Cognitive mediated eye movements during the SDMT reveal the challenges with processing speed faced by people with MS

BACKGROUND: The Symbol Digit Modalities Test (SDMT) is regarded as the cognitive test of choice for people with MS (pwMS). While deficits are linked to impaired processing speed, the mechanisms by which they arise are unclear. Cognitive-mediated eye movements offer one putative explanation. The objective of this study was to determine the association between eye movements and performance on the SDMT. METHODS: Thirty-three people with confirmed MS and 25 matched healthy control subjects (HC) were administered the oral SDMT while eye movements were recorded. RESULTS: Mean SDMT scores were significantly lower in pwMS (p < 0.038). Shorter mean saccade distance in the key area (p = 0.007), more visits to the key area per response (p = 0.014), and more total number of fixations in the test area (p = 0.045) differentiated pwMS from HCs. A hierarchical regression analysis revealed that the number of visits to the key area per response (p < 0.001; ΔR(2) = 0.549) and total number of fixations in the test area (p < 0.001; ΔR(2) = 0.782) were the most robust predictors of SDMT scores. CONCLUSION: Cognitive-mediated eye movements help elucidate the processing speed challenges confronted by people with MS. Mechanistic insights such as these can potentially help inform new cognitive rehabilitation strategies.