The default mode network and related right hemisphere structures may be the key substrates of dementia.

We have employed structural equation models to explicitly distinguish dementia-relevant variance in cognitive task performance (i.e., δ) from the variance that is unrelated to a dementing process (i.e., g'). Together g' and d comprise Spearman's "g". Although d represents only a minor fraction of the total variance in cognitive task performance, it is more strongly associated with dementia severity than is g'. In this analysis, we replicate δ in a new dataspace, the University of Kansas Brain Aging Project, and associate it specifically with regional grey matter atrophy by voxel-based morphometry of magnetic resonance imaging data. The latent variable d localizes to elements of the default mode network and related structures in the R hemisphere.