Shortening intertrial intervals in event-related cognitive studies with near-infrared spectroscopy

Functional near-infrared spectroscopy (fNIRS) enables imaging of brain activation by measuring changes in the concentration of oxy- and deoxy-hemoglobin (Hb). Shortening the length of the intertrial interval (ITI) seems to be a precondition for further cognitive event-related fNIRS experiments because it leads to higher statistical power. Therefore, this study investigated whether the ITI may be reduced from 12, 6, 4 s to at least 2 s. Brain activation was examined with a NIRO-300 spectrometer at the lateral prefrontal cortex in 17 healthy subjects during a randomized event-related color-word matching Stroop task. In the left lateral prefrontal cortex, the concentration of deoxy-Hb decreased significantly stronger during incongruent than neutral trials for an ITI of 12, 6, and 2 s due to coping with interference. For 4 s of ITI, no hemodynamic interference effect was detected, which was paralleled by low behavioral interference. Further, we examined whether the length of the ITI influenced the mean hemodynamic response. Shortening the ITI reduced the amplitude of oxy-Hb in contrast to deoxy-Hb, which remained almost unaltered. Summarizing results, randomized event-related cognitive fNIRS studies enable short ITIs particularly if changes in deoxy-Hb are considered.