC, Heartbeat detection – traditional index (); HBDGA, Heartbeat detection – alternative index ().
Procedure
We used a heart rate recording to determine participants’ vagally mediated heart rate variability and heartbeat detection. Each recording session was scheduled during the daytime (at least 2 h after wakening time and 5 h before sleeping time) to control for circadian and diurnal variations in participants’ heart rate (; ). Before we started with the recording session, we asked the participants to use the bathroom. This allowed us to rule out that bladder filling and gastric digestion had an effect on participants’ heart rate (; ). The participants completed the recording session in a comfortable chair that was located in a dimly lit room. The heart rate recording was performed with a mobile heart rate monitor (RS 800, Polar Electro Oy; Kempele, Finland) that has been shown to record heartbeats as accurate as mobile electrocardiogram systems ().
Heartbeat Detection
Participants’ heartbeat detection was determined during the first part of the recording session. Following an established procedure (, ), we asked the participants to count their heartbeats during 25, 35, and 45 s lasting time intervals. They were not informed about the length of the time intervals and they were not allowed to use any measure that may have facilitated the heartbeat detection. We used the number of counted and recorded heartbeats to compute two different indices of participants’ heartbeat detection, a traditional heartbeat detection index1 () and an alternative heartbeat detection index2 () that has been shown to be to be less sensitive against outliers than the traditional heartbeat detection index (). We used both heartbeat detection indices in our analyses to test the robustness of our findings.
Heart Rate Variability
Participants’ vagally mediated heart rate variability was determined during the second part of the recording session. Following an established procedure (, ), we asked the participants to sit still and to stay awake during a 300 s lasting time interval. The heartbeats that were recorded during this time interval were analyzed with Kubios HRV 2.2 (). The analysis followed the guidelines of the : The recordings were detrended (smoothn priors: λ = 500), visually inspected and artifact corrected (adaptive filtering: cubic spline interpolation) before they were subjected to a time-domain and spectral analysis. The time-domain analysis was used for the determination of a heart rate index (meanHR) and for the determination of two vagally mediated heartrate variability indices (the root mean square of successive differences between consecutive heartbeats, RMSSD; the number of successive heartbeat interval pairs that differ more than 50 ms divided by the total number of all heartbeat intervals, pNN50). The values of these indices were in the range of values that have been reported in comparable samples of participants (). The spectral analysis was used to determine a respiration index (), the peak frequency of the high frequency band (pHF). We used the respiration index to adjust the vagally mediated heartrate variability indices for respiration-induced alterations (). Using both vagally mediated heartrate variability indices in our analyses allowed us to test the robustness of our findings.
Statistical Analyses
We performed all analyses with the bootstrapping module of SPSS 27 (SPSS Inc., Chicago, IL, United States). Preliminary analyses revealed that the datasets of four participants were incomplete or invalid due to a recording error. We, thus, used the datasets of the remaining 109 participants for the main analyses. The main analyses comprised regression and correlation analyses. Multiple regressions were run to analyze the association between participants’ vagally mediated heart rate variability and heartbeat detection. The vagally mediated heart rate variability indices constituted the predictor variables and the heartbeat detection indices constituted the criterion variables. Participant characteristics that may distort the association between vagally mediated heartrate variability and heartbeat detection were used as additional predictor variables (age, sex, body mass index, physical activity, respiratory activity, smoking status, and medication status). The regression analyses were complemented by correlation analyses. Partial correlations were run to quantify the association between participants’ vagally mediated heart rate variability and heartbeat detection. The vagally mediated heart rate variability indices constituted the dependent variable, the heartbeat detection indices the independent variables and the aforementioned participant characteristics the control variables. We set the significance level for all analyses at α ≤ 0.05 and determined significance values (p), effect size measures (r, R2, ΔR2, and B) and 95% confidence intervals (CIs).
Results
Associations Between the Vagally Mediated Heart Rate Variability Indices (RMSSD, pNN50) and the Traditional Heartbeat Detection Index (HBDSC)
In the first set of regression models, the traditional heartbeat detection index constituted the criterion variable. Entering the participant characteristics as predictor variables in a first step into the regression models accounted for a significant proportion of the variance in the traditional heartbeat detection index [RMSSD: R2 = 0.15, F(8,100) = 2.13, p = 0.040; pNN50: R2 = 0.15, F(8,100) = 2.13, p = 0.040, see Table 2]. However, sex was the only predictor variable that turned out be significant in the regression models [RMSSD: B = 0.14, SE B = 0.05, 95% CI [0.03, 0.23], t(100) = 2.37, p = 0.015; pNN50: B = 0.13, SE B = 0.05, 95% CI [0.04, 0.23], t(100) = 2.37, p = 0.018; see Table 2]. Entering the vagally mediated heart rate variability indices as predictor variables in a second step into the regression models also accounted for a significant proportion of the variance in the traditional heartbeat detection index [RMSSD: ΔR2 = 0.04, ΔF(1,99) = 4.46, p = 0.037; pNN50: ΔR2 = 0.05, F(1,99) = 6.13, p = 0.015, see Table 2]. The vagally mediated heart rate variability indices were, besides sex [RMSSD: B = 0.13, SE B = 0.05, 95% CI [0.03, 0.23], t(99) = 2.33, p = 0.013; pNN50: B = 0.12, SE B = 0.05, 95% CI [0.02, 0.22], t(99) = 2.27, p = 0.020; see Table 2], the only significant predictors in the regression models [RMSSD: B = 0.15, SE B = 0.06, 95% CI [0.02, 0.28], t(99) = 2.11, p = 0.024; pNN50: B = 0.09, SE B = 0.04, 95% CI [0.02, 0.16], t(99) = 2.48, p = 0.009; see Table 2]. Taken together, the regression models suggested that there was a small to medium sized association between the vagally mediated heart rate variability indices and the traditional heartbeat detection index [RMSSD: r(99) = 0.21, 95% CI [0.05, 0.37], p = 0.037; pNN50: r(99) = 24, 95% CI [0.05, 0.41], p = 0.015; see Figure 1].
TABLE 2
| Heartbeat detection (HBDSC) | Heartbeat detection (HBDSC) | ||||||||||
| Model One | B | SE B | 95% CI | t | p | Model two | B | SE B | 95% CI | t | p |
| Step one | Step one | ||||||||||
| Sex | 0.13 | 0.05 | [0.03, 0.23] | 2.37 | 0.015* | Sex | 0.13 | 0.05 | [0.04, 0.23] | 2.37 | 0.018* |
| Age | 0.00 | 0.01 | [−0.01, 0.01] | –0.09 | 0.943 | Age | 0.00 | 0.01 | [−0.01, 0.01] | –0.09 | 0.915 |
| Tobacco use | –0.03 | 0.04 | [−0.11, 0.06] | –0.64 | 0.552 | Tobacco use | –0.03 | 0.04 | [−0.11, 0.06] | –0.64 | 0.545 |
| Medication use | 0.04 | 0.08 | [−0.10, 0.21] | 0.70 | 0.610 | Medication use | 0.04 | 0.08 | [−0.10, 0.20] | 0.70 | 0.574 |
| Contraceptive use | 0.03 | 0.05 | [−0.08, 0.14] | 0.54 | 0.545 | Contraceptive use | 0.03 | 0.05 | [−0.07, 0.14] | 0.54 | 0.559 |
| Body mass index | –0.01 | 0.01 | [−0.02, 0.01] | –0.97 | 0.342 | Body mass index | –0.01 | 0.01 | [−0.02, 0.01] | –0.97 | 0.312 |
| Physical activity | 0.01 | 0.01 | [0.00, 0.02] | 1.79 | 0.066 | Physical activity | 0.01 | 0.01 | [0.00, 0.02] | 1.79 | 0.068 |
| Respiratory activity (log pHF) | –0.03 | 0.16 | [−0.32, 0.30] | –0.15 | 0.891 | Respiratory activity (log pHF) | –0.03 | 0.16 | [−0.35, 0.30] | –0.15 | 0.870 |
| Step two | Step two | ||||||||||
| Sex | 0.13 | 0.05 | [0.03, 0.23] | 2.33 | 0.013** | Sex | 0.12 | 0.05 | [0.03, 0.22] | 2.27 | 0.020* |
| Age | 0.00 | 0.01 | [−0.01, 0.01] | 0.00 | 1.000 | Age | 0.00 | 0.00 | [−0.01, 0.01] | –0.07 | 0.946 |
| Tobacco use | –0.02 | 0.04 | [−0.10, 0.07] | –0.44 | 0.661 | Tobacco use | –0.02 | 0.04 | [−0.1, 0.07] | –0.41 | 0.694 |
| Medication use | 0.04 | 0.08 | [−0.1, 0.20] | 0.66 | 0.610 | Medication use | 0.04 | 0.07 | [−0.08, 0.19] | 0.72 | 0.551 |
| Contraceptive use | 0.05 | 0.05 | [−0.06, 0.15] | 0.81 | 0.374 | Contraceptive use | 0.04 | 0.05 | [−0.06, 0.15] | 0.69 | 0.452 |
| Body mass index | 0.00 | 0.01 | [−0.02, 0.01] | –0.50 | 0.669 | Body mass index | 0.00 | 0.01 | [−0.02, 0.01] | –0.41 | 0.687 |
| Physical activity | 0.01 | 0.00 | [0.00, 0.02] | 1.41 | 0.120 | Physical activity | 0.01 | 0.01 | [0.00, 0.02] | 1.24 | 0.185 |
| Respiratory activity (log pHF) | –0.11 | 0.16 | [−0.40, 0.22] | –0.59 | 0.486 | Respiratory activity (log pHF) | –0.10 | 0.16 | [−0.41, 0.25] | –0.53 | 0.558 |
| Heart rate variability (log RMSSD) | 0.15 | 0.06 | [0.02, 0.28] | 2.11 | 0.024* | Heart rate variability (log pNN50) | 0.09 | 0.04 | [0.02, 0.16] | 2.48 | 0.014** |
Associations between the vagally mediated heart rate variability indices (RMSSD and pNN50) and the traditional heartbeat detection index (HBDSC).
Model one: step one: R2 = 0.15, F(8,100) = 2.13, p = 0.040*, step two: ΔR2 = 0.04, ΔF(1,99) = 4.46, p = 0.037*; model two: step one: R2 = 0.15, F(8,100) = 2.13, p = 0.040*, step two: ΔR2 = 0.05, ΔF(1,99) = 6.13, p = 0.015*. HBDSC, Heartbeat detection – traditional index (); pHF, peak of high frequency band (); RMSSD, root mean square of successive differences between consecutive heartbeats (); pNN50, number of successive heartbeat interval pairs that differ more than 50 ms divided by the total number of all heartbeat intervals (). *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001.
FIGURE 1
Associations Between the Vagally Mediated Heart Rate Variability Indices (RMSSD, pNN50) and the Alternative Heartbeat Detection Index (HBDGA)
In the second set of regression models, the alternative heartbeat detection index constituted the criterion variable. Entering the participant characteristics as predictor variables in a first step into the regression models accounted for a significant proportion of the variance in the alternative heartbeat detection index [RMSSD: R2 = 0.14, F(8,100) = 2.12, p = 0.041; pNN50: R2 = 0.14, F(8,100) = 2.12, p = 0.041; see Table 3]. However, sex was the only predictor variable that turned out be significant in the regression models [RMSSD: B = 0.18, SE B = 0.08, 95% CI [0.03, 0.23], t(100) = 2.36, p = 0.017; pNN50: B = 0.18, SE B = 0.07, 95% CI [0.04, 0.33], t(100) = 2.36, p = 0.009; see Table 3]. Entering the vagally mediated heart rate variability indices as predictor variables in a second step into the regression models also accounted for a significant proportion of the variance in the alternative heartbeat detection index [RMSSD: ΔR2 = 0.04, ΔF(1,99) = 4.79, p = 0.031; pNN50: ΔR2 = 0.05, F(1,99) = 6.70, p = 0.011; see Table 3]. The vagally mediated heart rate variability indices were, besides sex [RMSSD: B = 0.18, SE B = 0.08, 95% CI [0.03, 0.34], t(99) = 2.32, p = 0.022; pNN50: B = 0.17, SE B = 0.07, 95% CI [0.04, 0.32], t(99) = 2.25, p = 0.015; see Table 3], the only significant predictors in the regression models [RMSSD: B = 0.23, SE B = 0.09, 95% CI [0.04, 0.40], t(99) = 2.19, p = 0.020; pNN50: B = 0.13, SE B = 0.05, 95% CI [0.03, 0.22], t(100) = 2.59, p = 0.007; see Table 3]. Taken together, the regression models suggested that there was a small to medium sized association between the vagally mediated heart rate variability indices and the alternative heartbeat detection index [RMSSD: r(99) = 0.21, 95% CI [0.05, 0.37], p = 0.031; pNN50: r(99) = 0.25, 95% CI [0.06, 0.42], p = 0.011; see Figure 2].
TABLE 3
| Heartbeat detection (HBDGA) | Heartbeat detection (HBDGA) | ||||||||||
| Model One | B | SE B | 95% CI | t | p | Model two | B | SE B | 95% CI | t | p |
| Step one | Step one | ||||||||||
| Sex | 0.18 | 0.08 | [0.03, 0.34] | 2.36 | 0.017* | Sex | 0.18 | 0.07 | [0.04, 0.33] | 2.36 | 0.009** |
| Age | 0.00 | 0.01 | [−0.02, 0.01] | –0.49 | 0.668 | Age | 0.00 | 0.01 | [−0.02, 0.01] | –0.49 | 0.656 |
| Tobacco use | –0.03 | 0.06 | [−0.14, 0.09] | –0.45 | 0.690 | Tobacco use | –0.03 | 0.06 | [−0.13, 0.09] | –0.45 | 0.696 |
| Medication use | 0.06 | 0.11 | [−0.15, 0.29] | 0.66 | 0.555 | Medication use | 0.06 | 0.11 | [−0.14, 0.28] | 0.66 | 0.583 |
| Contraceptive use | 0.04 | 0.08 | [−0.12, 0.18] | 0.53 | 0.579 | Contraceptive use | 0.04 | 0.08 | [−0.1, 0.20] | 0.53 | 0.565 |
| Body mass index | –0.01 | 0.01 | [−0.03, 0.01] | –1.05 | 0.290 | Body mass index | –0.01 | 0.01 | [−0.03, 0.01] | –1.05 | 0.318 |
| Physical activity | 0.01 | 0.01 | [0.00, 0.03] | 1.86 | 0.063 | Physical activity | 0.01 | 0.01 | [0.00, 0.03] | 1.86 | 0.064 |
| Respiratory activity (log pHF) | 0.02 | 0.23 | [−0.42, 0.49] | 0.07 | 0.930 | Respiratory activity (log pHF) | 0.02 | 0.23 | [−0.42, 0.45] | 0.07 | 0.933 |
| Step two | Step two | ||||||||||
| Sex | 0.18 | 0.08 | [0.03, 0.34] | 2.32 | 0.022* | Sex | 0.17 | 0.07 | [0.04, 0.32] | 2.25 | 0.015* |
| Age | 0.00 | 0.01 | [−0.02, 0.01] | –0.40 | 0.726 | Age | 0.00 | 0.01 | [−0.02, 0.01] | –0.47 | 0.688 |
| Tobacco use | –0.01 | 0.06 | [−0.13, 0.10] | –0.23 | 0.825 | Tobacco use | –0.01 | 0.06 | [−0.12, 0.1] | –0.20 | 0.842 |
| Medication use | 0.05 | 0.10 | [−0.15, 0.27] | 0.61 | 0.568 | Medication use | 0.06 | 0.10 | [−0.13, 0.27] | 0.68 | 0.552 |
| Contraceptive use | 0.07 | 0.08 | [−0.09, 0.21] | 0.82 | 0.384 | Contraceptive use | 0.06 | 0.08 | [−0.09, 0.2] | 0.70 | 0.468 |
| Body mass index | –0.01 | 0.01 | [−0.03, 0.01] | –0.56 | 0.553 | Body mass index | 0.00 | 0.01 | [−0.03, 0.02] | –0.47 | 0.665 |
| Physical activity | 0.01 | 0.01 | [0.00, 0.03] | 1.47 | 0.123 | Physical activity | 0.01 | 0.01 | [0.00, 0.02] | 1.29 | 0.162 |
| Respiratory activity (log pHF) | –0.10 | 0.24 | [−0.57, 0.4] | –0.40 | 0.647 | Respiratory activity (log pHF) | –0.08 | 0.23 | [−0.54, 0.35] | –0.32 | 0.734 |
| Heart rate variability (log RMSSD) | 0.23 | 0.09 | [0.04, 0.4] | 2.19 | 0.020* | Heart rate variability (log pNN50) | 0.13 | 0.05 | [0.03, 0.22] | 2.59 | 0.007** |
Associations between the vagally mediated heart rate variability indices (RMSSD and pNN50) and the alternative heartbeat detection index (HBDGA).
Model one: step one: R2 = 0.14, F(8,100) = 2.12, p = 0.031*, step two: ΔR2 = 0.04, ΔF(1,99) = 4.79, p = 0.031*; model two: step one: R2 = 0.14, F(8,101) = 2.12, p = 0.041*, step two: ΔR2 = 0.05, ΔF(1,99) = 6.70, p = 0.011*. HBDGA, Heartbeat detection – alternative index (); pHF, peak of high frequency band (); RMSSD, root mean square of successive differences between consecutive heartbeats (); pNN50, number of successive heartbeat interval pairs that differ more than 50 ms divided by the total number of all heartbeat intervals (). *p < 0.05, **p < 0.01, ***p < 0.001.
FIGURE 2
Discussion
To explore the possibility that our interoceptive accuracy depends on our ability to engage prefrontal and (para-)limbic brain regions for this matter (; , ; ), we administered cardiac measures of interoceptive accuracy and prefontal-(para-)limbic engagement to a sample of young adults. Interoceptive accuracy was measured with a heartbeat detection task and prefrontal-(para-)limbic engagement with a heart rate recording. We used the heartbeat detection task to determine two different heartbeat detection indices and the heart rate recording to determine two different vagally mediated heart rate variability indices (). The vagally mediated heart rate indices were both positively associated with the heartbeat detection indices, regardless whether the traditional or alternative heartbeat detection index were considered in the regression and correlation analyses. The regression and correlation analyses were well-powered and well-controlled, ruling out that the association between the vagally mediated heart rate variability index and the heartbeat detection indices was spurious. We, thus, found the expected association between vagally mediated heart rate variability and heartbeat detection. The association between vagally mediated heart rate variability and heartbeat detection supports the idea that our interoceptive accuracy depends on our ability to engage prefrontal and (para-)limbic brain regions for the perception and interpretation of cardiac changes.
To understand the association between vagally mediated heart rate variability and heartbeat detection, we have to delineate the processes that determine the performance on the heartbeat detection task. Heartbeat detection relies on executive control processes (), in particular on those that are related to attention. For an accurate perception and interpretation of cardiac changes, attention has to be shifted from the outside to the inside of the body, to be shielded against sensations from the outside of the body and to be focused on sensations inside the body. Attention shifting, attention shielding and attention focusing are executive control processes that are driven by activity changes in prefrontal brain and (para-)limbic regions (). Activity changes in prefrontal and (para-)limbic brain regions are associated with vagally mediated heart rate variability (; ), indicating that vagally mediated heart rate variability may also be associated with these executive control processes. Vagally mediated heart rate variability is, in fact, associated with executive control processes (; ), including attention shifting, attention shielding and attention focusing (; ; ; ). The association between vagally mediated heart rate variability and heartbeat detection may, thus, be mediated by executive control processes that are driven by activity changes in prefrontal and (para-)limbic brain regions.
There are several prefrontal and (para-)limbic brain regions that may mediate the association between vagally mediated heart rate variability and heartbeat detection through executive control processes. These brain regions are organized in networks that are implicated in the up- and downregulation of cardiac changes (), the perception and interpretation of cardiac changes () and the execution of externally and internally oriented attention changes (). Some brain regions are part of more than one network. These brain regions provide functional and structural connections between the networks (). Most connections are provided by the anterior cingulate cortex and the insula (). These connections make the anterior cingulate cortex and the insula to central network hubs that coordinate the interplay between the networks (; ). The anterior cingulate cortex and the insula monitor and regulate the activity of all brain regions in the networks (), which explains why activity changes in the anterior cingulate cortex and insula are closely associated with vagally mediated heart rate variability (; ; ), heartbeat detection (; ; ) and attention (; ; ). Activity changes in the anterior cingulate cortex and the insula may, thus, trigger executive control processes that mediate the association between vagally mediated heart rate variability and heartbeat detection.
To illustrate the importance of the anterior cingulate cortex and the insula for mediating the association between vagally mediated heart rate variability and heartbeat detection through executive control processes, we only have to take a look at some of the most common mental disorders (). Mood and anxiety disorders are characterized by alterations in vagally mediated heart rate variability (, ), heartbeat detection (; ) and executive control processes (, ; ). The alterations in vagally mediated heart rate variability and heartbeat detection are related to alterations in anterior cingulate cortex and insula activity during the perception and interpretation of cardiac changes (; ; ; ; DeVille et al., 2020), presumably via alterations in executive control processes (; ). The alterations in anterior cingulate cortex and insula activity account for severe alterations in emotion, cognition and behavior (; ; ; ; DeVille et al., 2020), indicating that interoceptive deficits play an important role in the etiology and pathogenesis of mood and anxiety disorders ().
Considering the importance of interoceptive deficits for the etiology and pathogenesis of mood and anxiety disorders (), we need measures that allow us to identify those of us whose interoceptive deficits put them at risk for these disorders. As we have shown, cardiac measures may be useful for this purpose. We combined a cardiac measure of interoceptive accuracy, the heartbeat detection index, with a cardiac measure of prefrontal-(para-)limbic engagement, the vagally mediated heart rate variability index. Combining these measures allowed us to demonstrate that our interoceptive accuracy depends on our prefrontal and (para-)limbic engagement during the perception and interpretation of cardiac changes. It should be noted, however, that we only employed cardiac but not neural measures in our investigation. We, thus, can only assume that vagally mediated heart rate variability reflected prefrontal-(para-)limbic engagement during the perception and interpretation of cardiac changes. Future investigations that supplement cardiac measures with neural measures may help to test this assumption with more rigor (e.g., measuring vagally mediated heart rate variability and heartbeat detection during functional or structural imaging). Future investigations should also employ a more rigorous control of participant characteristics that affect cardiac measures than we did in our investigation (e.g., excluding participants with medication use or caffeine use). We hope that our investigation opens an avenue for these types of investigations because we believe that cardiac measures are a promising tool for researchers in the field of interoception. Cardiac measures can be obtained from unobtrusive heart rate recordings that do not require dedicated staff or equipment. These measures may, thus, be interesting for researchers who need to investigate the association between interoception and prefrontal-(para-)limbic engagement in a time- and cost-efficient manner.
Statements
Data availability statement
The datasets presented in this article are not readily available because of ethical restrictions. Requests to access the datasets should be directed to AL, alexander.lischke@uni-greifswald.
Ethics statement
This study was reviewed and approved by the Ethics Committee of the University of Rostock. The participants provided their written informed consent to participate in the study.
Author contributions
AL, RP, and MW designed the study. AM-M and MW collected the data. AL and MW analyzed the data. AL and RP wrote the manuscript. AM-M, MW, and RP contributed to writing, reviewing, and editing of the manuscript. All authors approved the final version of the manuscript.
Funding
Funding for this study was supported by an Open Access Publishing grant that was provided by the German Research Foundation (DFG) and the University of Rostock. AL was supported by a grant provided by the DFG (LI 2517/2-1). The funding source had no further role in the design of the study, in the collection, analysis and interpretation of the data; in the writing of the manuscript; and in the decision to submit the manuscript for publication.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
References
1
AllenB.JenningsJ. R.GianarosP. J.ThayerJ. F.ManuckS. B. (2015). Resting high-frequency heart rate variability is related to resting brain perfusion.Psychophysiology52277–287. 10.1111/psyp.12321
2
AveryJ. A.DrevetsW. C.MosemanS. E.BodurkaJ.BarcalowJ. C.SimmonsW. K. (2014). Major depressive disorder is associated with abnormal interoceptive activity and functional connectivity in the insula.Biol. Psychiatry76258–266. 10.1016/j.biopsych.2013.11.027
3
BarrettL. F.SimmonsW. K. (2015). Interoceptive predictions in the brain.Nat. Rev. Neurosci.16419–429. 10.1038/nrn3950
4
BonnemeierH.RichardtG.PotratzJ.WiegandU. K.BrandesA.KlugeN.et al (2003). Circadian profile of cardiac autonomic nervous modulation in healthy subjects: differing effects of aging and gender on heart rate variability.J. Cardiovasc. Electrophysiol.14791–799. 10.1046/j.1540-8167.2003.03078.x
5
BullmoreE.SpornsO. (2012). The economy of brain network organization.Nat. Rev. Neurosci.13336–349. 10.1038/nrn3214
6
CaserasX.MurphyK.Mataix-ColsD.Lopez-SolaM.Soriano-MasC.OrtrizH.et al (2013). Anatomical and functional overlap within the insula and anterior cingulate cortex during interoception and phobic symptom provocation.Hum. Brain Mapp.341220–1229. 10.1002/hbm.21503
7
ChangC.MetzgerC. D.GloverG. H.DuynJ. H.HeinzeH. J.WalterM. (2013). Association between heart rate variability and fluctuations in resting-state functional connectivity.Neuroimage6893–104. 10.1016/j.neuroimage.2012.11.038
8
CraigA. D. (2002). How do you feel? Interoception: the sense of the physiological condition of the body.Nat. Rev. Neurosci.3655–666. 10.1038/nrn894
9
CritchleyH. D.GarfinkelS. N. (2017). Interoception and emotion.Curr. Opin. Psychol.177–14. 10.1016/j.copsyc.2017.04.020
10
CritchleyH. D.WiensS.RotshteinP.OhmanA.DolanR. J. (2004). Neural systems supporting interoceptive awareness.Nat. Neurosci.7189–195. 10.1038/nn1176
11
CuiH.ZhangB.LiW.LiH.PangJ.HuQ.et al (2020). Insula shows abnormal task-evoked and resting-state activity in first-episode drug-naive generalized anxiety disorder.Depress Anxiety37632–644. 10.1002/da.23009
12
DantasE. M.KempA. H.AndreaoR. V.da SilvaV. J. D.BrunoniA. R.HoshiR. A.et al (2018). Reference values for short-term resting-state heart rate variability in healthy adults: Results from the Brazilian Longitudinal Study of Adult Health-ELSA-Brasil study.Psychophysiology55:e13052. 10.1111/psyp.13052
13
DeVilleD. C.KuplickiR.StewartJ. L.TulsaI.AupperleR. L.BodurkaJ.et al (2020). Diminished responses to bodily threat and blunted interoception in suicide attempters.Elife9:51593.
14
DosenbachN. U.FairD. A.MiezinF. M.CohenA. L.WengerK. K.DosenbachR. A.et al (2007). Distinct brain networks for adaptive and stable task control in humans.Proc. Natl. Acad. Sci. U S A.10411073–11078. 10.1073/pnas.0704320104
15
DunnB. D.StefanovitchI.EvansD.OliverC.HawkinsA.DalgleishT. (2010). Can you feel the beat? Interoceptive awareness is an interactive function of anxiety- and depression-specific symptom dimensions.Behav. Res. Ther.481133–1138. 10.1016/j.brat.2010.07.006
16
EckertM. A.MenonV.WalczakA.AhlstromJ.DenslowS.HorwitzA.et al (2009). At the heart of the ventral attention system: the right anterior insula.Hum. Brain Mapp.302530–2541. 10.1002/hbm.20688
17
EtkinA.PraterK. E.HoeftF.MenonV.SchatzbergA. F. (2010). Failure of anterior cingulate activation and connectivity with the amygdala during implicit regulation of emotional processing in generalized anxiety disorder.Am. J. Psychiatry167545–554. 10.1176/appi.ajp.2009.09070931
18
FagiusJ.KarhuvaaraS. (1989). Sympathetic activity and blood pressure increases with bladder distension in humans.Hypertension14511–517. 10.1161/01.hyp.14.5.511
19
FaulF.ErdfelderE.BuchnerA.LangA. G. (2009). Statistical power analyses using G∗Power 3.1: tests for correlation and regression analyses.Behav. Res. Methods411149–1160. 10.3758/brm.41.4.1149
20
FustosJ.GramannK.HerbertB. M.PollatosO. (2013). On the embodiment of emotion regulation: interoceptive awareness facilitates reappraisal.Soc. Cogn. Affect. Neurosci.8911–917. 10.1093/scan/nss089
21
GarfinkelS. N.SethA. K.BarrettA. B.SuzukiK.CritchleyH. D. (2015). Knowing your own heart: distinguishing interoceptive accuracy from interoceptive awareness.Biol. Psychol.10465–74. 10.1016/j.biopsycho.2014.11.004
22
GrossJ. J.JazaieriH. (2014). Emotion, emotion regulation, and psychopathology: An affective science perspective.Clin. Psychol. Sci.2387–401. 10.1177/2167702614536164
23
HansenA. L.JohnsenB. H.ThayerJ. F. (2003). Vagal influence on working memory and attention.Int. J. Psychophysiol.48263–274. 10.1016/s0167-8760(03)00073-4
24
HartN.McGowanJ.MinatiL.CritchleyH. D. (2013). Emotional regulation and bodily sensation: interoceptive awareness is intact in borderline personality disorder.J. Pers. Disord.27506–518. 10.1521/pedi_2012_26_049
25
HerbertB. M.HerbertC.PollatosO. (2011). On the relationship between interoceptive awareness and alexithymia: is interoceptive awareness related to emotional awareness?J. Pers.791149–1175. 10.1111/j.1467-6494.2011.00717.x
26
HolzmanJ. B.BridgettD. J. (2017). Heart rate variability indices as bio-markers of top-down self-regulatory mechanisms: a meta-analytic review.Neurosci. Biobehav. Rev.74(Pt A), 233–255. 10.1016/j.neubiorev.2016.12.032
27
JenningsJ. R.SheuL. K.KuanD. C.ManuckS. B.GianarosP. J. (2016). Resting state connectivity of the medial prefrontal cortex covaries with individual differences in high-frequency heart rate variability.Psychophysiology53444–454. 10.1111/psyp.12586
28
KreibigS. D. (2010). Autonomic nervous system activity in emotion: A review.Biol. Psychol.84394–421. 10.1016/j.biopsycho.2010.03.010
29
KuehnE.MuellerK.LohmannG.Schuetz-BosbachS. (2016). Interoceptive awareness changes the posterior insula functional connectivity profile.Brain Struct. Funct.2211555–1571. 10.1007/s00429-015-0989-8
30
LichtC. M.de GeusE. J.van DyckR.PenninxB. W. (2009). Association between anxiety disorders and heart rate variability in The Netherlands Study of Depression and Anxiety (NESDA).Psychosom. Med.71508–518. 10.1097/psy.0b013e3181a292a6
31
LichtC. M.de GeusE. J.ZitmanF. G.HoogendijkW. J.van DyckR.PenninxB. W. (2008). Association between major depressive disorder and heart rate variability in the Netherlands Study of Depression and Anxiety (NESDA).Arch. Gen. Psychiatry651358–1367. 10.1001/archpsyc.65.12.1358
32
LimS. L.KimJ. H. (2005). Cognitive processing of emotional information in depression, panic, and somatoform disorder.J. Abnorm. Psychol.11450–61. 10.1037/0021-843x.114.1.50
33
LischkeA.JacksteitR.Mau-MoellerA.PahnkeR.HammA. O.WeippertM. (2018a). Heart rate variability is associated with psychosocial stress in distinct social domains.J. Psychosom. Res.10656–61. 10.1016/j.jpsychores.2018.01.005
34
LischkeA.Mau-MoellerA.JacksteitR.PahnkeR.HammA. O.WeippertM. (2018b). Heart rate variability is associated with social value orientation in males but not females.Sci. Rep.8:7336.
35
LischkeA.PahnkeR.Mau-MoellerA.JacksteitR.WeippertM. (2020a). Sex-specific relationships between interoceptive accuracy and emotion regulation.Front. Behav. Neurosci.14:67. 10.3389/fnbeh.2020.00067
36
LischkeA.WeippertM.Mau-MoellerA.JacksteitR.PahnkeR. (2020b). Interoceptive accuracy is associated with emotional contagion in a valence- and sex-dependent manner.Soc. Neurosci.15227–233. 10.1080/17470919.2019.1690573
37
LischkeA.WeippertM.Mau-MoellerA.PaschkeS.JacksteitR.HammA. O.et al (2019). Sex-specific associations between inter-individual differences in heart rate variability and inter-individual differences in emotion regulation.Front. Neurosci.12:1040. 10.3389/fnins.2018.01040
38
MedfordN.CritchleyH. D. (2010). Conjoint activity of anterior insular and anterior cingulate cortex: awareness and response.Brain Struct. Funct.214535–549. 10.1007/s00429-010-0265-x
39
MitterschiffthalerM. T.WilliamsS. C.WalshN. D.CleareA. J.DonaldsonC.ScottJ.et al (2008). Neural basis of the emotional Stroop interference effect in major depression.Psychol. Med.38247–256. 10.1017/s0033291707001523
40
MoggK.BradleyB. P.WilliamsR. (1995). Attentional bias in anxiety and depression: the role of awareness.Br. J. Clin. Psychol.3417–36. 10.1111/j.2044-8260.1995.tb01434.x
41
MoggK.MathewsA.EysenckM. (1992). Attentional bias to threat in clinical anxiety states.Cogn. Emot.6149–159. 10.1080/02699939208411064
42
OwensA. P.FristonK. J.LowD. A.MathiasC. J.CritchleyH. D. (2018). Investigating the relationship between cardiac interoception and autonomic cardiac control using a predictive coding framework.Auton. Neurosci.21065–71. 10.1016/j.autneu.2018.01.001
43
PaulusM. P.SteinM. B. (2010). Interoception in anxiety and depression.Brain Struct. Funct.214451–463. 10.1007/s00429-010-0258-9
44
PetersenS. E.PosnerM. I. (2012). The attention system of the human brain: 20 years after.Annu. Rev. Neurosci.3573–89. 10.1146/annurev-neuro-062111-150525
45
PollatosO.KirschW.SchandryR. (2005). Brain structures involved in interoceptive awareness and cardioafferent signal processing: a dipole source localization study.Hum. Brain Mapp.2654–64. 10.1002/hbm.20121
46
PollatosO.SchandryR.AuerD. P.KaufmannC. (2007a). Brain structures mediating cardiovascular arousal and interoceptive awareness.Brain Res.1141178–187. 10.1016/j.brainres.2007.01.026
47
PollatosO.Traut-MattauschE.SchandryR. (2009). Differential effects of anxiety and depression on interoceptive accuracy.Depress Anxiety26167–173. 10.1002/da.20504
48
PollatosO.Traut-MattauschE.SchroederH.SchandryR. (2007b). Interoceptive awareness mediates the relationship between anxiety and the intensity of unpleasant feelings.J. Anxiety Disord.21931–943. 10.1016/j.janxdis.2006.12.004
49
RossiP.AndriesseG. I.OeyP. L.WienekeG. H.RoelofsJ. M.AkkermansL. M. (1998). Stomach distension increases efferent muscle sympathetic nerve activity and blood pressure in healthy humans.J. Neurol. Sci.161148–155. 10.1016/s0022-510x(98)00276-7
50
Ruiz VargasE.SorosP.ShoemakerJ. K.HachinskiV. (2016). Human cerebral circuitry related to cardiac control: A neuroimaging meta-analysis.Ann. Neurol.79709–716. 10.1002/ana.24642
51
SchandryR. (1981). Heart beat perception and emotional experience.Psychophysiology18483–488. 10.1111/j.1469-8986.1981.tb02486.x
52
SchulzS. M. (2016). Neural correlates of heart-focused interoception: a functional magnetic resonance imaging meta-analysis.Philos. Trans. R. Soc. Lond. B Biol. Sci.371:20160018. 10.1098/rstb.2016.0018
53
SeeleyW. W.MenonV.SchatzbergA. F.KellerJ.GloverG. H.KennaH.et al (2007). Dissociable intrinsic connectivity networks for salience processing and executive control.J. Neurosci.272349–2356. 10.1523/jneurosci.5587-06.2007
54
ShafferF.GinsbergJ. P. (2017). An overview of heart rate variability metrics and norms.Front. Public Health5:258. 10.3389/fpubh.2017.00258
55
ShahP.CatmurC.BirdG. (2017). From heart to mind: Linking interoception, emotion, and theory of mind.Cortex93220–223. 10.1016/j.cortex.2017.02.010
56
ShahP.HallR.CatmurC.BirdG. (2016). Alexithymia, not autism, is associated with impaired interoception.Cortex81215–220. 10.1016/j.cortex.2016.03.021
57
ShulmanG. L.AstafievS. V.FrankeD.PopeD. L.SnyderA. Z.McAvoyM. P.et al (2009). Interaction of stimulus-driven reorienting and expectation in ventral and dorsal frontoparietal and basal ganglia-cortical networks.J. Neurosci.294392–4407. 10.1523/jneurosci.5609-08.2009
58
SiennickaA.QuintanaD. S.FedurekP.WijataA.PalecznyB.PonikowskaB.et al (2019). Resting heart rate variability, attention and attention maintenance in young adults.Int. J. Psychophysiol.143126–131. 10.1016/j.ijpsycho.2019.06.017
59
SmithR.ThayerJ. F.KhalsaS. S.LaneR. D. (2017). The hierarchical basis of neurovisceral integration.Neurosci. Biobehav. Rev.75274–296. 10.1016/j.neubiorev.2017.02.003
60
SorensenL.WassS.OsnesB.SchancheE.AdolfsdottirS.SvendsenJ. L.et al (2019). A psychophysiological investigation of the interplay between orienting and executive control during stimulus conflict: A heart rate variability study.Physiol. Behav.211:112657. 10.1016/j.physbeh.2019.112657
61
SridharanD.LevitinD. J.MenonV. (2008). A critical role for the right fronto-insular cortex in switching between central-executive and default-mode networks.Proc. Natl. Acad. Sci. U S A.10512569–12574. 10.1073/pnas.0800005105
62
TarvainenM. P.NiskanenJ. P.LipponenJ. A.Ranta-AhoP. O.KarjalainenP. A. (2014). Kubios HRV–heart rate variability analysis software.Comput. Methods Programs Biomed.113210–220.
63
Task Force of the European Society of Cardiology (1996). Heart rate variability standards of measurement, physiological interpretation, and clinical use.Eur. Heart J.17354–381. 10.1093/oxfordjournals.eurheartj.a014868
64
ThayerJ. F.AhsF.FredriksonM.SollersJ. J.IIIWagerT. D. (2012). A meta-analysis of heart rate variability and neuroimaging studies: implications for heart rate variability as a marker of stress and health.Neurosci. Biobehav. Rev.36747–756. 10.1016/j.neubiorev.2011.11.009
65
ThayerJ. F.LaneR. D. (2009). Claude Bernard and the heart-brain connection: further elaboration of a model of neurovisceral integration.Neurosci. Biobehav. Rev.3381–88. 10.1016/j.neubiorev.2008.08.004
66
ThayerJ. F.SollersJ. J.IIIRuiz-PadialE.VilaJ. (2002). Estimating respiratory frequency from autoregressive spectral analysis of heart period.IEEE Eng. Med. Biol. Mag.2141–45. 10.1109/memb.2002.1032638
67
WeippertM.BehrensK.RiegerA.KumarM.BehrensM. (2015). Effects of breathing patterns and light exercise on linear and nonlinear heart rate variability.Appl. Physiol. Nutr. Metab.40762–768. 10.1139/apnm-2014-0493
68
WeippertM.KumarM.KreuzfeldS.ArndtD.RiegerA.StollR. (2010). Comparison of three mobile devices for measuring R-R intervals and heart rate variability: Polar S810i. Suunto t6 and an ambulatory ECG system.Eur. J. Appl. Physiol.109779–786. 10.1007/s00421-010-1415-9
69
WiebkingC.de GreckM.DuncanN. W.TempelmannC.BajboujM.NorthoffG. (2015). Interoception in insula subregions as a possible state marker for depression-an exploratory fMRI study investigating healthy, depressed and remitted participants.Front. Behav. Neurosci.9:82. 10.3389/fnbeh.2015.00082
70
WilliamsD. P.ThayerJ. F.KoenigJ. (2016). Resting cardiac vagal tone predicts intraindividual reaction time variability during an attention task in a sample of young and healthy adults.Psychophysiology531843–1851. 10.1111/psyp.12739
71
YamasakiY.KodamaM.MatsuhisaM.KishimotoM.OzakiH.TaniA.et al (1996). Diurnal heart rate variability in healthy subjects: effects of aging and sex difference.Am. J. Physiol.271(1 Pt 2), H303–H310.
72
ZahnD.AdamsJ.KrohnJ.WenzelM.MannC. G.GomilleL. K.et al (2016). Heart rate variability and self-control–A meta-analysis.Biol. Psychol.1159–26. 10.1016/j.biopsycho.2015.12.007
Summary
Keywords
heartbeat detection, heart rate variability, interoception, attention, vagal tone, emotion
Citation
Lischke A, Pahnke R, Mau-Moeller A and Weippert M (2021) Heart Rate Variability Modulates Interoceptive Accuracy. Front. Neurosci. 14:612445. doi: 10.3389/fnins.2020.612445
Received
30 September 2020
Accepted
02 December 2020
Published
11 January 2021
Volume
14 - 2020
Edited by
Julian F. Thayer, The Ohio State University, United States
Reviewed by
Elena Makovac, King’s College London, United Kingdom; Veronica Dusi, University of Pavia, Italy
Updates
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© 2021 Lischke, Pahnke, Mau-Moeller and Weippert.
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*Correspondence: Alexander Lischke, alexander.lischke@uni-greifswald.deMatthias Weippert, matthias.weippert@uni-rostock.de
This article was submitted to Autonomic Neuroscience, a section of the journal Frontiers in Neuroscience
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