Recently abstinent methamphetamine-dependent adults showed altered brain responses to gentle touch without significantly different ratings of how pleasant it felt. Lower activation should therefore not be described as proof that the participants could not experience pleasure.
May and colleagues compared 25 recently abstinent methamphetamine-dependent adults with 17 comparison participants. During functional MRI, a soft brush stroked the palm or forearm. Cues signaled when touch would arrive, allowing anticipation to be examined separately from stimulation. The investigators controlled the sensory procedure; they did not randomly assign people to methamphetamine dependence or give methamphetamine during the experiment.
The dependence group showed lower activation in regions including anterior insula, dorsal striatum and thalamus across the task. Striatal responses also depended on phase: activation was relatively higher during anticipation but lower during the touch itself. By contrast, pleasantness and intensity ratings did not significantly differ between groups. Accuracy likewise showed no significant group difference.
Functional MRI records changes related to blood oxygenation. A group difference can suggest altered processing without specifying the intensity of an individual’s experience. Here, the predicted reduction in reported pleasantness was not supported. It would be inaccurate to replace that negative behavioral result with the stronger claim that methamphetamine dependence eliminated enjoyment of ordinary sensations.
The groups differed in education and use of other substances, and the sample was predominantly male. The design cannot distinguish pre-existing characteristics from consequences of drug use or determine whether the observed pattern resolves with prolonged abstinence. It also did not measure impairment during driving or another real-world event. Its value is methodological as well as clinical: anticipation, sensory stimulation, subjective experience and neural response should remain separate outcomes when interpreting reward-related research.
See also what brain imaging measurements can and cannot represent.
Read the original study (DOI: 10.1016/j.drugalcdep.2013.05.029).