Incidental learning during rapid information processing on the symbol-digit modalities test

The Symbol--Digit Modalities Test (SDMT) is widely used to assess processing speed in MS patients. We developed a computerized version of the SDMT (c-SDMT) that scored participants' performance during subintervals over the course of the usual 90-s time period and also added an incidental learning test (c-ILT) to assess how well participants learned the symbol-digit associations while completing the c-SDMT. Patients with MS (n = 65) achieved lower scores than healthy controls (n = 38) on both the c-SDMT and c-ILT, and the scores on the two tests were correlated. However, no increase in the rate of item completion occurred for either group over the course of the c-SDMT, and the difference between groups was the same during each subinterval. Therefore, it seems implausible that controls completed more items on the c-SDMT because they were more adept at learning the symbol-digit associations as the test ensued. Instead, MS patients' poorer incidental learning performance appears to reflect the greater attentional burden that tasks requiring rapid serial processing of information impose upon them.