Processing Speed and Working Memory in Multiple Sclerosis: Comparison of the Relative and Independent Consequence Models
Anderson, Carolyn Rae
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Objective: The relationship between reduced processing speed and impaired working memory commonly noted in patients with multiple sclerosis (MS) is currently unclear. DeLuca et al. (2004) proposed two models to explain this relationship: the Relative Consequence Model, which states that impairments in working memory are secondary to reduced processing speed, and the Independent Consequence Model, which states that these deficits are independent of one another. Research examining the application of these models has been inconsistent, perhaps due to sex differences complicating this relationship, as well as varying methodologies and statistical techniques used among studies. The present study was designed to employ a novel statistical technique, path analysis, to directly compare these models using the experimental n-back task to parse processing speed and working memory abilities. The role of sex in explaining processing speed and working memory impairments in MS was also investigated. Primary hypotheses were that MS patients would demonstrate impaired processing speed and working memory relative to healthy controls, while analyses comparing the models were exploratory. Secondary hypotheses were that relative to women with MS, men with MS would exhibit impaired working memory along with slowed simple and complex processing speed on the n-back task. Design: Forty-five MS patients and 29 healthy controls completed the n-back task as part of a larger neuropsychological battery. Questionnaires addressing fatigue and depression were also completed. Results: After controlling for simple processing speed, MS patients did not demonstrate impaired processing speed or working memory relative to controls. Path models demonstrated that, for MS patients, processing speed both directly and indirectly predicted working memory performance on the n-back task. The importance of examining task approach was also demonstrated (i.e., whether people performed the task as instructed), as this partially mediated the relationship between processing speed and working memory. Sex of participant was not predictive of processing speed or working memory performance. Conclusions: Results of the present study provide support for the Relative Consequence Model, as processing speed was found to predict working memory performance. Future research should explore rehabilitative strategies that may help to alleviate problems caused by reduced processing speed in MS patients.