When we see individual or group differences in the magnitude of activations in brain-imaging studies, we frequently assume that these differences arise from differences in the level of brain activity in a given region. However, this assumption can be problematic. For instance, in fMRI, the magnitude of the BOLD response to visual stimulation is associated with levels of hematocrit in the blood (Levin et al., 2001). Because individuals differ in hematocrit levels, and men have higher overall hematocrit levels than women, these differences can easily confound interpretation of differences in BOLD magnitude. Attention to such variables becomes especially important if functional activations are going to be used as an assessment measure.
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