Cognition, Gamma Oscillations and Neuronal Synchrony

Abstract

Neuroanatomical and electrophysiological evidence emphasizes the distributed nature of brain processes. There is no single convergence center in the brain for the representation of the final results of cognitive operations and for the coordination of executive functions. This raises the question how the numerous, simultaneously occurring subprocesses are bound together in order to permit coherent interpretations ofpolymodal sensory signals and coordination of adapted behaviour. At a smaller scale the same problems arise in sensory processing and motor coordination because representations of perceptual objects and movements appear to consist of ensembles of widely distributed interacting neurons rather than of individual, object-specific cells and movement specific command neurons (1). Following the discovery of stimulus-induced synchronization and oscillatory patterning of sensory responses in the visual cortex (2) it has been suggested that binding of distributed responses into coherent representations is achieved by precise synchronization of neuronal responses. Since then numerous experiments in anesthetized and awake animals have shown that this synchronization results from cooperative interactions among selectively coupled cortical neurons and changes in a context-dependent way. Evidence indicates that synchronization correlates with feature binding in sensory processing, with shifts in attention and with the preparation and execution of movements, suggesting that it serves as a general mechanism for the definition of functional relations among distributed neuronal responses. A conspicuous feature of these processing related synchronization phenomena is that they very often go along with an oscillatory patterning of the synchronized neuronal discharges in the betaand gamma-frequency range (20 60 Hz), in particular when synchronization is established over larger distances. This suggests that oscillatory patterning supports the establishment of precise temporal relations among neuronal responses. Fortunately, these synchronization phenomena are sufficiently robust so that epochs of synchronized oscillatory activity can be detected in EEG and MEG recordings that average over large populations of neurons. This permitted examination of hypotheses derived from animal experiments in human subjects. Evidence accumulated over the past years suggests close relations between the occurrence and interareal coherence of gamma oscillations on the one hand and a variety of cognitive processes on the other. These processes include feature binding, sensory-motor coordination, target selection by attention, short-term memory and associative learning, suggesting again that synchronization processes in the gamma frequency range play an important and rather general role in the coordination of distributed processes in the brain.