Mental workload and neural efficiency quantified in the prefrontal cortex using fNIRS

Mickaël Causse,Zarrin Chua,Natalia Del Campo,Nadine Matton,Vsevolod Peysakhovich

doi:10.1038/s41598-017-05378-x

Abstract

An improved understanding of how the brain allocates mental resources as a function of task difficulty is critical for enhancing human performance. Functional near infrared spectroscopy (fNIRS) is a field-deployable optical brain monitoring technology that provides a direct measure of cerebral blood flow in response to cognitive activity. We found that fNIRS was sensitive to variations in task difficulty in both real-life (flight simulator) and laboratory settings (tests measuring executive functions), showing increased concentration of oxygenated hemoglobin (HbO2) and decreased concentration of deoxygenated hemoglobin (HHb) in the prefrontal cortex as the tasks became more complex. Intensity of prefrontal activation (HbO2 concentration) was not clearly correlated to task performance. Rather, activation intensity shed insight on the level of mental effort, i.e., how hard an individual was working to accomplish a task. When combined with performance, fNIRS provided an estimate of the participants’ neural efficiency, and this efficiency was consistent across levels of difficulty of the same task. Overall, our data support the suitability of fNIRS to assess the mental effort related to human operations and represents a promising tool for the measurement of neural efficiency in other contexts such as training programs or the clinical setting.

Highlights

An improved understanding of how the brain allocates mental resources as a function of task difficulty is critical for enhancing human performance
The main goals of this research were to contribute to the ongoing validation of the sensitivity of Functional near infrared spectroscopy (fNIRS) measurements to various mental effort levels, and to better understand how variation in prefrontal cortex activity correlates with task performance
The results confirmed that increased task difficulty in both laboratory setting and flight simulator setting was associated with degraded performance, higher self-reported measurements of mental workload, as well as HbO2 increase and HHb decrease in the prefrontal cortex

Summary

Introduction

An improved understanding of how the brain allocates mental resources as a function of task difficulty is critical for enhancing human performance. Functional near infrared spectroscopy (fNIRS) is a relatively new and promising imaging technique that meets such measurement requirements, and the important advantage of being portable and field-deployable This technique measures the oxygenated (HbO2) and deoxygenated (HHb) hemoglobin in the blood supply of the brain, and has been shown to discriminate between various mental effort levels (e.g. Ayaz et al.[6]). An accurate measurement of the neurovascular coupling with fNIRS can be a valuable neurophysiological marker for quantifying changes in brain activity in specific areas such as the prefrontal cortex This can be achieved by linking the measure of the blood flow with the concept of mental workload, in line with previous work showing that mentally demanding tasks require resources in prefrontal-cortex-dependent functions[16,17,18,19]. It remains to be seen whether individuals who demonstrate high neural efficiency in laboratory cognitive tasks can demonstrate the same level of efficiency in real-world activities

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