Effects of the combined letter fluency task on frontal cortex regional and dynamic oxygenation patterns

Verbal fluency tests (VFTs) are commonly used tasks for analyzing frontal cortex activation imagined by functional near-infrared spectroscopy (fNIRS). Some new VFT versions have been developed to strengthen the test's executive character. However, these modified tasks were not used to recognize whether they activate cortical areas significantly differently compared with typical tasks. We introduced a modified VFT version consisting of the necessity to produce words according to one phonemic criterion and exclude others according to another phonemic criterion to establish frontal cortex task-dependent hemodynamic activity for the proposed new task. The analysis was conducted by comparison with a typical initial letter fluency task in a group of 35 students. Behavioral results showed between-tasks differences in productivity during the initial phase of performance. Increased oxygenated hemoglobin (oxy-Hb) concentration in lateral and frontopolar regions was noted in both analyzed tasks, but oxy-Hb suppression has been observed in the middle/superior frontal cortex only for the new one. Such an arrangement of selective metabolic activation and inactivation suggests that the brain's adaptation to increased processing demands may consist of restraining the oxygen intake of the neuronal region, which activity is not necessary to perform a task.