fNIRS used to investigate mental fatigue from traumatic brain injury

Simon Skau, PhD student at Gothenburg University and MedTech West, is the main author of an article published in the scientific journal Frontiers in Human Neuroscience in April, 2019. The article with the title ”Mental fatigue and functional near-infrared spectroscopy (fNIRS)-based assessment of cognitive performance after mild traumatic brain injury” is about pathological mental fatigue after mild traumatic brain injury (TBI-MF), which is characterized by pronounced mental fatigue after cognitive activity. The neurological origin of the mental fatigue is unknown, and this study was aiming to investigate how prolonged mental activity affects cognitive performance and it´s neural correlates in individuals with TBI-MF.

In this study, Simon Skau and his collegues, Lina Bunketorp-Käll, Hans-Georg Kuhn och Birgitta Johansson, examined 20 individuals that were identified by neurologists because they were still suffering from mental fatigue at least 5 months after having a mild traumatic brain injury (TBI-MF). They also examined 20 age-matched healthy individuals. The technology used was functional near-infrared spectroscopy (fNIRS). A battery of six neuropsychological tests was used and then repeated once. Altogether, the test lasted for approx. 2 ½ hours. The self-assessed mental energy level was measured before and after the experiment.

Simon Skau, what´s fNIRS?
– We have a lab at MedTech West where the fNIRS equipment is normally set up, so that´s where this research was conducted during 2016-2017. Functional Infrared Spectroscopy (fNIRS) is a non-invasive functional brain imaging technique that uses harmless infrared light to investigate activity in the brain or other tissues by assessing hemodynamic changes in the frontal cortex. fNIRS measures the same physiological signal as fMRI, but since fNIRS uses light, it doesn’t have the same problems with disturbances as fMRI. The fNIRS signals can only be disturbed by other light sources, which is easy to avoid. fNIRS is not a new technology, but it has developed a lot in recent years. In addition to the fNIRS device, glass fiber optics are required which are attached to the head. Half of the optodes transmit infrared light on separate wavelengths, which gets captured by the remaining optodes. By detecting the light, we can see the brain activity where the light traveled. We measure ten times per second. Right now the measuring doesn´t go deeper than two centimeters into the cerebral cortex. The fNIRS equipment has a significantly lower cost than both fMRI and PET. It´s very easy to move the equipment to different locations, and it requires very little from the premises. While fNIRS has better temporal resolution than fMRI, spatial resolution is lower.

Birgitta Johansson, what is mental fatigue?
– Mental fatigue is a pathological lack of energy mainly evident during mental activities, and which one do not recover from even by resting or sleeping. It´s common after diseases and injuries that affects the brain, such as traumatic brain injury, stroke, multiple sclerosis and other neurological disorders. It´s also common during long-term stress, e.g. burnout. Mental fatigue can be a huge problem during all forms of work and studies, but it can also be a big obstacle in everyday life, for example when participating in conversations or when in situations where attention or multitasking is needed.

Those who suffer from metal fatigue are drained on their energy abnormally fast and it requires an abnormally long recovery time to get back to the starting position- which was already low. This means that they cannot be active for as long, or as often, as before, because in order to live a normal life one must be able to perform activities over and over again.

How is it cured?
– Today, we lack effective treatment, and rehabilitation can take many years. Many patients won´t be able to ever work again. It´s easy to understand that the individual suffering is big, but the costs for society are also very large.

What causes it?
– We currently don´t know enough about what causes mental fatigue, but our theory is that the supply of energy isn´t functioning well and the energy balance cannot be restored. In this study, we have investigated how the brain is activated in people suffering from mental fatigue after a traumatic brain injury, and how mental work over a longer time span of a total of 2.5 hours affects them. We have used functional near-infrared spectroscopy (fNIRS), which can measure superficial cortical activity while performing cognitive tests. We found that the brain’s frontal activity was clearly impaired in the mental fatigue group compared to the healthy control group. It became especially clear when the tasks became more mentally demanding. This difference between the group suffering from mental fatigue and the control group was already present at the start. In addition, the group suffering from mental fatigue was unable to improve their cognitive performance in repeating tests, which the control group did. This indicates that the brain does not work effectively for those who suffer from mental fatigue after a traumatic skull injury, which can be a cause of a perceived energy shortage after a period of mental stress.

What will happen now?
– We hope that in future studies, we will deepen our knowledge of the brain’s energy supply in mental fatigue and also get a better understanding of how the brain works. Today, very little research has been done on underlying causes of mental fatigue. Better knowledge of underlying factors for mental fatigue could contribute to the development of more effective treatment methods in the future.

Simon, what other studies are you involved in right now?
– I´m using fNIRS in several other projects. We are investigating mental fatigue caused by hypothyroidism and mental fatigue in exhaustion disorder. Another study is about whether general fatigue can affect the cognitive control among healthy subjects. We are also studying cognition among 8-9-year-old children solving mathematical problems in a school environment.

Funding

This study was founded by the Local Research and Development Council Göteborg and Södra Bohuslän. Swedish Research Council, Swedish Childhood Cancer foundation, and Swedish governmental support under the ALF agreement.