Cardiac Interoceptive Accuracy Research Articles

Exploring sex differences in cardiac interoceptive accuracy using the phase adjustment task.

Previous evidence suggests males and females differ with respect to interoception-the processing of internal bodily signals-with males typically outperforming females on tasks of interoceptive accuracy. However, interpretation of existing evidence in the cardiac domain is hindered by the limitations of existing tools. In this investigation, we pooled data from several samples to examine sex differences in cardiac interoceptive accuracy on the phase adjustment task, a new measure that overcomes several limitations of the existing tools. In a sample of 266 individuals, we observed that females outperformed males, indicative of better cardiac interoceptive accuracy, but had lower confidence than males. These results held after controlling for sex differences in demographic, physiological and engagement factors. Importantly, these results were specific to the measure of cardiac interoceptive accuracy. No sex differences were observed for individuals who completed the structurally identical screener task, although a similar pattern of results was observed in relation to confidence. These surprising data suggest the presence of a female advantage for cardiac interoceptive accuracy and potential differences in interoceptive awareness (metacognition). Possible reasons for mixed results in the literature, as well as implications for theory and future research, are discussed.

Individual differences in interoception and autistic traits share altered facial emotion perception, but not recognition per se

While alterations in both physiological responses to others’ emotions as well as interoceptive abilities have been identified in autism, their relevance in altered emotion recognition is largely unknown. We here examined the role of interoceptive ability, facial mimicry, and autistic traits in facial emotion processing in non-autistic individuals. In an online Experiment 1, participants (N = 99) performed a facial emotion recognition task, including ratings of perceived emotional intensity and confidence in emotion recognition, and reported on trait interoceptive accuracy, interoceptive sensibility and autistic traits. In a follow-up lab Experiment 2 involving 100 participants, we replicated the online experiment and additionally investigated the relationship between facial mimicry (measured through electromyography), cardiac interoceptive accuracy (evaluated using a heartbeat discrimination task), and autistic traits in relation to emotion processing. Across experiments, neither interoception measures nor facial mimicry accounted for a reduced recognition of specific expressions with higher autistic traits. Higher trait interoceptive accuracy was rather associated with more confidence in correct recognition of some expressions, as well as with higher ratings of their perceived emotional intensity. Exploratory analyses indicated that those higher intensity ratings might result from a stronger integration of instant facial muscle activations, which seem to be less integrated in intensity ratings with higher autistic traits. Future studies should test whether facial muscle activity, and physiological signals in general, are correspondingly less predictive of perceiving emotionality in others in individuals on the autism spectrum, and whether training interoceptive abilities might facilitate the interpretation of emotional expressions.

Blind individuals' enhanced ability to sense their own heartbeat is related to the thickness of their occipital cortex.

Blindness is associated with heightened sensory abilities, such as improved hearing and tactile acuity. Moreover, recent evidence suggests that blind individuals are better than sighted individuals at perceiving their own heartbeat, suggesting enhanced interoceptive accuracy. Structural changes in the occipital cortex have been hypothesized as the basis of these behavioral enhancements. Indeed, several studies have shown that congenitally blind individuals have increased cortical thickness within occipital areas compared to sighted individuals, but how these structural differences relate to behavioral enhancements is unclear. This study investigated the relationship between cardiac interoceptive accuracy and cortical thickness in 23 congenitally blind individuals and 23 matched sighted controls. Our results show a significant positive correlation between performance in a heartbeat counting task and cortical thickness only in the blind group, indicating a connection between structural changes in occipital areas and blind individuals' enhanced ability to perceive heartbeats.

Maternal interoceptive focus is associated with greater reported engagement in mother-infant stroking and rocking.

Parental caregiving during infancy is primarily aimed at the regulation of infants' physiological and emotional states. Recent models of embodied cognition propose that interoception, i.e., the perception of internal bodily states, may influence the quality and quantity of parent-infant caregiving. Yet, empirical investigations into this relationship remain scarce. Across two online studies of mothers with 6- to 18-month-old infants during Covid-19 lockdowns, we examined whether mothers' self-reported engagement in stroking and rocking their infant was related to self-reported interoceptive abilities. Additional measures included retrospective accounts of pregnancy and postnatal body satisfaction, and mothers' reports of their infant's understanding of vocabulary relating to body parts. In Study 1 (N = 151) and Study 2 (N = 111), mothers reported their engagement in caregiving behaviours and their tendency to focus on and regulate bodily states. In a subsample from Study 2 (N = 49), we also obtained an objective measure of cardiac interoceptive accuracy using an online heartbeat counting task. Across both studies, the tendency to focus on and regulate interoceptive states was associated with greater mother-infant stroking and rocking. Conversely, we found no evidence for a relationship between objective interoceptive accuracy and caregiving. The findings suggest that interoception may play a role in parental engagement in stroking and rocking, however, in-person dyadic studies are warranted to further investigate this relationship.

Cardiac Contractility Drives Cardiac Interoception

Introduction: Many of the receptors for the exteroceptive senses (e.g. vision, hearing, smell, touch, pain, etc.) are known. Interoception allows us to sense threats to our “milieu interior.” The mechanisms by which threats induce a feeling of anxiety manifested by heart palpitations, shortness of breath (dyspnea), and gastric awareness (queasiness) are unknown. Using photoplethysmography, we reported that individual variation in cardiac interoception, i.e. the ability to feel a change in one’s heartbeat without taking one’s pulse, correlated with contrasts in heart rate and stroke volumes among the population. Hypothesis: Because increases in heart rate and stroke volume raise cardiac contractility, we postulated that stretch activated piezo channels in the heart are responsible for cardiac interoception. Methods: Interoceptive awareness was determined with a ten point Likert scale in which volunteers rated their ability to feel their heartbeat. Interoceptive accuracy was measured by asking subjects to count their heartbeats without taking their pulse over four time intervals in random order. Simultaneously, heart rates were surreptitiously recorded with a two lead electrocardiogram (EKG). Cardiac interoceptive accuracy was calculated using the formula 1/4 Σ [1 − (|EKG measured heartbeats − subjectively reported heartbeats|) / EKG measured heartbeats]. We used 2D speckle tracking ultrasound to measure the relationship between cardiac strain patterns, normalized for heart rate (HR) and body surface area (BSA), and cardiac interoception in 12 resting men and women. We also measured gastric interoception by determining the percent increase in the volume of water some subjects could drink after reporting water satiety. Results: Cardiac interoceptive awareness correlated with interoceptive accuracy ( r = 0.73, p = 0.009). Speckle tracking of cardiac chambers with echocardiography demonstrated that global longitudinal left ventricular (LV) strain correlated with total left ventricular preload (atrial reservoir strain, r = 0.83, p = 0.0009) and early diastolic filling (atrial conduit strain, r = 0.88, p = 0.0002). LV strain also correlated with interoceptive awareness ( r = 0.58, p = 0.046). When stratified, individuals with the highest interoceptive accuracy (n = 6), compared to those with the lowest interoceptive accuracy (n = 6), had less LV strain, [values expressed as (percent change/BSA)*HR ± S.E.M., 828 ± 81 vs. 582 ± 47, p = 0.026]. Cardiac interoceptive awareness also correlated with gastric interoception accuracy, (n = 6) r = 0.90, p = 0.015. Conclusions: Changes in the activation of cardiac stretch receptors by cardiac filling and contractility could modify interoceptive awareness in men and women at rest. Contrasts in stretch receptor activation in other contractile organs (e.g. stomach) could explain variation in interoceptive awareness in the population. Henry M. Jackson Foundation for the Advancement of Military Medicine. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Delving into the relationship between regular physical exercise and cardiac interoception in two cross-sectional studies

Cardiac interoception, the ability to sense and process cardiac afferent signals, has been shown to improve after a single session of acute physical exercise. However, it remains unclear whether repetitive engagement in physical exercise over time leads to long-term changes in cardiac interoceptive accuracy. It is also unknown whether those changes affect the neural activity associated with the processing of afferent cardiac signals, assessed by the heart-evoked potential (HEP). In this study, we aimed to investigate this hypothesis through two cross-sectional studies, categorizing participants as active or inactive based on physical fitness (Study I; N = 45) or self-reported physical activity levels (Study II; N = 60). Interoception was assessed at rest using the HEP (Studies I and II), the Heartbeat Counting task (Study II), and the Rubber Hand Illusion (RHI) (Study II). Study I showed strong evidence of better cardiovascular fitness in the active group than in the inactive group as well as robust between-group differences in electrocardiogram (ECG) recordings. Study 2 replicated the clear differences in ECG as a function of regular physical activity. Those results were expected due to clear differences in physical activity habits. In contrast, our analysis revealed no robust differences between groups across cardiac interoception tasks and the RHI, although the direct relevance of these measures to interoception remains under investigation. In sum, our results do not provide convincing evidence to support a strong version of the notion that regular physical exercise is associated with an enhanced in cardiac interoception.

Heightened interoception in adults with fibromyalgia

Previous research suggests that the processing of internal body sensations (interoception) affects how we experience pain. Some evidence suggests that people with fibromyalgia syndrome (FMS) – a condition characterised by chronic pain and fatigue – may have altered interoceptive processing. However, extant findings are inconclusive, and some tasks previously used to measure interoception are of questionable validity. Here, we used an alternative measure – the Phase Adjustment Task (PAT) – to examine cardiac interoceptive accuracy in adults with FMS. We examined: (i) the tolerability of the PAT in an FMS sample (N = 154); (ii) if there are differences in facets of interoception (PAT performance, PAT-related confidence, and scores on the Private Body Consciousness Scale) between an FMS sample and an age- and gender-matched pain-free sample (N = 94); and (iii) if subgroups of participants with FMS are identifiable according to interoceptive accuracy levels. We found the PAT was tolerable in the FMS sample, with additional task breaks and a recommended hand posture. The FMS sample were more likely to be classified as ‘interoceptive’ on the PAT, and had significantly higher self-reported interoception compared to the pain-free sample. Within the FMS sample, we identified a subgroup who demonstrated very strong evidence of being interoceptive, and concurrently had lower fibromyalgia symptom impact (although the effect size was small). Conversely, self-reported interoception was positively correlated with FMS symptom severity and impact. Overall, interoception may be an important factor to consider in understanding and managing FMS symptoms. We recommend future longitudinal work to better understand associations between fluctuating FMS symptoms and interoception.

Are blind individuals immune to bodily illusions? Somatic rubber hand illusion in the blind revisited

Multisensory awareness of one’s own body relies on the integration of signals from various sensory modalities such as vision, touch, and proprioception. But how do blind individuals perceive their bodies without visual cues, and does the brain of a blind person integrate bodily senses differently from a sighted person? To address this question, we aimed to replicate the only two previous studies on this topic, which claimed that blind individuals do not experience the somatic rubber hand illusion, a bodily illusion triggered by the integration of correlated tactile and proprioceptive signals from the two hands. We used a larger sample size than the previous studies and added Bayesian analyses to examine statistical evidence in favor of the lack of an illusion effect. Moreover, we employed tests to investigate whether enhanced tactile acuity and cardiac interoceptive accuracy in blind individuals could also explain the weaker illusion. We tested 36 blind individuals and 36 age- and sex-matched sighted volunteers. The results show that blind individuals do not experience the somatic rubber hand illusion based on questionnaire ratings and behavioral measures that assessed changes in hand position sense toward the location of the rubber hand. This conclusion is supported by Bayesian evidence in favor of the null hypothesis. The findings confirm that blind individuals do not experience the somatic rubber hand illusion, indicating that lack of visual experience leads to permanent changes in multisensory bodily perception. In summary, our study suggests that changes in multisensory integration of tactile and proprioceptive signals may explain why blind individuals are “immune” to the nonvisual version of the rubber hand illusion.

Is That My Heartbeat? Measuring and Understanding Modality-Dependent Cardiac Interoception in Virtual Reality.

Measuring interoception ('perceiving internal bodily states') has diagnostic and wellbeing implications. Since heartbeats are distinct and frequent, various methods aim at measuring cardiac interoceptive accuracy (CIAcc). However, the role of exteroceptive modalities for representing heart rate (HR) across screen-based and Virtual Reality (VR) environments remains unclear. Using a PolarH10 HR monitor, we develop a modality-dependent cardiac recognition task that modifies displayed HR. In a mixed-factorial design (N=50), we investigate how task environment (Screen, VR), modality (Audio, Visual, Audio-Visual), and real-time HR modifications ( ±15%, ±30%, None) influence CIAcc, interoceptive awareness, mind-body measures, VR presence, and post-experience responses. Findings showed that participants confused their HR with underestimates up to 30%; environment did not affect CIAcc but influenced mind-related measures; modality did not influence CIAcc, however including audio increased interoceptive awareness; and VR presence inversely correlated with CIAcc. We contribute a lightweight and extensible cardiac interoception measurement method, and implications for biofeedback displays.

Revisiting the relationship between illusory hand ownership induced by visuotactile synchrony and cardiac interoceptive accuracy

Multisensory integration plays an important role in the experience of the bodily self. Recently, the relationship between exteroception and interoception has been actively debated. The first evidence was a report that the susceptibility of the sense of ownership over a fake hand (i.e., illusory hand ownership: IHO) in the typical rubber hand illusion is negatively modulated by the accuracy of the heartbeat perception (i.e., cardiac interoceptive accuracy: CIA). If reliable, this would suggest an antagonism between the exteroceptive and interoceptive cues underlying the bodily self. However, some inconsistent data have been reported, raising questions about the robustness of the initial evidence. To investigate this robustness, we estimated the extent of the modulatory effect of CIA on IHO susceptibility by applying Bayesian hierarchical modeling to two independent datasets. Overall, our results did not support that IHO susceptibility is modulated by CIA. The present estimates with high uncertainty cannot exclude the hypothesis that the relationship between IHO susceptibility and CIA is so weak as to be negligible. Further studies with larger sample sizes are needed to reach a conclusion about the extent of the modulatory effect. These findings highlight the lack of robustness of key evidence supporting the “antagonism hypothesis”.

Interoception, alexithymia, and anxiety among individuals with alcohol use disorder.

Interoception (i.e., the ability to recognize bodily signals), alexithymia (i.e., the inability to recognize emotional states) and negative affect (i.e., unpleasant feelings such as anxiety) have been associated with alcohol use disorder (AUD). Previous research suggests that interoception may underlie alexithymia, which in turn may be associated with negative affectivity. However, this remains to be empirically tested. This study investigates whether alexithymia mediates the association between interoception and anxiety and whether this association differs across individuals with AUD and a healthy control (HC) comparison group. The AUD group consisted of 99 participants enrolled in an 8-week abstinence-based inpatient treatment program. The HC group included 103 healthy individuals. The heartbeat counting task (HCT) was used to assess interoception (cardiac interoceptive accuracy). The Toronto Alexithymia Scale (TAS-20) was used to assess alexithymia. The Brief Symptom Inventory (BSI) was used to assess anxiety. The moderated mediation model with interoception as the predictor, alexithymia as the mediator, and negative affect (i.e., state anxiety) as the dependent variable was tested. The analysis showed that the conditional indirect effect of interoception on anxiety via alexithymia was significant for individuals with AUD [ab = -0.300, bootstrap 95% CI = (-0.618, -0.088)], as well as for HCs [ab = -0.088, bootstrap 95% CI = (-0.195, -0.014)]; however, the conditional indirect effect significantly differed across HCs and individuals with AUD. Namely, the mediated effect was greater among individuals with AUD compared to the HC group. The results suggests that interoceptive impairment contributes to greater negative affect (i.e., state anxiety) via alexithymia especially for individuals with AUD. Improving emotion recognition via therapeutic methods focused on strengthening interoceptive abilities could improve outcomes for individuals receiving treatment for AUD.

Visual respiratory biofeedback to improve visuospatial cognition and cardiac interoception in migraineurs: a study protocol for a randomized controlled trial.

Migraine is a complex neurological disorder that typically presents with unilateral cephalgia associated with cognitive impairment and reduced interoception. These symptoms result in socio-economic repercussions due to reduced productivity, efficiency, and work performance. Therefore, along with headache management, improving cognition and interoception should also be significant therapeutic targets to effectively manage migraine. To achieve this, we propose to explore the role of a yoga-based visual respiratory biofeedback (VRB) as a possible therapeutic strategy. At least 64 participants will be recruited for the trial after screening for eligibility criteria, using the migraine screening questionnaire and Montreal cognitive assessment test. They will be randomly allocated (1:1) to either the experimental group receiving a 20-min session of yoga-based VRB or the control group who will be asked to watch a documentary film for the same duration. Visuospatial cognition will be assessed by the Corsi block-tapping task, and cardiac interoceptive accuracy will be assessed by the heartbeat counting task at baseline and immediately after the intervention. Based on the distribution and variance of the data obtained, analysis will be conducted based on linear mixed models using SPSS version 28.0.1.0, with a two-sided p-value of < 0.05 considered to be statistically significant. To the best of our knowledge, this is the first study to design and assess the effects of yoga-based biofeedback therapy on cognition and cardiac interoception in migraineurs. Furthermore, we postulated that pranayama's therapeutic effects might be enhanced by using visual yogic respiratory biofeedback. Considering the socio-economic burden of migraine, if found effective, VRB investigated in the trial could be considered as a therapeutic strategy. ClinicalTrials.gov CTRI, CTRI/2023/03/050430.

Interoceptive abilities facilitate taking another’s spatial perspective

Information can be perceived from a multiplicity of spatial perspectives, which is central to effectively understanding and interacting with our environment and other people. Interoception, the sense of the physiological state of our body, is also a fundamental component contributing to our perception. However, whether the perception of our inner body signals influences our ability to adopt and flexibly change between different spatial perspectives remains poorly understood. To investigate this, 90 participants completed tasks assessing multiple dimensions of interoception (interoceptive sensibility, cardiac interoceptive accuracy and awareness) and the Graphesthesia task to assess tactile spatial perspective-taking and its flexibility. The results revealed that higher cardiac interoceptive awareness is associated with greater consistency in adopting a perspective decentred from the self. Second, higher cardiac interoceptive accuracy was associated with slower and less accurate performance in switching from a decentred to an egocentred perspective. These results show that interoceptive abilities facilitate decentred spatial perspective-taking, likely reflecting stronger perceived boundaries between internal states and the external world.

594-P: Untrained Men and Women with Diabetes Mellitus Have Impaired Interoceptive Accuracy

Background: The ADA recommends 150 minutes/week of physical activity for patients with diabetes mellitus (DM). Training-induced reductions in autonomic reactivity could impair awareness of hypoglycemia (IAH) in some patients with DM unless there are compensatory signals of low blood glucose not dependent upon the autonomic nervous system. The receptors for exteroceptive senses (e.g. temperature, pain, vision, hearing) are known, interoceptive signal transduction is unexplored. This mechanism allows us to detect the cardiac, respiratory, and gastric sensations from stress (“My heart is pounding,” and “I can’t breathe,” and I feel queasy”). Hypothesis: We posit cardiac interoceptive accuracy (IA) is impaired in patients with DM, but exercise training could enhance IA and reduce IAH. We assessed IA in healthy, fit subjects, and also, untrained men and women with DM. Methods: IA was determined by comparing the number of heartbeats a subject could count without taking their pulse while their EKG was surreptitiously recorded. Heart rate variability (HRV) was assessed by measuring the root mean square of successive differences of the interval between sequential heartbeats. Results: Although MAP was not significantly different between these two cohorts, when compared to DM (N=8), the fit subjects (N=9) had (all values in mean ± SEM) lower resting HR (63±3 vs. 85±4 bpm, p&amp;lt;0.001) and greater HRV (85±4 vs. 69±11 ms, p=0.01) and IA (82±7 vs. 50±12 in percent, p=0.02). Furthermore, HR was highly correlated with both HRV (r=−0.63, p=0.007) and IA (r=−0.72, p=0.001). Conclusion: Untrained DM have diminished IA which may be improved by fitness training without fear of worsening IAH; resting HR should be used to track changes in HRV and IA as DM subjects undergo fitness training. Disclosure J.Summers: None. Z.Miklja: None. W.Lockette: None.

Interoception: Where do we go from here?

In recent years, there has been a significant rise in interest in interoception, the processing of internal bodily signals. This interest has been coupled by increased concerns regarding the measurement and conceptualisation of interoception. Focusing on cardiac interoceptive accuracy, I outline what I believe to be the most pressing issues in the field of interoception-specifically the continued reliance on the heartbeat counting task. I then provide an overview of what I believe to be more general limitations concerning how we measure and conceptualise individual differences in interoception and suggestions for a way forward. Specifically, I believe that by moving beyond single measurements, establishing optimal levels of interoceptive accuracy, and refocusing from accuracy to propensity, we may be able to uncover the real-life relevance of interoceptive abilities.

The actor's insight: Actors have comparable interoception but better metacognition than nonactors.

Both accurately sensing our own bodily signals and knowing whether we have accurately sensed them may contribute to a successful emotional life, but there is little evidence on whether these physiological perceptual and metacognitive abilities systematically differ between people. Here, we examined whether actors, who receive substantial training in the production, awareness, and control of emotion, and nonactor controls differed in interoceptive ability (the perception of internal bodily signals) and/or metacognition about interoceptive accuracy (awareness of that perception), and explored potential sources of individual differences in and consequences of these abilities including correlational relationships with state and trait anxiety, proxies for acting ability, and the amount of acting training. Participants performed a heartbeat detection task in which they judged whether tones were played synchronously or delayed relative to their heartbeats, and then rated their metacognitive confidence in that judgment. Cardiac interoceptive accuracy and metacognitive awareness of interoceptive accuracy were independent, and while actors' and controls' interoceptive accuracy was not significantly different, actors had consistently superior metacognitive awareness of interoception. Exploratory analyses additionally suggest that this metacognitive ability may be correlated with measures of acting ability, but not the duration of acting training. Interoceptive accuracy and metacognitive insight into that accuracy appear to be separate abilities, and while actors may be no more accurate in reading their bodies, their metacognitive insight means they know better when they're accurate and when they're not. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Higher cardiovascular activation, but normal heartbeat-evoked potentials and cardiac interoceptive accuracy in somatoform disorders and major depressive disorder

The perception-filter model posits that the generation of medically-unexplained symptoms is associated with (I.) more intense afferent bodily signals, (II.) impaired filter system activity to differentiate relevant from irrelevant signals, and (III.) altered perception of bodily signals. We tested these assumptions for cardiac perception in patients with somatoform disorders (SFD), patients with major depressive disorder (MDD) and healthy control (HC) individuals. Heart rate (variability; HR/HRV) and blood pressure served as indicators of bodily signals (I.); heartbeat-evoked potentials (HEPs) assessed during a heartbeat counting task (HCT) and a distraction task indicated filter system activity (II.); interoceptive accuracy (IAc) in the HCT was interpreted as an index of perception (III.). All indicators were assessed before and after a socially-evaluated cold pressor stress task (SECPT) and a control intervention. SFD patients (n ​= ​24) showed higher average HR and diastolic blood pressure, as well as lower HRV than HC individuals (n ​= ​22), but there were no differences in HEPs or IAc. Neither were there significant differences between the SFD and the MDD groups (n ​= ​24), nor any stress effect on HEPs or IAc. Our findings suggest that increased intensity of bodily signals (I.) is the only model assumption that could be supported for patients with fully-developed SFD. • We investigated interoception in somatoform disorders (SFD) and depression (MDD). • SFD and MDD patients had higher heart rate (HR) and blood pressure than controls. • Heartbeat-evoked potentials did not differ between SFD, MDD patients and controls. • Cardiac interoception did not differ between SFD, MDD patients and controls. • Acute stress did not affect heartbeat-evoked potentials and cardiac interoception.

The Effects of a 3-Week Heartbeat Perception Training on Interoceptive Abilities.

Recent studies showed promising short-term effects of heartbeat perception training on interoceptive abilities. Research on the effects of heartbeat perception training on interoceptive abilities over time is sparse. Therefore, the aim of this study was to examine the short-term effects and the effects of a 3-week heartbeat perception training over time on interoceptive abilities, namely, cardiac interoceptive accuracy (IAc) and interoceptive sensibility (IS). A total of 40 healthy participants were randomized to the intervention group (n = 20) or the control group (n = 20). The intervention group conducted three cardiac biofeedback sessions (one per week) at the laboratory, whereas the control group watched a documentary instead. Interoceptive abilities were assessed via the heartbeat perception task (IAc) and confidence ratings (IS) at baseline, after each laboratory session, and 1 week after the last session (post-measurement). IAc was significantly increased in the intervention group compared to the control group after the first training session (short-term effect). There were no significant improvements in IS due to the first session, and neither on IAc nor IS over time. Descriptive trends of improved interoceptive abilities over time were found in both groups. Single session of heartbeat perception training seems to be a promising approach to improve IAc. Future research should further investigate the long-term effects of diverse heartbeat perception training varying in frequency and intensity of the training sessions in diverse samples aiming to improve interoceptive abilities.

Hemodynamic Determinants of Cardiac Interoception

Background. Many of the sensory receptors that encode touch, temperature, pain, and visual or auditory cues are well known. These exteroceptive receptors allow us to respond to external threats from the environment. The interoceptive sensory receptors that allow us to identify threats within the internal milieu, such as changes of our heart rate brought about by fright, gastric fullness with satiety, or breathing difficulty from increased respiratory resistance are not known. We postulated that the mechanosensitive piezo receptors in the heart are responsible for cardiac interoception in humans. If true, individual contrasts in cardiac contractility among the population sensed by these stretch receptors in the heart could explain the ability of some men and women to “feel” their heart rate without taking their pulse.Hypothesis. We postulate that individual differences in ventricular contractility assessed by measurements of stroke volume in subjects would correlate with cardiac interoceptive accuracy.Methods. First, to confirm whether the gene encoding Piezo 1 is highly expressed in ventricular tissue, we assessed Piezo 1 transcripts in rodent hearts. Subsequently, heart rates, blood pressures, and stroke volumes were measured non‐invasively in 18 healthy volunteers, without a history of hypertension, using a commercial photoplethysmography device (Finapres,™ Nova Instruments). Measurements were taken in resting subjects while supine, upright, and after three minutes of standing. Cardiac interoceptive accuracy (“IA”) was assessed using a heartbeat tracking task where participants counted their heartbeats during four separate trials after being instructed not to take their pulse or engage in any manipulation that could facilitate heartbeat detection while their EKG was surreptitiously being recorded.Results. Using RT‐PCR, we confirmed that heart, lung, and stomach had the highest expression of the Piezo 1 channel. Next, we found that “IA” correlated inversely with resting heart rate (r = ‐0.71, p = 0.0009, n = 18) and directly with resting heart rate variability as determined by the standard deviation of the inter‐beat interval (r = 0.68, p = 0.005, n = 15). As expected, high “IA” was associated with higher resting stroke volumes (r = 0.62, p = 0.01, n = 15). Most significantly, “IA” was highly correlated with standing‐induced changes in heart rate (r = 0.72, p = 0.0023, n = 15), and also, standing induced changes in stroke volume (r = 0.72, p = 0.0025, n = 15). The increased cardiac contractility was not associated with increased systemic vascular resistance (r = ‐0.50, p = 0.051, n = 16) or increased work of the heart. Indeed, the resting rate pressure product was inversely correlated “IA” (r = ‐0.65, p = 0.009, n = 15).Conclusions. Individuals with lower resting heart rates, lower resting rate‐pressure products, and higher heart rate variability and stroke volumes have greater interoceptive accuracy. These characteristics mimic the phenotype that results from exercise training. Similar to fitness trained individuals, lower heart rates allow increased ventricular filling and greater stroke volumes. Indeed, the inter‐beat intervals in our subjects at rest were positively correlated with greater stroke volumes. (r = 0.75, p=0.001, n=15). On the basis of these findings, we suggest that enhanced interoceptive accuracy will correlate with individuals having higher levels of fitness within the population.

Cardiovascular Fitness Improves Cardiac Interoceptive Accuracy

Background. Many of the sensory receptors encoding touch, temperature, pain, and visual or auditory cues are known; these exteroceptive receptors allow us to respond to external threats. The interoceptive sensory receptors that identify stimuli within the internal milieu, e.g. changes in our heart rate, gastric fullness, or breathing difficulty are not known. We postulated mechanosensitive piezo channels in the heart are responsible for cardiac interoception. We found that individuals with lower resting heart rates and resting rate‐pressure products, and higher heart rate variability and stroke volumes have greater interoceptive accuracy. These characteristics often result from exercise training.HypothesisWe suggest that individual contrasts in interoceptive accuracy will correlate with genotypic differences in the PIEZO 1 channels in the population and individual variation in fitness levels. Furthermore, activities that increase stroke volume, such as thermal warming, would enhance interoceptive accuracy.MethodsHeart rates, blood pressures, and stroke volumes were measured non‐invasively in 12 healthy men and women using a commercial photoplethysmography device (Finapres,™ Nova Instruments). Fitness levels were assessed using a modified Harvard step test. Cardiac interoceptive accuracy (“IA”) was assessed using a heartbeat tracking task where participants counted their heartbeats during four separate trials after being instructed not to take their pulse or engage in any manipulation that could facilitate heartbeat detection while their EKG was being recorded. “IA” was calculated from 1/4 Σ [1 – (|EKG heartbeats – reported heartbeats|) ÷ EKG heartbeats]; higher scores represent greater interoceptive accuracy. “IA” was assessed while subjects rested supine in a room at 68o F and after warming with thermal blankets for 25‐30 min. Because of confounding effects of menstrual cycle on body temperature, we studied the effect of thermal warming only on men. Because PIEZO 1 channels also mediate regulatory volume changes to osmotic stress, we assessed PIEZO 1 channel‐mediated hemolysis of erythrocytes during hypotonic stress.Results. Cardiac “IA” correlated positively with fitness levels (n = 9, r = 0.59, p &lt; 0.05). Furthermore, the EC50 calculated from an osmotic fragility test on erythrocytes from our subjects correlated with their “IA” (n = 6, r = 0.91, p = 0.01). Thermal heating raised forearm skin temperatures (values expressed as mean oC + SEM) from 36.3 + 0.2 to 37.9 + 0.18 (n = 10, p &lt; 0.0001) which was accompanied by an increase in “IA” scores from (values in arbitrary units + SEM) 83.3 + 3.2 to 89.5 + 2.6 (n = 10, p = 0.002). Surprisingly, mild thermal heating did not affect resting heart rate or vascular resistance, but stroke volume tended to fall (values expressed in ml/beat + SEM) from 102 + 5.6 to 92 + 5.6 (n = 6, p = 0.03).Conclusions. Increasing fitness levels and endogenous PIEZO 1 channel activity correlate with enhanced “IA.” Although the individual’s stroke volume may be a determinant of their “IA,” this self‐awareness of heart beats and other interoceptive clues are sensitive to environmental factors such as mild thermal heating.