Vigilance and Alertness

Just as alertness can be distinguished from arousal, it can also be distinguished from vigilance. Vigilance has also been described as the focused readiness to detect and respond to a given stimulus. When a warning is given prior to a stimulus, the response to the stimulus is improved—the warning is said to "alert" the subject to a forthcoming event. This "alerting" effect of cues actually refers to an increase in the state of vigilance. Vigilance, therefore, refers to directed alertness—one can be vigilant for a specific event, whereas alertness is a state of more generalized readiness.

Vigilance also carries connotations of effort, in contrast with alertness, which does not carry such connotations. Thomas Jefferson did not say the price of freedom is eternal alertness but rather, eternal vigilance. With vigilance, one is alert to particular possibilities, threats or dangers, and signs or signals. Eternal vigilance is possible to imagine, whereas eternal alertness is impossible. However, eternal vigilance is not easy because vigilance requires effort. The cost of vigilance is clearly seen in the laboratory as the ''vigilance decrement"—a decline in performance over time. In a classic vigilance task, the target stimuli to which the subject must respond are temporally unpredictable. They may also be spatially unpredictable or of low intensity. When a subject performs such a task, over time the detection rate of targets decreases and the reaction time to targets increases. This is the vigilance decrement and it is greatest under conditions in which the targets are most difficult to detect and the requirement for vigilance is greatest. Not only is adequate performance of a target detection task difficult to maintain over time but also the effort involved is tiring.

Motor readiness is also called phasic alertness. This refers to the short duration (typically measured within a second rather than over minutes or even hours) performance enhancement, which is related to temporal expectation of targets. Reaction times to targets are faster as the temporal probability of the target increases. Phasic alertness is related to vigilance and may indeed reflect operation of similar processes.

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