Right Hemisphere Recruitment During Language Processing in Frontotemporal Lobar Degeneration and Alzheimer’s Disease

Right hemisphere recruitment of areas homotopical to affected left-sided language areas has classically been described in aphasia following stroke or brain tumors. It may also be a clinically significant mechanism in frontotemporal lobar degeneration (FTLD) and Alzheimer’s disease (AD). In a pooled analysis of previous functional magnetic resonance imaging studies of a modified version of the Pyramids and Palm Trees test, we probed the language network in 19 patients with primary progressive aphasia (nine semantic (SV) and ten agrammatic variant; neuropathologically confirmed FTLD in three cases to date), 15 patients with AD (14 clinically probable and one neuropathologically definite AD to date), and 37 healthy controls. The upper and lower bank of the left posterior superior temporal sulcus (STS) was affected in AD and the left anterior temporal pole (ATP) in primary progressive aphasia (PPA; mainly driven by SV). In the right hemisphere, the posterior STS showed an activity increase in both patient groups compared with controls. In AD, this activity increase correlated positively with naming accuracy. Both in AD and in PPA, the connection strength between right STS and right ATP was decreased compared with controls and this correlated with naming and comprehension scores, respectively. Only in PPA did the right anterior temporal pole show an activity increase, which correlated negatively with comprehension. Right-hemispheric recruitment and disconnections within the right temporal lobe may affect the degree of aphasia in cortical neurodegenerative disease.