Frontal activity during the digit symbol substitution test determined by multichannel near-infrared spectroscopy

The digit symbol substitution test (DSST) is a clinically useful and widely accepted tool for the detection of various psychiatric disorders. Investigating neural activity during the DSST is useful when considering the relationship between the poor performance on the DSST and neurocognitive deficits. However, obtaining reliable functional imaging of the neural mechanisms associated with this test is challenging due to motion artifacts.

To circumvent this problem, we examined frontal lobe activity during the DSST using multichannel near-infrared spectroscopy, a noninvasive functional imaging technique that does not interfere with the DSST procedure.

Twenty-five healthy volunteers were enrolled in this study. Changes in the concentration of oxygenated hemoglobin (oxyHb) during the DSST were determined bilaterally in 52 measurement points (channels) on the frontal area.

We found significant increases in oxyHb in more than 70% of the channels, with the intensity of the increase being more pronounced in the left hemisphere. Several channels showed significant positive correlations between changes in oxyHb and DSST performance. Some of the channels with a significant increase in oxyHb during the DSST did not show a correlation with the DSST performance.

Our findings indicate that the DSST could prove useful as a frontal lobe stimulating task. Further examinations of DSST/near-infrared spectroscopy analyses of neural mechanisms in patients with psychiatric and neurological diseases are necessary to assess its effectiveness in clinical practice for the evaluation of neuropsychopathology.