Individual Variability in Speed of Information Processing: An Index of Cognitive Impairment in Multiple Sclerosis

Objective: The principal cognitive domain impacted by multiple sclerosis (MS) appears to be information processing speed. Studies have documented mean differences between MS patients and healthy controls on numerous measures of information processing speed, but the present study is one of the first to examine individual variability (i.e., inconsistency) in processing speed. Method: Thirty-nine patients with relapsing-remitting or secondary progressive MS and 32 healthy controls completed a series of reaction time (RI) tests, the Stroop Test, and the Rey Auditory Verbal Learning Test. In addition to mean differences, appropriate measures of individual variability (i.e., coefficients of variability) were obtained for the tests yielding response times. Results: MS patients performed more poorly on all response time measures, responding more slowly and with greater inconsistency than controls. Logistic regression analyses showed that inconsistency measures contributed independently to the discrimination between groups beyond that resulting from measures of mean processing speed. The so-called complexity effect was also demonstrated; greater between-groups differences on both mean RTs and inconsistency generally occurred on tasks placing greater demand on central executive resources. Conclusion: Processing speed in MS patients is characterized by greater individual variability as well as overall declines. This variability should be examined further in relation to neuroimaging indices of MS.