Brainstem mechanisms for analyzing temporal patterns of echolocation sounds: a model for understanding early stages of speech processing?

Because of their stereotyped audio--vocal behavior and highly accessible brainstem circuitry, echolocating bats provide a good model system in which to study the neural mechanisms that underlie the analysis of temporal features of sound. This paper reviews the lower brainstem auditory circuitry and describes selected forms of information processing that are performed in the pathways of the lower brainstem and auditory midbrain (inferior colliculus (IC)). Several examples of neural circuits in echolocating bats point out the ways in which inputs with different properties converge on IC neurons to create selectivity for specific temporal features of sound that are common to speech and echolocation. The initial transformations of auditory nerve input that occur in the lower brainstem pathways include a change in sign from excitatory input to inhibitory output, changes in discharge pattern, and the creation of delay lines. Convergence of multiple inputs on neurons in the IC produces tuning for temporal features of sound including duration, the direction of frequency sweeps, modulation rate, and interstimulus interval. The auditory cortex exerts control over some of this processing by sharpening or shifting neuronal filter properties. The computational processes that occur in the IC result in integration across a time scale that is consistent with the rate at which biological sounds are produced, whether they be echolocation signals or human speech components.