Feeling your body or feeling badly: evidence for the limited validity of the Somatosensory Amplification Scale as an index of somatic sensitivity

OBJECTIVE

The Somatosensory Amplification Scale (SSAS) purports to measure the extent to which individuals are sensitive to their bodies. The present study examined the psychometric properties of the SSAS in two studies with university students.

METHODS

Participants completed the SSAS, various cross-sectional measures of somatic and psychological distress, longitudinal measures of somatic symptoms, daily hassles and mood, and participated in a heartbeat detection task (Study 2 only).

RESULTS

The SSAS was correlated with cross-sectional measures of somatic symptom reporting, but not with somatic symptoms reported on a daily basis nor with an index of interoceptive sensitivity. The SSAS was also correlated with several indices of general distress including anxious and depressive symptoms, daily hassles, and negative emotionality.

CONCLUSION

Taken together, the results suggest that the SSAS is more likely an index of negative emotionality and general distress than a valid measure of somatic sensitivity per se.

Gastric myoelectrical and autonomic cardiac reactivity to laboratory stressors

We evaluated the effects of two laboratory stressors (speech preparation and isometric handgrip) on gastric myoelectrical and autonomic cardiac activity, and the extent to which autonomic responses to these stressors and somatization predict reports of motion sickness during exposure to a rotating optokinetic drum. Both stressors prompted a decrease in preejection period (PEP) and respiratory sinus arrhythmia (RSA), and an increase in a dysrhythmic pattern of gastric myoelectrical activity, termed gastric tachyarrhythmia. Stressor-induced decreases in RSA and higher somatization scores predicted increased reports of motion sickness during drum rotation. These results demonstrate that laboratory stressors concurrently affect gastric myoelectrical activity and autonomic control of the heart, and that stressor-induced decreases in RSA and higher levels of somatization predict motion sickness susceptibility.

Autonomic origins of a nonsignal stimulus-elicited bradycardia and its habituation in humans

The purposes of the present study were to determine the autonomic origins of a bradycardiac response to a moderate intensity nonsignal auditory stimulus and the changes in autonomic cardiac control of this response as a function of habituation. Pure tone stimuli were repeatedly presented to participants while phasic changes in heart period (HP), preejection period (PEP), and respiratory sinus arrhythmia (RSA) were observed. Tone stimuli initially elicited an increase in HP, an increase in RSA, and a decrease in PEP, suggesting a coactivation of the parasympathetic and sympathetic inputs mediating changes in the bradycardiac HP response. As expected, HP responses habituated with repeated presentations of the tones. PEP and RSA responses, however, demonstrated different habituation rates than HP. These data demonstrate that cardiodeceleratory responses to nonsignal stimuli can arise from changes in activity of both autonomic divisions and document the importance of considering the autonomic origins of habituating cardiac responses in order to fully understand the process of response habituation.

Autonomic interactions and chronotropic control of the heart: heart period versus heart rate

Autonomic control of the heart varies more linearly with heart period than with rate. Relative linearity confers a greater independence of basal autonomic activation and heart period changes. Thus, heart period appears to be more appropriate for characterizing cardiac phenomena such as autonomic interactions that involve significant baseline shifts. Simulated and published empirical data were used to demonstrate the importance of the chronotropic metric for characterizing autonomic interactions. Simulations revealed a significant autonomic interaction when heart rate, but not heart period, was the chronotropic metric. Published heart rate data also show a substantial autonomic interaction, whereas heart period data do not. These findings suggest that the choice of chronotropic metric can overstate the extent of autonomic interactions on cardiac chronotropic function.

Parasympathetic control of heart period during early postnatal development in the rat

Basal autonomic control of the heart period (HP) changes considerably during the early postnatal period in the rat. Although studies in the developing animal have examined the ability of the sympathetic branch to decrease HP during physiological challenge, few studies have examined the emerging capabilities of the parasympathetic branch to alter HP during early development. To determine the extent of parasympathetic control of HP in the young rat, we used a modified dive reflex procedure and electrical stimulation of the vagal nerve to examine the range of parasympathetic effects on HP in postnatal day 3-24 rats. Modified dive reflex manipulations produced maximal parasympathetically-mediated HPs that were longer just after birth and at weaning than at intervening ages. Direct vagal nerve stimulation studies revealed significant decreases with age in the HP at maximal vagal activation and in the intrinsic HP. The dynamic range, or difference between minimal and maximal parasympathetic effects on HP was similar across ages when assessed from the results of vagal stimulation. Nerve stimulations also revealed age-independent and relatively linear transfer functions relating parasympathetic stimulation frequency and HP during early life. Therefore, several parameters characterizing parasympathetic control of HP, including the dynamic range and transfer function, remain reasonably stable throughout the early postnatal period in the rat. These data provide a framework delineating the autonomic limits within which cardiac responses operate in the young rat. Knowledge of changes in these limits across time affords a firmer physiological basis for cross-age comparisons of autonomically-mediated cardiac changes.

The metrics of cardiac chronotropism: biometric perspectives

The selection of heart period versus heart rate as a chronotropic metric has been considered from quantitative and statistical perspectives, which have not yielded a universal preference for either metric. In the present paper, we discuss biometric considerations that bear on the selection of the optimal chronotropic metric. Biometric evidence reveals that the transfer functions relating autonomic nerve traffic to chronotropic effects on the heart are more nearly linear for heart period than for heart rate. This confers considerable advantage on heart period as a chronotropic metric and can facilitate the study of psychophysiological relationships. We further show that heart period offers greater flexibility, because heart period data can be evaluated in cardiac time units (beats) or in real-time units (s), whereas heart rate data can only be analyzed in real time. These considerations suggest clear advantages to heart period as a chronotropic metric.

Autonomic cardiac control. II. Noninvasive indices and basal response as revealed by autonomic blockades

Heart period, systolic time intervals, low and high frequency heart period variability, blood pressure, and respiration were measured in female subjects under three drug conditions (saline, atropine sulfate, metoprolol) while sitting and standing on three consecutive days. Following preinfusion baseline recordings, saline, metoprolol (14 mg), or atropine sulfate (2 mg) was infused for 15 min (by using a double-blind procedure). Recordings were taken during a postinfusion baseline and in response to an orthostatic stressor (standing versus sitting postures). At the end of the metoprolol session, atropine sulfate was infused and responses were monitored during the postinfusion (i.e., double blockade) baseline and during orthostatic stressor. Analyses of the blockade data revealed that the preejection period (PEP) reflected sympathetic but not vagal influences on the heart, and high frequency (HF, 0.12-0.40 Hz) heart rate variability (respiratory sinus arrhythmia) reflected vagal but not sympathetic influences on the heart. No other measure provided a specific index of the tonic sympathetic or vagal activation of the heart. Postinfusion PEP under saline predicted individual differences in postinfusion cardiac sympathetic activation, whereas postinfusion heart period (but not HF variability) under saline predicted individual differences in postinfusion cardiac vagal activation.

Autonomic cardiac control. III. Psychological stress and cardiac response in autonomic space as revealed by pharmacological blockades

Behavioral contexts can evoke a variety of autonomic modes of response, characterized by reciprocal, coactive, or independent changes in the autonomic divisions. In the present study, we investigated the reactive autonomic control of the heart in response to psychological stressors, using quantitative methods for analyzing single and double autonomic blockades, and through the use of noninvasive indices based on heart period variability and systolic time intervals. Analysis of the effects of pharmacological blockades revealed an overall pattern of increased sympathetic and decreased parasympathetic control of the heart during speech stress, mental arithmetic, and a reaction-time task. Unlike the classical reciprocal sympathetic-parasympathetic response to orthostatic challenge, however, the responses of the autonomic branches to stress were uncorrelated. This reflected notable individual differences in the mode of autonomic response to stress, which had considerable stability across stress tasks. The putative noninvasive indices of sympathetic (preejection period) and parasympathetic (respiratory sinus arrhythmia) control changed in accord with the results of pharmacological blockades. Together, these results emphasize the substantial individual differences in the mode of autonomic response to stress, the advantages of a quantitative approach to analyzing blockade data, and the importance of validity estimates of blockade data.

Autonomic cardiac control. I. Estimation and validation from pharmacological blockades

Pharmacological blockades have been used to estimate the relative contributions of the autonomic branches to cardiac chronotropic control. Systematic biases in these estimates, however, can arise from both methodological and physiological factors. Selective blockades can be interpreted by two inferential models, and a single blockade condition can yield estimates of autonomic control for both autonomic branches. The residual autonomic control of the heart after blockade of a single division provides an index of the functional contribution of the unblocked branch. In contrast, the change in chronotropic state of the heart after blockade of the same division reflects the subtractive loss of that branch and thus provides an index of the normal contribution of the blocked branch. We demonstrate that the systematic biases that can arise in blockade studies introduce distortions of the subtractive and residual estimates that are of equal magnitude but opposite sign. Consequently, the discrepancy between the subtractive- and residual-model estimates provides a measure of bias in blockade studies and permits the derivation of validity indices that can facilitate interpretations of blockade data.

Cardiovascular effects of the benzodiazepine receptor partial inverse agonist FG 7142 in rats

Effects of the benzodiazepine receptor (BZR) partial inverse agonist FG 7142 (FG) on basal and reactive cardiovascular measures were examined in freely moving rats. FG (8 mg/kg) modestly increased basal heart period, but had no effects on basal blood pressure. More notably, however, FG augmented the cardioacceleratory response to an auditory stimulus relative to vehicle controls. Selective blockade of sympathetic (atenolol, 1 mg/kg) or parasympathetic (scopolamine methylnitrate, 0.1 mg/kg) effects on the heart under control conditions revealed that the stimulus-evoked cardiac response originated from a concurrent (reciprocal) sympathetic activation and vagal withdrawal. Following FG pretreatment, both atenolol and scopolamine blocked the cardioacceleratory response to the auditory stimulus. Thus, although FG minimally increased basal heart period, FG significantly enhanced a reactive cardioacceleration. More importantly, these results demonstrate that the cardiovascular effects of BZR inverse agonists are more fully characterized by an assessment of both tonic and reactive cardiovascular responses.

Autonomic space and psychophysiological response

Contemporary findings reveal that autonomic control of dually innervated target organs cannot adequately be viewed as a continuum extending from parasympathetic to sympathetic dominance. Rather, a two-dimensional autonomic space, bounded by sympathetic and parasympathetic axes, is the minimal representation necessary to characterize the multiple modes of autonomic control. We have previously considered the theoretical implications of this view and have developed quantitative conceptual models of the formal properties of autonomic space and its translation into target organ effects. In the present paper, we further develop this perspective by an empirical instantiation of the quantitative autonomic space model for the control of cardiac chronotropy in the rat. We show that this model (a) provides a more comprehensive characterization of cardiac response than simple measures of end-organ state, (b) permits a parsing of the multiple transformations underlying psychophysiological responses, (c) illuminates and subsumes psychophysiological principles, such as the Law of Initial Values, (d) reveals an interpretive advantage of expressing cardiac chronotropy in heart period rather than heart rate, and (e) has fundamental implications for the direction and interpretation of a broad range of psychophysiological studies.

Cardiac psychophysiology and autonomic space in humans: empirical perspectives and conceptual implications

Contemporary findings reveal that autonomic control of dually innervated visceral organs does not lie along a single continuum extending from parasympathetic to sympathetic dominance. Rather, a bivariate autonomic space bounded by sympathetic and parasympathetic axes is the minimal representation necessary to capture the modes of autonomic control. We here empirically instantiate a quantitative bivariate model for the chronotropic control of the heart in humans. This model provides a more comprehensive characterization of psychophysiological response than simple measures of end-organ state and permits a differentiation of behavioral states and processes that would otherwise remain obscure. The model also illuminates and subsumes general principles such as the law of initial values and reveals a fundamental physiological rationale for the selection of heart period over heart rate as a metric for cardiac chronotropy. The present article also considers strategies for psychophysiological investigations within the autonomic space model, the limitations of these methods, and analytical tools for assessing their validity.

Processing of ordinality and transitivity by chimpanzees (Pan troglodytes)

Three chimpanzees (Pan troglodytes) were trained to discriminate among pairs of boxes in an ABCDE-ordered series. The 2nd member of each pair was reinforced, until all 4 training pairs were learned. During novel tests the nonadjacent BD pair was presented, and all 3 animals reliably selected D. In Experiment 2, numerals 1-5 served as stimuli. One chimpanzee reliably selected the larger numeral 4 during testing with a nonadjacent pair (2-4), and 2 chimps showed no preference. In a 2nd phase, the same chimp demonstrated proficiency at reversing the task, reliably selecting the smaller of the 2-4 pair. In Experiment 4, after additional training, a 2nd test, which included novel test pairs composed of numbers that had not been used during training, was completed. Two of 3 animals were 100% correct on Trial 1 for all novel pairs. The results suggest that chimpanzees with experience in number concepts may recognize the ordinal character of numbers.

Respiratory sinus arrhythmia: autonomic origins, physiological mechanisms, and psychophysiological implications

Respiratory sinus arrhythmia (RSA) is being used increasingly in psychophysiological studies as an index of vagal control of the heart and may be among the most selective noninvasive indices of parasympathetic control of cardiac functions. A comprehensive understanding of RSA, however, requires an appreciation of its multiple autonomic and physiological origins. We review the physiological bases of RSA and show that RSA arises from multiple tonic and phasic processes of both central and peripheral origin. These underlying mechanisms are at least partially differentiated, have distinct dynamics and consequences, and may be differentially sensitive to behavioral and cognitive events. These multiple mechanisms are relevant for psychophysiological studies of RSA, and a thorough understanding of RSA can only be achieved through an appreciation of the dynamics of its underlying origins. There is a distinction between the psychophysiological and neurophysiological domains, and conceptual and empirical bridges between these domains are needed.

Vagal stimulation and cardiac chronotropy in rats

Increasing frequencies of electrical stimulation of the right vagus nerve in the rat yielded a progressive lengthening of heart periods. Stimulation was capable of driving vagal control of cardiac chronotropy over its full physiological dynamic range, to the point of sinus block. The steady-state transfer function between vagal stimulation frequency and cardiac chronotropy was approximately linear, with a slope of 7.4 ms/Hz. The linearity of the stimulation-heart period function is consistent with previous reports in dogs, rabbits and humans, although the slope of the function was considerably lower in the rat.

Autonomic determinism: the modes of autonomic control, the doctrine of autonomic space, and the laws of autonomic constraint

Contemporary findings reveal that the multiple modes of autonomic control do not lie along a single continuum extending from parasympathetic to sympathetic dominance but rather distribute within a 2-dimensional space. The physiological origins and empirical documentation for the multiple modes of autonomic control are considered. Then a formal 2-dimensional conception of autonomic space is proposed, and a quantitative model for its translation into a functional output surface is derived. It is shown that this model (a) accounts for much of the error variance that has traditionally plagued psychophysiological studies, (b) subsumes psychophysiological principles such as the law of initial values, (c) gives rise to formal laws of autonomic constraint, and (d) has fundamental implications for the direction and interpretation of a wide array of psychophysiological studies.

Autonomic origins of cardiac responses to nonsignal stimuli in the rat

Heart rate (HR) and blood pressure responses to nonsignal auditory stimuli were measured in rats after saline or pharmacological blockade of the sympathetic or vagal innervation of the heart. HR responses to the low-intensity stimulus were predominantly deceleratory, whereas responses to the high-intensity stimulus were more notably acceleratory. Both stimuli elicited a biphasic pressor-depressor response, although potential baroreflex influences accounted for only a small proportion of the HR response variance. Deceleratory responses to the low-intensity stimulus were eliminated by scopolamine and thus appeared to be predominantly of vagal origin. Acceleratory response to the high-intensity stimulus appeared to be mediated primarily by sympathetic activation because it was substantially attenuated by the beta 1 antagonist atenolol. Furthermore, HR responses to the low-intensity stimulus appeared to reflect coactivation of both sympathetic and vagal systems.

An approach to artifact identification: application to heart period data

A rational strategy for the automated detection of artifacts in heart period data is outlined and evaluated. The specific implementation of this approach for heart period data is based on the distribution characteristics of successive heart period differences. Because beat-to-beat differences generated by artifacts are large, relative to normal heart period variability, extreme differences between successive heart periods serve to identify potential artifacts. Critical to this approach are: 1) the derivation of the artifact criterion from the distribution of beat differences of the individual subject, and 2) the use of percentile-based distribution indexes, which are less sensitive to corruption by the presence of artifactual values than are least-squares estimates. The artifact algorithms were able to effectively identify artifactual beats embedded in heart period records, flagging each of the 1494 simulated and actual artifacts in data sets derived from both humans and chimpanzees. At the same time, the artifact algorithms yielded a false alarm rate of less than 0.3%. Although the present implementation was restricted to heart period data, the outlined approach to artifact detection may also be applicable to other biological signals.

Autonomic origins of cardiac responses to nonsignal stimuli in the rat

Heart rate (HR) and blood pressure responses to nonsignal auditory stimuli were measured in rats after saline or pharmacological blockade of the sympathetic or vagal innervation of the heart. HR responses to the low-intensity stimulus were predominantly deceleratory, whereas responses to the high-intensity stimulus were more notably acceleratory. Both stimuli elicited a biphasic pressor-depressor response, although potential baroreflex influences accounted for only a small proportion of the HR response variance. Deceleratory responses to the low-intensity stimulus were eliminated by scopolamine and thus appeared to be predominantly of vagal origin. Acceleratory response to the high-intensity stimulus appeared to be mediated primarily by sympathetic activation because it was substantially attenuated by the beta 1 antagonist atenolol. Furthermore, HR responses to the low-intensity stimulus appeared to reflect coactivation of both sympathetic and vagal systems.