Entrainment

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Brainwave-entrainment.jpg

Origin

Middle French entrainer, from en- + trainer to draw, drag

Definition

  • 1 : to draw along with or after oneself
  • 2 : to draw in and transport (something, such as solid particles or gas) by the flow of a fluid
  • 3 : to incorporate (air bubbles) into concrete
  • 4 : to determine or modify the phase or period of circadian rhythms entrained by a light cycle

Description

  • Brainwave entrainment

Brainwave entrainment, also referred to as brainwave synchronization and neural entrainment, refers to the hypothesized capacity of the brain to naturally synchronize its brainwave frequencies with the rhythm of periodic external stimuli, most commonly auditory, visual, or tactile.

It is widely accepted that patterns of neural firing, measured in Hz, correspond with states of alertness such as focused attention, deep sleep, etc. It is hypothesized that listening to these beats of certain frequencies one can induce a desired state of consciousness that corresponds with specific neural activity.

  • Biomusicology

Entrainment in the biomusicological sense refers to the synchronization of organisms (only humans as a whole, with some particular instances of a particular animal) to an external perceived rhythm, such as human music and dance such as foot tapping.

  • Chronobiology

Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period to that of an environmental oscillation. It is ultimately the interaction between circadian rhythms and the environment. A central example is the entrainment of circadian rhythms to the daily lightdark cycle, which ultimately is determined by the Earth's rotation. Exposure to certain environmental stimuli will cue a phase shift, and abrupt change in the timing of the rhythm. Entrainment helps organisms maintain an adaptive phase relationship with the environment as well as prevent drifting of a free running rhythm. This stable phase relationship achieved is thought to be the main function of entrainment.

There are two general modes of entrainment: Phasic vs. Continuous. The phasic model is when there is limited interaction with the environment to "reset" the clock every day by the amount equal to the "error", which is the difference between the environmental cycle and the organism's circadian rhythm. The continuous model is where the circadian rhythm is continuously adjusted by the environment usually by constant light. Two properties, the free running period of an organism, and the phase response curve, are the main pieces of information needed to investigate individual entrainment. There are also limits to entrainment. Although there may be individual differences in this limit, most organisms have a +/- 3 hours limit of entrainment. Due to this limit, it may take several days for re-entrainment.

The term entrainment is applied because the biological rhythms are endogenous: The rhythm persists even in the absence of environmental cues because it is not a learned behavior but something that is inherently in organisms. Of the several possible cues, called zeitgebers (German for 'time-givers', 'synchronizers'), which can contribute to entrainment, light has the largest impact. Units of Circadian Time (CT) are used to describe entrainment to refer to the relationship between the rhythm and the light signal/pulse.

The activity/rest (sleep) cycle in animals is one of the circadian rhythms that normally are entrained by environmental cues. In mammals, such endogenous rhythms are generated by the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. Entrainment is accomplished by altering the concentration of clock components through altered gene expression and protein stability.

Circadian oscillations occur even in the cells of isolated organs such as the liver/heart as peripheral oscillators, and it is believed that they sync up with the master pacemaker in the mammalian brain, the SCN. Such hierarchical relationships are not the only ones possible: Two or more oscillators may couple in order to assume the same period without either being dominant over the other(s). This situation is analogous to Huygens' pendulum clocks.

The phase of entrainment refers to the relative timing of any circadian event within the objective 24-hour day.

When good sleep hygiene is insufficient, a person's lack of synchronization to night and day can have health consequences. There is some variation within normal chronotypes' entrainment; it is normal for humans to awaken anywhere from about 5 a.m. to 9 a.m. However, patients with DSPD, ASPD and non-24-hour sleep–wake disorder are improperly entrained to light/dark.

Further reading

Pittendrigh CS (1981) Circadian systems: Entrainment. In Handbook Behavioral Neurobiology, Vol. 4. Biological Rhythms, J. Aschoff, ed. pp. 239–68, University of California Press, New York.

Category: Biology]]