The increased specificity of multiple regression is notable for many of the tests – for example, a significant univariate association of T2* with cholesterol disappears after controlling for the other covariates. Similarly, for T2* in hippocampus and amygdala, many of the associations with age, sex, BMI and other factors become much weaker after controlling for all variables, in particular amount of head motion. Despite the fact that this motion was recorded from the functional data (not the T2* data), it is likely a general indicator of head motion, and these results illustrate why interpretation of imaging associations requires care. For example, BMI could be predictive of head motion (e.g., comfort in the scanner) while also potentially relating to biophysical parameters of deeper interest.
