Torsten Marquardt

Precise inhibition is essential for microsecond interaural time difference coding

Microsecond differences in the arrival time of a sound at the two ears (interaural time differences, ITDs) are the main cue for localizing low-frequency sounds in space. Traditionally, ITDs are thought to be encoded by an array of coincidence-detector neurons, receiving excitatory inputs from the two ears via axons of variable length (‘delay lines’), to create a topographic map of azimuthal auditory space. Compelling evidence for the existence of such a map in the mammalian lTD detector, the medial superior olive (MSO), however, is lacking. Equally puzzling is the role of a—temporally very precise—glycine-mediated inhibitory input to MSO neurons. Using in vivo recordings from the MSO of the Mongolian gerbil, we found the responses of ITD-sensitive neurons to be inconsistent with the idea of a topographic map of auditory space. Moreover, local application of glycine and its antagonist strychnine by iontophoresis (through glass pipette electrodes, by means of an electric current) revealed that precisely timed glycine-controlled inhibition is a critical part of the mechanism by which the physiologically relevant range of ITDs is encoded in the MSO. A computer model, simulating the response of a coincidence-detector neuron with bilateral excitatory inputs and a temporally precise contralateral inhibitory input, supports this conclusion.

Detection of static and dynamic changes in interaural correlation

This study examines the relation between a static and a dynamic measure of interaural correlation discrimination: (1) the just noticeable difference (JND) in interaural correlation and (2) the minimum detectable duration of a fixed interaural correlation change embedded within a single noise-burst of a given reference correlation. For the first task, JNDs were obtained from reference interaural correlations of + 1, -1, and from 0 interaural correlation in either the positive or negative direction. For the dynamic task, duration thresholds were obtained for a brief target noise of +1, -1, and 0 interaural correlation embedded in reference marker noise of +1, -1, and 0 interaural correlation. Performance with a reference interaural correlation of +1 was significantly better than with a reference correlation of -1. Similarly, when the reference noise was interaurally uncorrelated, discrimination was significantly better for a target correlation change towards +1 than towards -1. Thus, for both static and dynamic tasks, interaural correlation discrimination in the positive range was significantly better than in the negative range. Using the two measures, the length of a binaural temporal window was estimated. Its equivalent rectangular duration (ERD) was approximately 86 ms and independent of the interaural correlation configuration.

Representation of interaural time delay in the human auditory midbrain

Interaural time difference (ITD) is a critical cue to sound-source localization. Traditional models assume that sounds leading at one ear, and perceived on that side, are processed in the opposite midbrain. Using functional magnetic resonance imaging we demonstrate that as the ITDs of sounds increase, midbrain activity can switch sides, even though perceived location remains on the same side. The data require a new model for human ITD processing.

Low-frequency characteristics of human and guinea pig cochleae.

Previous physiological studies investigating the transfer of low-frequency sound into the cochlea have been invasive. Predictions about the human cochlea are based on anatomical similarities with animal cochleae but no direct comparison has been possible. This paper presents a noninvasive method of observing low frequency cochlear vibration using distortion product otoacoustic emissions (DPOAE) modulated by low-frequency tones. For various frequencies (15-480 Hz), the level was adjusted to maintain an equal DPOAE-modulation depth, interpreted as a constant basilar membrane displacement amplitude. The resulting modulator level curves from four human ears match equal-loudness contours (ISO226:2003) except for an irregularity consisting of a notch and a peak at 45 Hz and 60 Hz, respectively, suggesting a cochlear resonance. This resonator interacts with the middle ear stiffness. The irregularity separates two regions of the middle ear transfer function in humans: A slope of 12 dB/octave below the irregularity suggests mass-controlled impedance resulting from perilymph movement through the helicotrema; a 6-dB/octave slope above the irregularity suggests resistive cochlear impedance and the existence of a traveling wave. The results from four guinea pig ears showed a 6-dB/octave slope on either side of an irregularity around 120 Hz, and agree with published data.

Emphasis of spatial cues in the temporal fine structure during the rising segments of amplitude-modulated sounds

Significance Sound localization in rooms is challenging, especially for hearing-impaired listeners or technical devices. Reflections from walls and ceilings make it difficult to distinguish sounds arriving direct from a source from the mixture of potentially confounding sounds arriving a few milliseconds later. Nevertheless, normal-hearing listeners show remarkable localization abilities under such challenging listening conditions. The current study investigates the underlying mechanisms responsible for accurate localization performance. Stimuli we named amplitude modulated binaural beats are introduced. These stimuli allow for a more controlled testing than speech stimuli. Results from two experimental procedures (psychoacoustics and the brain-imaging technique magnetoencephalography) reveal that listeners appear to “glimpse” reliable spatial information during the early, rising portions of modulated sounds, ignoring later-arriving, potentially conflicting, spatial cues. The ability to locate the direction of a target sound in a background of competing sources is critical to the survival of many species and important for human communication. Nevertheless, brain mechanisms that provide for such accurate localization abilities remain poorly understood. In particular, it remains unclear how the auditory brain is able to extract reliable spatial information directly from the source when competing sounds and reflections dominate all but the earliest moments of the sound wave reaching each ear. We developed a stimulus mimicking the mutual relationship of sound amplitude and binaural cues, characteristic to reverberant speech. This stimulus, named amplitude modulated binaural beat, allows for a parametric and isolated change of modulation frequency and phase relations. Employing magnetoencephalography and psychoacoustics it is demonstrated that the auditory brain uses binaural information in the stimulus fine structure only during the rising portion of each modulation cycle, rendering spatial information recoverable in an otherwise unlocalizable sound. The data suggest that amplitude modulation provides a means of “glimpsing” low-frequency spatial cues in a manner that benefits listening in noisy or reverberant environments.

Action sounds update the mental representation of arm dimension: contributions of kinaesthesia and agency.

Auditory feedback accompanies almost all our actions, but its contribution to body-representation is understudied. Recently it has been shown that the auditory distance of action sounds recalibrates perceived tactile distances on one’s arm, suggesting that action sounds can change the mental representation of arm length. However, the question remains open of what factors play a role in this recalibration. In this study we investigate two of these factors, kinaesthesia, and sense of agency. Across two experiments, we asked participants to tap with their arm on a surface while extending their arm. We manipulated the tapping sounds to originate at double the distance to the tapping locations, as well as their synchrony to the action, which is known to affect feelings of agency over the sounds. Kinaesthetic cues were manipulated by having additional conditions in which participants did not displace their arm but kept tapping either close (Experiment 1) or far (Experiment 2) from their body torso. Results show that both the feelings of agency over the action sounds and kinaesthetic cues signaling arm displacement when displacement of the sound source occurs are necessary to observe changes in perceived tactile distance on the arm. In particular, these cues resulted in the perceived tactile distances on the arm being felt smaller, as compared to distances on a reference location. Moreover, our results provide the first evidence of consciously perceived changes in arm-representation evoked by action sounds and suggest that the observed changes in perceived tactile distance relate to experienced arm elongation. We discuss the observed effects in the context of forward internal models of sensorimotor integration. Our results add to these models by showing that predictions related to action sounds must fit with kinaesthetic cues in order for auditory inputs to change body-representation.

Towards objective measures of speech intelligibility for Cochlear Implant users in reverberant environments

This study validates a novel approach to predict speech intelligibility for Cochlear Implant users (CIs) in reverberant environments. More specifically, we explore the use of existing objective quality and intelligibility metrics, applied directly to vocoded speech degraded by room reverberation, here assessed at ten different reverberation time (RT60) values: 0 s, 0.4 s - 1.0 s (0.1 s increments), 1.5 s and 2 s. Eight objective speech intelligibility predictors (SIPs) were investigated in this study. Of these, two were non-intrusive (i.e. did not require a reference signal) audio quality measures, four were intrusive, and two were intrusive speech intelligibility indexes. Three types of vocoders were implemented to examine how speech intelligibility predictions depended on the vocoder type. These were: noise-excited vocoder, tone-excited vocoder and a FFT-based N-of-M vocoder. Experimental results show that several intrusive quality and intelligibility measures were highly correlated with exponentially fit CI intelligibility data. On the other hand, only a recently - developed non-intrusive measure showed high correlations. These evaluations suggest that CI intelligibility may be accurately assessed via objective metrics applied to vocoded speech, thus may reduce the need for expensive and time-consuming listening tests.

A π-Limit for Coding ITDs: Implications for Binaural Models

Interaural time difference (ITD) is an important sound localization cue, arising from the different travel time of a sound from its source to the left and right ears for sources located to either side of the head. Neural extraction of ITDs occurs in the superior olivary complex (SOC). SOC neurons receive binaural input and are thought to perform a process of cross-correlation between the spike trains arriving from left and right ear. Since the acoustic signals are bandpass filtered by the cochlea, the ITD tuning curves of SOC neurons to noise stimuli exhibit shapes similar to cross-correlation functions of bandpass noise (Yin and Chan 1990). They are generally sinusoidal in shape and their amplitude is symmetrically damped either side of their tuning maximum (Fig. 1A). The current view of binaural processing, based on Jeffress’ hypothesis (Jeffress 1948), lays down a system of various axonal travel time differences to individual coincidence detecting SOC neurons. This results in a shift of the entire tuning curves along the ITD axis, as illustrated in Fig. 1A for several neurons with increasing best ITD (tuning maximum). There is no principal limit to the shift in Jeffress-based models. The range of best ITD is, rather, determined by the range of naturally occurring ITDs, the so-called ecological range. Many physiological recordings are not symmetric about the peak. A wellknown extension of Jeffress model therefore includes neurons innervated by excitation from one ear and inhibition from the other (e.g. Durlach 1963; Breebaart et al. 2001). This leads theoretically to a horizontal inversion of the cross-correlation function, i.e. to tuning curves symmetric about their tuning minimum, or trough. However, many recorded tuning curves still do not fit in either the “peaker” or the “trougher” category but are somewhere in between (McAlpine et al. 1996; Fitzpatrick et al. 2000). These neurons have been referred to as “tweeners” and are currently ignored in binaural models.

The Influence of the Envelope Waveform on Binaural Tuning of Neurons in the Inferior Colliculus and Its Relation to Binaural Perception

Recently, Klein-Hennig et al. (J Acoust Soc Am 129:3856–3872, 2011) suggested a design for envelope waveforms that allows for independent setting of the duration of the four segments of an envelope cycle – pause, attack, sustain, and decay. These authors conducted psychoacoustic experiments to determine the threshold interaural time differences (ITDs) for different waveforms and revealed that a steep attack flank and at least 4 ms of pause duration prior to the attack are optimal for discrimination performance, whilst sustained and decay durations were of only minor influence. The current study tests the sharpness of rate-ITD-functions recorded in the inferior colliculus of guinea pigs in response to a similar set of waveforms, examining their relationship to the psychoacoustic data. Particular focus is applied to temporally asymmetric envelope waveforms: a long 15-ms attack and a short 1.5-ms decay envelope and the temporally inverted envelope with a short 1.5-ms attack and a long 15-ms decay.

A Comparison of Two Objective Measures of Binaural Processing: The Interaural Phase Modulation Following Response and the Binaural Interaction Component.

There has been continued interest in clinical objective measures of binaural processing. One commonly proposed measure is the binaural interaction component (BIC), which is obtained typically by recording auditory brainstem responses (ABRs)—the BIC reflects the difference between the binaural ABR and the sum of the monaural ABRs (i.e., binaural − (left + right)). We have recently developed an alternative, direct measure of sensitivity to interaural time differences, namely, a following response to modulations in interaural phase difference (the interaural phase modulation following response; IPM-FR). To obtain this measure, an ongoing diotically amplitude-modulated signal is presented, and the interaural phase difference of the carrier is switched periodically at minima in the modulation cycle. Such periodic modulations to interaural phase difference can evoke a steady state following response. BIC and IPM-FR measurements were compared from 10 normal-hearing subjects using a 16-channel electroencephalographic system. Both ABRs and IPM-FRs were observed most clearly from similar electrode locations—differential recordings taken from electrodes near the ear (e.g., mastoid) in reference to a vertex electrode (Cz). Although all subjects displayed clear ABRs, the BIC was not reliably observed. In contrast, the IPM-FR typically elicited a robust and significant response. In addition, the IPM-FR measure required a considerably shorter recording session. As the IPM-FR magnitude varied with interaural phase difference modulation depth, it could potentially serve as a correlate of perceptual salience. Overall, the IPM-FR appears a more suitable clinical measure than the BIC.