What kind of a person volunteers for a free brain scan?

When psychologists scan the brains of a group of people, they usually do so in the hope that the findings will generalise more widely. For example, if they find that there are correlations between localised brain shrinkage and mental performance in a group of healthy older participants, they will usually infer that such correlations apply in healthy older people more generally. But there's an important problem with this logic (one that applies to other fields of psychology): what if the people who volunteer for brain scans are systematically different from those who don't?

To explore this issue, Mary Ganguli and her colleagues turned to 1,982 older participants (aged 65+) who were participating in a large, long-running study into ageing. This study excluded participants who were severely mentally impaired or in long-term care.

Helpfully, the researchers already had a good deal of data from all the participants, including their demographics, health and mental skills. Next they asked the participants if they'd be interested in taking part in a free brain scan study at their local hospital in return for a cash incentive.

Nearly half the sample (46.2 per cent) stated flat out that they would not be interested. The others gave answers ranging from definitely to maybe. Those who expressed an interest in volunteering for a brain scan differed from those who were definitely not keen in many ways: the willing were more likely to be younger, male, better educated, married, employed, free from depressive symptoms, mentally fitter, subjectively healthier, on fewer meds and living unsupervised. There were no differences between the groups in terms of subjective memory concerns or ethnicity.

Next, the researchers conducted an actual brain scan on 48 of those participants who'd expressed an earlier interest. This revealed the expected correlations between grey matter volume in specific brain areas and cognitive performance.

Now the researchers made some adjustments so that the results from each brain scan participant were weighted according to how similar they were to the averaged group of 1,982 participants involved in the larger ageing study. This was a proof of principle, to see if it's possible to correct for the bias introduced by relying on volunteers rather than truly random samples. The adjustment certainly made a difference to the findings – now grey matter volume in fewer regions showed correlations with cognitive test scores, which the researchers attributed to a reduction in bias.

This isn't the most exciting brain scan study you'll read about this year, and the specific findings might only apply to older adults, but it addresses an important issue in neuroimaging and contributes to the gradual refining of psychological methods, helping our science become more reliable by avoiding biased results.

Further reading

Ganguli, M., Lee, C., Hughes, T., Snitz, B., Jakubcak, J., Duara, R., & Chang, C. (2015). Who wants a free brain scan? Assessing and correcting for recruitment biases in a population-based sMRI pilot study Brain Imaging and Behavior, 9 (2), 204-212 DOI: 10.1007/s11682-014-9297-9