Neural processing of natural sounds

Natural sounds include animal vocalizations, environmental sounds such as wind, water and fire noises, and non-vocal sounds made by animals and humans for communication. These natural sounds have characteristic statistical properties that make them perceptually salient and that drive auditory neurons in optimal regimes for information transmission.Recent advances in statistics and computer sciences have enabled neurophysiologists to extract the stimulus–response function of complex auditory neurons from responses to natural sounds. These studies have shown a hierarchical processing that leads to the neural detection of progressively more complex natural sound features and have demonstrated the importance of the acoustical and behavioural context for the neural responses.High-level auditory neurons have been shown to be exquisitely selective for conspecific calls. This fine selectivity could have an important role in species recognition, vocal learning in songbirds and, in the case of the bats, the processing of the sounds used in echolocation. Research that investigates how communication sounds are categorized into behaviourally meaningful groups (for example, call types in animals and words in human speech) remains in its infancy.Animals and humans also excel at separating communication sounds from each other and from background noise. Neurons that detect communication calls in noise have been found, but the neural computations involved in sound source separation and natural auditory scene analysis remain overall poorly understood. Thus, future auditory research will have to focus not only on how natural sounds are processed by the auditory system but also on the computations that enable this processing to occur in natural listening situations.The complexity of the computations needed in the natural hearing task might require a high-dimensional representation provided by an ensemble of neurons, and the use of natural sounds might be the best solution for understanding the ensemble neural code.