The term vigilance is used frequently in a wide variety of research areas. The British neurologist Sir Henry Head introduced the term to refer to a state of high consciousness. Nowadays, ‘vigilance’ is used in neurophysiological research, but also in the experimental psychological field. Related terms, such as arousal, sustained attention, and tonic alertness are often used jointly with or instead of the term vigilance. It may seem that all these designations can be interchanged freely, but this is not the case. Many investigators differentiate these terms and the distinctions made are not always subtle. The terminological confusion of vigilance involves on the one hand its definition (i.e. clarification of the theoretical construct) with reference to different processes, and on the other hand different measuring procedures. The original definition of vigilance: “… a high state of physiological efficiency” is rather physiological in nature, but the “efficiency” part points to behavior. Head’s clarification that a vigilant state differs from a pure condition of raised excitability appears to be of major importance. The combination of physiological activity and efficient behavior is of great relevance and underlines the difference between vigilance and more basic energetic conditions.
The topic of this paper is the exploration of procedures for measuring vigilance. EEG-measures are very popular and are described first. More specifically, the spectral content of the EEG is investigated. Second, behavioral measures are presented. These concern performance on vigilance tasks. Finally, subjective questionnaires are explored.