<h4>Introduction</h4>Over the past few decades, research on affective touch has clarified its impact on key psychological functions essential for environmental adaptation, such as self-awareness, self-other differentiation, attachment, and stress response. These effects are primarily driven by the stimulation of C-tactile (CT) fibers. Despite significant advancements in understanding the fundamental mechanisms of affective touch, its clinical applications in mental health remain underdeveloped. This systematic review aims to rigorously assess the scientific literature on the relationship between CT fiber stimulation and psychological disorders, evaluating its potential as a therapeutic intervention.<h4>Methods</h4>This systematic review was conducted in accordance with PRISMA guidelines. A search was performed in the EMBASE, PubMed, and Web of Science databases for articles published in the last 10 years. The review focused on two main aspects: (1) potential dysregulation of CT fibers in individuals with psychological disorders, and (2) psychological treatments based on CT fiber stimulation and their psychological and functional outcomes.<h4>Results</h4>Most studies investigating CT fiber dysregulation in psychological disorders reported sensory alterations, with patients rating affective touch as less pleasant than healthy controls. These differences were often associated with dysregulation in the reward network and interoceptive processing, with several studies suggesting reduced insular cortex activation as a contributing factor. Regarding psychological treatments, only a limited number of studies analyzed therapies based on CT fiber stimulation. Despite methodological variations and differences in psychological diagnoses, the available evidence suggests that affective touch therapies can effectively reduce symptom severity and improve interoception across different psychological conditions.<h4>Discussion</h4>The findings underscore the potential of affective tou 2.3 Inclusion and exclusion criteria The PICOS model was used to determine the inclusion criteria: – P (population): “individuals with a diagnosis of psychological disorder.” – I (intervention): “CT fiber Materials and Methods Article Stimulation Protocol Psychological Assessment Neuroimaging Electrophysiologic Activity Recording Other Cascio et al., 2016 ( 52 ) Passive touch through three textures (unpleasant, pleasant, and social) at three different body sites: perioral face, dorsal forearm, and thenar palm. Experimenter administered all combinations twice (18 trials) Cognitive evaluation: Stanford-Binet Intelligence Scales (SB5), Mullen Scales of Early Learning (MSEL). Sensory evaluation: Sensory Experiences Questionnaire (SEQ) Not applicable (NA) NA Subjective evaluation of the pleasantness of tactile stimulation through a 5-point Likert scale. Experiment 2: subjective evaluation of the pleasantness and the intensity of tactile stimulation through a 10-point scale Zoltowski et al., 2023 ( 65 ) Passive touch in the right forearm (5 cm/s); stimulation occurred with three texture type: pleasant (soft brush), neutral (burlap), unpleasant (mesh); two runs including two block per texture type Cognitive evaluation: Wechsler Abbreviated Scales of Intelligence (WASI). These studies examined neuroimaging evidence ( 57 , 58 , 62 , 65 ) and subjective perceptions of pleasantness ( 52 , 58 , 62 , 65 ) in response to standardized tactile stimulation, employing various experimental protocols. Specifically, the studies differed in the tactile texture of stimulation and the velocity of touch. Researchers explored pleasant and unpleasant touch ( 52 , 58 , 65 ), as well as neutral touch using burlap fabric ( 65 ). Additionally, self-touch and object-touch ( 57 ), along with social-touch interactions ( 52 , 57 ), were assessed. Perini et al. (2021) ( 62 ) administered CT stimulations at velocities of 3 and 30 cm/s, while Zoltowski et al. Moreover, greater activation was observed in the parahippocampal gyrus, superior and middle temporal gyri, middle frontal gyrus, cingulate cortex, insula, and putamen in patients with AN compared to those with ASD. During other-touch conditions, subjects with AN showed significantly higher activity in S1 and the superior temporal gyrus compared to both patients with ASD and controls. Davidovic et al. (2018) ( 56 ) also utilized fMRI to examine brain responses during CT fiber stimulation, which was administered via a linear tactile stimulator. Moreover, a significant decrease in the left caudate nucleus’ activity in subjects with AN was found as compared to controls ( 56 ). Several selected studies corroborated the hypoactivation of the reward circuit and its components, involving patients diagnosed with AN ( 56 ), MDD ( 61 ), and PTSD ( 60 , 64 ). According to Nestler and Carlezon (2006) ( 69 ), another study ( 61 ) investigated the association between reward network, affective touch, and MDD. The shared involvement of the reward circuit across AN, MDD, and PTSD highlights a potential transdiagnostic mechanism linking affective dysregulation and maladaptive social functioning. Future research should explore whether targeting reward system dysfunction could enhance treatment outcomes. Understanding the interplay between reward processing and affective touch may offer novel therapeutic insights, particularly for individuals resistant to conventional treatments. As regards patients with PTSD, Maier et al. The majority of existing studies focus on the short-term effects of affective touch, leaving its long-term potential effects unexplored. It could be useful to improve the research on this topic to build longitudinal studies with extended follow-up periods to evaluate if the observed beneficial effects persists over time. Despite the growing interest in the potential benefits of affective touch, the knowledge on the interactions of CT fiber stimulation with brain networks remains incomplete.

In everyday interaction we touch different materials, which we experience along a limited number of perceptual and emotional dimensions: For instances, a furry surface feels soft and pleasant, whereas sandpaper feels rough and unpleasant. In a previous study, younger adults manually explored a representative set of solid, fluid and granular materials. Their ratings were made along six perceptual dimensions (roughness, fluidity, granularity, deformability, fibrousness, heaviness) and three emotional ones (valence, arousal, dominance). Perceptual and emotional dimensions were systematically corr * E-mail: Knut.Drewing@psychol.uni-giessen.de 22 1 2024 2024 19 1 452145 e0296633 26 6 2023 16 12 2023 22 01 2024 22 01 2024 21 10 2024 © 2024 Knut Drewing 2024 Knut Drewing https://creativecommons.org/licenses/by/4.0/ This is an open access article In particular, we wondered whether decline in perception of detail would come along with a dedifferentiation of perceptual space and emotional responses or whether young peoples’ complex world of touch would be preserved. The dimensions along which humans discriminate have been investigated in several studies. In the studies, (younger) people judged materials according to a list of perceptual adjectives, such as “smooth” or “hard” [ 2 ] or rated perceptual similarities of different materials [ 3 , 4 ]. Perceptual dimensions have then been determined by factor analyses or by multidimensional scaling methods. With respect to somatosensory processing we know about decline in skin elasticity, skin moisture, receptor density and spatial accuracy as assessed by two-point discrimination [ 26 – 30 ]. Dinse [ 32 ], e.g., gives two-point discrimination thresholds of 1.5 mm for younger adults (20–30 years) and 3.4 mm for older adults (66–86 years). These changes come along with reduced haptic perception of fine detail in texture, material, shape and spatial perception [ 30 – 35 ] and reduced abilities to detect light touch or vibration [ 27 , 36 ]. The decline can well be observed after an age of around 60 years [ 29 ]. Methods Participants The sample consisted of 15 older adults (age range 61–77 years, M = 66.2; 9 females) and 30 young adults (age range 18–29 years, M = 22.0 years). 15 of the young adults were females (19–29 years, M = 22.0), 15 were males (18–29 years, M = 22.1). The sample size of 15 per group was appropriate to detect small to moderate interaction effects (Cohen’s f = 0.15; power of 0.95) between the judgment patterns of either two groups. Participants were recruited in November 2017, data was collected in 2017 and beginning of 2018. Data was pseudonymized in accordance with Europe’s General Data Protection Regulation (GDPR). The final perceptual list included 18 adjectives: elastisch (elastic), faserig (fibrous), feucht (moist), fluffig (fluffy), glatt (smooth), glitschig (slippery), grobkörnig (coarse), haarig (hairy), hart (hard), klebrig (sticky), körnig (grainy), leicht (light), pulverig (powdery), rau (rough), schroff (jagged), schwer (heavy), verformbar (deformable), weich (soft). In the final emotional list were 9 adjectives: angenehm (pleasant), aufmerksamkeitserregend (attention-grabbing), aufregend (exciting), dominant (dominant), entspannend (relaxing), erfreulich (enjoyable), langweilig (boring), mächtig (mighty), schwach (weak). For the second component relevant adjectives are “fluffy”, “fibrous” and “hairy” (even if in the old group “hairy” explains 26% of variance per adjective only, and thus marginally fails the criterion of 30%), and can be labelled Fibrousness. Also components five and six are similar in all groups: Component five covers the adjectives “heavy” and “light”—plus “soft” in the older group, plus “hard” in the young males and plus “hard”, “smooth” in the young females. The component might be called Heaviness. Component six contains the adjectives “elastic” and “deformable” in all groups and can be identified with Deformability. Given the power calculations above, we can thus confirm that judgment patterns across materials do not differ between genders to a more than small-to-moderate degree. Note that we also did not find any significant interaction of gender with materials when analyzing the full sample including the older adults. 10.1371/journal.pone.0296633.g003 Fig 3 Bartlett scores for each component, group and material. Materials are ordered according to their average score. In case of a significant interaction age X material, materials where old age’s score exceeds the young age groups’ average by more than 0.2 are placed at the left and materials where young groups’ average is higher by the same amount are placed to the right. 10.1371/journal.pone.0296633.t003 Table 3 Gender comparison: Two-way ANOVA statistics of Bartlett scores. 19.775 <0.001* 0.32 0.671 0.417 0.02 1.221 0.268 0.03 However, we found several significant interactions of age group with material in the full sample: Roughness, Fluidity/Friction and Granularity are the perceptual components that younger and older participants judged differently for different materials, and Valence is the emotional component. In Fig 3 one can see how judgment patterns across materials differ between age groups. In particular, a large number of materials tend to be perceived as less rough by older as compared to younger participants. In particular, many materials were perceived as less rough by older as compared to younger participants ( Fig 3 ), older adults tended to judge granular stuff with larger parts (pebbles, polished stone) as more granular and stuff with smaller particles (flour, sand) as less granular than younger people, and there are differentiated age effects for valence. It is actually, two of three dimensions on fine texture or material detail that are differently judged by older as compared to younger people, while the three perceptual dimensions that deal less with detail (heaviness, deformability, fluidity/friction) are unaffected.