Parallel Coding of What and Where in the Auditory Brainstem of the Barn Owl

Abstract

The brainstem of the barn owl has a network of nuclei which process auditory information. Sound localization studies have been able to define two processing pathways that originate at the cochlear nuclei. The first pathway, which originates at nucleus magnocellularis (NM), has a proclivity for processing information related to interaural time differences (ITDs), a sound localization cue associated with the azimuth location of a sound source. The second pathway originates at nucleus angularis (NA) and has been shown to process information related to interaural level difference (ILD), which cues the vertical location of a sound source. These two pathways merge at the level of the inferior colliculus where a map of auditory space is formed in the external nucleus of the inferior colliculus (ICx). These pathways must also encode information about the spectrotemporal features of sound, which are important for sound identification and discrimination. We have found that the ILD pathway encodes spectrotemporal information of sounds with higher fidelity than the ITD pathway. Specifically, using in vivo recording of neural responses and computational modeling, we have demonstrated that NA more reliably encodes spectrotemporal information and that this is quantitatively related to specific features of neurons' spectrotemporal tuning. Furthermore, we show that the ILD pathway's superior ability to encode spectrotemporal information is maintained downstream in the processing pathway at the nucleus LLDp through stimulus driven spectrotemporally tuned inhibition which increases signal-noise-ratios in these cells through spectral surround suppression. We hypothesize that the ILD pathway plays an essential role conveying information about stimulus identity to downstream neural structures involved in decision making in the mid- and forebrain.