Gabor Analysis of Auditory Midbrain Receptive Fields: Spectro-Temporal and Binaural Composition

Abstract

The spectro-temporal receptive field (STRF) is a model representation of the excitatory and inhibitory integration area of auditory neurons. Recently it has been used to study spectral and temporal aspects of monaural integration in auditory centers. Here we report the properties of monaural STRFs and the relationship between ipsi- and contralateral inputs to neurons of the central nucleus of cat inferior colliculus (ICC) of cats. First, we use an optimal singular-value decomposition method to approximate auditory STRFs as a sum of time-frequency separable Gabor functions. This procedure extracts nine physiologically meaningful parameters. The STRFs of approximately 60% of collicular neurons are well described by a time-frequency separable Gabor STRF model, whereas the remaining neurons exhibited obliquely oriented or multiple excitatory/inhibitory subfields that require a nonseparable Gabor fitting procedure. Parametric analysis reveals distinct spectro-temporal tradeoffs in receptive field size and modulation filtering resolution. Comparisons between an identical model used to study spatio-temporal integration areas of visual neurons further shows that auditory and visual STRFs share numerous structural properties. We then use the Gabor STRF model to compare quantitatively receptive field properties of contra- and ipsilateral inputs to the ICC. We show that most interaural STRF parameters are highly correlated bilaterally. However, the spectral and temporal phases of ipsi- and contralateral STRFs often differ significantly. This suggests that activity originating from each ear share various spectro-temporal response properties such as their temporal delay, bandwidth, and center frequency but have shifted or interleaved patterns of excitation and inhibition. These differences in converging monaural receptive fields expand binaural processing capacity beyond interaural time and intensity aspects and may enable colliculus neurons to detect disparities in the spectro-temporal composition of the binaural input.

Spectral and Temporal Modulation Tradeoff in the Inferior Colliculus

“…During the 20-min presentation of the DMR, each ear was stimulated with an independent DMR. This allowed us to concurrently measure independent STRFs for the contralateral and ipsilateral ears (Qiu et al 2003). For the purpose of this study, only the STRF for the contralateral ear are considered as it characterizes the dominant phase locked response of CNIC neurons (Qiu et al 2003).…”

“…2, B and D). Neuronal responses were classified as either band-pass or low-pass tuned response pattern for both the spectral and temporal modulation dimensions (Qiu et al 2003). To do this, the lower 3-dB cutoff was measured for the sMTF and tMTF independently.…”

“…Spectral modulation sensitivity has been tested in the IC with spectral ripple sounds ) and a variety of broadband noises (Keller and Takahashi 2000;Lesica and Grothe 2008;Yu and Young 2000). CNIC units can exhibit tuned responses to spectral modulations with selectivity for spectral modulation extending up to ϳ3 cycle/octave (Qiu et al 2003).…”

The Consequences of Response Nonlinearities for Interpretation of Spectrotemporal Receptive Fields

“…However, in higher levels of the auditory system, white noise tends to be ineffective at eliciting neuronal responses (Wang et al, 2005). This has led auditory researchers to use other stimuli, such as dynamic random chords (DRCs) (deCharms et al, 1998;Schnupp et al, 2001;Rutkowski et al, 2002;Linden et al, 2003), natural sounds Theunissen et al, 2000;Machens et al, 2004), and a family of stimuli whose basic element is a ripple, a sound modulated sinusoidally in both the temporal and spectral domains (Kowalski et al, 1996a,b;Calhoun and Schreiner, 1998;Klein et al, 2000;Escabí and Schreiner, 2002;Miller et al, 2002;Qiu et al, 2003;Fritz et al, 2005). For many of these stimulus classes, the power at any two points in spectrotemporal space is uncorrelated by design, which simplifies estimation of the STRF.…”

Increasing Spectrotemporal Sound Density Reveals an Octave-Based Organization in Cat Primary Auditory Cortex

“…The stimulus ensemble used to derive STRFs is typically composed of random broadband sounds that do not contain any autocorrelation structure in the time or frequency domains (Eggermont et al, 1983;Schreiner and Calhoun, 1994;Blake and Merzenich, 2002;Rutkowski et al, 2002). Stimulus ensembles composed of random overlapping tone pips (deCharms et al, 1998;Blake and Merzenich, 2002;Valentine and Eggermont, 2004) and dynamic ripple sounds have been used extensively because they simulate elementary features of complex sounds (Depireux et al, 2001;Miller et al, 2001Miller et al, , 2002Qiu et al, 2003;Escabí and Read, 2005). Both multi-tone pip and ripple stimuli, although still artificial (i.e., not present in the natural environment), are presumably more natural than single nonoverlapping tone pips because they contain spectrotemporal modulations.…”