The Symbol-Digit Modalities Test in Mild Cognitive Impairment: Evidence from Parkinson's Disease Patients.

BACKGROUND: The evaluation process of the performance of the symbol-digit modalities test (SDMT) has focused much on numerical scores paying only little attention to the qualitative aspects of performance. Incorporating the gaze analysis technique, we aimed to investigate the performance of Parkinson's disease (PD) patients on the written SDMT task.

METHODS: Twelve patients with PD and normal cognition (PD-N), 11 with PD and mild cognitive impairment (PD-MCI), and 13 healthy participants (NC) controlled for age, sex and education were recruited.

RESULTS: PD-MCI participants achieved significantly lower scores than NC and PD-N participants. Eye-movement parameters, however, did not differ among the study groups, and were not correlated with task performance.

CONCLUSIONS: Impaired performance on the SDMT by PD-MCI participants despite relatively preserved oculomotor performance indicates that lower SDMT scores are not due - even in part - to visuomotor impairments otherwise seen in PD patients.