Tracking Sound Dynamics in Human Auditory Cortex: New macroscopic perspectives from MEG
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Both the external world and our internal world are full of changing activities , and the question of how these two dynamic systems are linked constitutes the most intriguing and fundamental question in neuroscience and cognitive science. This study specifically investigates the processing and representation of sound dynamic information in human auditory cortex using magnetoencephalography (MEG), a non-invasive brain imaging technique whose high temporal resolution (on the order of ~1ms) makes it an appropriate tool for studying the neural correlates of dynamic auditory information. The other goal of this study is to understand the essence of the macroscopic activities reflected in non-invasive brain imaging experiments, specifically focusing on MEG. Invasive single-cell recordings in animals have yielded a large amount of information about how the brain works at a microscopic level. However, there still exist large gaps in our understanding of the relationship between the activities recorded at the microscopic level in animals and at the macroscopic level in humans, which have yet to be reconciled in terms of their different spatial scales and activities format, making a unified knowledge framework still unsuccessful. In this study, natural speech sentences and sounds containing speech-like temporal dynamic features are employed to probe the human auditory system. The recorded MEG signal is found to be well correlated with the stimulus dynamics via amplitude modulation (AM) and/or phase modulation (PM) mechanisms. Specifically, oscillations at various frequency bands are found to be the main information-carrying elements of the MEG signal, and the two major parameters of these endogenous brain rhythms, amplitude and phase, are modulated by incoming sensory stimulus dynamics, corresponding to AM and PM mechanism, to track sound dynamics. Crucially, such modulation tracking is found to be correlated with human perception and behavior. This study suggests that these two dynamic and complex systems, the external and internal worlds, systematically communicate and are coupled via modulation mechanism, leading to a reverberating flow of information embedded in oscillating waves in human cortex. The results also have implications for brain imaging studies, suggesting that these recorded macroscopic activities reflect brain state, the more close neural correlate of high-level cognitive behavior.