Differential brain response to metrical accents in isochronous auditory sequences

Even within equitonal isochronous sequences, listeners report perceiving differences among the tones, reflecting some grouping and accenting of the sound events. In a previous study, we explored this phenomenon of "subjective rhythmization" physiologically through brain event-related potentials (ERPs). We found differences in the ERP responses to small intensity deviations introduced in different positions of isochronous sequences, even though all sound events were physically identical. These differences seemed to follow a binary pattern, with larger amplitudes in the response elicited by deviants in odd-numbered than in even-numbered positions. The experiments reported here were designed to test whether the differences observed corresponded to a metrical pattern, by using a similar design in sequences of a binary (long-short) or a ternary (long-short-short) meter. We found a similar pattern of results in the binary condition, but a significantly different pattern in the ternary one. Importantly, the amplitude of the ERP response was largest in positions corresponding to strong beats in all conditions. These results support the notion of a binary default metrical pattern spontaneously imposed by listeners, and a better processing of the first (accented) event in each perceptual group. The differences were mainly observed in a late, attention-dependent component of the ERPs, corresponding to rather high-level processing.