Top-down control of rhythm perception modulates early auditory responses.

Our perceptions are shaped by both extrinsic stimuli and intrinsic interpretation. The perceptual experience of a simple rhythm, for example, depends upon its metrical interpretation (where one hears the beat). Such interpretation can be altered at will, providing a model to study the interaction of endogenous and exogenous influences in the cognitive organization of perception. Using magnetoencephalography (MEG), we measured brain responses evoked by a repeating, rhythmically ambiguous phrase (two tones followed by a rest). In separate trials listeners were instructed to impose different metrical organizations on the rhythm by mentally placing the downbeat on either the first or the second tone. Since the stimulus was invariant, differences in brain activity between the two conditions should relate to endogenous metrical interpretation. Metrical interpretation influenced early evoked neural responses to tones, specifically in the upper beta range (20-30 Hz). Beta response was stronger (by 64% on average) when a tone was imagined to be the beat, compared to when it was not. A second experiment established that the beta increase closely resembles that due to physical accents, and thus may represent the genesis of a subjective accent. The results demonstrate endogenous modulation of early auditory responses, and suggest a unique role for the beta band in linking of endogenous and exogenous processing. Given the suggested role of beta in motor processing and long-range intracortical coordination, it is hypothesized that the motor system influences metrical interpretation of sound, even in the absence of overt movement.