Task-load manipulation in the Symbol Digit Modalities Test: An alternative measure of information processing speed

Objective: To evaluate the utility of an oral fMRI-adapted version of the Symbol Digit Modalities Test (SDMT) to assess information processing speed (IPS) using three different interstimulus intervals (ISI). Methods: Nineteen right-handed healthy controls performed the adapted version of the SDMT, consisting of a block design that had a total of 6 control/activation block pairs with 3 different ISIs (1.5, 2 and 2.5. s) presented in two different runs: in ascending and descending orders. The brain activation patterns during different ISIs were assessed by effective functional connectivity analysis based on independent component analysis. Results: As expected, all conditions yielded activations in the fronto-parietal networks (FPNs) related to attention processes. Shorter ISIs (1.5 and 2. s) not only yielded greater patterns of connectivity within fronto-parietal and occipital regions such as the FPN and fronto-occipital network (FON), but also recruited more functional networks overall. Task performance at the shortest ISI was negatively correlated with connectivity at the FPN and activity of the pre-supplementary motor area extending to the cingulate gyrus. Conclusion: Increasing IPS demands due to shorter ISIs resulted in an increased level and number of functional networks required, increased connectivity within the FPN and FON, and enhancement of the prefrontal cortex. IPS does not arise from activity of a single b area but from affective information transfer among distant cortical regions of the frontal and parietal cortices. This adapted version of the SDMT may be useful for studying alterations of IPS in clinical and nonclinical populations.