Measuring prefrontal brain activity during verbal fluency tasks using functional near infrared spectroscopy.

Introduction: Tasks of verbal fluency (VF) are common neuropsychological tests used for assessing executive function, language, and processing speed to identify impairments and aid in differential diagnosis. VF is usually divided into letter fluency (LF), quickly generating words beginning with a certain letter, and semantic fluency (SF), quickly naming words belonging to a category. Although both tasks utilize some executive function and language abilities, they have unique cognitive and neuroanatomical correlates, with LF traditionally being associated with more frontal and executive patterns while SF is seen as more temporal and linguistic. The current study aims to compare LF and SF cortical activation within the prefrontal cortex (PFC) in a non-clinical, English-speaking sample to explore how these VF tasks may both be relevant to frontal systems and how this can be applied clinically. Methods: This study uses functional near-infrared spectroscopy (fNIRS), a functional imaging method that utilizes near-infrared light to detect hemodynamic changes in cortical regions of the brain, to examine PFC activation during SF and LF tasks. Twenty-six English-speaking undergraduate participants performed 3 SF and 3 LF tasks. Differences in oxygenated and deoxygenated responses were compared between SF, LF, and individual baselines. Results: Results indicate that LF tasks required more PFC anterior resources and SF required more left posterior PFC resources, with both showing increased diffuse activation when compared to baseline. Discussion: These results (1) show activation of the PFC during SF and LF tasks and (2) have implications for clinical work, including potential for combining fNIRS with cognitive tests.