Timescales for Sparseness of Natural Sound: Implications for Auditory-Symbols Processing

The statistical structure of the natural visual environment influences the strategies for sensory processing in the retina and the thalamus of primates, where second-order redundancy is reduced, and in the primary visual areas of cortex, where filters with high output-kurtosis, which expose the “sparseness” of the visual environment, are believed to facilitate scene parsing and object segmentation. Here we examine the hypothesis of a similar link between the sparseness of natural auditory ensembles and their respective symbolic structures. We find that second-order redundancy reducing filters, which presumably operate at the cochlear level, have output kurtoses, much larger than those for images. Moreover, the kurtosis depends strongly on the length of the filter time-window. We find that the difference between the characteristic timescales at which such sparseness is maximal across species-specific ensembles—human speech and bird song—correlate with the known differences in the duration of their respective symbols and inter-symbol transitions. We discuss the implications for the design of sensory systems that would code efficiently such natural auditory stimuli.