alpha rhythms Electroencephalogram (EEG)/magnetoencephalo-gram (MEG) rhythmic activity at 8-13 Hz occurring during wakefulness, mainly over the posterior regions of the head. It is predominant when the subject has closed eyes and is in a state of relaxation. It is attenuated by attention, especially visual, and by mental effort. There are other rhythmic activities within the same frequency range, such as the mu rhythm and the tau or temporal alphoid rhythm, that in MEG recordings have been associated with cortical auditory function, but these are less clear in EEG recordings.

beta/gamma rhythms These rhythmic activities represent frequencies higher than 13 Hz; typically, the beta rhythmic activity ranges from 13 to approximately 30-40 Hz and the gamma extends beyond the latter frequency. Rhythmical beta/gamma activities are encountered mainly over the frontal and central regions and are not uniform phenomena. These rhythmic activities can occur in relation to different behaviors, such as during movements or relaxation after a movement.

desynchronization A state in which neurons are randomly active. Desynchronization is reflected in the absence of a preferred EEG/ MEG frequency component. Event-related desynchronization of ongoing EEG/MEG is a state characterized by a decrease in the power of spectral peaks at specific frequency components, elicited by an event.

electroencephalogram, magnetoencephalogram Electrical potentials or magnetic fields recorded from the brain, directly or through overlying tissues.

excitatory synaptic potential or current An active electrical response of the postsynaptic membrane of a neuron to the release of a neurotransmitter that consists ofa local, graded depolarization or of the corresponding ionic current.

event-related potential or field A change in electrical or magnetic activity related to an event, either sensory or motor. The event may precede or follow the event-related potential or field.

evoked potentials or fields A change in electrical or magnetic activity in response to a stimulus. Typically, these transients last tens or hundreds of milliseconds.

inhibitory postsynaptic potential/current Consists of a local, graded hyperpolarization or the corresponding ionic current.

mu or rolandic (central) rhythm Rhythmic activity within the same frequency range as the posterior alpha rhythm but with a topographic distribution that is predominant over the central sensorimotor areas. It is attenuated or blocked by movements.

nonlinear dynamics and brain oscillations Systems with nonlinear elements, such as neuronal networks, may exhibit complex dynamics. Typically, these systems may have different kinds of evolution in time and may switch from one oscillatory mode to another. A qualitative change in the dynamics that occurs as a system's parameter varies is called a bifurcation. Some brain oscillations appear to be generated by systems of this kind, with complex nonlinear dynamics.

sleep or sigma spindles Waxing and waning spindle-like waves occurring during the early stage of sleep at 7-14 Hz within sequences lasting 1 or 2 sec. Sleep spindles typically recur at a slow rhythm of about 0.2-0.5 Hz.

synchronization State in which neurons oscillate in phase as a result of a common input and/or of mutual influences. Event-related synchronization of the ongoing EEG/MEG signals is a state characterized by an increase in power of specific frequency components.

theta rhythms or rhythmical slow activity This term denotes rhythmic activity in the frequency range from 4 to 7 Hz in humans, although macro-osmatic animals show a powerful limbic, and especially hippocampi, rhythm from 3 to 12 Hz that is activated by arousal and motor activity.

The recording of the electrical activity of the brain [i.e., the electroencephalogram (EEG)], either the ongoing activity or the changes of activity related to a given sensory or motor event [the event-related potentials (ERPs)], provides the possibility of studying brain functions with a high time resolution but with a relatively modest spatial resolution. The latter, however, has been improved recently with the development of the magnetoencephalogram (MEG) and more sophisticated source imaging techniques. These new methods allow an analysis of the dynamics of brain activities not only of global brain functions, such as sleep and arousal, but also of cognitive processes, such as perception, motor preparation, and higher cognitive functions. Furthermore, these methods are essential for the characterization of pathophysiological processes, particularly those with a paroxysmal character such as epilepsy. In this article, the possibilities offered by EEG/MEG recordings to analyze brain functions, and particularly the neural basis of cognitive processes, are discussed. In this respect, special attention is given to the basic mechanisms underlying the generation of functional neuronal assemblies (i.e., the processes of synchronization and desynchronization of neuronal activity) and their modulation by exogenous and endogenous factors. Therefore, a brief overview of the neurophysiology of the dynamics of neuronal networks and of the generation of brain oscillations, an account of the phenomenology of EEG/MEG activities, and a discussion of the main aspects of ERPs and/or magnetic fields are presented. The article concludes with a short discussion of the roles that brain oscillations may play in the processing of neural information. In this respect, the need to understand what happens at the level of neuronal elements within brain systems in relation to different types of oscillations is emphasized. Only in this way is it possible to obtain insight into the functional processes that occur in the brain, at the level of information transfer or processing, during the occurrence of brain oscillations and the transition between different states. The relevance of electrophysiological studies of the human brain regarding neurocognitive investigations is also discussed in light of recent advances in brain imaging techniques.

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