Temporal Lobe and Frontal-Subcortical Dissociations in Non-Demented Parkinson’s Disease with Verbal Memory Impairment

OBJECTIVE The current investigation examined verbal memory in idiopathic non-dementia Parkinson's disease and the significance of the left entorhinal cortex and left entorhinal-retrosplenial region connections (via temporal cingulum) on memory impairment in Parkinson's disease. METHODS Forty non-demented Parkinson's disease patients and forty non-Parkinson's disease controls completed two verbal memory tests--a wordlist measure (Philadelphia repeatable Verbal Memory Test) and a story measure (Logical Memory). All participants received T1-weighted and diffusion magnetic resonance imaging (3T; Siemens) sequences. Left entorhinal volume and left entorhinal-retrosplenial connectivity (temporal cingulum edge weight) were the primary imaging variables of interest with frontal lobe thickness and subcortical structure volumes as dissociating variables. RESULTS Individuals with Parkinson's disease showed worse verbal memory, smaller entorhinal volumes, but did not differ in entorhinal-retrosplenial connectivity. For Parkinson's disease entorhinal-retrosplenial edge weight had the strongest associations with verbal memory. A subset of Parkinson's disease patients (23%) had deficits (z-scores < -1.5) across both memory measures. Relative to non-impaired Parkinson's peers, this memory-impaired group had smaller entorhinal volumes. DISCUSSION Although entorhinal cortex volume was significantly reduced in Parkinson's disease patients relative to non-Parkinson's peers, only white matter connections associated with the entorhinal cortex were significantly associated with verbal memory performance in our sample. There was also no suggestion of contribution from frontal-subcortical gray or frontal white matter regions. These findings argue for additional investigation into medial temporal lobe gray and white matter connectivity for understanding memory in Parkinson's disease.