Mnemonic Similarity Task to study episodic memory in Parkinson's disease.

Introduction: Parkinson's disease (PD) patients commonly experience episodic memory impairments, which are associated with an increased risk of dementia. The Mnemonic Similarity Task (MST) is a well-validated test to investigate episodic memory changes in healthy aging and in neurodegenerative diseases but has not been studied in PD patients.

Methods: In the MST task, participants respond during a testing phase whether visualized images are "repeat", "similar", or "new", compared to images previously shown during an encoding phase. We tested 17 PD without cognitive impairment (level-II criteria), both off (PD-OFF) and on (PD-ON) dopaminergic medications; and compared PD-OFF with 17 age- and education-matched healthy controls (HC).

Results: We found no influence of dopaminergic medications nor of disease on MST reaction time for any responses ("repeat", "similar", and "new") during the test phase. However, response probabilities showed that the MST is sensitive to subtle PD-related memory impairments. Specifically, PD-OFF responded more frequently with 'repeat', instead of 'similar' during lure trials, compared to HC ( = 0.030). This finding was still significant after correcting for response bias using the Recognition Index ( = 0.005).

Conclusions: PD patients perform the MST without interference from bradykinesia or other PD-related motor symptoms. Our findings suggest that PD patients who do not meet criteria for mild cognitive impairment can have subtle recall or recognition impairments, which can be identified using the MST. We propose the MST as a well-tolerated and sensitive cognitive task in future studies of episodic memory impairment and progressive memory dysfunction in people with PD.