فرآیندهای ادراکی لمسی و ارتباط آنها با علائم پزشکی غیرقابل توضیح و اضطراب سلامت

Objective The Somatic Signal Detection Task (SSDT; Lloyd, Manson, Brown and Poliakoff, 2008) is an innovative paradigm to study perceptual processes related to physical symptoms. It allows examining touch illusions as a laboratory analog of medically unexplained symptoms (MUS) according to the cognitive model of MUS proposed by Brown (2004). The present study compared psychopathologic measures of MUS and health anxiety with SSDT parameters. Furthermore, we aimed to define a reliable measurement of tactile perception threshold. Methods 67 participants of a student population reported whether they detected tactile stimuli at their fingertip which were presented in half of the test trials. An additional brief visual stimulus was displayed with a probability of 50%. The rate of false-positive perceptions of the tactile stimulus in its absence, response bias, tactile sensitivity, and tactile perception thresholds was recorded. Questionnaires were used to assess MUS and health anxiety. Results The visual stimulus led to a more liberal response criterion (i.e., the tendency to report tactile perceptions irrespective of whether a stimulus was presented or not) and a non-significant increase in tactile sensitivity. The false-alarm rate when reporting the tactile stimulus was correlated with MUS (r=.26). Tactile perception thresholds were measured reliably (rtt=.84). Conclusion Some of the SSDT parameters, especially the response criterion (c), were related to self-report-measures of MUS and health anxiety. Previous SSDT results were replicated and extended. Further SSDT studies with clinical samples are needed.

Introduction Medically unexplained symptoms (MUS) are a common, widespread phenomenon in the general population and in primary health care settings [1]. Concerned persons suffer from bodily complaints that cannot be sufficiently explained by known medical conditions. Although treatments have been developed on the basis of bio-psycho-social models [e.g., [2] and [3]], the precise etiology of MUS and somatoform disorders is still unknown [4] and [5]. In an innovative model of MUS [4], based on cognitive psychological principles [e.g., 6], the development of MUS is described as the result of alterations in the cognitive system. Two different hypothetical attentional systems (i.e., a primary and a secondary attentional system) are differentiated. These systems select so called “rogue representations” which refer to information related to physical symptoms. The specific contents of these multimodal representations in memory depend on prior experiences (e.g., illness concerning oneself or family members). According to the model, symptom experiences arise from the automatic activation of symptom representations in the primary attentional system (PAS). However, the selection of these symptom representations by the PAS can be moderated and facilitated by the secondary attentional system, e.g., via negative affect, via the generation of disease-confirming information, or via an extensive body-focused attentional style. These processes in the secondary attentional system consecutively facilitate the reactivation of rogue representations. In essence, Brown [4] conceptualizes MUS as illusory somatosensory phenomena that are subjectively real and that are based on cognitive psychological principles. According to Lloyd et al. [7] distortions in bodily experience could be created simply by raising the activation of corresponding representations in memory. The researchers argue that this would allow not only to support the validity of the integrative cognitive model [4], but also to create a laboratory analog of MUS that may be investigated under controlled conditions. Such illusory perceptions are frequent in the normal population and subject of research in cross-modal integration of sensory stimuli. In this regard, Violentyev, Shimojo, and Shams [8] showed that tactile stimuli provoked visual illusions in healthy individuals and also altered sensitivity (d′) when detecting visual stimuli. Vice versa, illusory tactile sensations and an enhancement of tactile sensitivity can be triggered by visual stimuli [9], [10] and [11]. The somatic signal detection paradigm The Somatic Signal Detection Task [SSDT; 7] integrates the findings previously described as well as Brown's conception of MUS [4]. It aims at constituting a laboratory model of MUS that allows studying somatoform symptoms as cognitively triggered illusory touch experiences. In the SSDT, a near-threshold tactile stimulus is presented at the fingertip on half of the trials. An auxiliary visual stimulus is presented during the observation interval, with a probability of .5. Thus, there are four types of trial: vibration only, vibration-plus-light, light-only, and no stimulus trials. Illusory touch perceptions are expected to be triggered in the light-only condition, because the visual stimulus is assumed to activate representations of the tactile stimulus [7]. The use of visual stimuli is not directly related to MUS, but it serves as an example in order to create a laboratory model of MUS. This process is understood as a phenomenon of normal multisensory integration, not as a consequence of conditioning [10]. Mirams, Poliakoff, Brown, and Lloyd [11] found that attending to the body had an effect on the number of false alarms (FAs) (i.e., illusory touch experiences) in the SSDT paradigm. Furthermore, the signal detection theory based paradigm [12] allows differentiating sensitivity (d′), the capability to detect the stimuli, and response bias (c), a pre-existing tendency to respond in a certain way to the presented stimuli [13]. Modifications to the original SSDT In previous studies using the SSDT paradigm [7], [10], [11] and [14], the vibration intensity was selected so that participants obtained between 40% and 60% correct responses in a block containing 10 vibration trials and 3 no-vibration trials. A potential problem of this method for selecting the vibration intensity is that due to the use of a one-interval (“yes/no”) task, the selected stimulus intensity depends not only on the sensitivity to detect the vibration, but also on the response bias (i.e., the tendency to respond “yes”). Thus, it is somewhat unclear how the vibration intensities presented in previous studies were positioned relative to the psychophysical detection threshold. To avoid effects of response bias on the selection of the vibration intensity, we measured vibrotactile perception thresholds in a two-alternative forced-choice task [e.g., [12] and [13]]. As Green and Swets [13] have noted, one potential reason for a smaller response bias in the two-interval as compared to a one-interval task is that responding, “yes, stimulus present” or “no, stimulus absent” represents a stronger difference in subjective value than responding either “stimulus in interval 1” or “stimulus in interval 2”. We applied one of the adaptive procedures most widely used in psychophysics, namely the transformed up–down adaptive procedure proposed by Levitt [15], to determine an individual vibration intensity corresponding to a clearly defined level of performance (70.7% correct) in the two-interval task. We thus for the first time combined the SSDT paradigm with a precise measurement of the tactile detection threshold, so that the detectability of the vibration signal presented in the SSDT could be expected to be identical for all participants. Additionally, we used acoustic start cues for signaling the observation intervals, rather than visual start cues [7], in order to prevent interference with the visual accessory stimulus presented in the SSDT or a direction of attention towards the visual auxiliary stimulus [10]. Note that a study published after the completion of the present experiment [10] found that an acoustic or a visual stimulus does not lead to different results. Aims and hypotheses of the present study The first aim of our study was to replicate the general findings of previous SSDT studies [7], [10], [11] and [14]. Most importantly, we expected an elevated FA rate in the light-only condition in which only the visual stimulus, but no tactile stimulus was presented. In the same line of reasoning, we expected a shift in response bias (c) towards “signal present” responses in trials presenting the visual stimulus. Sensitivity (d′) was expected to be augmented in the light-present condition, compatible to the small to medium effects [16] reported in previous SSDT-studies [10] and [11]. In a subclinical sample, Brown, Brunt, Poliakoff, and Lloyd [14] found that experiencing illusory perceptual events was more likely in subjects with a tendency to somatoform dissociation despite perceptual abilities comparable to normal subjects. Consequently, the second aim of our study was to extend these findings by exploring the relationship between SSDT parameters and MUS in general. Additionally, interoceptive apperceptions (i.e., subjective sensations of pulses within the index finger (finger pulse) resulting from physiological processes) were addressed. Our aim was to explore if such an unspecific interoceptive feature would be linked to response behavior within the SSDT-paradigm, especially whether false alarms in the light-only condition may be misattributed tactile perceptions due to a tendency to interoceptive sensations. We also analyzed the relationships between the SSDT parameters, tactile thresholds, finger pulse perceptions, and self-report data. Apart from MUS, health anxiety was chosen as similar models are proposed in this domain [5]. With an explorative approach, we aimed at examining whether the SSDT might be helpful in this context as well. Method Participants 68 volunteer participants were recruited at the University of Mainz, Germany. They all provided written informed consent according to the Declaration of Helsinki prior to participation. The study was approved by the Ethics Committee of the German Psychological Society (DGPs; MWWHAK28082008DGPS). Data of one participant had to be removed from the final analysis because of current psychotropic drug intake. Finally, 67 participants (14 men, 20.9%) remained in the sample. All of them were students from different faculties. Their mean age was 23.2 (SD= 4.8) years. Those who completed the study were paid 10 Euro for participation or received course credits. A session lasted about three hours. Participants were naïve about the purpose of the study until having passed all stages of the investigation. Experimental measures Participants were tested individually in a dimly lit room in front of a console containing a red light emitting diode (LED) and a 1.4 cm×2.3 cm surface which delivered vibrations to the dominant hand's index fingertip. The vibrotactile stimuli were brief pulses (2 ms) presented with a rate of 50 Hz, addressing Pacinian and Meissner mechanoreceptors [17]. The intensity of the applied vibrotactile stimuli was adjusted by a second console panel. The experimenter sat in an angle of 90° to the participant in front of a LCD monitor in order to give instructions and record the participant's responses. The experiment was run with the software Inquisit [18]. Circumaural head-phones (Sennheiser HD 201) were used to apply acoustic signals at a comfortable loudness level at the beginning and the end of the trials. As these head-phones enclose the listener's ear completely with a foam-padded material they provided a good attenuation of ambient noise and of potential sounds produced by the vibration device. Tactile perception thresholds First, the dominant hand was determined by using the Edinburgh Handedness Inventory [19]. Tactile perception thresholds were measured by an adaptive procedure [15] in a two-alternative forced-choice task which avoids effects of response bias [e.g., [20] and [21]]. On each trial, two observation intervals of 1330 ms were preceded by a beep of 25 ms duration, which indicated the beginning of an interval (see Fig. 1). Full-size image (21 K) Fig. 1. Schematic depiction of the temporal structure of a trial presented in the threshold measurements (two-interval task). Figure options In one of the observation intervals (selected randomly), the vibrotactile signal was applied for 20 ms following 660 ms after the beep onset. In the other interval, no vibrotactile signal was presented. The participant's task was to indicate the interval in which the tactile stimulus had occurred. A new trial started after the experimenter had recorded the subject's response. The measurement of the tactile perception thresholds involved three blocks: In the practice block, first ten trials were presented at the maximum vibration intensity in order to familiarize the subject with the tactile stimulus. Afterwards, when registering two consecutive correct responses, the intensity of the vibrating stimulus was decreased by 10 units on a scale that ranged from 0 to 100, with 0 representing no stimulation, and 100 representing the initial, maximum intensity of the vibration. If one incorrect response was given, the intensity was increased by 10 units. This procedure converges at a stimulus intensity corresponding to 70.7% correct [15]. The practice block contained 40 trials in total. After the practice block, a tactile threshold was determined using the same adaptive procedure as for the practice block but without presenting the initial ten trials with maximum intensity. In this type of adaptive procedure, a trial on which the direction of the stimulus level sequence changes from up to down or vice versa is termed a reversal [15]. The block was terminated when eight reversals had occurred. The number of trials was not limited. The arithmetic mean of the intensities of the tactile stimulus at the eight reversals was taken as the individual tactile threshold that was used in the subsequent SSDT. After the SSDT, a second measurement of the tactile threshold followed to assess the reliability. In this block, the same adaptive procedure as described above was used. Somatic Signal Detection Task In the SSDT task [7], there was only one observation interval. The beginning and the end of the interval was signaled by a 25 ms tone. In total, a trial lasted 2300 ms. On vibration-only trials, the tactile stimulus was presented for 20 ms in the middle of the time period of 2300 ms. The intensity of the previously established individual tactile threshold was used when presenting the vibrotactile stimulus. Four different trial types were presented: vibration only, vibration-plus-light, light only, and no stimulus. The temporal structure of the four trial types is displayed in Fig. 2. Full-size image (28 K) Fig. 2. Schematic depiction of the temporal structure of a trial presented in the four different trial types in the Somatic Signal Detection Task. Figure options In vibration-plus-light trials, light was presented simultaneously with the tactile stimulus. In light-only trials, only the visual stimulus was presented for 20 ms, at the same temporal position as in the vibration-plus-light condition. In no-stimulus trials neither a tactile stimulus nor light was presented. Participants rated their confidence concerning the perception of the vibrotactile stimulus on a four-point scale with the response categories “definitely yes”, “probably yes”, “probably not”, or “definitely not”. The new trial was started by the experimenter after registering the participant's response. The SSDT [7] was administered in four consecutive test blocks of 40 trials. Each trial type was presented 10 times within each block of 40 trials, in random order. Thus, 80 trials were obtained in the light-present condition (vibration-plus-light and light-only trials), and 80 trials in the light-absent condition (vibration-only and no-stimulus trials). Each subject was tested in all conditions. Subsequent to the experiment participants were asked about interoceptive pulses in their index finger during the experiment. This finger pulse interoception task was about reported finger pulses in general. Participants were asked if they had perceived their finger pulse during the experiment. They were asked to rate if they had felt the finger pulse never, sometimes, or all the time. Responses were coded on an ordinal scale ranging from 0 (“never”) to 2 (“all the time”). We did not check if these subjective reports were related objectively to physiological features. Diagnostic procedures The somatic symptom index of the PHQ-15 [22] assesses physical symptoms that were found to be relevant for somatisation disorder. It comprises 13 items from the German version of the Patient Health Questionnaire (PHQ-D) somatic symptom module. Three response categories ranging from 0 (“not bothered at all”) to 2 (“bothered a lot”) indicate symptom severity during the last four weeks. Furthermore, two items of the PHQ depression scale [23] describing physical symptoms were added (feeling tired; trouble sleeping). The categories of these items were 0 (“not at all”), 1 (“several days”) and 2 (“more than half the days” or “nearly every day”). Good reliability and validity of the PHQ-15 has been demonstrated (Cronbach's α=.80) [22]. In the current sample α was .56. The Whitley Index (WI) with 14 dichotomous items was used to assess health anxiety. Whereas the dimensionality of the 14 items is still under debate [24] and [25], acceptable reliability and validity have been demonstrated. Hinz, Rief, and Brähler [26] reported a Cronbach's α coefficient of .83. In the current sample Cronbach's α for the total score was .68. The subscales according to Schwarz, Witthöft and Bailer [25] showed α coefficients of .64 (health anxiety) and .61 (health beliefs and complaints) in the present study. The Multidimensional Inventory of Hypochondriacal Traits [MIHT; [27] is an instrument for the dimensional assessment of health anxiety in community samples. It assesses cognitive, behavioral, affective, and perceptual aspects of health anxiety. For the German version Cronbach's α coefficients took values within the range of .75 to .89 [28]. In the current sample Cronbach's α concerning MIHT total score was .81. The internal consistencies for the subscales were satisfying: α=.84 (behavioral); α=.89 (affective,); α=.88 (cognitive) and α=.87 (perceptual). Trait anxiety was measured by the German version of the State Trait Anxiety Inventory [STAI; [29]. It consists of 20 items with answer categories ranging from 1 (“almost never”) to 4 (“almost ever”). The Cronbach's α coefficient of STAI-trait in the current sample was .92. The STAI-trait questionnaire was used as a covariate to test the specificity of possible relations between somatoform symptoms and the SSDT since anxiety and somatoform symptoms are related phenomena. We added the STAI trait solely to test for the impact of general anxiety on the perceptual phenomena. Statistical analysis of the SSDT data Responses in the SSDT were categorized into “yes” and “no” responses because not all participants had used all of the four response categories (definitely yes, probably yes, probably no, and definitely no). Therefore, further analysis of the rating data (e.g., estimation of a receiver operating characteristic curve) was impossible. Following signal detection theory [12], the responses were classified as hits (“Yes, signal present” responses on trials presenting the vibration stimulus), misses (“No” responses on trials presenting the vibration stimulus), FAs (“Yes” responses on trials presenting no vibration stimulus), and correct rejections (“No” responses on trials presenting no vibration stimulus). All proportions were calculated separately for light-present and light-absent trials. The log-linear correction [30] was applied by adding .5 to each cell and each marginal mean in the 2×2 classification table. The signal detection theory statistics d′= z(hit)−z(FA) was computed as an index of sensitivity, and c=−.5[z(hit)+z(FA)] as an index of response bias. The index d′ represents a measure of each participant's sensitivity to correctly recognize the tactile stimulus. The tendency to report vibrotactile sensation is estimated by the parameter c, which is a measure of response bias. Values of c smaller than 0 indicate a liberal response criterion (i.e., a tendency towards “yes”-responses), and c scores higher than 0 indicate a conservative response criterion (i.e., a tendency towards “no”-responses) [12]. As the FA rates were not normally distributed (light-present condition: Kolmogorov–Smirnov-Z= 1.42, p<.05; light-absent condition: Kolmogorov–Smirnov-Z= 1.45, p<.05), the statistical tests and correlational analyses were conducted on arcsine-square-root transformed FA rates [31]. To test our a-priori hypotheses, the signal detection theory parameters were analyzed by repeated-measures t-tests. We used an alpha-level of .05 and report two-tailed p-values. Measures of effect sizes (Cohen's d) are based on means and variances of the d′ and c parameters. Following Cohen [16], d≥.30 is considered a small, d≥.50 a medium, and d≥.80 a large effect. For d′, c, and the FA rate, we calculated difference scores between the light-present and the light-absent condition in order to examine whether the light-induced changes are related to any of the psychopathological measures. Additional analyses were conducted on the SSDT parameters averaged across the two experimental conditions. Correlational analyses were conducted in order to investigate the relationship of SSDT parameters and psychopathological measures. We report two-tailed tests of significance for the Pearson product-moment correlation coefficients. Statistical analyses were conducted by PASW Statistics 18 for Windows (SPSS Inc.,

Somatic Signal Detection Task parameters Table 1 shows the average FA and hit rates in light-absent and light-present trials for the 160 test trials, and displays the signal detection theory statistics d′ and c. Table 1. Means (and standard deviations) of hit rates, false-alarm rates, d′ (sensitivity) and c (response bias) in light-present and light-absent trials of the SSDT (N=67) Hit rate False alarm rate d′ a c b Light-present condition .63 (.21) .15 (.13) 1.61 (.92) .40 (.44) Light-absent condition .57 (.20) .13 (.10) 1.49 (.81) .55 (.39) a Tactile sensitivity-scores differ significantly from 0 indicating an acceptable detection rate [t(66)=14.39, p<.001, for the light condition; t(66)=15.13, p<.001, for the without-light condition]. b Response bias-scores differ significantly from 0 indicating a generally conservative response style [t(66)=7.43, p<.001, for the light condition; t(66)=11.47, p<.001, for the without-light condition]. Table options The presence of the concurrent visual stimulus significantly increased the hit rate by 6.2% in comparison to tactile stimuli presented without a light [t(66)=3.85, p<.001]. The light also triggered illusory tactile perceptions in trials without vibration, leading to an increase of 2.2% in FA rates [t(66)=1.94, p=.058]. Tactile sensitivity (d′) in the light-present condition was slightly higher than in the light-absent condition [t(66)=1.91, p=.06, Cohen's d=.14], but it failed significance. Response bias (c) was significantly more liberal in experimental conditions with light [t(66)=3.74, p<.001, Cohen's d=.36]. Correlational analyses of SSDT parameters and self-report measures We analyzed the correlations between SSDT parameters (FA rate (“illusory perceptions”), response bias (c), tactile sensitivity (d′)), ratings of interoceptive perceptions, tactile threshold, and psychopathological self-report measures (PHQ-15, WI, MIHT, STAI). Descriptive statistics for the questionnaire data is presented in Table 2. Table 2. Descriptive statistics for the questionnaire data N Minimum Maximum Mean Standard deviation PHQ-15a 67 1.00 15.00 6.55 3.05 WIb Total score 67 .00 7.00 1.43 1.73 WI Health anxiety 67 .00 4.00 .94 1.15 WI Health beliefs and complaints 67 .00 4.00 .49 .96 STAIc 67 33.00 55.00 45.70 4.97 a PHQ-15=Patient Health Questionnaire. b WI=Whiteley Index. c STAI=State-Trait Anxiety Inventory. Table options As the difference scores between SSDT parameters of light-present and light-absent conditions were not correlated to any of the self-report measures (for all correlations, p>.05), we used SSDT scores averaged across the light-absent and the light-present trials. Table 3 shows their correlations and partial correlations with trait anxiety as a control variable. Table 3. Overview of Pearson product-moment correlations (and partial correlationsa; control variable STAIb) between SSDT-parameters and psychopathological measures (N=67) False-alarm ratec Response bias (c) d Sensitivity (d′) d Interoceptive pulse rating Tactile threshold PHQ-15e .26⁎ (.29⁎) − .36⁎⁎ (− .32⁎⁎) − .02 (− .09) .26⁎ (.23) − .17 (− .12) WIf Total score .21 (.24⁎) − .35⁎⁎ (− .30⁎) .04 (− .06) .40⁎⁎ (.37⁎⁎) − .06 (.02) WI Health anxiety .20 (.22) − .29⁎ (− .26⁎) − .01 (− .06) .40⁎⁎ (.38⁎⁎) − .15 (− .11) WI Health beliefs and complaints .13 (.17) − .29 ⁎ (− .23) .09 (− .02) .23 (.18) − .07 (.17) a df=64. b STAI=State-Trait Anxiety Inventory. c Correlations are based on averaged scores of arcsine-square-root-transformed false-alarm rates in the light-absent and in the light-present condition. d Correlations are based on averaged scores. e PHQ-15=Patient Health Questionnaire. f WI=Whiteley Index. ⁎ p<.05. ⁎⁎ p<.01. Table options Illusory perceptions of tactile stimuli (FA) In the correlational analyses, we used the arcsine-square-root transformed FA rates. Illusory perceptions were positively related to PHQ-15 scores (r=.26, p=.03). When controlling for trait anxiety the partial correlation increased slightly (r=.29; p=.02). The correlation between interoceptive finger pulses and illusory tactile perceptions just failed to reach significance (r=.23, p=.06). The correlation between the WI total score and illusory tactile perception (r=.21, p=.10) became significant after controlling for trait anxiety (r=.24, p=.048, see also Table 3). The other self-report measures did not correlate significantly with FA rates. Response bias The response bias (c) was found to be negatively associated to PHQ-15 scores (r=− .36, p=.003). The more symptoms participants reported, the more liberally they reported perceptions of tactile stimulation in the SSDT. Similar results were found for the WI total score (r=− .36, p=.003) and WI subscale scores (r=− .29, p=.02, WI anxiety scale; r=− .29, p=.02, WI somatic score). Dimensions of health anxiety as measured by the MIHT did not consistently correlate with response bias. Only the affective scale showed a significant negative correlation (r=− .34, p=.005). All other scales (STAI, interoception ratings) did not yield significant results (all p-values >.05). Tactile sensitivity The average tactile sensitivity (d′) was positively related to STAI scores (r=.28, p=.02), indicating that higher levels of trait anxiety were associated with greater sensitivity for tactile stimuli. Somatic symptoms (PHQ-15), health anxiety (WI, MIHT), and finger pulse ratings were unrelated to tactile sensitivity (all p-values >.05). Stability and reliability of the tactile perception threshold The first tactile threshold measured before the SSDT (M= 26.51, SD= 14.38) did not differ significantly from the second one (M= 27.09, SD= 14.80) measured after the SSDT [t(66)=.57, p=.57]. More importantly, the test–retest correlation was rtt=.84 (p<.001), indicating that the tactile perception threshold was reliably determined by the presented two-down/one-up procedure. Across all participants and conditions, the average percentage of correct responses in the SSDT was 73.0% (SD= 11.5%). Tactile threshold yielded no substantial correlations with measures of MUS or health anxiety (all p-values >.05). Interoceptive finger pulse perception Finger pulse perception correlated positively with the PHQ-15 (r=.26, p=.04, two-tailed). Participants reporting to be more concerned by their complaints more frequently reported interoceptive pulses within the experiment. Furthermore, interoception ratings were significantly related to WI scores, while correlations were medium to large in size (r=.40, p=.001, WI total score; r=.40, p=.001, WI anxiety scale; r=.23, p=.06, WI somatic score), i.e., the more subjects reported feeling anxious in health context, the more interoceptions were reported. All other self-report measures did not yield significant results (all p-values >.05). Discussion The primary aim of the current study was to evaluate whether the recently developed SSDT paradigm can be used to induce illusory touch experiences in the laboratory [7] and whether there is an association with psychopathology, especially with MUS and facets of health anxiety. Secondly, we intended to combine the SSDT method with a reliable measure of tactile perception thresholds and reports of interoceptive perceptions. Illusory and interoceptive perceptions One of the most important SSDT parameter is the FA rate in the light-only condition because it is thought to represent a laboratory analog of MUS in the sense of Brown's theory [4] and [7]. We found a 2.2% increase in the FA rate induced by the visual stimulus that was not significant but comparable to the results of other studies [10] and [11] which found an increase of about 2.4%. Only in the first published SSDT study this effect had been stronger [7]. While associations between somatoform dissociation and a tendency to experience tactile illusions were shown [14], averaged FA rates also correlate with the broader concept of somatoform symptoms as measured by the PHQ-15. This finding supports that the SSDT may be helpful in studying basic mechanisms of somatoform disorders and encourages research with clinical samples. Furthermore, it appears possible that correlations between the SSDT parameters and the PHQ-15 are even underestimated in our study because of the comparatively low reliability of the PHQ-15 in our sample. We attribute this low reliability to a restriction of range problem in our non-clinical sample of college students with a comparatively low rate of MUS. Illusory tactile perceptions (i.e., FA rates) did not correlate with health anxiety scores except after controlling for trait anxiety. This might suggest that the induced illusory perceptions may be specifically related to MUS which are often part of the clinical phenomenology of hypochondria as well. The observed correlations between the FA rate and interoceptive finger pulse perceptions as well as MUS encourage integrating interoceptive aspects in further studies. Inconsistently, interoception ratings correlated with WI scores but not with MIHT ratings. However, completely unspecific bodily sensations seem to be related to health anxiety. When comparing our results to previously published SSDT studies, comparable hit and FA rates were found despite using an acoustic instead of a visual start cue. This result is completely in line with a recent study that varied the start cue systematically and concluded that the start cue did not lead to different SSDT parameter values [10].