The entrainment of biological rhythms is a complex subject.
Menaker reports that cyanobacteria, simple organisms that originated at least 3 billion years ago, “have fully functional circadian clocks”, which may give support to the suggestion that biological rhythms and their entrainment are fundamental to life in any form (2002:2). Some have gone so far as to characterize any organism “as a (loosely coupled) ‘population of oscillators’.” (Pittendrigh 1975, quoted by Warner 1988:68-9).
Chronobiologists are turning their attention increasingly to ultradian rhythms, i.e. those with periods of less than a day. The term ‘ultradian’ covers a wide range of possible periods, from milliseconds up to 12 hours, although it tends to refer particularly to the range from a few minutes up to a few hours: faster rhythms are sometimes referred to as supra-ultradian.
The electrical activity that is recorded in electroencephalograms (EEGs) is largely attributable to postsynaptic potentials (PSPs; i.e. graded potentials produced by synaptic activities that eventually lead to firing of neurons) in cell bodies and dendrites of cortical neurons (Lopes da Silva and Storm van Leeuwan 1978). Neurons of the human brain, the ‘gray matter’, come in two principal arrangements: layered they form a cortex and in non-layered agglomerations they form a nucleus. Two cortices, the cerebral and the cerebellar cortex form the surface layer of the human brain; nuclei are located beneath the cortex and in the brain stem. The columnar arrangement of neurons in the cerebral cortex facilitates summation of these potentials and their registration at the scalp. However, other geometric arrangements of neuronal assemblies can lead to extracellular attenuation or even cancellation and therefore not all activities of brain cells can be recorded in the EEG. The regular spontaneous EEG components are thought to be due to PSPs synchronized by discharges from deep nuclei (thalamus) and the degree of synchronicity is reflected in the amplitude and form of the EEG (Lopes da Silva 1991). If cortical activity is synchronous over a larger area it produces larger potentials (e.g. Cooper et al. 1965). Desynchronisation of the EEG and reduction of its amplitudes presumably reflects increased interaction of several neuronal sub-populations engaging in cooperative activities.
As Bluedorn puts it, then:
(Condon 1976:305), which he interpreted as evidence of “entrainment or stimulus tracking” (309). Although in some respects Condon’s methodology raised suspicions of researcher bias (his method for dealing with the key problem of the segmentation of behaviour “constituted re-viewing a sound film of human communication over and over for many hours until forms of order began to be seen” [1976:288]), his findings that this synchrony could not be observed in subjects “with severe psychopathology or communication disorders” such as autism, dyslexia and schizophrenia (1982:61), and that the detailed features of the interactional synchrony varied with the ethnic origin of his subjects, do suggest that his work should not be casually dismissed. At one point, Condon even suggested that the periodicities he observed in behaviour were uncannily close to the frequency ranges of the different types of brain waves, and in light of this he coined the term “behavioural waves” (1986: 63-66).
In terms of the mechanics of entrainment in human cognition, Jones theorizes that there are three primary stages: (1) perception, which primes the listener to form expectations; if expectations are met, (2) synchronization; and if expectations are not met, (3) adjustment or assimilation. Perception and the priming of expectations are nearly instantaneous occurrences. Cues from events unfolding around the attender are taken as indicators of where to focus attentional energies in order to ‘catch’ upcoming events. Anticipation of future events is facilitated by the presence of highly coherent (i.e., regularly patterned) temporal events, such as a steady beat (see Jones and Boltz 1989:466). Synchronization follows priming and occurs as our expectations are met. As such, synchronization is itself a verification of the correctness of our expectations. If our expectations do not match what happens next, then synchronization has not occurred. It should be noted, however, that the discrepancies between our expectations and the actual unfolding of events can cause arousal that in turn heightens attention and results in learning.