Sensor Location Optimization of Wireless Wearable fNIRS System for Cognitive Workload Monitoring Using a Data-Driven Approach for Improved Wearability

Abstract

Functional Near-Infrared Spectroscopy (fNIRS) is a hemodynamic modality in human cognitive workload assessment receiving popularity due to its easier implementation, non-invasiveness, low cost and other benefits from the signal-processing point of view. Wearable wireless fNIRS systems used in research have promisingly shown that fNIRS could be used in cognitive workload assessment in out-of-the-lab scenarios, such as in operators' cognitive workload monitoring. In such a scenario, the wearability of the system is a significant factor affecting user comfort. In this respect, the wearability of the system can be improved if it is possible to minimize an fNIRS system without much compromise of the cognitive workload detection accuracy. In this study, cognitive workload-related hemodynamic changes were acquired using an fNIRS system covering the whole forehead, which is the region of interest in most cognitive workload-monitoring studies. A machine learning approach was applied to explore how the mean accuracy of the cognitive workload classification accuracy varied across various sensing locations on the forehead such as the Left, Mid, Right, Left-Mid, Right-Mid and Whole forehead. The statistical significance analysis result showed that the Mid location could result in significant cognitive workload classification accuracy compared to Whole forehead sensing, with a statistically insignificant difference in the mean accuracy. Thus, the wearable fNIRS system can be improved in terms of wearability by optimizing the sensor location, considering the sensing of the Mid location on the forehead for cognitive workload monitoring.

Keywords: Functional Near-Infrared Spectroscopy (fNIRS); cognitive workload monitoring; hemodynamics; linear SVM; machine learning; sensor location optimization; wireless wearable fNIRS.

Figure 1

Graphical abstract of the study.

Figure 2

Positional 2-back test. Each event is 2 s long, and the task state lasts for 48 s. Afterward, a 25 s Rest state followed, when the subjects did not move and remained visually affixed to the blank computer screen.

Figure 3

(a) Photodetector placement and channel positions. All the distances between the detector and LED are the same, 3 cm. The distances between the adjacent detectors are 5.5 cm horizontally and 4.5 cm vertically. Similarly, the distance between adjacent LEDs is 5.5 cm. (b) Sensitivity map for the depicted channel arrangement.

Figure 4

Mean accuracy of classifications across various location for different segmentation window lengths. The standard errors of the mean classification accuracies are presented by the error bars. The four classification accuracy means whose differences are statistically significant are highlighted for significance with stars.