The Problem of Neural Integration: Induced Rhythms and Short-Term Correlations

Since the beginnings of neurophysiology and electroencephalography, various rhythmic patterns of brain activity have been recorded, differing in frequency, location, and relationship to behavior or cognitive activity. Recently, the cortical frequency band around 40 Hz (gamma range) has become a focus of attention (Bressler, 1990). To be sure, this band has already been studied in the past (cf. Sheer, 1970; Sheer and Grandstaff, 1970; Bassr, 1980); for instance, 40-Hz electroencephalogram (EEG) activity has been shown to be related to focused arousal (Sheer, 1976; Bouyer et al., 1981, 1987), and sensory (Galambos et al., 1981) as well as cognitive 40-Hz event-related potentials (ERPs) (Bauer and Jones, 1976; Spydell et al., 1985) have been widely examined. Rhythmic neuronal activity can show varying degrees of stimulus dependence. Ongoing background rhythms tend to be relatively independent of specific stimuli. By contrast, in stimulus-driven rhythms, temporal fluctuations of the neuronal activity are tightly locked to temporal fluctuations in the stimulus itself. What characterizes induced rhythms is that, although they are triggered by an external stimulus, their temporal structure is largely determined by interactions within the participating neuronal circuits.