Background: Mechano-receptive C-fiber (MR-CF) stimulation via slow stroking of C-fiber rich skin areas can be used to probe the relationship between reward and interoception. Individuals with substance use disorders show impaired reward processing, and dysfunctional interoceptive processing of MR-CF may contribute to this dysfunction. This study predicted that methamphetamine dependent (MD) individuals would exhibit altered responses to MR-CF stimulation in brain regions important for interoception.
Methods: Recently abstinent MD (n=25) and comparison (CTL, n=17) subjects received a pleasant interoceptive stimulus ("Soft Touch" consisting of a slow brush stroke) to the palm or forearm during functional magnetic resonance imaging. Subjects were provided with cues signaling stimulation to examine anticipatory and stimulus-related processing. Subjective responses were measured using visual analog scales (VAS).
Results: Groups were similar on behavioral performance and ratings of the interoceptive stimuli, yet MD exhibited lower anterior insula, dorsal striatum, and thalamus activation than CTL, across anticipation and soft touch conditions. The lower the anterior insula activation, the faster the reaction time across conditions in MD, whereas the opposite pattern was evident in CTL. Striatal activation in MD was greater than CTL during anticipation, but lower during soft touch. Greater striatal attenuation was associated with higher VAS pleasantness ratings of soft touch.
Conclusions: MD expend fewer brain processing resources during soft touch, a form of positively-valenced interoceptive stimuli, in brain areas that are important for both interoception and reward. Future studies will ascertain if sustained abstinence from methamphetamine use can normalize aberrant neural interoceptive processing.
Keywords: Interoception; Methamphetamine; Reward; fMRI.
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