Objective: To examine whether processing speed or working memory is the primary information processing deficit in persons with MS.
Design: Case-control study.
Setting: Hospital-based specialty clinic.
Participants: 215 adults with clinically definite MS.
Main outcome measure: Mean demographically corrected T-scores, prevalence rates of impairment and relative risk of impaired Processing Speed and Working Memory Index Scores from the WAIS-WMS III.
Results: Deficits in Processing Speed were much more common than Working Memory in all comparisons. This was observed for both relapsing remitting (RRMS) and secondary progressive MS (SPMS) subjects, but accentuated in the latter group.
Conclusions: Results strongly suggest that the primary information processing deficit in persons with MS is in speed of processing.