Spectral information encoding in the cochlear nucleus and inferior colliculus: A study based on the random spectral shape method

Within the auditory system, neural populations with distinct morphologies, physiological response properties, and projection patterns participate in the parallel processing of acoustic information. To varying degrees, these populations encode behaviorally relevant information about the stimulus spectrum---spectral cues that are required for accurate sound identification and localization. This work describes the development of the random spectral shape (RSS) method---a numerical technique that can be used to investigate how different neurons represent spectral information within a complex acoustic environment. The RSS method, which postulates a linear relationship between the stimulus spectrum and discharge rate, is used to (1) extract the frequency response properties of single auditory neurons and (2) test the validity of low-order spectral encoding models.; The RSS method is first applied to a study of the cochlear nucleus---a processing center where the anatomy and physiology have already been studied extensively. In most principal neurons of the ventral cochlear nucleus (VCN), energy at best frequency (BF) produces an excitatory response that is accurately predicted by a linear model. Thus, VCN principal neurons possess the properties necessary to generate rate-place representations of spectral shape. By contrast, frequency response properties of dorsal cochlear nucleus (DCN) principal neurons are consistent with a stronger inhibitory influence. In these neurons, complex inhibitory interactions are likely responsible for spectral notch sensitivity---a sensitivity to wideband features that is not related to spectral energy in any simple way. These results confirm what is already known about CN neurons, thus establishing the RSS method as a legitimate tool for the analysis of spectral encoding across different parallel pathways. A binaural version of the RSS method also proves useful when tested in the inferior colliculus (central nucleus; ICC), a higher-level auditory structure where spectral encoding properties are not as well understood. For units within the ICC study, frequency response properties and spectral encoding results are often consistent with those of lower brainstem inputs to the ICC. Future RSS-based analyses within the ICC and other complex binaural nuclei will likely provide insight into the representation of spectral information across auditory pathways that are yet to be explored.