Spatial modulation: Hearing the environment

Auditory processing of complex sources, after an initial peripheral spectro-temporal stage, is thought to have a more central stage identify in the output time segments and frequency regions of higher activity by way of a temporal and spectral modulation analysis. Such analysis broadens the view on perception, both that of complex signals and of auditory scene analysis (ASA). When resolution of temporal and spectral modulations is adequate, the auditory system can decode complex signals and separate simultaneous sources in a scene. Although research in the modulation domain has uncovered important properties of the central (cortical) mechanism active in such analysis, so far it has bypassed the spatial dimension. The present study proposes to include spatial modulation in the horizontal plane into this mechanism. The signal emanating from multiple and diverse sources at different azimuths will first undergo peripheral binaural processing using known methods, consisting of frequency analysis, phasecompensated rectification, left-right cross-correlation, straightening, and weighted frequency integration. The output will represent azimuthal activity between -π and +π radians as a function of time. This analysis stage will be followed by the modulation analysis stage: convolution of the magnitudes, across the azimuth activity axis, with a kernel function that signifies resolution of nearby simultaneous sources. Results of the spatial modulation analysis will be shown as a function of the same input frequency analyzed and put through a stage of temporal modulation processing. Spatial and temporal modulation analysis results viewed side-by side will predict the temporal fluctuation rate and spatial source density at which perception of multiple sources should be optimal.