Objective: To assess alterations in functional brain activation patterns related to mental slowing in patients with Parkinson’s disease (PD).

Background: Attentional deficits, namely mental slowing, are frequent in PD. However, the mechanisms at the origin of mental slowing have been poorly investigated in PD. A MRI-compatible version of the Symbol Digit Modalities Test (SDMT) has been developed to explore these mechanisms in healthy individuals but has never been used in PD.

Method: 17 PD patients and 11 sex- and age-matched healthy controls (HC) were enrolled. Patients were assessed after receiving their usual antiparkinsonian treatment. Subjects were scanned with a 3T Philips Achieva (Philips, Netherlands). High- resolution 3D T1-weighted images were acquired as well as BOLD images during the SDMT activation sequence. The task consisted of 3 blocks of 48 stimuli. Each stimulus represented a SDMT item where at the top, an answer key with nine symbols and their correspondent numbers was presented and at the bottom a symbol/number combination. The subject was asked to indicate, by pressing a button on a joystick, whether or not the symbol/number combination was right or false. SDMT-related brain networks were extracted from one-sample T-test maps for HC and PD. In each hemisphere, regions were identified by selecting 100 voxels around the peak of each significant cluster (p<0.05 after correction for the false discovery rate), additional ROI analyses were performed using MarsBaR.

Results: Mean score at the MoCA was 26.4 (4.1), mean SDMT performance was 42.9 (18.4) correct responses. Performance at both versions of the SDMT versions highly correlated (r=0.920). When performing the SDMT, both groupes displayed activation in frontal, parietal and occipital regions which were previously shown to be involved in attention. In PD, activation was lower in several parts of the cerebellum, bilateral occipital cortex and in the right supramarginal gyrus, compared with HC.

Conclusion: Mental slowing is observed even in cognitively-intact PD patients. This is related to an alteration of the brain activation patterns when performing task assessing speed of processing. Our results suggest an involvement of the right supramarginal gyrus, an important interface intergrating multiple information, in mental slowing in PD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/functional-mri-activation-during-the-symbol-digit-modalities-test-in-parkinsons-disease/