A review of the arousal literature reveals that a variety of terms are used to define aspects of arousal. Arousal and alerting are often used interchangeably, as are arousal and behavioral state. In this article, we take the position that the changes in nervous system excitability may differ significantly in duration. We refer to changes lasting a few milliseconds to a few seconds as shifts in alertness. We refer to changes lasting from seconds to minutes as vigilance, for lack of a better word. Our use of vigilance is different than that in the signal detection literature, in which it refers to the readiness to detect infrequent events in noise. We use vigilance to describe the physiological reactions to environmental events that accompany shifts in focal attention, an increased readiness to respond, and increased activity in the sympathetic nervous system. The events are often cognitive demands on the organism, including emotional valence, task difficulty, and stimulus novelty. Relatively speaking, we consider alerting to be a phasic process and vigilance a tonic process. The term arousal is used in a more general sense to refer to any changes in nervous system excitability regardless of duration.

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