Naturalistic stimuli in touch research

Endocannabinoid contributions to the perception of socially relevant, affective touch in humans

Social relationships are central to well-being. A subgroup of afferent nerve fibers, C-tactile (CT) afferents, are primed to respond to affective, socially relevant touch and may mitigate the effects of stress. The endocannabinoid ligand anandamide (AEA) modulates both social reward and stress. We thus hypothesized that AEA levels would be associated with the perceived pleasantness of affective touch in humans. Across two studies, we explored perceptions of affective, socially relevant touch and general affective stimuli. In study 1, adult participants (N = 101) were recruited based on presence (CM+) or absence (CM-) of documented childhood maltreatment (N = 52 CM+; N = 49 CM-). In study 2, healthy individuals were randomized to receive an inhibitor of fatty acid amide hydrolase (FAAH; PF-04457845) to increase AEA levels (n = 16) or placebo (n = 29). Outcomes included self-report ratings of touch pleasantness and intensity, valence and arousal ratings of affective images, and plasma levels of endocannabinoids AEA and 2-AG, cortisol, and oxytocin. In study 1, higher AEA levels were associated with a reduced preference for affective, CT-optimal touch. In study 2, pharmacological elevation of AEA resulted in reduced preference for affective touch. These effects were specific to social processing, as AEA levels were not related to ratings of affective images. In contrast to our hypothesis, elevated AEA was associated with reduced pleasantness ratings of CT-optimal, affective touch. This provides novel, in-human data linking AEA to social processing, adding nuance to the rationale for its use as a potential novel therapeutic target in disordered in social processing.

The Impact of Relaxation Massage Prior to Bedtime on Sleep Quality and Quantity in People with Symptoms of Chronic Insomnia: A Home-Based Sleep Study

Background/Objectives: Manual massage is an effective treatment approach for reducing general stress and promoting an overall sense of well-being. Relaxation massage aims to alleviate psychophysiological tension, enhance both blood and lymphatic circulation, and promote mental and physical relaxation. It is particularly beneficial for those with anxiety-related symptoms (such as generalized anxiety disorder and social anxiety) and sleep disorders, aiming to improve calmness and promote sleepiness. Aims: The purpose of the present study was to assess the effectiveness of a single session of relaxation massage prior to bedtime on sleep quality and quantity indices in individuals with symptoms of chronic insomnia. Methods: In total, 20 (N = 20) healthy individuals (aged 25.5 ± 12.0 years; 6F/14M) with a score on the Athens Insomnia Scale of ≥16 participated under three different conditions over one week apart: (1) a 45 min relaxation massage condition (REL), (2) a 45 min sham massage condition (PLA), and (3) a control condition with no massage. Sleep activity was monitored using a portable polysomnographic system. Results: A statistically significant effect was observed between sleep efficiency across the three sessions (p = 0.034), with a notable effect in the relaxation massage (REL) session (p = 0.045). Additionally, sustained sleep efficiency showed a statistically significant difference among the sessions (p = 0.005). Conclusions: Relaxation massage prior to bedtime could be used as an effective and safe non-pharmacological approach for improving sleep efficiency and potentially restoring the fragmented sleep of individuals with symptoms of insomnia. Trial registration number: The trial was registered at clinicaltrials.gov as NCT06781866.

Brain Encoding of Naturalistic, Continuous, and Unpredictable Tactile Events

Studies employing EEG to measure somatosensory responses have been typically optimized to compute event-related potentials in response to discrete events. However, tactile interactions involve continuous processing of nonstationary inputs that change in location, duration, and intensity. To fill this gap, this study aims to demonstrate the possibility of measuring the neural tracking of continuous and unpredictable tactile information. Twenty-seven young adults (females, 15) were continuously and passively stimulated with a random series of gentle brushes on single fingers of each hand, which were covered from view. Thus, tactile stimulations were unique for each participant and stimulated fingers. An encoding model measured the degree of synchronization between brain activity and continuous tactile input, generating a temporal response function (TRF). Brain topographies associated with the encoding of each finger stimulation showed a contralateral response at central sensors starting at 50 ms and peaking at ∼140 ms of lag, followed by a bilateral response at ∼240 ms. A series of analyses highlighted that reliable tactile TRF emerged after just 3 min of stimulation. Strikingly, topographical patterns of the TRF allowed discriminating digit lateralization across hands and digit representation within each hand. Our results demonstrated for the first time the possibility of using EEG to measure the neural tracking of a naturalistic, continuous, and unpredictable stimulation in the somatosensory domain. Crucially, this approach allows the study of brain activity following individualized, idiosyncratic tactile events to the fingers.

Out of touch? How trauma shapes the experience of social touch - Neural and endocrine pathways

Trauma can shape the way an individual experiences the world and interacts with other people. Touch is a key component of social interactions, but surprisingly little is known about how trauma exposure influences the processing of social touch. In this review, we examine possible neurobiological pathways through which trauma can influence touch processing and lead to touch aversion and avoidance in trauma-exposed individuals. Emerging evidence indicates that trauma may affect sensory touch thresholds by modulating activity in the primary sensory cortex and posterior insula. Disturbances in multisensory integration and oxytocin reactivity combined with diminished reward-related and anxiolytic responses may induce a bias towards negative appraisal of touch contexts. Furthermore, hippocampus deactivation during social touch may reflect a dissociative state. These changes depend not only on the type and severity of the trauma but also on the features of the touch. We hypothesise that disrupted touch processing may impair social interactions and confer elevated risk for future stress-related disorders.

Phantom Illusion Based Vibrotactile Rendering of Affective Touch Patterns

Physically accurate (authentic) reproduction of affective touch patterns on the forearm is limited by actuator technology. However, in most VR applications a direct comparison with actual touch is not possible. Here, the plausibility is only compared to the user's expectation. Focusing on the approach of plausible instead of authentic touch reproduction enables new rendering techniques, like the utilization of the phantom illusion to create the sensation of moving vibrations. Following this idea, a haptic armband array (4x2 vibrational actuators) was built to investigate the possibilities of recreating plausible affective touch patterns with vibration. The novel aspect of this work is the approach of touch reproduction with a parameterized rendering strategy, enabling the integration in VR. A first user study evaluates suitable parameter ranges for vibrational touch rendering. Duration of vibration and signal shape influence plausibility the most. A second user study found high plausibility ratings in a multimodal scenario and confirmed the expressiveness of the system. Rendering device and strategy are suitable for a various stroking patterns and applicable for emerging research on social affective touch reproduction.