Affective responses to electrocutaneous stimulation in three groups of participants with different levels of skin-picking severity

Abstract Some individuals pick their skin excessively which induces skin lesions. These individuals with high severity of skin-picking (SOSP) usually experience their picking not as painful, although this behavior causes physical harm. This may point to lowered pain sensitivity in individuals with elevated SOSP. A total of 144 participants characterized by three different levels of self-reported SOSP received electrocutaneous pulses of different intensities to the skin surface of their left forearm. A mixed-model analysis of variance tested the effects of SOSP group (no/medium/high skin-picking severity) and stimulus intensity (6,16,26,36 mA) on participants’ ratings for perceived intensity/unpleasantness of stimuli and their own affective state (valence, arousal). Relative to participants who never pick their skin, the medium and high SOSP groups reported feeling more unpleasant and aroused during electrocutaneous stimulation. The three SOSP groups did not differ in the evaluation of the stimuli. It has to be noted that only a limited set of stimulation parameters (one location of stimulation; four stimulus intensities) were included. The present findings suggest that emotional responsivity to skin stimulation (but not pain sensitivity) is an important factor in understanding skin-picking behavior.


Introduction
Many people pick at their skin from time to time.While occasional skin-picking that does not cause any damage to the skin or interference with daily activities is considered normal, excessive or pathological skin-picking is classified as a mental disorder (skin-picking disorder (SPD), Diagnostic and Statistical Manual of Mental Disorders: DSM-5, APA, 2013).SPD is associated with clinically significant distress and impairment in social, occupational, or other important areas of functioning.The recurrent skin-picking in patients diagnosed with SPD results in open wounds and scars; some even show serious physical disfigurement (Grant et al., 2012).Although this type of behavior causes physical harm, most patients with SPD report that they experience the picking not as painful, but even as pleasant (Gallinat et al., 2021;Schienle et al., 2018).This phenomenology of ABOUT THE AUTHORS Anne Schienle is a professor of Clinical Psychology and the head of the outpatient clinic at the University of Graz, Austria Florian Osmani is a technical assistant at the department of Clinical Psychology at the University of Graz, Austria SPD may point to heightened pain thresholds and/or pain tolerance in individuals with an elevated severity of skin-picking (SOSP).
Altered pain processing has been observed in individuals who deliberately harm themselves in the absence of an intent to die.So-called non-suicidal self-injury (NSSI) includes behaviors such as excessive scratching, cutting, burning the skin and/or preventing a wound from healing.According to the integrated model of self-injury by Nock et al. (2009), those who engage in NSSI have a lowered pain sensitivity, which reduces their aversion to painful stimuli.Moreover, individuals with a propensity for NSSI display difficulties in emotion regulation.Self-injury is used as a dysfunctional strategy to deal with emotional distress (Klonsky et al., 2014).The same function has been recognized for episodes of pathological skin-picking that are often triggered by negative affective states (Gallinat et al., 2021;Schienle et al., 2018).Thus, although skin-picking is not a primary NSSI behavior (because the patients do not engage in skin-picking to injure themselves but to remove skin irregularities or calm themselves), there are functional similarities between the two types of symptoms.
To the best of our knowledge, only two studies (Grant et al., 2017;Lochner et al., 2022) have examined whether high SOSP is associated with atypical pain processing.In the first study (Grant et al., 2017), the cold pressor test was implemented, while heart rate, blood pressure, and pain intensity were recorded simultaneously.The participants were asked to immerse their hands in a container with ice water for three minutes and rate the pain intensity every 15 seconds.Individuals with SPD (n = 14) demonstrated an attenuated autonomic response (reduced heart rate) to cold pressor pain, compared to healthy controls (n = 14).The reported pain intensity did not differ between the two groups.In a recent study by Lochner et al. (2022), which likewise used the cold pressor test, the reported pain intensity and pain tolerance (the time with immersed hands in the ice water) also did not differ between 24 participants with SPD and 26 healthy controls.
Since cold pressor represents a specific type of noxious stimulation that activates temperaturemediating Aδ-and C-nerve fibers (followed by enhanced sympathetic outflow), the findings mentioned above might be specific to this method (Hilz et al., 2002).Therefore, the present study implemented another type of noxious stimulation and examined whether responses to electrocutaneous stimulation (which directly activates afferent nerve fibers rather than sensory end structures) are associated with SOSP.Electrocutaneous stimulation evokes (painful) tactile sensations by using an electric current flowing through the skin via electrodes placed on the skin surface.
The participants in this investigation were assigned to three groups (no, medium, and high SOSP) based on a questionnaire for the assessment of skin-picking severity (Gallinat et al., 2016).The high SOSP group was characterized by questionnaire scores above the clinical cut-off indicating a provisional SPD diagnosis.All participants received four electrical pulses to the skin surface on their left forearm, using different intensities applied in a random order (6, 16, 26, 36 mA).Each stimulus was rated according to its perceived intensity and unpleasantness.Moreover, the participants rated their affective state (valence, arousal), and the urge to pick their skin, after the administration of each stimulus.Based on clinical observations (reports of patients with SPD that skin-picking is not painful despite causing skin lesions) and previous findings on reduced pain sensitivity in individuals who engage in non-suicidal self-injury (e.g., excessive scratching of the skin; Nock et al., 2009), it was hypothesized that the groups with moderate/high SOSP would experience the stimuli as less intense and unpleasant than the group with no skin-picking (with the most pronounced reduction in the high SOSP group).Further, an exploratory analysis was planned to compare the three SOSP groups concerning the reported urge to pick the skin after electrocutaneous stimulation.

Participants
A total of 144 participants (84% female; mean age = 23.98 years, SD = 5.04) were recruited at the university and through social media.The participants were university students (90%) or whitecollar workers (10%).Exclusion criteria were self-reported diagnoses of neurological disorders and dermatological conditions (e.g., psoriasis).Moreover, reported diagnoses of mental disorders that are associated with self-injury (e.g., borderline personality disorder), psychosis, and substance abuse/dependence led to exclusion.
Based on their sum scores on the Skin-Picking Scale revised (SPS-R; Gallinat et al., 2016; Cronbach's α = .96in the present sample), the participants were assigned to one of three groups.One group reported no skin-picking at all (SPS-R = 0), while the other groups reported moderate skin-picking (SPS-R: 1-6) or excessive skin-picking with SPS_R scores above the clinical cut-off (SPS-R ≥ 7).The SPS-R is a questionnaire to assess the severity of skin-picking symptoms.The scale contains eight items covering the following domains: (1) frequency of the urge to pick one's skin, (2) intensity of the urge to pick one's skin, (3) time spent picking, (4) control over picking, (5) functional impairment, (6) emotional distress, (7) avoidance behavior, (8) skin damage due to picking.Each item is rated on a 5-point scale from 0 (none) to 4 (extreme).The item scores can be summed to a total score reflecting the severity of skin-picking (SOSP).The three groups did not differ in mean age and gender distribution (p > .15).
An apriori power analysis (G-Power, Faul et al., 2009) indicated that a total sample size of 102 participants would be required if three groups (no/medium/high SOSP) were compared with each other for four electrocutaneous stimuli (f = 0.25, alpha = .05,correlation among rep.measures = .50).

Electrocutaneous stimulation
The participants received electrocutaneous stimulation to the skin on the medial side of their left forearm (using two self-adhesive Ag/AgCl surface electrodes (Ternimed; 2 cm) with the DS8R biphasic direct current stimulator (Digitimer, Letchworth Garden City, SG6 9BL, UK).Four rectangular pulses (duration = 2 ms) were applied (inter-trial interval: 30 s) in randomized order with four different intensities: 6 mA, 16 mA, 26 mA, and 36 mA.Pilot tests showed that the level of 6 mA was rated as just noticeable (as a tactile sensation) by the majority of participants, whereas a level of 36 mA was rated as unpleasant or even painful.

Procedure
The participants completed an online survey (questionnaires, checking of exclusion criteria, assessment of sociodemographic information).Eligible participants were invited to the lab; 144 participants followed the invitation (none of the participants dropped out of the lab experiment).
Before applying the electrodes, the skin was cleaned with ethanol.The electrodes were attached and the stimuli were administered in random order.Each stimulus was evaluated according to the perceived intensity and unpleasantness (0 = very low intensity/not unpleasant; 100 = extremely intense/unpleasant).In addition, the participants rated their affective state after each stimulus (valence, arousal: 0 = not positive, not aroused; 100 = very positive, very aroused) and whether they felt an urge to pick their skin (0 = not at all; 100 = extremely).Across all ratings, the percentage of missing data was 2%.
The experiment complied with all relevant ethical guidelines and regulations involving human participants and was approved by the ethics committee of the University (GZ.39/50/63 ex 2020/ 21).All participants provided written informed consent.The study was registered on the German Clinical Trials Register (DRKS00029927; 28.07.2022).

Statistical analysis
Sum scores for the SPS-R (Gallinat et al., 2016) were computed for each participant.Based on these scores, participants were assigned to one of three groups: high severity of skin-picking (SOSP; SPS-R ≥ 7; n = 34), medium SOSP (SPS-R: 1-6; n = 46), and no SOSP (SPS-R = 0; n = 64).We computed 3 × 4 mixed-model analyses of variance (ANOVAs) to test the effects of the three levels of skin-picking severity (between-subjects factor "SOSP group") and intensities of the electrocutaneous stimulation (within-subjects factor "stimulus intensity" with four levels: 6, 16, 26, 36 mA) on participants' ratings concerning stimulus evaluation (perceived stimulus intensity/pleasantness) and evaluation of the own affective state (valence, arousal).Because of the low average level and restricted variance of ratings concerning the urge to pick one's skin, we refrained from computing an ANOVA.Note: overall ratings = ratings for valence, and arousal averaged across the four stimulus intensities (6,16,26,36 mA) Mean ratings and standard deviations for this variable (and all other variables) are displayed in Figure 1.Significant main/interaction effects were followed up via post-hoc t-tests.We report partial eta squared as the effect size measure.Analyses were conducted with SPSS version 28 (IBM Corp).

Discussion
Individual differences in pain sensitivity may help to clarify the reason why some people, but not others, engage in excessive skin-picking.Therefore, the present study compared responses to electrocutaneous stimuli across individuals with different levels of reported severity of skinpicking (SOSP).In line with previous investigations, skin-picking severity was not associated with the intensity ratings for the noxious stimuli.Similar results were found in two previous studies (Grant et al., 2017;Lochner et al., 2022), both of which implemented the cold pressor test as a type of noxious stimulation; it was observed in those studies that pain intensity ratings did not differ between participants with or without a diagnosis of skin-picking disorder (SPD).In the present study, participants recognized the different intensity levels of the stimuli (6, 16, 26, 36 mA) correctly, and all groups assigned higher levels of stimulus unpleasantness to higher intensity levels.Thus, SOSP was not associated with the sensations evoked by the electrocutaneous stimuli.
However, the three SOSP groups did differ in their affective responses to the electrocutaneous stimulation.Participants who reported never picking their skin (SPS-R score = 0) felt less aroused and unpleasant during the stimulation than the other two SOSP groups.This SOSP-related difference in affective ratings was present for all levels of stimulus intensity.The electrocutaneous stimulation was perceived as generally more negative and more arousing by participants with medium and high SOSP.This finding suggests that not perceptual but affective processing of tactile stimulation may be associated with skin-picking behavior.In line with this interpretation, two other studies have found altered affective touch processing in individuals with elevated SOSP (Schienle & Wabnegger, 2022;Schienle et al., 2018).In one of those studies, which used functional magnetic resonance imaging, individuals with SPD displayed reduced neural activity when they caressed their forearms with their hands (Schienle et al., 2018).In the other study (Schienle & Wabnegger, 2020), soft-slow brushing of the participants' forearms from an experimenter (affective touch) induced the urge to pick the skin in participants with elevated SOSP.This reaction was opposite-of-expected since skin-picking has been shown to have the function of managing negative affective states (and not positive ones).
Other authors have introduced the concept of sensory over-responsivity (SOR) in the context of excessive skin-picking (e.g., Houghton et al., 2018).People with SOR react adversely to ordinary sensory stimuli that others find neutral or even pleasant.For example, they are bothered when touching certain materials or when hearing certain sounds.In a study by Houghton et al. (2018), individuals with pathological skin-picking showed greater sensory sensitivity and sensory avoidance than healthy controls.In the context of the current study, this over-responsivity might have played a role in the SOSP-related differences in affective responses to the electrocutaneous stimulation.
The following limitations need to be considered when interpreting the present results.This study used four specific stimulus intensities ranging from just noticeable to unpleasant (painful) tactile sensations and one location for stimulation (forearm).Therefore, the results cannot be generalized to other stimulus parameters.Moreover, in a future study, qualitative ratings for the stimuli could also be assessed, since the application of electrocutaneous stimuli at the perception threshold level can be perceived differently by participants; for example, for some as "knocking" or for others as "tickling" sensations (see Dölker et al., 2022).This variation in the quality of the perceived sensation might be associated with SOSP.
The study had been advertised as an investigation with electrocutaneous stimulation.This might have introduced a self-selection bias.Studies have shown that lower levels of fear of pain and higher levels of sensation-seeking predict the perceived likelihood of participating in pain research (Karos et al., 2018).However, it has to be noted that the three SOSP groups did not differ in their scores on the Pain Sensitivity Questionnaire (PSQ, Ruscheweyh et al., 2009), which assesses assumed pain intensity for daily life situations (e.g., pain associated with a sunburn).
Finally, even in the group with high SOSP (with SPS_R scores above the clinical cut-off), the reported urge to carry out skin-picking was very low (on average M = 5 on a scale with a maximum value of 100).This evaluation might have been subject to bias (social desirability), which could be assessed in a future study.Subsequent studies should also include clinical interviews to identify participants with a diagnosis of SPD.Those participants receiving the diagnosis may be characterized by more pronounced alterations in affective responses to electrocutaneous stimulation than the high SOSP group investigated in the present experiment.

Figure
Figure 1.Comparison of ratings (means, standard errors) between the three groups with no, medium, or high skinpicking severity.