Neuronal oscillations are postulated to play a fundamental role in top-down processes of expectation. We used magnetoencephalography (MEG) to investigate whether expectation of a tactile event involves a pre-stimulus modulation of neuronal oscillations in human somatosensory cortex. In a bimodal attention paradigm, participants were presented with a predictable spatio-temporal pattern of lateralized tactile stimulations and simultaneously occurring non-lateralized auditory stimuli. Before the onset of a series of such combined audio-tactile stimuli, a cue was presented that indicated the sensory stream that had to be attended. By investigating lateralized patterns of oscillatory activity, we were able to study both attentive (when the tactile stream was attended) and non-attentive (when the auditory stream was attended) tactile expectations. For both attention conditions, we observed a lateralized modulation of the amplitude of beta band oscillations prior to a predictable - and accordingly lateralized - tactile stimulus. As such, we show that anticipatory modulation of ongoing oscillatory activity is not restricted to attended sensory events. Attention did enlarge the size of this modulation. We argue that this modulation constitutes a suppression of beta oscillations that originate at least partly from primary somatosensory cortex (S1) contralateral to the expected stimulation. We discuss our results in the light of the hypothesis that ongoing beta oscillations over sensorimotor cortex reflect a brain state in which neuronal processing efficacy is low. Pre-stimulus suppression of these oscillations then prepares the system for future processing. This shows that perception is an active process that starts even prior to sensation.
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