Quiet Connections: Reduced Fronto-Temporal Connectivity in Nondemented Parkinson's Disease During Working Memory Encoding

Parkinson's disease ( PD) is a common neurodegenerative disorder characterized primarily by motor symptoms such as bradykinesia, muscle rigidity, and resting tremor. It is now broadly accepted that these motor symptoms frequently co-occur with cognitive impairments, with deficits in working memory and attention being among the most common cognitive sequelae associated with PD. While these cognitive impairments are now recognized, the underlying neural dynamics and precise regions involved remain largely unknown. To this end, we examined the oscillatory dynamics and interregional functional connectivity that serve working memory processing in a group of unmedicated adults with PD and a matched group without PD. Each participant completed a high-load, Sternberg-type working memory task during magnetoencephalography ( MEG), and we focused on the encoding and maintenance phases. All data were transformed into the time-frequency domain and significant oscillatory activity was imaged using a beamforming approach. Phase-coherence ( connectivity) was also computed among the brain subregions exhibiting the strongest responses. Our most important findings were that unmedicated patients with PD had significantly diminished working memory performance ( i.e., accuracy), and reduced functional connectivity between left inferior frontal cortices and left supramarginal-superior temporal cortices compared to participants without PD during the encoding phase of working memory processing. We conclude that patients with PD have reduced neural interactions between left prefrontal executive circuits and temporary verbal storage centers in the left supramarginal/ superior temporal cortices during the stimulus encoding phase, which may underlie their diminished working memory function. (C) 2016 Wiley Periodicals, Inc.