Developing neurophysiological metrics for the assessment of mental workload and the functional state of the brain

OF DOCTORAL DISSERTATION AALTO UNIVERSITY SCHOOL OF SCIENCE AND TECHNOLOGY P.O. BOX 11000, FI-00076 AALTO http://www.aalto.fi Author Anu Holm Name of the dissertation DEVELOPING NEUROPHYSIOLOGICAL METRICS FOR THE ASSESSMENT OF MENTAL WORKLOAD AND THE FUNCTIONAL STATE OF THE BRAIN Manuscript submitted 15.12.2009 Manuscript revised 09.04.2010 Date of the defence 12.05.2010 Monograph Article dissertation (summary + original articles) Faculty Faculty of Information and Natural Sciences Department Department of Biomedical Engineering and Computational Science Field of research Neuroergonomics Opponent(s) Docent Minna Huotilainen, PhD Supervisor Professor Risto Ilmoniemi Instructor Docent Kiti Müller, MD Abstract Modern working environments are often information intensive and work performance requires acting on multiple tasks simultaneously, i.e., multitasking. Also, irregular and prolonged work schedules, shift work and night work are typical in many work sectors. This causes both acute and chronic sleep loss, which results in performance impairment, such as increased reaction times, memory difficulties, cognitive slowing, and lapses of attention. Long lasting sleep loss and sustained overloading increase the risk of human errors and may cause work related stress and even occupational burn-out.Modern working environments are often information intensive and work performance requires acting on multiple tasks simultaneously, i.e., multitasking. Also, irregular and prolonged work schedules, shift work and night work are typical in many work sectors. This causes both acute and chronic sleep loss, which results in performance impairment, such as increased reaction times, memory difficulties, cognitive slowing, and lapses of attention. Long lasting sleep loss and sustained overloading increase the risk of human errors and may cause work related stress and even occupational burn-out. According to the Finnish Occupational Safety and Health Act (738/2002, Työturvallisuuslaki), Section 25 Avoiding and reducing workloads, an employer should assess the workload the employee is exposed to. Despite the fact that this important issue is enacted in the law, the objective measures to assess the workload are lacking. This Thesis reviews neurophysiologic methods for assessment of cognitive workload and sleep loss. Then it describes experimental studies where the feasibility of conventional event related potential (ERP) and electroencephalography (EEG) methods were tested both in assessment of internal state of participants during challenging task performance after sleep debt and in diagnostic of work-related central nervous system disorder. After that, methodological improvements both on ERPs and EEG metrics are shown: ERPs were analysed with a single-trial method, and EEG methodology was developed for estimation of both internal (caused by sleep loss) and external (caused by task demands) load. The methods were tested in healthy controls. The most promising metric to study overall brain load, including both cognitive workload as well as sleep loss, is suggested to be theta Fz / alpha Pz -ratio. It increases both with growing cognitive workload level and time spent awake, being sensitive also to sleep loss. This metric is possible to measure both in laboratory and in the field conditions. Measurements may be carried out even during real work tasks, at least in professions where most work is done in office-like environments. As the ratio increases with cognitive brain load similar to the heartbeat with increasing physical load, the ratio was named "brainbeat".