The link between depression and performance on the Symbol Digit Modalities Test: Mechanisms and clinical significance

Objective: To determine the mechanism and clinical significance of depression-related differences in performance on the Symbol Digit Modalities Test (SDMT). Methods: The influence of depression on two versions of a computerized SDMT (i.e. fixed versus variable code) was assessed. Both versions involve processing speed, but the fixed c-SDMT also encompasses incidental visual memory. Results: Depression was associated with a 19.06% slowing on the variable (p = 0.002) and an 8.10% slowing on the fixed (p = 0.219) c-SDMT. Conclusion: Depression-associated differences in performance on the SDMT appear linked more to a reduction in processing speed than a decline in incidental visual memory and exceed the 10% threshold considered clinically significant.