Modulations in the degree of synchronization during ongoing oscillatory activity in the human brain.

When a subject is asked to respond as quickly as possible to a stimulus he/she responds much faster if this stimulus is preceded by a warning cue. This cue enables the subject to anticipate the forthcoming stimulus, initiating neural processes subserving the future perception and processing of the target stimulus and the motor preparation of the associated response action. It has recently been suggested that neuronal activity before such an anticipated target stimulus could be associated with modulations in neuronal synchronization and oscillatory activity. Here we recorded electrical brain activity whilst subjects performed a choice reaction time task, in which one of the stimuli could be predicted with 90% certainty. We show that the prediction of a forthcoming stimulus was associated with an increase in gamma oscillations overlying occipital areas and a decrease in beta oscillations overlying sensorimotor cortex before the stimulus was presented. We suggest that these regionally specific modulations in oscillatory activity reflect the establishment of neural networks that are 'primed' for the future processing of the forthcoming predictable visual stimulus.