Etsuko Kanai

Effect of mental fatigue on the central nervous system: an electroencephalography study

BackgroundFatigue can be classified as mental and physical depending on its cause, and eachtype of fatigue has a multi-factorial nature. We examined the effect of mentalfatigue on the central nervous system using electroencephalography (EEG) in eighteenhealthy male volunteers.MethodsAfter enrollment, subjects were randomly assigned to two groups in a single-blinded,crossover fashion to perform two types of mental fatigue-inducing experiments. Eachexperiment consisted of four 30-min fatigue-inducing 0- or 2-back test sessions andtwo evaluation sessions performed just before and after the fatigue-inducingsessions. During the evaluation session, the participants were assessed using EEG.Eleven electrodes were attached to the head skin, from positions F3, Fz, F4, C3, Cz,C4, P3, Pz, P4, O1, and O2.ResultsIn the 2-back test, the beta power density on the Pz electrode and the alpha powerdensities on the P3 and O2 electrodes were decreased, and the theta power density onthe Cz electrode was increased after the fatigue-inducing mental task sessions. Inthe 0-back test, no electrodes were altered after the fatigue-inducing sessions.ConclusionsDifferent types of mental fatigue produced different kinds of alterations of thespontaneous EEG variables. Our findings provide new perspectives on the neuralmechanisms underlying mental fatigue.

Two types of mental fatigue affect spontaneous oscillatory brain activities in different ways

BackgroundFatigue has a multi-factorial nature. We examined the effects of two types of mental fatigue on spontaneous oscillatory brain activity using magnetoencephalography (MEG).MethodsParticipants were randomly assigned to two groups in a single-blinded, crossover fashion to perform two types of mental fatigue-inducing experiments. Each experiment consisted of a 30-min fatigue-inducing 0- or 2-back test session and two evaluation sessions performed just before and after the fatigue-inducing mental task session.ResultsAfter the 0-back test, decreased alpha power was indicated in the right angular gyrus and increased levels in the left middle and superior temporal gyrus, left postcentral gyrus, right superior frontal gyrus, left inferior frontal gyrus, and right medial frontal gyrus. After the 2-back test, decreased alpha power was indicated in the right middle and superior frontal gyrus and increased levels in the left inferior parietal and superior parietal lobules, right parahippocampal gyrus, right uncus, left postcentral gyrus, left middle frontal gyrus, and right inferior frontal gyrus. For beta power, increased power following the 0-back test was indicated in the left middle temporal gyrus, left superior frontal gyrus, left cingulate gyrus, and left precentral gyrus. After the 2-back test, decreased power was suggested in the left superior frontal gyrus and increased levels in the left middle temporal gyrus and left inferior parietal lobule. Some of these brain regions might be associated with task performance during the fatigue-inducing trials.ConclusionsTwo types of mental fatigue may produce different alterations of the spontaneous oscillatory MEG activities. Our findings would provide new perspectives on the neural mechanisms underlying mental fatigue.

Neural effects of prolonged mental fatigue: A magnetoencephalography study

Mental fatigue, manifest as a reduced efficiency for mental work load, is prevalent in modern society. It is important to understand the neural mechanisms of mental fatigue and to develop appropriate methods for evaluating mental fatigue. In this study we quantified the effect of a long-duration mental fatigue-inducing task on neural activity. We used magnetoencephalography (MEG) to examine the time course change of neural activity over the long duration of the task trials. Nine healthy male volunteers participated in this study. They performed two mental fatigue-inducing tasks on separate days. The order of task presentation was randomized in a single-blinded, crossover fashion. Each task consisted of 25-min mental fatigue-inducing 0- or 2-back task session for three times. Subjective rating of mental fatigue sensation and electrocardiogram, and resting state MEG measurements were performed just before and after each task session. MEG data were analyzed using narrow-band adaptive spatial filtering methods. Alpha band (8-13 Hz) power in the visual cortex decreased after performing the mental fatigue-inducing tasks, and the decrease of alpha power was greater when they performed 2-back task trials. The decrease in alpha power was positively associated with the self-reported level of mental fatigue sensation and sympathetic nerve activity level. These results demonstrate that performing the prolonged mental fatigue-inducing task causes overactivation of the visual cortex, manifest as decreased alpha power in this brain region. Our results increase understanding of the neural mechanisms of mental fatigue and can be used to develop new quantitative methods to assess mental fatigue.

Fatigue-Associated Alterations of Cognitive Function and Electroencephalographic Power Densities

Fatigue is a common problem in modern society. We attempted to identify moderate- to long-term fatigue-related alterations in the central nervous system using cognitive tasks and electroencephalography (EEG) measures. The study group consisted of 17 healthy male participants. After saliva samples were collected to measure copy number of human herpesvirus (HHV)-6 DNA to assess the level of moderate- to long-term fatigue, subjects were evaluated using EEG, with their eyes open for 2 min, then closed for 1 min sitting quietly. Thereafter, they completed cognitive task trials to evaluate simple selective attention for 3 min (Task 1) and conflict-controlling selective attention for 6 min (Task 2, which included Stroop trials). The percent error of Task 2 for Stroop trials was positively associated with the copy number of saliva HHV-6 DNA, although the simple selective attention measures in Task 1 did not differ significantly. EEG power densities (especially the alpha power density) during the eye-closed condition were negatively associated with the saliva HHV-6 DNA level. Impaired high-level information processing such as that required for conflict-controlling selective attention in the central nervous system may be a characteristic feature of moderate- to long-term fatigue.

Effects of daily levels of fatigue and acutely induced fatigue on the visual evoked magnetic response

Fatigue is a common complaint in modern society. As photosensitivity is associated with fatigue, this study aimed to clarify the relationship between neural response to visual stimuli and fatigue using a 160-channel whole-head-type magnetoencephalographic system. Twelve healthy male volunteers were enrolled. Participants were randomly assigned to two groups in a single-blinded, crossover fashion to perform acute fatigue-inducing mental task sessions, i.e., 0-back or 2-back test for 30 min. Visual evoked magnetic field (VEF) intensities were evaluated by standardized low-resolution brain electromagnetic tomography modified for a quantifiable method. VEF consisted of two phases, and although acute fatigue did not alter the VEF intensities and the intensities before the acute fatigue-inducing mental task sessions were not correlated with the Chalders Fatigue Scale scores in either of the two phases, the intensities after the 0-back test trials for 30 min in Phase 1 and those after the 2-back test trials in Phase 2 were significantly correlated with the fatigue scale scores. The daily level of fatigue was related to VEF intensity after the acute mental fatigue loads. Our findings provide new perspectives to evaluate our daily level of fatigue as well as to clarify the neural mechanisms underlying it.