The Specific Role of Rhythmicity in Neuroregulation
It has been argued above that in the extreme cases of EEG synchronization and desynchronization, an obvious correlation with low and high activation and arousal, respectively, exists. It is also well known that arousal correlates with dominant frequency in the EEG. It falls readily to hand to argue that the degree of rhythmicity, together with changes in the EEG frequency spectrum, manages the entire range of activation and arousal in the bio-electrical domain. The EEG, then, reflects a parameter that the brain tightly constrains in the ordinary course of events. An appeal to dominant frequency or to the amplitude at a given frequency by operant conditioning could therefore be expected to serve as a powerful external forcing function on the brain's management of arousal. The whole matter of the role of frequency, however, bears further discussion.

One role advocated for rhythmic activity is that of time binding, the need for harnessing brain electrical activity which is spatially distributed while maintaining it as a single entity. The need for this kind of function is apparent when it is recognized that visual processing, for example, must occur by parallel processing over large areas of cortical real estate. The integrity and stability of the image must be maintained over time. Simultaneity of firing of the various neurons participating in the mapping of an image may be the relevant criterion of "belonging". The transient organization of such distributed, correlated neuronal activity may be the role of the thalamocortical rhythmic generators. At the lower frequency regimes, say less than 30 Hz, this organization ranges broadly over the cortex, and manages activation and arousal with relatively long persistence. At higher frequency regimes, above 30 Hz, and peaking in the 40-60 Hz regimes, the brain manages specific cognitive processes that are of a more transient nature, and more spatially localized.

A recent study beautifully exhibits both of these roles of rhythmic activity (Munk, 1996). In this study, a visual image was moved across the visual cortex under two conditions: normal, and under electrical stimulation of the mesencephalon (brain stem region in which the nuclei reside which source the neuromodulator substances that control attention and arousal.) With stimulation, a global coherence became prominent in which the firing rates of neurons in different regions became more coincident. This coherence was observed over the region of visual cortex that was involved in mapping the moving image. If the moving image was then changed into two images, moving in opposite directions, the coherence was still present, but was restricted to the neurons belonging to each moving target. This beautiful experiment illustrates the influence of global activating mechanisms directed from the brainstem. However, this mechanism was not sufficient to guarantee time binding. That requires augmentation by information derived from the image itself, and processed 'locally' in cortex, in order to define the specific cohort to which each participating neuron belonged. This is a process of which the brainstem remains ignorant. Hence, time binding requires both brainstem and cortical governance, and both may be mediated by thalamo- cortical networks, and may also be modulated by direct cortical-cortical interaction.

It must be kept in mind that most of the signal processing we do in the brain involves very transient events taking place on small time scales. The analogy to dynamic RAMs or to the refresh on your computer screen (every 17 milliseconds) comes to mind. Further, it is apparent that the real information content in neural signals (action potentials) relates in first order (and trivially) to the presence or absence of a particular signal, and, more significantly, to the actual timing of the signal. The magnitude of an action potential is not a function of the size of the stimulus that gives rise to it. Only the timing matters. And even the timing gains significance only in the context of other events. All "mental activity" must ultimately have its basis in particular neuronal firing patterns that become discernible from the ambient noise background by virtue of timing coincidences or at least correlations. It is this timing which appears to be managed by thalamocortical circuitry. Rhythmicity may be one of the key ways in which such timing is organized. Recent research by Pfurtscheller (1990) and Sterman (1996), show that the brain's ability to locally desynchronize in a timely manner defines its capacity to process the next stage of an ongoing task. The ability to resynchronize quickly allows it to reenter a state of readiness for the next task. The process breaks down when synchronization or desynchronization of specific frequencies persists or is disregulated, decoupled from the demands of the moment. EEG biofeedback is then to be seen as a challenge to the mechanisms that underlie the management of this rhythmic activity, and in application to neuromodulation of arousal and activation its natural domain is the frequency range less than thirty Hz. Training is similar to stimulation, and constitutes a push that invokes the brain's capacity for restoring homeostasis. Over the longer term, this results in a long-term increase in stability. Training at a specific frequency is then a push in a very specific direction, which can be chosen in light of specific arousal disregulation or attentional deficits found in each case.