Modulation change detection in human auditory cortex: Evidence for asymmetric, non-linear edge detection.

Changes in modulation rate are important cues for parsing acoustic signals, such as speech. We parametrically controlled modulation rate via the correlation coefficient (r) of amplitude spectra across fixed frequency channels between adjacent time frames: broadband modulation spectra are biased toward slow modulate rates with increasing r, and vice versa. By concatenating segments with different r, acoustic changes of various directions (e.g., changes from low to high correlation coefficients, that is, random-to-correlated or vice versa) and sizes (e.g., changes from low to high or from medium to high correlation coefficients) can be obtained. Participants listened to sound blocks and detected changes in correlation while MEG was recorded. Evoked responses to changes in correlation demonstrated (a) an asymmetric representation of change direction: random-to-correlated changes produced a prominent evoked field around 180 ms, while correlated-to-random changes evoked an earlier response with peaks at around 70 and 120 ms, whose topographies resemble those of the canonical P50m and N100m responses, respectively, and (b) a highly non-linear representation of correlation structure, whereby even small changes involving segments with a high correlation coefficient were much more salient than relatively large changes that did not involve segments with high correlation coefficients. Induced responses revealed phase tracking in the delta and theta frequency bands for the high correlation stimuli. The results confirm a high sensitivity for low modulation rates in human auditory cortex, both in terms of their representation and their segregation from other modulation rates.

The effect of conditional probability of chord progression on brain response: an MEG study.

BACKGROUND: Recent electrophysiological and neuroimaging studies have explored how and where musical syntax in Western music is processed in the human brain. An inappropriate chord progression elicits an event-related potential (ERP) component called an early right anterior negativity (ERAN) or simply an early anterior negativity (EAN) in an early stage of processing the musical syntax. Though the possible underlying mechanism of the EAN is assumed to be probabilistic learning, the effect of the probability of chord progressions on the EAN response has not been previously explored explicitly. METHODOLOGY/PRINCIPAL FINDINGS: In the present study, the empirical conditional probabilities in a Western music corpus were employed as an approximation of the frequencies in previous exposure of participants. Three types of chord progression were presented to musicians and non-musicians in order to examine the correlation between the probability of chord progression and the neuromagnetic response using magnetoencephalography (MEG). Chord progressions were found to elicit early responses in a negatively correlating fashion with the conditional probability. Observed EANm (as a magnetic counterpart of the EAN component) responses were consistent with the previously reported EAN responses in terms of latency and location. The effect of conditional probability interacted with the effect of musical training. In addition, the neural response also correlated with the behavioral measures in the non-musicians. CONCLUSIONS/SIGNIFICANCE: Our study is the first to reveal the correlation between the probability of chord progression and the corresponding neuromagnetic response. The current results suggest that the physiological response is a reflection of the probabilistic representations of the musical syntax. Moreover, the results indicate that the probabilistic representation is related to the musical training as well as the sensitivity of an individual.