Sensitivity and vagal reactivity to C-tactile-mediated affective touch in mild cognitive impairment due to Alzheimer's disease

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Abstract
Background: C-tactile (CT) afferents preferentially activate in response to slow caress-like touch, evoking a diffuse pleasant sensation and promoting autonomic regulation. According to Braak's classic model, the neurodegenerative process in Alzheimer's disease (AD) only affects somatosensory cortices in advanced disease stages; however, the processing of complex sensory stimuli, such as CT-mediated Affective Touch, may be impacted in earlier phases. Objective: The present study aimed to assess the behavioral and psychophysiological response to CT-targeted touch in a group of biomarker-tested patients with mild cognitive impairment (MCI) due to AD, compared to a group of age, sex, and education matched controls. Methods: The preference for CT-optimal stimulations (3 cm/s) over non-CT-optimal stimulations (30 cm/s), in terms of perceived pleasantness, was first assessed in both groups. The behavioral (i.e., perceived pleasantness) and psychophysiological (i.e., heart rate and heart rate variability) responses to 16 min of continuous CT-targeted stimulations were then evaluated. All participants also underwent quantitative sensory testing for the screening of tactile sensitivity, tactile acuity, nociception, and interoception. Results: Despite both groups rating CT-optimal stimulations as more pleasant than non-CT-optimal ones, MCI patients specifically exhibited an altered response to prolonged CT-targeted stimulations, characterized by a progressive reduction of their perceived pleasantness and by an unvaried Heart Rate Variability. No group-differences emerged in other somatosensory-related functions. Conclusions: A specific multi-level alteration of the pleasant and soothing features of CT-optimal touch was identified as a prodromal marker of AD, providing the first indications of how the CT system degrades in conditions of pathological aging.

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