Sensory Coding and the Second Spectra of Natural Signals

Natural auditory and visual stimuli are shown to demonstrate consistent non-Gaussian signatures: their phase-only second spectra behave approximately as the inverse square root of frequency, a property which is due to a predominance of difference beats between Fourier components at slightly offset frequencies. These second spectra also demonstrate statistical scale invariance, a hypothesis which is tested explicitly by computing phase-only second spectra from bandpass-filtered data. The bandpass frequency selectivity observed experimentally in sensory cells is shown to be qualitatively consistent with a strategy of exploiting the non-Gaussian structure of natural signals.