The Validation of a Portable Functional NIRS System for Assessing Mental Workload

Abstract

Portable functional near-infrared spectroscopy (fNIRS) systems have the potential to image the brain in naturalistic settings. Experimental studies are essential to validate such fNIRS systems. Working memory (WM) is a short-term active memory that is associated with the temporary storage and manipulation of information. The prefrontal cortex (PFC) brain area is involved in the processing of WM. We assessed the PFC brain during n-back WM tasks in a group of 25 college students using our laboratory-developed portable fNIRS system, WearLight. We designed an experimental protocol with 32 n-back WM task blocks with four different pseudo-randomized task difficulty levels. The hemodynamic response of the brain was computed from the experimental data and the evaluated brain responses due to these tasks. We observed the incremental mean hemodynamic activation induced by the increasing WM load. The left-PFC area was more activated in the WM task compared to the right-PFC. The task performance was seen to be related to the hemodynamic responses. The experimental results proved the functioning of the WearLight system in cognitive load imaging. Since the portable fNIRS system was wearable and operated wirelessly, it was possible to measure the cognitive load in the naturalistic environment, which could also lead to the development of a user-friendly brain–computer interface system.

Load-Dependent Prefrontal Cortex Activation Assessed by Continuous-Wave Near-Infrared Spectroscopy during Two Executive Tasks with Three Cognitive Loads in Young Adults

“…Finally, the sample size for the present study was quite small. Although this sample size is similar or even greater than the one of the previous studies using a similar protocol [7,8,[16][17][18][19][20], our study may lack statistical power, and future studies should use a larger sample size to validate the robustness and generalizability of the present results.…”

“…The sample size estimation was based on the reviewed literature reported in Section 1 , which examined the prefrontal hemodynamic activity using fNIRS during the performance of n-back or RNG tasks [ 7 , 8 , 16 , 17 , 18 , 19 , 20 ]. Nineteen right-handed young adults participated in the study.…”

“…Indeed, the hemodynamic activity of the PFC was shown to be bilateral [ 7 ], but with a strong activation of the left hemisphere, particularly for the verbal version of the n-back task [ 14 ]. In addition, other studies have shown that the hemodynamic activity of the PFC increases substantially with an increasing working memory load [ 15 , 16 , 17 ]. According to Vermeij and collaborators, the hemodynamic activity of the right PFC in young participants was slightly dominant at low levels of working-memory load (0-back, 1-back), while it became more bilateralized at a higher cognitive load (e.g., two-back) [ 17 ].…”

“…According to Vermeij and collaborators, the hemodynamic activity of the right PFC in young participants was slightly dominant at low levels of working-memory load (0-back, 1-back), while it became more bilateralized at a higher cognitive load (e.g., two-back) [ 17 ]. On the contrary, Saikia and collaborators also showed an increased bilateral PFC activation as a function of an increasing cognitive load (from one-back to three-back) and reported a slightly greater overall activation over the left PFC [ 16 ]. Further studies should address these discrepancies, acknowledging that these two fNIRS studies did not differentiate between the DLPFC and VLPFC.…”

“…The effect of the cognitive load on the hemodynamic activity has been widely demonstrated in the n-back task using fNIRS [ 7 , 8 , 15 , 16 , 17 , 19 , 20 ] and fMRI [ 21 ], and in others working memory tasks, using fMRI [ 22 , 23 ]. For the most part, these studies relied on neurocognitive models describing the augmented hemodynamic activity in supplementary brain areas other than the preliminary specialized ones, as a function of an increasing cognitive load e.g., [ 24 , 25 , 26 ].…”