Responses of auditory-cortex neurons to structural features of natural sounds

Sound-processing strategies that use the highly non-random structure of natural sounds may confer evolutionary advantage to many species. Auditory processing of natural sounds has been studied almost exclusively in the context of species-specific vocalizations, although these form only a small part of the acoustic biotope. To study the relationships between properties of natural soundscapes and neuronal processing mechanisms in the auditory system, we analysed sound from a range of different environments. Here we show that for many non-animal sounds and background mixtures of animal sounds, energy in different frequency bands is coherently modulated. Co- modulation of different frequency bands in background noise facilitates the detection of tones in noise by humans, a phenomenon known as co-modulation masking release (CMR). We show that comodulation also improves the ability of auditory-cortex neurons to detect tones in noise, and we propose that this property of auditory neurons may underlie behavioural CMR. This correspondence may represent an adaptation of the auditory system for the use of an attribute of natural sounds to facilitate real-world processing tasks.