Development of a test for spatial working memory in Parkinson's disease: sensitivity to medication induced periodic performance changes

Introduction: Spatial working memory is susceptible to impairment early on in Parkinson’s disease (PD). Later on, the wearing off phenomenon of motor function during dopaminergic treatment seems to carry over into working memory.

Aim: To initialize development of a test capable of capturing the variation in spatial working memory within a patient’s daily functioning.

Methods: The test consisted of 192 instances of n-back, yes/no tasks, with correctness and latency of response automatically recorded. We collected data from two persons, one PD patient and one healthy control, relatively evenly sampled regarding time-of-day. The patient took levodopa once every three hours and performed tests on 70 occasions (the healthy control on 33 occasions). A frequency band containing period lengths from 2 to 4 hours was defined as the region of interest in regard to the medication cycle. The resulting  time  series  were  digitally  band-pass  filtered allowing only the relevant frequencies to pass. Reasoning that filtering would preserve essential information if the time series were periodic, but eradicate it if they were not, we used as our primary outcome measure the correlations between filtered and unfiltered data.

Results: Correlations between the pass-band relating to the levodopa intake schedule and the full data were positive and significantly larger for the patient than for the control. These differences applied to both latency and accuracy.

Conclusion: This supports the hypothesis that the test is able to detect levodopa-dependent variations in spatial working memory.