Evaluation of Affective Touch: A Comparison Between Two Groups of Younger and Older Females

ABSTRACT

Background

Caress-like touch activates C-tactile (CT) afferents present in the skin of humans. It is generally assumed that this type of touch provides beneficial effects throughout the lifespan. However, little is known about its processing in later adulthood. Moreover, emotional responses to interpersonal vs. self-touch have not been investigated in older adults.

Method

A total of 132 females from two age groups (older: 60–96 years; younger: 18–30) participated in a well-validated method of tactile stimulation, in which CT-optimal vs. nonoptimal soft brushing (3 cm/s vs. 30 cm/s) was administered on the forearms of the participants, once by a female experimenter and once by the participant themselves. All participants rated their affective state in each of the conditions and tactile acuity was assessed.

Results

Relative to the younger group, older participants rated touch as more pleasant, independent of type (CT-optimal, non-optimal) and origin (experimenter, self). In the older group, the emotional differentiation of CT-optimal vs. non-optimal touch was reduced, which was associated with reduced tactile acuity.

Conclusion

This study revealed an age-dependent enhancement of the perceived pleasantness of (self)touch. Interventions with auditory-guided CT-optimal self-touch could be easily implemented as part of training programs for older adults.

Introduction

Touch has discriminative as well as affective properties. The neurophysiological system that mediates affective touch responds to soft tactile stimulation of the skin with velocities between 1–10 cm/sec (e.g., Ackerley, Carlsson, Wester, Olausson, & Backlund Wasling, 2014; Löken, Wessberg, Morrison, McGlone, & Olausson, 2009; McGlone, Wessberg, & Olausson, 2014). This type of touch (“caressing”) activates slowly conducting, unmyelinated peripheral nerve fibers (so-called C-tactile (CT) afferents), which transmit the information to somatosensory and limbic regions of the brain (Morrison, 2016a)).

CT-optimal touch is typically perceived as pleasant and can reduce the intensity of negative affective states (Cruciani et al., 2021; Kirsch et al., 2018; Liljencrantz et al., 2017). The stress-buffer hypothesis of affective touch holds that being gently touched by another person positively influences the downregulation of negative emotions (Morrison, 2016b). Moreover, affective touch plays an important role in social communication. It supports social bonding and is a central component of intimate relationships. This effect is evident throughout the lifespan. For example, affective touch is one of the key mediators of early parental bonding and helps to reduce discomfort in children (Field, 2010).

In adulthood, affective touch can also alleviate the intensity of emotional distress and pain (Kirsch et al., 2018; Liljencrantz et al., 2017). This effect is most pronounced when touch is administered by significant others (e.g. Jakubiak, 2022). However, the rewarding function of gentle touch is also present in other types of social relationships apart from parents/children and partners. It has been shown that, even when the toucher is not acquainted with the person being touched, CT-optimal touch is still perceived as pleasant and can change behavioral as well as neurobiological reactions in adverse situations (Schienle, Scheucher, & Zorjan, 2022; Schienle, Schlintl, & Wabnegger, 2022).

Although the beneficial effects of interpersonal touch seem to be important throughout the lifespan, research on the effects of CT-optimal stimulation has focused primarily on younger adults thus far. According to a review article of the previously published research on affective touch by Cruciani et al. (2021), only 3% of the investigations carried out in this area have included older individuals (>60 years), while only one single study has assessed the perception of affective touch (soft brushing of the skin) in even later stages of adulthood (Sehlstedt et al., 2016). This is surprising since the tactile system undergoes pronounced changes with aging. For example, the density of neurons in the skin decreases with age (e.g., Chang, Lin, & Hsieh, 2004), which is associated with lowered discriminative tactile abilities (e.g., Stevens & Choo, 1996).

As mentioned above, only one study has thus far investigated the perception of affective touch across a wide range of ages including later adulthood (Sehlstedt et al., 2016). The authors investigated 60 males (age range: 13–74 years, median = 31 years), and 60 females (age range: 14–82 years, median = 31 years). The participants rated the pleasantness of CT-optimal stroking (velocity: 1, 3, 10 cm/s) and non-optimal stroking (0.1, 0.3, 30 cm/s) of their forearms. Pleasantness ratings for touch (CT-optimal as well as non-optimal) correlated with age. The authors concluded that the affective responsivity to touch increases with age. However, in this experiment, only a small number of participants were older than 60 years. The findings therefore may not be representative of older adulthood.

It is also noteworthy, that all of the mentioned studies focused on touch administered by others. To the best of our knowledge, no research exists on the evaluation of CT-optimal self-touch in different age groups. This is surprising, since humans frequently touch themselves, particularly during exposure to stressors (Boehme & Olausson, 2022). Different authors have demonstrated that self-touch during aversive stimulation can reduce experienced arousal and the neurobiological stress response (e.g., Dreisoerner et al., 2021; Grunwald, Weiss, Mueller, & Rall, 2014). Moreover, self-touch can improve well-being. For example, self-brushing of the whole body practiced over four weeks increased positive affect, emotional awareness, and self-regulation (e.g., Matiz, Guzzon, Crescentini, Paschetto, & Fabbro, 2020; Schienle, Schlintl, & Wabnegger, 2022).

The current study compared two groups of older (age range: 60–96 years) and younger adults (18–30 years). All participants were female, as well as the experimenter who administered the tactile stimulation. We chose this approach for two main reasons. First, research has consistently identified sex differences in pleasantness ratings for affective touch, with females giving higher pleasantness ratings than males (see meta-analysis by Russo, Ottaviani, & Spitoni, 2020). Second, the attribution of meaning to affective touch (and resulting pleasantness ratings) can differ in dyads of the same vs. different sex (Russo, Ottaviani, & Spitoni, 2020). For the present investigation, we, therefore, decided that the sender and receiver of the affective touch signals should be of the same sex in order to not further increase the complexity of our experimental design.

We attempted to replicate findings on the enhanced affective responsivity to touch (administered by another person) in older adults. Additionally, we compared affective responses to self-touch with CT-optimal and non-optimal velocities between older and younger adults. Finally, we assessed tactile acuity (two-point differentiation) and tested whether this measure would be associated with the evaluation of affective touch.

Method

Sample

An a priori power analysis with G*Power 3.1.9.7 (Faul et al., 2007) indicated that a minimum sample size of N = 128 would be necessary to detect an effect size of part.η² = 0.06 (i.e. medium effect) with a probability of 1 – β = 0.80, α = 0.05 for the main effects and interaction effects of Type of Touch (CT-optimal vs. nonoptimal), Toucher (participant vs. experimenter), and Age Group (younger vs. older) on the emotional state. A total of 132 females from two age groups (older: n = 64; M = 74.8 years, SD = 10.7; range = 60–96 vs. younger: n = 68; M = 23.7 years, SD = 3.1; range = 18–30) participated in the study.

The majority of younger females (96%) had a high school diploma compared to 27% in the older group. Thirty-six percent of the older participants were in assisted living. The conducted Mini-Mental-State Examination (MMSE; Folstein, Folstein, & McHugh, 1975) indicated normal cognitive functioning (M = 27.6, SD = 3.04) in the older group. The MMSE is a 30-point questionnaire that is used in clinical and research settings to measure cognitive impairment. A score of ≥ 24 indicates normal cognitive functioning.

Both age groups did not differ in reported psychological problems as indicated by the Brief Symptom Inventory (BSI; Spitzer et al., 2011), which screens for symptoms related to anxiety, depression, and somatization. In the present sample, the T-score of the mean BSI score was in the normal range for younger (T = 45) as well as older participants (T = 44).

Touch Paradigm

Tactile stimulation was administered with a hand-held soft boar bristle brush to the non-dominant dorsal forearm. CT-optimal touch had a velocity of 3 cm/s (stroking in proximal to distal direction, 6 cm region), whereas CT-nonoptimal touch had a velocity of 30 cm/s. The stroking was guided by a metronome.

The stroking was administered by a trained female research assistant (condition: experimenter-touch) and by the participant (condition: self-touch). Each of the four resulting touch conditions lasted for 30 seconds. The sequence of the conditions was randomized. After each condition, participants rated their emotional state (valence and arousal) on a 9-point scale (9 = very pleasant; very aroused).

Two-Point Discrimination Test

Tactile acuity (two-point discrimination) was assessed with the W54670 (Baseline) sensitivity tester (Fabrication Enterprises Inc.; model number: 12–1492), which was used to measure the ability of the participants to discern two nearby points (distances ranging between 2–5 mm) at the tip of their dominant index finger. The test used a forced-choice technique; the two-point threshold was defined as the smallest distance at which 7 out of 10 tactile stimulations were correctly identified. According to Veit and Zumhasch (2021), average scores for the two-point threshold at the fingertips vary between 2 and 3 mm.

Procedure

The study complied with all relevant ethical guidelines and regulations involving human participants and was approved by the ethics committee of the University (GZ. 39/8/63 ex 2021/22). All participants provided written informed consent before participating. The study was preregistered on January 17^th, 2022 (Open Science Framework: https://doi.org/10.17605/OSF.IO/6GCQP). Participants were individually tested in a quiet room.

Statistical Analyses

We computed an analysis of variance (ANOVA) to test the effects of Type of Touch (CT-optimal/nonoptimal), Toucher (experimenter, participant), and Age Group (younger, older) on the valence ratings (pleasantness). We could not compute an ANOVA for the arousal ratings due to the restricted variance of this dependent measure. The majority of older adults (77%) and 35% of the younger adults reported minimal arousal (rating of 1) across all brushing conditions. The mean arousal ratings (M, SD) were as follows: older group: 1.51 (1.25); younger group: 2.20 (1.27).

Moreover, we compared the tactile acuity between the Age Groups (younger, older) via an independent sample t-test. Older adults with high vs. low tactile acuity (two-point differentiation: < 4 mm vs. ≥4 mm) were compared concerning their pleasantness ratings for CT-optimal and nonoptimal touch. In the younger group, only six participants had a two-point differentiation score ≥4 mm. Therefore, no further analyses were conducted.

Additional regression analyses were carried out according to the preregistration. The statistically nonsignificant results are provided in the Supplementary Material.

Results

Evaluation of CT-Optimal Vs. Nonoptimal Touch

The ANOVA revealed significant main effects for Type of Touch (F(1,130) = 136.43, p < .001, partial η² = .512), Age Group (F(1, 130) = 12.32, p < .001, partial η² = .087) and Toucher (F(1,130) = 49.06, p < .001, partial η² = .274). CT-optimal touch was rated as more pleasant (M = 7.4, SD = 1.5) than CT-nonoptimal touch (M = 5.8, SD = 2.0). The older group (M = 7.1, SD = 1.3) rated touch as more pleasant than the younger group (M = 6.1, SD = 1.7). Touch administered by the experimenter (M = 7.0, SD = 1.6) was perceived as more pleasant than self-touch (M = 6.2, SD = 1.9).

Moreover, the interaction effects Toucher x Type of Touch (F(1,130) = 8.07, p = .005, partial η² = .058) and Age Group x Type of Touch x Toucher were significant (F(1,130) = 4.29, p = .040, partial η² =.032). The interaction Type of Touch x Age Group was not significant (F(1,130) = 1.20, p = .275, partial η² =.009). Posthoc comparisons revealed that the older and younger group differed in all touch conditions (CT-optimal, CT-nonoptimal, experimenter, self-touch; all p < .02, for descriptives, see Figure 1).

Figure 1. Means and standard deviations for the valence ratings of the younger (18–30 years) and older females (60–90 years).

exp = touch administered by experimenter; part = touch administered by participant; affective = touch with CT-optimal velocity (3 cm/s); nonaffective = touch with CT-nonoptimal velocity (30 cm/s); 9 = very pleasant

Further group differences concerned the difference scores for the valence ratings (CT-optimal minus CT-nonoptimal touch) in the two Toucher conditions (touch by experimenter vs. self). The younger group had a higher difference score (M_diff = 2.25, SD = 1.93) than the older group (M_diff = 1.56, SD = 2.05; t(130) = 1.99, p = .049, Cohen’s d = 0.35) when touched by the experimenter. The group difference was not significant in the self-touch condition (M_{diff_younger} = 1.35, SD = 1.52; M_{diff_older} = 1.42, SD = 2.18; t(111.9) = −.209; p = .835, Cohen’s d = 0.04).

Tactile Acuity

Younger adults obtained a lower score (M = 2.63 mm, SD = .64) in the two-point discrimination test indicating greater tactile acuity than older adults (M = 3.45 mm, SD = 1.10; t(82.85) = −4.90, p < .001, Cohen’s d = .89). In the older group, nine participants (14%) abandoned the test and were not able to distinguish two points with a distance of 5 mm (largest distance of the test).

We compared two groups of older adults with high vs. low tactile acuity (two-point differentiation: < 4 mm vs. ≥4 mm) in their affective ratings. Participants with lower tactile acuity rated CT-nonoptimal touch as more pleasant (n = 34; M = 7.56, SD = 1.91) than participants with higher tactile acuity (n = 30; M = 5.77, SD = 2.05; t(62) = 3.62, p < .001). The two acuity groups did not differ in their evaluation of CT-optimal touch (high: M = 8.20, SD = 1.47 vs. low: M = 8.35, SD = 1.01; t(62) = .489, p = .626), see Figure 2).

Figure 2. Means and standard deviations for the valence ratings of older females with lower and higher tactile acuity in the CT-optimal and CT-nonoptimal touch conditions.

lower tactile acuity: two point-discrimination ≥4 mm; higher tactile acuity: two point-discrimination <4 mm; CT-optimal velocity (3 cm/s); CT-nonoptimal velocity (30 cm/s); 9 = very pleasant

Discussion

The present study compared affective touch processing and tactile acuity between an older group (60–96 years) and a younger group (18–30 years) of female participants. Consistent with a previous study (Sehlstedt et al., 2016), the older group perceived being touched as generally more pleasant than the younger group independent of the brushing velocity (CT-optimal vs. non-optimal). Moreover, interpersonal as well as self-touch received higher pleasantness ratings from the older participants compared to the younger participants. Thus, the older group always rated their affective state as more positive after being touched than the younger group.

In addition, we detected pronounced age-related effects on tactile acuity. This is in line with previous research (for a summary see McIntyre, Nagi, McGlone, & Olausson, 2021). The younger participants outperformed the older participants in the two-point discrimination task. Some of the participants in the older group even had to abandon the discrimination test and were not able to detect a two-point distance of 5 mm (the maximal distance of the test). It has been shown that tactile acuity is associated with the number of nerve fibers in the skin which decreases with age (Chang, Lin, & Hsieh, 2004). For example, in older adults, degenerative processes have been shown to affect myelinated A-beta, mechanoreceptive afferents that mediate discriminative aspects of tactile function (McIntyre, Nagi, McGlone, & Olausson, 2021). These afferents conduct temporally accurate information to the brain that is relevant for the decoding of location, duration, and intensity of touch.

Further age-related effects on touch processing concerned the emotional differentiation between CT-optimal and non-optimal touch that had been administered by the experimenter. Older participants perceived both types of touch as being more similar in valence than younger participants. Very interestingly, emotional differentiation was associated with tactile acuity in the older group. Those participants with reduced tactile acuity (≥4 mm) evaluated CT-nonoptimal (fast) brushing of their forearms more positively than those with higher acuity (<4 mm).

This deficit could be related to specific problems of older adults in the processing of higher-frequency tactile stimulation. In younger individuals, there is an inverted U-shaped relationship between the reported pleasantness of arm stroking and stroking velocity (Schirmer, Cham, Lai, Le, & Ackerley, 2023). Very slow and very fast stroking is experienced as less positive. However, when a specific spatiotemporal pattern of tactile stimulation with a high frequency cannot be identified correctly anymore (e.g., due to lowered tactile acuity), this should affect the quality of perceived touch. Thus, the elevated positive valence of CT-nonoptimal touch in the older group might be a consequence of a reduced ability to correctly identify the high velocity of brushing. In line with this interpretation, other studies have also demonstrated that aging is associated with difficulties in frequency identification of tactile information (e.g., vibration; for a summary see McIntyre, Nagi, McGlone, & Olausson, 2021).

Moreover, it has been suggested that the age-related degeneration of CT afferents is less pronounced compared to the degeneration of A-beta fibers (McIntyre, Nagi, McGlone, & Olausson, 2021). Thus, while the discriminative touch system loses its capacity over time, the affective system is more temporally stable. This would suggest that the perceived pleasantness of affective touch remains similar throughout the lifespan. In contrast to this assumption, the present data as well as previous findings point to an increase in the pleasantness of affective touch signals with age (Sehlstedt et al., 2016).

Therefore, additional factors have to be relevant. Within this context, it has to be noted that touch has an emotional meaning and that the regularity of being touched decreases with age on average (Cruciani et al., 2021). Older adults may therefore experience a higher emotional significance attached to touch experiences. They typically have fewer social contacts, which makes interpersonal touch more special. However, it has to be noted, that the older group of the current experiment not only rated touch by the experimenter as more pleasant than the younger group but also self-touch (which is always available).

In summary, the present study identified a complex pattern of changes in tactile processing in older adults that referred to both discriminative as well as affective functions of touch. Moreover, discriminative and affective processing of tactile information were functionally related. This merits further research.

The following limitations of the present study deserve consideration. The investigation included an exclusively female sample. Thus, our results cannot be generalized to other groups. A subsequent study should attempt to replicate the present findings in a sample of younger and older males.

Moreover, a cross-sectional design was implemented in our study. For future projects, it could be beneficial to examine the same participants repeatedly to monitor changes in affective touch processing over time.

In conclusion, the present study provides important findings on tactile processing in later adulthood. These findings have practical implications. The older group evaluated interpersonal touch as well as self-touch as generally very pleasant. Therefore, CT-optimal self-touch could be used to induce positive affect, particularly when social contacts are limited. Further, this suggests possible beneficial effects of interventions (similar to a form of relaxation training) that deliver auditory instructions for affective self-touch (Schienle, Scheucher, & Zorjan, 2022). Such interventions should be the subject of further research since they could be easily implemented as part of a training program for older adults.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/0361073X.2023.2250225