Neural coherence and the content of consciousness.

On the one hand Dr. John's paper is a reintegration of previous animal work originally published during the period 1959 to 1977, and on the other hand, it is a refocusing on a subset of this work translated to human subjects. The subissue in Dr. John's paper is the content of consciousness and not, specifically, the operation of consciousness. Previously, Dr. John helped to show why it is anatomically and functionally helpful to distinguish between the operation of consciousness and the content of consciousness (Thatcher and John, 1977). To briefly review, the "general operation of consciousness" (GOC) is characterized by longer time intervals and relatively time invariant processes such as sleep, wakefulness, arousal, and coma. The GOC is a relatively tonic state, with slow adjustments of the levels of arousal (i.e., on the order of minutes and hours), such as occur during changes in levels of awareness and/or drowsiness and mediated by reticular-limbic and thalamic excitatory control systems. In contrast, the "content of consciousness" (COQ is defined as the momentary collection of sensations and thoughts that we would call the "present moment," which is temporally and hierarchically nested within the GOC and is mediated by the coherent activation of large ensembles of neurons. William James referred to the content of consciousness as the "specious present," which he defined as that "interval of time (about 1 to 7 seconds) when an event that belonged to the present is distinctly perceived as the past" (James, 1890). A finer temporal grain of the content of consciousness is provided by the psychophysical experiments of Efron (1967, 1970a,b) and others (Fraisse, 1963; Shallice, 1964) that demonstrated an approximately 20to 200-ms "perceptual frame" in which nearly simultaneous events were integrated into a perceptual whole. Subsequent EP studies have indicated that the duration of the frame increases as a function of the complexity of the integration, especially as shown in P400 studies (Thatcher, 1976, 1977a). As Dr. John pointed out in 1977, the running average summation of the content of each of the approximately 20to 200-ms perceptual frames may represent a temporal limit of the momentary content of consciousness (Thatcher and John, 1977). Experimentally, the duration of the integrative peceptual frame varies de