Near-Infrared Spectroscopy as a Useful Research Tool to Measure Prefrontal Cortex Activity During Visually Demanding Near Work

Abstract

OCCUPATIONAL APPLICATIONS Effort, which overrides the need for rest and recuperation to avoid deterioration in visual performance, is commonplace in many contemporary workplaces. The prefrontal cortex is capable of allocating more mental/physical resources to carry out a given work task in the presence of fatigue. However, adverse musculoskeletal and mental health consequences are an anticipated outcome of this since fatigue is a physiological sign of the need for recuperative rest. This article outlines the current evidence suggesting that probing into prefrontal brain activity with near-infrared spectroscopy could advance progress in visual ergonomic research, and pilot data are provided that exemplify the proposed approach. Improved understanding of the age-related health consequences of straining the brain and/or the eyes “to see well” will likely help ergonomic stakeholders find better methods to meet the needs of safer and more productive work environments.

TECHNICAL ABSTRACT Background: Unlike the usual skeletal muscles, ciliary muscles responsible for focusing the crystalline eye lens and extraocular muscles responsible for convergence eye movements appear resistant to fatigue. Purpose: The dual goals of this article are to briefly outline the current evidence that suggests that probing into blood flow and hemodynamic prefrontal brain activity with near-infrared spectroscopy could advance progress in visual ergonomic research and to provide pilot data exemplifying the proposed approach. Methods: The vision task consisted of sustained focusing on a contrast-varying black-and-white Gabor grating. Four participants with a median age of 46 (interquartile range 44–50) fixated the grating from a distance of 65 cm. Three counterbalanced 10-minute tasks required central fixation and accommodation/convergence on the grating target through (i) 0.0 diopter lenses, (ii) −1.5 diopter lenses, and (iii) −3.5 diopter lenses while maintaining maximal focus. Non-invasive measurements of local oxyhemoglobin (HbO₂) were quantified with a one-channel near-infrared spectrometer. The near-infrared spectroscopy probe was placed on the prefrontal cortex in the vicinity of the right dorsolateral prefrontal cortex or Brodmann area 46. Accommodation response and contrast threshold were measured in parallel. Results: General estimating equation analyses showed that baseline-subtracted dorsolateral prefrontal cortex blood flow (ΔHbO₂) increased significantly over time in all three lens conditions. The effect of time may be caused by a continuous increase in mental effort to compensate for progressively more mental fatigue induced by increased visual attention. The increase of dorsolateral prefrontal cortex ΔHbO₂ was also larger in magnitude in participants with larger amplitudes accommodation response (i.e., in participants who minimized deterioration in visual performance). Conclusion: The results from this study indicate that oxyhemoglobin changes recorded over the dorsolateral prefrontal cortex with near-infrared spectroscopy can be used to assay the degree to which the visual system is strained during demanding near work.

KEYWORDS:

• Accommodation
• attention fatigue
• compensatory effort
• discomfort
• electromyography
• mental fatigue
• near-infrared spectroscopy
• neck pain
• productivity
• visual ergonomics

ACKNOWLEDGMENT

Barbara Rosborg is acknowledged for her skilled proofreading.

Funding

This study was supported by grants from the Swedish Council for Working Life, Social Research Grant 2009–1761, to author Richter.