Representation of the Interaural Level Difference Cue in Auditory Cortex of the Rat

Date of Completion

January 2012


Psychology, Psychobiology




The Interaural level difference (ILD) is one of three primary cues mammals use for sound localization and the primary cue for azimuth localization of high frequencies. It is created when a sound comes from off the midline and the shadow of the head decreases the sound level at the ear opposite, or contralateral to the sound source. Then, what begins as a monaural sound level signal from each ear is integrated to create a binaural code representing the ILD in the lateral superior olive (LSO). In this initial stage ILD spike rate functions are strongly shaped by excitation from the ipsilateral ear and inhibition from the contralateral ear in a shape strongly characterized with a sigmoid function. From the LSO the primary pathway crosses to the opposite side of the brain and goes to the inferior colliculus (IC) where the signal is inverted such that contralateral input is excitatory and ipsilateral inhibitory. This profile is retained in the majority of ILD spike-rate functions found in the medial geniculate body (MGB) of the thalamus, and the primary auditory cortex (A1). One study investigating the response properties in A1 found approximately 75% of neurons are accurately described as a linear filter that combines sound level and frequency band from monaural inputs. with the remaining 25% providing nonlinear binaural responses specialized for specific features of the stimuli (Schnupp et al. 2001). The implication of this result is that A1 may provide a gateway to processing cortices specialized to extract features of the stimuli such as spatial or frequency specific information. In a previous publication we showed a cortical region ventral to A1 (VAF) that responds to significantly narrower frequency cues and has a nonmonotonic response to increasing sound level. From these results we hypothesized that ventral cortices may also be specialized for encoding binaural responses. ^ The literature concerning spatial cue representation in the auditory cortex of the rat is limited; in fact only one study found attempted to quantify ILD responsiveness and organization (Kelly and Sally 1988). Therefore the initial goal of this research is to comprehensively describe how the rat cortex represents ILD cues over a wide range of average binaural levels (ABL) and is addressed in Chapter 1. Specifically, this chapter covers development of a technique that enables systematic and objective comparison of response topographies and designation of cortical regions, followed by multi-unit cortical response recordings using stimuli that cover a wide range of ILD and ABL conditions. This chapter also includes characterization of spike rate response areas and binaural sensitivity. Chapter 2 investigates the relationship between spike rate response tuning, and the ability to discriminate ILD position. This method uses an approximation of a 2-alternative forced-choice paradigm to assign discrimination ability to individual responses, and examines response variation to changes in sound level. Finally, Chapter 3 provides a brief analysis testing the hypothesis that the ILD cue is encoded in the temporal response. ^