A lower imaging response to a pleasant stimulus does not automatically mean a person experienced less pleasure. In a 2013 study of recently abstinent people with methamphetamine dependence, researchers found differences in brain activation during gentle touch while reported pleasantness and simple task accuracy did not significantly differ from controls.
Who was studied?
May and colleagues compared 25 treatment participants meeting then-used DSM-IV criteria for methamphetamine dependence with 17 healthy comparison participants. The dependence group included 19 men and six women; the comparison group included ten men and seven women.
The treatment participants were required to be 15–120 days sober, with a reported mean of approximately 41 days. No participants reported withdrawal symptoms during study participation. This was a historical observational comparison during abstinence, not an experiment administering a measured methamphetamine dose.
The stimulus and the task were different components
Researchers used a soft brush to stroke marked areas on the forearm or palm at about two centimeters per second. Visual cues indicated when touch was approaching. During imaging, participants also responded to arrows pointing left or right in a simple continuous-performance task.
The design separated anticipation from receipt of touch and compared the two skin locations. Functional MRI measured blood-oxygenation-level-dependent responses. Participants rated the experience using visual analog scales, including pleasantness and intensity.
The hypothesized difference in pleasantness was not found
| Measured endpoint | Reported finding |
|---|---|
| Subjective pleasantness | No significant group difference |
| Subjective intensity | No significant group difference |
| Arrow-task accuracy | No significant group difference |
| Regional imaging responses | Group differences in selected regions and conditions |
The investigators had expected the dependence group to describe the touch as less pleasant. That prediction was not supported. A summary saying participants “could not enjoy normal touch” would replace a null subjective finding with an unsupported behavioral claim.
The simple arrow task also did not establish an overall accuracy deficit. Imaging differences and measured task performance should be reported separately.
The imaging findings depended on the phase
Across the examined conditions, the dependence group showed lower activation in regions including anterior insula, dorsal striatum and thalamus. The striatal comparison also changed with task phase: activation was higher in the dependence group during anticipation but lower during touch itself.
That phase-specific pattern does not fit a claim that every reward-related region was simply less active at all times. The three-way interaction involving group, phase and skin location was not significant. The study also found no significant correlations between days of abstinence and the examined task, rating or regional activation outcomes.
Those null results do not establish that abstinence has no effect. The study observed people at one assessment, rather than repeatedly measuring recovery within each person.
Alternative explanations remain important
All treatment participants had a history of regular nicotine use, compared with 29% of controls. The groups also differed in education and histories of other substance use. These differences limit attribution of the imaging pattern specifically to methamphetamine.
The authors noted the limited repetitions of touch, the simplicity of the performance task, the predominantly male sample and the timing of subjective ratings after scanning. Such features affect what the design could detect.
What can this evidence establish in a case?
The study supports an association between treatment-group membership and selected brain responses during this experimental task. It did not validate a diagnostic scan for an individual, determine permanent brain injury, measure acute intoxication or establish driving impairment. No administered dose or blood-concentration rule can be recovered from this observational design.
Its useful lesson is the need to keep neural responses, subjective experience and objective performance distinct. A difference on one level should not be promoted into an unmeasured deficit on another.
Primary scientific source
May AC, Stewart JL, Migliorini R, Tapert SF, Paulus MP. Methamphetamine Dependent Individuals show Attenuated Brain Response to Pleasant Interoceptive Stimuli. Drug and Alcohol Dependence. 2013;131(3):238–246. doi:10.1016/j.drugalcdep.2013.05.029. Historical human observational imaging study; original commentary on the supplied manuscript.
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