Natural sounds are represented as spectrotemporal modulations in human auditory cortex
The question of how the human auditory cortex represents complex natural sounds is one of the most fundamental ones in cognitive neuroscience. While previous studies have documented a number of tonotopically organized areas occupying the primary and non-primary auditory cortices, there are additionally studies that have shown preference to other sound features, such as sound location and speech sound category, in specific auditory cortical areas. Furthermore, there are findings in animal models suggesting that auditory cortical neurons are selective to various types of spectrotemporal sound features, however, it has not been known whether there are topographic representations of spectrotemporal features, a model that could potentially explain how complex natural sounds are represented in the human auditory cortex.
In their recent study, Santoro et al. (2014), analyzed data from two previous functional magnetic resonance imaging experiments where a rich array of natural sounds had been presented to healthy volunteers. They then tested between three computational models, where the first model assumed that auditory cortex represents sounds as spectral/frequency information, the second model assumed that auditory cortex represents sounds as temporal information and third model assumed that sounds are represented as sets of spectrotemporal modulations. The results indicate that natural sounds are represented with frequency-specific analysis of spectrotemporal modulations. Furthermore, in anterior auditory cortex regions analysis of spectral information was found to be more fine-grained than in posterior auditory cortical areas, wherein temporal information was, in turn, found to be represented more accurately with rather coarse representation of spectral information.
In sum, the authors provide a very exciting approach to testing how well alternative computational models, inspired by neurophysiological findings obtained in animal research, can predict hemodynamic data collected during presentation of a various natural sounds. Taken together, the results offer a very interesting vantage point into how natural sounds could be represented in the human auditory cortex. It is easy to predict that the approach and findings will generate wide interest and help further research efforts to significantly step forward, especially given the increasing popularity of the use of naturalistic stimuli in neuroimaging research.
Reference: Santoro R, Moerel M, De Martino F, Goebel R, Ugurbil K, Yacoub E, Formisano E. Encoding of natural sounds at multiple spectral and temporal resolutions in the human auditory cortex. PLoS Computational Biology 10: e1003412. http://dx.doi.org/10.1371/journal.pcbi.1003412