Cues for Sound Localization Are Encoded in Multiple Aspects of Spike Trains in the Inferior Colliculus

Abstract

To localize sound, information from three cues--interaural timing differences (ITDs), interaural level differences (ILDs), and spectral notch cues (SNs)--must be properly integrated. The inferior colliculus (IC) receives convergent input from neurons encoding all three cues. Using virtual space stimuli and information theoretic techniques, we investigated the coding of the various localization cues in single neurons of the IC under different encoding schemes. Here we focus on the analysis of information encoded by first-spike latency, in comparison to previous results on discharge rate and ongoing spike timing. The results show that the localization cues converge to different degrees in particular neurons. ITD information is conveyed most strongly by spike rate, with small amounts of independent information in latency and ongoing spike timing. ILD information shows a similar pattern, with larger mutual information values for all three cues. For these cues, ongoing spike timing does not typically contribute independent information over that captured by a joint rate/first-spike latency code. SNs are coded by both rate and first-spike latency, but ongoing spike timing significantly enhances their representation in a best frequency-dependent manner, as long as the temporal envelope of the stimulus can be used in the decoder. The differential coding of the localization cues suggests that information about multiple cues could be multiplexed onto the responses of single neurons.

Intrinsic variability of latency to first-spike

“…One key advantage of this form of neuronal coding over, say, rate coding, is the speed with which it allows information transfer and processing (Thorpe et al 1996;Guyonneau et al 2005). Latency coding may be ubiquitous in nervous systems, as indicated by a wide range of studies in diverse neuronal structures such as the auditory system (Furukawa and Middlebrooks 2002;Heil 2004;Chase and Young 2008), somato-sensory system (Panzeri et al 2001;Johansson and Birznieks 2004), or the visual system (Kjaer et al 1996;Rullen 2003) both for primary (retina gollish) and higher-level (Kiani et al 2005;Mormann et al 2008) functions. For efficient latency coding, as in other forms of information encoding by spike timing, the precise timing of spike generation is essential.…”

Insensitivity to the coherence of interaural-time-difference modulation across frequency channels

“…Both of the nuclei have tonotopic structures, indicating frequency-by-frequency processing. Single neurons in the central nucleus of the inferior colliculus (ICc), a higher-level processing, are known to represent both ITD and ILD information simultaneously [15]. The ICc is, again, tonotopic, indicating that the integration of ITD and ILD is performed also in a frequency specific manner.…”

An experiment set-up for analysis of lateralization judgments of binaural stimuli

“…The kind of stimulus used in this work distinguishes our methodology from most usual experimental techniques (Chait et al, 2006;Chase and Young, 2008;Delgutte et al, 1999;Kollmeier, 2002a, 2002b;Zhang and Hartmann, 2006), which in general adopt either single sinusoidal signals or unfiltered clicks or tone bursts comprising harmonic components above the auditory system bandwidth.…”