Responses of inferior colliculus neurons to time-varying interaural phase disparity: effects of shifting the locus of virtual motion

Abstract

1. Motion of sound sources results in temporal variation of the binaural cues for sound localization. We evaluated the influence of virtual motion on neural tuning to one of these cues, interaural phase disparity (IPD). Responses to dichotic stimuli were recorded from single units in the inferior colliculus of the anesthetized cat and gerbil (Meriones unguiculatus). Static IPDs were generated by presenting dichotic tone pairs with a constant phase offset maintained for the duration of the stimulus. Time-varying IPDs were generated by simultaneously presenting a pure tone to one ear and a phase-modulated tone to the other ear. Sets of time-varying stimuli consisted of modulations through partially overlapping ranges of IPD, corresponding to movement of a sound source through partially overlapping arcs in the horizontal plane. 2. In agreement with previous results, neuronal discharge was typically a peaked function of static IPD resulting from both binaural facilitation at favorable IPDs and binaural suppression at unfavorable IPDs. Responses to time-varying IPD stimuli appeared to be shaped by the same facilitative and inhibitory mechanisms that underlie static IPD tuning. Modulation toward the peak of binaural facilitation increased the probability of discharge, and modulation toward the peak of binaural suppression decreased the probability of discharge. However, it was also clear that IPD tuning could be significantly altered by the temporal context of the stimulus. For the vast majority of units in response to modulation through partially overlapping ranges of IPD the discharge rate profiles were generally nonoverlapping. This shift in IPD tuning induced by the virtual motion reflects the fact that the binaural interaction associated with a given IPD depends on the recent history of stimulation. In addition, modulation in opposite directions through the same range of IPDs often elicited asymmetric responses. These nonlinearities imply that most inferior colliculus neurons do not unambiguously encode a specific IPD, but instead may encode small changes of IPD occurring virtually anywhere within their receptive fields. In a few cases modulation through overlapping ranges of IPD elicited contiguous response profiles, indicating that for these units responses were determined entirely by instantaneous IPD. 3. The nonlinearity of responses to time-varying IPD stimuli could not be attributed to monaural entrainment to the phase-modulated signals, did not depend on the phase modulation waveform, and occurred irrespective of which ear received the phase-modulated signal. Responses were similar in cats and gerbils, suggesting that the underlying mechanisms are common to binaural processing in diverse mammalian species.(ABSTRACT TRUNCATED AT 400 WORDS)

Processing Temporal Modulations in Binaural and Monaural Auditory Stimuli by Neurons in the Inferior Colliculus and Auditory Cortex

“…That is, adaptation will occur if a stimulus is on for a sufficient time period (i.e., a low frequency of modulation) but will not occur in the case of a rapidly changing stimulus that does not allow the adaptation to build. The more pronounced high-pass behavior of cortical compared to IC neurons to the BB stimuli is consistent with a greater magnitude of adaptation of cortical neurons, as has been previously reported (Spitzer and Semple 1993;McAlpine et al 2000;Malone et al 2002;Ingham and McAlpine 2004;TerMikaelian et al 2007).…”

“…Another possibility is that temporal sensitivity in the binaural system aids in the detection and discrimination of multiple source locations or provides information about the acoustical properties of spaces, both of which cause dynamic changes in interaural time and correlation (Bradley et al 2000;Faller and Merimaa 2004;Goupell and Hartmann 2006). Because of the broad tuning to ITDs, a population code is most likely to be used for ITD determination of source location (Fitzpatrick et al 1997;McAlpine et al 2001;Stecker et al 2005). In this case, locations of multiple sources are not directly available during any epoch of the stimulus stream, but must be obtained sequentially during epochs where a single source dominates the interaural time and correlation (Faller and Merimaa 2004).…”

Reversible Inactivation of the Dorsal Nucleus of the Lateral Lemniscus Reveals Its Role in the Processing of Multiple Sound Sources in the Inferior Colliculus of Bats

“…These features suggest that a moving sound source that begins in the contralateral sound field and moves with a certain velocity into the ipsilateral sound field should evoke more vigorous responses in an ICc cell than would be evoked when the sound source moved from the ipsilateral into the contralateral sound field. This type of sensitivity for the direction of movement has been reported for neurons in the IC of several mammals, including bats (Spitzer and Semple, 1993;Kleiser and Schuller, 1995;Wilson and O'Neill, 1998;McAlpine et al, 2000). Moreover, Keller and Takahashi (1996) proposed a similar mechanism to explain directionally selective motion sensitivity in IC neurons of the barn owl.…”

Dynamic changes in level influence spatial coding in the lateral superior olive

“…Therefore, stable ILD and ITD processing has been assumed at the earliest level of binaural coding in the SOC. At higher levels, dynamics had been assumed to be related to motion direction selectivity (Spitzer and Semple, 1993;. However, a recent study challenged the explanation that dynamics in ITD processing of IC neurons is related specifically to motion detection, and argued instead that dynamics might be a result of general adaptation to overall firing rate (McAlpine, et al, 2003).…”