Disentangling phonology from phonological short-term memory in Alzheimer's disease phenotypes

Impaired phonological short-term memory is a core feature of the logopenic variant of primary progressive aphasia (lvPPA), but it is not clear whether a core phonological processing deficit is also present.

We asked three questions: (i) beyond short-term memory impairment, do lvPPA patients have an impairment within phonology itself?; (ii) is their performance in working memory and naming reflective of this phonological impairment?; and (iii) is their repetition performance related to structural and functional differences in key language-dominant regions? We compared non-word and word repetition and short-term memory performance in patients with typical Alzheimer's disease (tAD), lvPPA per consensus criteria, and others who previously satisfied definitions of lvPPA but had progressed with multi-domain cognitive impairments (lvPPA+).

Bayesian analyses revealed no group differences in phonological tasks of word and non-word repetition. We found very strong evidence for an effect of self-reported hearing loss on word and non-word repetition, but not multi-syllabic word/phrase repetition. A comparison of phonological versus working memory and naming tasks produced either no evidence or evidence for no correlation. Beyond the expected grey matter reductions in patients relative to controls, there was anecdotal evidence for an association between non-word repetition and functional connectivity between dorsal premotor and posterior superior temporal gyrus regions in patients.

Our results indicated that, in the absence of self-reported hearing loss, patients did not exhibit impairments in tasks tapping "pure" phonological processing. Our results suggest that instead of having a core phonological impairment, lvPPA patients have a working memory/buffering impairment.