Vigilance is an undirected, permanent state of wakefulness that can take various forms. Clinical symptoms and syndromes that manifest in the form of severely decreased vigilance are called quantitative disorders of consciousness and occur in the context of numerous neurologic, mental, and other diseases.
What is vigilance?
Vigilance is an undirected, permanent state of wakefulness. Neuroscience defines vigilance as a form of attention that is a component of neural information processing. Vigilance describes the activation state of the nervous system and is not categorically either present or absent, but varies in intensity. Vigilance is distinguished from other forms of attention because it is tonic, that is, it persists permanently rather than occurring only in brief periods. Moreover, vigilance is always undirected. In the context of physical and mental illness, severely reduced vigilance can manifest as somnolence, sopor, or coma, among other symptoms.
Function and task
A healthy person who is not focused on any specific task is in a state of conscious readiness: specific stimuli may attract the person’s attention, sudden hazards trigger a state of alert, and, in general, consciousness is open to various sensory inputs. When the person consciously relaxes, he or she enters the conscious resting state and possibly one of the various sleep stages. A sleep laboratory can determine and record vigilance during sleep; particularly in the EEG, diagnosticians can see how pronounced a person’s tonic undirected activation is. Vigilance is subject to natural variations throughout the day, which can vary from person to person. Cognitive neuroscience also refers to such cycles as circadian rhythms; they underlie the biological or molecular clock and are based on biochemical interactions that are genetically determined: An individual does not learn these cycles but intuitively follows them. Typically, neuronal activation peaks in the course of the morning: physicians and psychologists often perform cognitive function tests during this period in order to be able to assess a person’s performance and to exclude, as far as possible, disturbing factors caused by time-of-day-dependent fluctuations in vigilance. In addition, vigilance also varies in the context of shorter cycles, so-called ultradian rhythms. These include the Basic Rest-Activity Cycle, or BRAC. A run of the BRAC lasts about 90 minutes and is characterized by different expressions of vigilance that repeat at the end of this cycle. The ascending reticular activating system (ARAS) represents that part of the nervous system that is responsible, among other things, for the control of vigilance. The ARAS possesses far-reaching influence on the human body: vigilance not only affects neuronal information processing, but also affects the hormonal system and other areas of the organism.
Diseases and ailments
Disorders of vigilance are referred to primarily by psychiatry as quantitative disorders of consciousness, diminished consciousness, or clouding of consciousness. In contrast, qualitative disorders of consciousness or shifts in consciousness preserve vigilance. Quantitative disorders of consciousness may indicate, among other things, impaired brain function, possibly due to organic, toxicological, or psychological causes. Medicine divides quantitative disorders of consciousness into different degrees of severity, with somnolence, sopor, precoma, and coma among the most important. Somnolence is characterized by clinically significant sleepiness and goes beyond the level of normal drowsiness. It may occur, for example, in the context of delirium in alcohol withdrawal, acute intoxication (for example, with psychotropic drugs). Somnolent persons appear and feel sleepy and give the impression of mental absence to outsiders. However, they can be awakened, show (possibly limited) responses to external stimuli, and their reflexes are usually still present. In the case of somnolence, intensive inpatient treatment is often necessary. The same applies to sopor.This term refers to the Latin word for “sleep,” but also denotes a clinically relevant state in the sense of a quantitative disturbance of consciousness. Persons in sopor are not only drowsy, but unconscious and appear to be asleep. However, affected persons often cannot be awakened by usual means such as shaking the shoulders, loud talking and similar measures. Usually, a strong pain stimulus or a comparably strong signal is necessary to elicit a response. Coma is the most severe form of clouding of consciousness, as there is no longer any wakefulness in this state: affected persons appear to be asleep, but cannot be awakened and are unresponsive. In addition, they no longer respond to external stimuli and often show no or reduced reflexes. Coma requires close medical monitoring in an intensive care unit. People suffering from epilepsy also experience a reduction in vigilance during a seizure, which cognitive neuroscientists sometimes refer to as epileptic alteration of consciousness. This form of vigilance impairment is transient and usually subsides after the seizure. Complications in some cases lead to possibly prolonged limitations of undirected tonic attention. Anesthesia, for example, in connection with surgery, describes an artificial reduction in vigilance induced with medication.