Cognitive processing speed deficits in multiple sclerosis: Dissociating sensorial and motor processing changes from cognitive processing speed.

BACKGROUND: The assessment of cognitive information processing speed (IPS) is complicated in MS, with altered performance on tests such as the Symbol Digit Modalities Test (SDMT) potentially representing changes not only within cognitive networks but in the initial sensorial transmission of information to cognitive networks, and/or efferent transmission of the motor response. OBJECTIVE: We aimed to isolate and characterise cognitive IPS deficits in MS using ocular motor tasks; a prosaccade task (used to assess and control for sensorial and motor IPS) which was then used to adjust performance on the Simon task (cognitive IPS). METHODS: All participants (22 MS patients with early disease, 22 healthy controls) completed the ocular motor tasks and the SDMT. The Simon task assessed cognitive IPS by manipulating the relationship between a stimulus location and its associated response direction. Two trial types were interleaved: (1) congruent, where stimulus location = response direction; or (2) incongruent, where stimulus location ≠ response direction. RESULTS MS patients did not perform differently to controls on the SDMT. For OM tasks, when sensorial and motor IPS was controlled, MS patients had significantly slower cognitive IPS (incongruent trials only) and poorer conflict resolution. SDMT performance did not correlate with slower cognitive IPS in MS patients, highlighting the limitation of using SDMT performance to interpret cognitive IPS changes in patients with MS. CONCLUSION: Cognitive IPS deficits in MS patients are dissociable from changes in other processing stages, manifesting as impaired conflict resolution between automatic and non-automatic processes. Importantly, these results raise concerns about the SDMT as an accurate measure of cognitive IPS in MS.