The discrimination of interaural level difference sensitivity functions: development of a taxonomic data template for modelling

Abstract

BackgroundA major cue for the position of a high-frequency sound source in azimuth is the difference in sound pressure levels in the two ears, Interaural Level Differences (ILDs), as a sound is presented from different positions around the head. This study aims to use data classification techniques to build a descriptive model of electro-physiologically determined neuronal sensitivity functions for ILDs. The ILDs were recorded from neurons in the central nucleus of the Inferior Colliculus (ICc), an obligatory midbrain auditory relay nucleus. The majority of ICc neurons (~ 85%) show sensitivity to ILDs but with a variety of different forms that are often difficult to unambiguously separate into different information-bearing types. Thus, this division is often based on laboratory-specific and relatively subjective criteria. Given the subjectivity and non-uniformity of ILD classification methods in use, we examined if objective data classification techniques for this purpose. Our key objectives were to determine if we could find an analytical method (A) to validate the presence of four typical ILD sensitivity functions as is commonly assumed in the field, and (B) whether this method produced classifications that mapped on to the physiologically observed results.MethodsThe three-step data classification procedure forms the basic methodology of this manuscript. In this three-step procedure, several data normalization techniques were first tested to select a suitable normalization technique to our data. This was then followed by PCA to reduce data dimensionality without losing the core characteristics of the data. Finally Cluster Analysis technique was applied to determine the number of clustered data with the aid of the CCC and Inconsistency Coefficient values.ResultsThe outcome of a three-step analytical data classification process was the identification of seven distinctive forms of ILD functions. These seven ILD function classes were found to map to the four “known” ideal ILD sensitivity function types, namely: Sigmoidal-EI, Sigmoidal-IE, Peaked, and Insensitive, ILD functions, and variations within these classes. This indicates that these seven templates can be utilized in future modelling studies.ConclusionsWe developed a taxonomy of ILD sensitivity functions using a methodological data classification approach. The number and types of generic ILD function patterns found with this method mapped well on to our electrophysiologically determined ILD sensitivity functions. While a larger data set of the latter functions may bring a more robust outcome, this good mapping is encouraging in providing a principled method for classifying such data sets, and could be well extended to other such neuronal sensitivity functions, such as contrast tuning in vision.

The Mathematically Describable ILD Patterns

“…Nearly 85% of the electrophysiological data were of the peak and sigmoidal type of ILD patterns. These analyses were completely congruent with the Cluster Analysis and the seven ILD pattern types from statistical analyses corresponded very well with the seven ILD pattern types determined by a previous Cluster Analysis [5]. Table 4.…”

“…The author has received ILD data set for normalization and classification studies from his PhD supervisor whom had been the co-author of both manuscripts in [5], [47].…”

Effect of background noise on neuronal coding of interaural level difference cues in rat inferior colliculus

“…The main focus of this study was to assess the effect of background noise on ILD sensitivity of the IC neurons. As this effect may differ between neurons coding for different azimuthal locations, we grouped the recorded ILD sensitivity functions into six categories based on their putative functional role in the coding of sound location, using descriptors that have been identified and used in previous studies in the IC and other auditory structures to define ILD sensitivity (Irvine, 1986 ; Irvine et al ., 1996 ; Uragun & Rajan, 2013 ).…”

“…Although despite a long history at all levels of the auditory system of describing ILD functions as being monotonic (sigmoidal-shaped) or non-monotonic (peaked), there is no strong theoretical basis to guide what form of functions should be used to fit to ILD sensitivity functions in IC or other areas in the auditory pathway (Uragun & Rajan, 2013 ). Therefore, to determine if an ILD function was monotonic or non-monotonic, initially we used a sigmoidal function (Boltzmann) and one bell-shaped function (Gaussian).…”