Touch is perceived as more pleasant when caressing touch is paired with attractive, compared to unattractive, faces; attractive faces particularly enhance subjective and autonomic responses to slow caresses

Hedonic responses to touch are modulated by the perceived attractiveness of the caresser. Giovanni Novembre, Roberta Etzi, India Morrison. Neuroscience, October 17 2020. https://doi.org/10.1016/j.neuroscience.2020.10.007

Highlights:

• Touch is perceived as more pleasant when caressing touch is paired with attractive, compared to unattractive, faces.

• Attractive faces particularly enhance subjective and autonomic responses to slow caresses.

• Heart rate variability increases only for slow touch paired with attractive faces.

Abstract: Previous research has shown that a specific type of C fiber, the C tactile afferents, are involved in detecting gentle, dynamic tactile stimuli on the skin, giving rise to affective responses in the central nervous system. Despite building on such bottom-up information flow, the hedonic perception and the physiological consequences of affective touch are influenced by various sources of top-down information. In the present study we investigated how perception of affective touch is influenced by the attractiveness of hypothetical caressers. Participants were stroked on the arm and the palm while looking at photos of high attractive and low attractive opposite-gender faces, and were instructed to imagine those people as the caressers. In a control condition no photo was paired with the touch. The stroking stimulation was delivered with a soft brush either on the forearm or on the palm, and either with a slower or faster speed. Participants rated the pleasantness of each stimulation, while electrocardiographic recordings were made to extract heart rate variability data. Results showed that participants preferred touch stimuli paired with high attractive faces; they also preferred palm stroking and slower stroking speed. Like subjective pleasantness ratings, heart rate variability responses to affective touch (slow) were higher for high attractive than for low attractive caressers, but were not selective for arm or palm stroking. Overall, the present study confirms that contextual social information plays a major role in affective touch experiences, influencing not only the hedonic quality of the experience but also the physiological state of the body.

Keywords: heart rate variabilityaffective touchCT fibers

Discussion

In this study we investigated how a contextual factor, namely attractiveness, shapes hedonic perception in tactile interactions with imagined unfamiliar touchers. We brushed participants’ skin while they viewed opposite gender faces and were instructed to imagine that that person was caressing them. By manipulating face attractiveness level (high vs low), velocity (CT-optimal vs CT-non-optimal) and site (forearm vs palm) of the touch, we were able to show that: 1) touch is most pleasant when it is paired with an attractive face; 2) attractive faces particularly enhance subjective responses to slow caresses; 3) touch delivered to the palm is preferred over touch delivered to the forearm, regardless of the imagined toucher’s attractiveness; 4) heart rate variability increases only for slow touch paired with attractive faces.

Tactile interactions are critical throughout human life and constitute an important platform for establishing social connections. Therefore, it is not surprising that research about touch and social interactions in the recent past has mostly focused on the beneficial effects of interpersonal touch and the detrimental consequences of the lack of it (Field, 2014, Gallace and Spence, 2010, Sailer and Ackerley, 2019). Nevertheless, the context in which social interactions occur may completely alter the valence of the affective experience of the touch from another person from positive to negative. The present study suggests that in tactile interactions with strangers, attractiveness might be an important factor determining the ultimate hedonic perception of the experience. The evidence for such a claim comes from both behavioral and psychophysiological data.

Behavior

On the behavioral level, touch paired with high attractive faces was preferred over touch paired with low attractive faces but was not hedonically discriminated from unpaired touch. This indicates that face information can influence the hedonic perception of touch, with touch experienced as less pleasant unless the face is perceived as attractive. Interestingly, despite slow touch being preferred over fast touch in each condition, the significant interaction between velocity of touch and context of touch (unpaired, high attractive toucher or low attractive toucher) shows that the emotional response to slow touch differentiates the three conditions. Namely, unpaired touch elicited more positive responses than touch paired with high attractive faces, which in turn was preferred over slow touch paired with low attractive faces. In contrast, affective responses to fast touch were significantly lower to touch paired with low attractive faces, but they did not differ between unpaired touch and touch from high attractive faces.

The slow touch used in this study (3 cm/s) is among the most used experimental stimulations in paradigms studying social touch and falls in the range of touch velocities (1-10 cm/s) that correspond to spontaneous affective touch interactions, e.g. caresses (Croy et al., 2016, Hertenstein et al., 2007, Morrison et al., 2010). It should be noticed, however, that a very recent study has shown that when asked to spontaneously stroke other people (e.g., their partner, friend or even a stranger), participants tend to do so with a mean velocity falling between 10 and 20 cm/s (Strauss et al., 2020). Since a consistent investigation on the psychophysical responses to touch within this interval is not currently available, future studies will address whether the slow touch used in this study is physiologically and affectively distinguishable from that delivered in the 10-20 cm/s range. Nevertheless, in the light of current evidence, it is not surprising that responses to the slow touch are well differentiated across conditions, and they indicate that when tactile interactions with imagined touchers occur with a velocity associated with higher affective value, the attractiveness of the toucher plays a major role.

One potential mediator of such effect is believed to be a class of afferent nerves so far found in the hairy skin only, but not on the glabrous skin of the palms and soles. These CT fibers, show greatest increases in firing frequency for stimuli in the aforementioned velocity range (1-10 cm/s), as well triggering the most positive affective responses, compared to very slow (<1 cm/s) or fast velocities (>10 cm/s). Accordingly, we expected to see a general preference for touch on the forearm (hairy skin) over touch on the palm (glabrous skin). This was not the case, and instead we found a preference for touch on the palm. Whereas the initial evidence had suggested a general preference for touch on the hairy skin over glabrous skin (Löken et al., 2009), a growing number of studies employing various paradigms have shown an undifferentiated hedonic response to arm and palm stroking (Ackerley, Carlsson, et al., 2014; Kirsch et al., 2018, Perini et al., 2015).

Furthermore, affective ratings for hairy and glabrous skin stimulation have been shown to heavily depend on the order of stimuli presentation, with preceding hairy skin stimulations positively influencing the perception of pleasantness of following palm stimulations, even in block designs (Löken et al., 2011). In our study there was a significant three-way interaction between the factors “Face”, “Site” and “Randomization” indicating that participants who started with palm touch rated touch on the palm paired with high attractive faces significantly more pleasant than participants who started with touch on the arm. Such an effect was not observed for arm touch. Therefore, if anything, we observed a facilitating effect of initial palm touch on subsequent palm stimulations, specific to high attractive faces and regardless of velocity. A possible explanation for such difference compared to the results of Löken and colleagues (Löken et al., 2011) might lie in the more complex structure of our task, possibly indicating that the presence of other factors like attractiveness, or more generally tactile interactions with unfamiliar touchers, might influence the intrinsic hedonic properties of forearm touch in favor of palm touch. Nonetheless, more controlled experiments which specifically look at how attractiveness and type of skin stimulated interact in affectively rich tactile experiences are needed to cast light on this aspect.

More generally, the crucial conditions of this study (high and low attractive faces) involved the presentation of two types of sensory stimuli (tactile and visual), and required participants to integrate them to produce a hedonic response, whereas another condition (no face) did not require such multisensory integration. This differential recruitment of sensory and attentional resources could explain the pattern of hedonic responses to touch in our study. There is indeed existing research showing that attention and allocation of cognitive resources influence tactile perception (Lier et al., 2018, Schubert et al., 2008), especially when attention must be directed to concomitant stimuli from other sensory modalities (Hanke et al., 2016). If the presentation of an additional stimulus (photo) competing for attention with the brushing stimulus affected behavioral responses, this would equally influence both high attractive and low attractive trials in comparison to unpaired trials. Nevertheless, whereas affective responses to touch paired with low attractive faces were consistently lower than unpaired touch, this was not the case for touch paired high attractive faces, which was rated as pleasant as unpaired touch when the touch was delivered at higher speed. Therefore, even if attention plays a general role in shaping hedonic responses to touch, the interaction between the attractiveness of the paired faces and the velocity of the touch speaks for the intervention an additional factor. Whether this factor is attributable to differential CT response or other mechanisms will require further investigations.

Another interesting behavioral observation comes from the comparison between males’ and females’ affective ratings. Indeed, males showed consistently higher ratings across all touch conditions. On a first glance, this result would seem to contradict the findings of a recent meta-analysis regarding sex differences in response to affective touch (Russo et al., 2020). Across 13 studies it was found that females perceive affective touch as more pleasant than males. Nevertheless, some of the included studies (Ackerley, Carlsson, et al., 2014; Jönsson et al., 2015, Triscoli et al., 2013) did not find such a difference. On the other hand, our result is corroborated from other evidence in the touch literature: for instance, recent work has shown that female touch is generally considered more pleasant than male touch across both genders (Suvilehto et al., 2015). Furthermore, our results can also be interpreted as a more specific gender difference when the considered variable is touch from strangers. Indeed, when Suvilehto et al. computed the ‘touchability index’ for their participants, the only figure whom males would allow to touch more than females were in fact female strangers (Suvilehto, pers comm)1. This applies regardless of all the other factors tested in our experiment, since no interactions between the gender and site, velocity or context of touch were found. Therefore, it appears that males’ and females’ affective responses to touch are differentially modulated by a broad range of contextual factors. In this regard, a potential confound in our experiment might be that the brushing stimuli were delivered by a female experimenter whom participants met before the experimental session started.

Heart Rate Variability

On the psychophysiological level, heart rate variability (HRV) results confirm that the attractiveness of the toucher plays a major role in the hedonic perception of tactile interactions. Slow touch was associated with higher HRV than fast touch only during trials with high attractive faces. Specifically, slow touch significantly increased HRV compared to faster touch, but only when the imagined toucher was perceived as attractive. This occurred regardless of the site of the stimulation (hairy or glabrous skin).

According to several theories (Porges, 1995, Thayer and Lane, 2000) heart rate variability is a reliable index of the capacity of the central nervous system to control cardiac activity through parasympathetic influence and in turn adjust metabolic strategies to adapt to constantly changing environmental demands (Thayer et al., 2010). Both resting and task related higher HRV values are generally associated with better performances across many domains, like emotion and cognitive regulation, possibly reflecting higher flexibility. For instance, psychological stressors often cause decreases in heart rate variability (Chandola et al., 2008, Dimitriev and Saperova, 2015). Accordingly, higher values of heart rate variability are thought to reflect better adaption to stressors (Kim et al., 2018). Hence, the observation of higher HRV values during slow touch received by an attractive person may indicate a more favorable reaction to a potentially stressful and affective meaningful interaction with a stranger, which is not observed for the touch of a low attractive person.

An alternative interpretation is related to the suggested link between HRV and social cognition. For instance, it has been shown that HRV is positively related to performances in social tasks (Quintana et al., 2012), whereas many psychiatric disorders that show impairment of social cognition skills are associated with reduced HRV (Chalmers et al., 2014, Kemp and Quintana, 2013). In this framework, the affective touch of an attractive person might have represented a highly salient stimulus which needed increased social attention compared to the other stimuli.

Furthermore, it should be noted that our data also showed a significant interaction between context of the touch and participants’ gender, which hinted at higher HRV values for females compared to males, especially for touch paired with low attractive faces. Heart rate variability has been shown to vary quite consistently between males and females across many experimental settings (Koenig & Thayer, 2016). Females are usually reported as having greater vagal tone, hence higher values of HRV, which is in keeping with what we observed in our data. In our sample females showed higher HRV values than males in a consistent manner across all conditions, though it did not reach significance probably due to high variability in the sample. Therefore, rather than a specific effect of our task, we interpret this result as reflecting a general tendency for female to have greater basal levels of vagal activity.

Although our results show a striking correspondence between pleasantness ratings and HRV values, the complex relationship between subjective affective touch and physiological responses remained to be clarified. To date, research investigating this relationship has produced inconclusive and contradicting results. For instance, studies investigating facial muscles reactivity to slow touch have either found an increased zygomatic response (associated with positive affect) unlinked to emotional ratings (Pawling et al., 2017), or relaxation in the corrugator muscles (activation of which is associated with negative affect) without effects on zygomatic activity (Mayo et al., 2018, Ree et al., 2019).

This uncertainty also applies to research investigating the relationship between subjective reports to touch and HRV response. On the one hand, prolonged slow touch has already been shown to decrease heart rate (Triscoli, Croy, Olausson, et al., 2017) and enhance HRV (Triscoli, Croy, Steudte-Schmiedgen, et al., 2017), thus providing evidence for a significant influence of slow touch on the autonomic nervous system. In the latter study, slow brushing delivered with a robot was compared to a vibratory stimulus and HRV was measured as SDNN (standard deviation of normal to normal R-R intervals). Unlike Triscoli and colleagues, we measured HRV as RMSSD, another time-domain measurement, that (compared to SDNN) is more influenced by the parasympathetic nervous system and is more adapted for measuring HRV in shorter intervals (Shaffer & Ginsberg, 2017). On the other hand, another study (Ree et al., 2020) in which healthy participants repeatedly received short skin-to-skin slow touch stimulations on their forearm for about 60 minutes did not find an increase of RMSSD values compared to a rest period.

Compared to both these studies, a chief novelty of our work lies in the evidence of how the manipulation in a variable which is “external” to the touch per se (attractiveness of the toucher) can modulate slow touch effects on a psychophysiological index like HRV. Altogether, this evidence points to the importance of the measures used for quantifying HRV and to the contextual manipulations occurring meanwhile touch is delivered. In fact, in line with the findings of Ree and colleagues (Ree et al., 2020), in the present study slow touch did not result in a general HRV increase, at least compared to fast touch. We found instead a specific interaction with the attractiveness factor, indicating that HRV might increase following slow touch only in specific circumstances. It must be also noted that unlike Ree and colleagues’ study, we did not measure baseline HRV. Therefore, our design is not suitable for detecting specific changes in HRV following slow touch. Interestingly, whereas we stimulated both forearm and palm in our study, the forearm was the only skin site stimulated with slow touch in both Triscoli and colleagues’ and Ree and colleagues’ works, but with two different modalities: in the former brushing was used, similar to the present study; in the latter instead, skin-to-skin touch was delivered by an experimenter. Despite hand and brush touch are rated similarly by healthy participants (Strauss et al., 2019), future studies will be needed to investigate whether touch modality plays a role in modulating HRV responses.

Finally, contrary to our hypothesis, arm touch did not elicit significantly higher HRV than palm touch. This is in keeping with the behavioral data, in which palm touch was preferred over arm touch. This result suggests that unlike touch velocity, skin type (hairy or glabrous) might not be a factor in the physiological response to touch. This lends support to the proposition that activation of CT-fibers is not a necessary component of affective touch responses (Ackerley, Carlsson, et al., 2014), with many cognitive factors like learning, motivation and expectation playing an important role (Ellingsen et al., 2016). Nevertheless, in our knowledge this is the first study collecting HRV responses while touch is delivered to both hairy and glabrous skin, therefore further studies focusing on this aspect will be needed to clarify this question.

Limitations

Overall, the present study provides a further evidence of how contextual factors, specifically attractiveness, change the pleasantness of social tactile interactions. However, it is limited by several considerations. First, we cannot rule out that participants’ knowledge of the identity of the brusher could have influenced their behavioral and psychophysiological responses. Though these touch interactions were mediated by a brush, males’ more than females’ responses might have been highly influenced by their impression of the experimenter. Furthermore, this may have represented an uncontrolled source of heterogeneity external to the task which could have overshadowed the real effects of the independent variables.

Secondly, knowing the identity of the brusher might have dampened the participants’ engagement in the imagination task, making more difficult for them to think that the touch they were receiving was in fact delivered by the person they saw on the screen. Future studies will implement real touch from strangers with different grades of attractiveness to refine our answer to the question of how much attractiveness of a person affects emotional and physiological responses to tactile interactions.

Finally, we should consider that some important confounding variables for measuring HRV were not considered in this study. For instance, Laborde and colleagues list a series of different stable (e.g., smoking, alcohol consumption, weight and height) and transient (e.g., normal sleep routine and no caffeine consumption prior to the experiment) variables which should be investigated in the sample (Laborde et al., 2017). Nevertheless, we believe that these confounds only apply to the group comparisons, since the male and female subgroups might differ significantly in some of those parameters. Otherwise, in our experimental design, the crucial comparisons are protected from the within-subjects design. Related to this last aspect, our study did not include a baseline measurement. Despite collecting a baseline measurement is encouraged to allow detection of changes in HRV after intervention (Shaffer & Ginsberg, 2017), our main goal was the comparison between conditions, rather than measuring changes from rest.