Does slowed processing speed account for executive deficits in multiple sclerosis? Evidence from neuropsychological performance and structural neuroimaging.

Abstract

Executive deficits and slow processing speed (PS) are observed in persons with multiple sclerosis (MS). The question of whether executive deficits can be explained by slow PS was examined with neuropsychological measures and a neurostructural measure (brain atrophy). Fifty MS patients were compared with 28 healthy controls (HCs) on tasks of executive functioning with and without a PS element (e.g., Trail Making Test and Wisconsin Card Sorting Test). The MS group performed worse than HCs on speeded tasks of executive function. However, after controlling for speed, group differences on executive tasks disappeared. There were also no group differences on executive tasks with no PS demands. The effect of disease progression on executive task performance was assessed in the MS group. Higher atrophy in MS participants was associated with greater deficits on speeded executive tasks, but this association disappeared when controlling for PS. There was no association between atrophy and performance on nonspeeded executive tasks. Our results support the notion that executive deficits in MS may be explained by slow PS. These findings highlight the role of slowed PS as a primary impairment underlying other cognitive functions. Disentangling the relative contribution of PS to executive function is an important step toward the development of appropriate rehabilitation strategies for persons with MS.