Each compressed envelope is further decomposed using a bank of 20 bandpass modulation filters.

Modulation filters are conceptually similar to cochlear filters, except that they operate on (compressed) envelopes rather than the sound pressure waveform, and are tuned to frequencies an order of magnitude lower, as envelopes fluctuate at relatively slow rates. A modulation filter bank is consistent with previous auditory models (Bacon and Grantham, 1989 and Dau et al., 1997) as well as reports of modulation tuning in midbrain and thalamic neurons (Baumann et al., 2011, Joris et al., 2004, Miller et al., 2002 and Rodríguez et al., 2010). Both the cochlear and modulation filters in our model had bandwidths that increased with their center frequency (such that they were approximately constant on a logarithmic scale), as is observed in biological auditory systems. From cochlear envelopes and their modulation bands, we derive a representation http://www.selleckchem.com/products/PD-0332991.html of texture by computing statistics (red symbols in Figure 1). The statistics are time-averages of nonlinear functions of either the envelopes or the modulation

bands. Such statistics are in principle suited to summarizing stationary signals like textures, whose properties are constant over some moderate timescale. A priori, however, it is not obvious whether simple, biologically plausible statistics would have much explanatory AZD2281 power as descriptors of natural sounds or of their perception. Previous attempts to model sound texture have come from the machine audio and sound rendering communities (Athineos and Ellis, 2003, Dubnov et al., 2002, Saint-Arnaud and Popat, 1995, Verron et al., 2009 and Zhu and Wyse, 2004) and have involved representations unrelated to those in biological auditory systems. Of all the statistics the brain could

compute, which might be used by the auditory system? Natural sounds can provide clues: in order for a statistic to be useful for recognition, it must produce different values for different sounds. We considered a set of generic statistics and verified that they varied substantially across a set of 168 natural sound textures. We crotamiton examined two general classes of statistic: marginal moments and pairwise correlations. Both types of statistic involve averages of simple nonlinear operations (e.g., squaring, products) that could plausibly be measured using neural circuitry at a later stage of neural processing. Moments and correlations derive additional plausibility from their importance in the representation of visual texture (Heeger and Bergen, 1995 and Portilla and Simoncelli, 2000), which provided inspiration for our work. Both types of statistic were computed on cochlear envelopes as well as their modulation bands (Figure 1). Because modulation filters are applied to the output of a particular cochlear channel, they are tuned in both acoustic frequency and modulation frequency.