Binaural modulation detection interference

When acoustic stimuli are presented over headphones listeners usually report that a sound image appears within their head. Interaural differences in time and intensity can move the position of this image within the head. This effect is usually referred to as lateralization. We will review some of the basic results on the lateralization of simple stimuli. The relationship between lateral position and the interaural differences of time and intensity will be summarized. These results are used to predict those values of interaural time and intensity that yield an equivalent lateral position. The results from experiments which measure the just discriminable differences in interaural time and intensity will also be reviewed. The relationship between these lateral discrimination results and those from the lateral position studies will be described. The various models of lateralization all assume some type of cross-correlation operation. The cross-correlation approach to modeling these data has some strengths and some weaknesses. Results concerning the interaural differences of onset time and ongoing time will be compared. An attempt will be made to provide a general framework for the basic data concerning the lateral position and lateral discrimination of simple stimuli. The extent to which results from these lateralization experiments can account for our ability to localize sound sources in the free field will be discussed.

Interaural time differences (ITD) and interaural level differences (ILD) both signal horizontal sound source location. To achieve a unified percept of our acoustic environment, these two cues require integration. In the present study, we tested this integration of ITD and ILD with electroencephalography (EEG) by measuring the mismatch negativity (MMN). The MMN can arise in response to spatial changes and is at least partly generated in auditory cortex. In our study, we aimed at testing for an MMN in response to stimuli with counter-balanced ITD/ILD cues. To this end, we employed a roving oddball paradigm with alternating sound sequences in two types of blocks: (a) lateralized stimuli with congruently combined ITD/ILD cues and (b) midline stimuli created by counter-balanced, incongruently combined ITD/ILD cues. We observed a significant MMN peaking at about 112–128 ms after change onset for the congruent ITD/ILD cues, for both lower (0.5 kHz) and higher carrier frequency (4 kHz). More importantly, we also observed significant MMN peaking at about 129 ms for incongruently combined ITD/ILD cues, but this effect was only detectable in the lower frequency range (0.5 kHz). There were no significant differences of the MMN responses for the two types of cue combinations (congruent/incongruent). These results suggest that—at least in the lower frequency ranges (0.5 kHz)—ITD and ILD are processed independently at the level of the MMN in auditory cortex.

Sensitivity for interaural time and intensity difference of auditory midbrain neurons in the grassfrog

1. Interaural level differences (ILDs), created by the head and pinna, have long been known to be the dominant acoustic cue for azimuthal localization of high-frequency tones. However, psychophysical experiments have demonstrated that human subjects can also lateralize complex high-frequency sounds on the basis of interaural time differences (ITDs) of the signal envelope. The lateral superior olive (LSO) is one of two pairs of binaural nuclei where the primary extraction of binaural cues for sound source location occurs. "IE" cells in LSO are inhibited by stimuli to the contralateral and excited by stimuli to the ipsilateral ear, and their response rate is therefore dependent on ILD. Anatomic specializations in the afferent pathways to the LSO suggest that this circuit also has a function in the detection of timing cues. We hypothesized that, besides ILD sensitivity, the IE property also conveys a sensitivity to ITDs of amplitude-modulated (AM) tones and could provide the physiological substrate for the psychophysical effect mentioned above. 2. In extracellular recordings from binaural LSO cells in barbiturate-anesthetized cats, response rate was a periodic function of ITDs of AM stimuli, i.e., all cells displayed ITD sensitivity. Binaural responses were smaller than responses to stimulation of the ipsilateral ear alone and were minimal when the envelopes in both ears were in-phase or nearly so. There was good correspondence between responses to ITDs and to dynamic interaural phase differences (IPDs), created by a difference in the envelope frequency to the two ears. Qualitatively, the responses were consistent with the outcome of an IE operation on temporally structured inputs. 3. To compare the relative importance of ILD and ITD, responses to combinations of the two cues were obtained. Despite robust ITD sensitivity in all binaural LSO cells encountered, the changes in response rate that would occur in response to naturally occurring ITDs were small in comparison with the changes expected for naturally occurring ILDs. The main limitation on ITD sensitivity was a steep decline in average discharge rate as the modulation frequency exceeded several hundred Hertz. 4. ITD sensitivity was also present to broadband stimuli, again with minimal rates occurring near 0 ITD. The sensitivity depended in a predictable fashion on the passband of filtered noise and was absent to binaurally uncorrelated noise bands. In response to clicks, ILDs interacted with ITD in a complicated fashion involving amplitude and latency effects. 5. Three low-characteristic frequency (CF) LSO cells were encountered that were IE and showed ITD sensitivity to the fine structure of low-frequency stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Barn owls perform sound localization based on analyses of interaural differences in arrival time and intensity of sound. Two kinds of neural signals representing the interaural time difference (ITD) and the interaural intensity difference (IID) are processed in parallel in anatomically separate pathway. We explain briefly the neural models of pathways of ITD and IID detection which we have already presented [1,2]. We present a neural network model of ICc ls in which the signals representing ITD and IID are first combined with each other. It is shown using our neural models how the neural map, in which ITD and IID are represented along the axes being perpendicular mutually, can be generated in ICc ls by the excitatory inputs from ICc core representing ITD and the inhibitory inputs from bilateral VLVps representing IID. We show that the firing rates of ICc ls neuron are well represented by a suppressive multiplication of ITD and IID inputs. This seems to be the origin of the multiplicative binding found in ICx by Pena and Konishi [3].

The sound localization of barn owl with respect to horizontal and vertical direction is made based on analysis of the interaural differences in arrival time and intensity of sound, respectively. The neural information processing of detection of interaural intensity difference (IID) is made on the neural pathway parallel with the pathway of interaural time difference (ITD) detection before both the signals arrive at the shell of central nucleus of the inferior colliculus (ICc shell). We have presented a neural model of ITD detection pathway (Inoue et al, Neurocomputing 2001, vol. 38-40, pp. 675-682). We present a neuronal model of the nucleus ventralis lemnisci lateralis pars posterior (VLVp) in which the signals of sound intensities coming from both the ears are first combined to be compared with each other. We clarify the functional role of mutual inhibitory connections between both VLVps observed experimentally in detection of IID. Outputs of IID sensitive neurons in both VLVps are projected bilaterally to ICc shell and the connections are inhibitory, whereas outputs of ITD sensitive neurons in ICc core projected to ICc shell are excitatory. It is shown using our neuronal models of VLVps, ICc core, and ICc shell that the neural map, in which ITD and IID are represented along the axes perpendicular mutually, can be generated in ICc shell by the excitatory inputs from ICc core and the inhibitory inputs from bilateral VLVps, as observed experimentally. We also show that the firing rate of ICc shell neuron are well represented by a suppressive multiplication of ITD and IID inputs.

The localization of low-frequency sounds (less than 3 kHz) along the azimuth involves comparing the ongoing difference in the time of arrival of a sound at the two ears. Information about interaural time differences (ITDs) is derived from an initial comparison performed in the superior olivary complex. However, little is known about which aspects of this information are transformed as it ascends the brainstem. To address this issue, we compared the ITD sensitivity of neurons in the inferior colliculus (IC) and auditory thalamus, successive stations in the auditory pathway. We found ITD sensitivity in the IC and thalamus to be similar in several respects. At both levels, the large majority of neurons responded maximally to ITDs within the range that a rabbit would normally encounter (+/- 300 microseconds) and preferred ipsilateral delays, delays that would be created by sounds in the contralateral sound field. The range of frequencies over which ITD sensitivity was expressed was also similar in the midbrain and thalamus. Several differences were also apparent. In comparison to IC neurons, neurons in the thalamus responded over more restricted ranges of ITD, responded at lower rates, and, in response to monaural stimulation, showed an increased influence of inhibition. Finally, a greater proportion of thalamic units had characteristic delays corresponding to intermediate discharge rates. The preservation of a bias for ipsilateral delays from IC to thalamus suggests that a representation of contralateral azimuths is present at both levels. Similarities between the two levels suggest that information about ITDs is faithfully transmitted from midbrain to thalamus. Differences in ITD sensitivity, such as the sharper tuning for ITDs, suggest that the thalamus is not a simple relay. Enhanced sensitivity to ITDs should translate to better-defined azimuthal receptive fields, and therefore may be a step toward achieving an optimal representation of azimuth within the auditory pathway.

Interaural time difference (ITDs) are the most important cue to the location of sounds containing low-frequency energy (Wightman and Kistler 1992). ITDs are encoded centrally in the medial (MSO) and lateral (LSO) superior olives which transmit the code to the inferior colliulus (IC) (Batra et al. 1997; Goldberg and Brown 1969). Each ITD-sensitive neuron is characterized by its best ITD (BD), the one producing maximal discharge rate. It is a longstanding view that these neurons are conceptually arranged in an array with best frequency (BF) on one axis and BD on the other to form a labeled-line code. According to this model, the stimulus ITD corresponds to the BD (i.e. the label) of the most active neuron in the array (Jeffress 1948). The labeled-line model is challenged by physiological data from guinea pig (confirmed in cat and gerbil) showing that the distribution of BD is highly dependent on BF, and in general does not correspond to the range of naturally-occurring ITDs (Brand et al. 2002; Hancock and Delgutte 2004; McAlpine et al. 2001). Instead, best interaural phase (BP = BD × BF) is more nearly independent of BF, such that the steepest slopes of neural rate-ITD curves tend to occur near the midline. Because the slopes, not the peaks, align near the midline (where perceptual ITD acuity is finest), it has been suggested that ITD is encoded by the discharge rate itself rather than by the locus of maximal activity (McAlpine et al. 2001). Thus, ITD may be represented by a population rate code, in which the activity of many neurons pool to form monolithic ITD channels on each side of the brain, and the stimulus ITD may be inferred by comparing the relative activity of the two channels (van Bergeijk 1962; von Bekesy 1960). Though the physiological data suggest the existence of a rate code, analysis of its viability has barely begun (Marquardt and McAlpine 2001). Here, we demonstrate that a population rate code model can account for the dependence of perceived laterality on stimulus bandwidth.

Interaural time difference (ITD) detection plays a critical role in sound localization. Some neurophysiology experiments found that the ITD discrimination threshold for single neuron in medial superior olive (MSO) is about 20 μs. In order to reveal the ITD coding mechanism essentially, an ITD detection model has been proposed in this paper based on the anatomical structure of the mammalian auditory system. Most previous research only discuss the relationship between the ITDs and the spike rate of the MSO neuron. In these results, the spike rate difference is under 1 spike/s when the ITD changes in 20 μs. It means the neuron needs more than 1 s to distinguish the ITD by spike rate coding. This explanation contradicts with the rapid and accurate sound localization response in animal psychoacoustics experiments. In our study, the neural coding in ITD detection model has been investigated via the approach of circle maps and symbolic dynamics. With these two methods, the ITDs under 20 μs can be detected in 1 s. This neural coding hypothesis agrees with the ITD detection response time in the animal behavior experiments.

Auditory evoked magnetic fields in relation to interaural time delay and interaural correlation

Neurons in the medial superior olive (MSO) perform one of the most time sensitive computations in the brain. Situated in the mammalian brainstem just one synapse removed from the auditory nerve, MSO neurons encode the location of low-frequency sounds in the horizontal plane by detecting the simultaneous arrival times of auditory inputs driven from the two ears, a process termed ‘binaural coincidence detection’. Given that sounds travel at ∼34 cm ms−1, the physiologically relevant range of interaural time differences (ITDs) is sub-millisecond for humans and smaller animals. Consequently, the reliable encoding of ITDs requires extraordinary temporal precision that must be maintained at sound frequencies approaching ∼2 kHz, the limit of phase locking exhibited by excitatory inputs to the MSO in most mammals. At first glance, a sensory system so heavily dependent on temporal precision would seem a poor candidate for neuromodulation, especially neuromodulation of presynaptic release. Yet a study in this issue of The Journal of Physiology provides compelling evidence that presynaptic GABAB receptors modulate neurotransmitter release at synapses onto MSO neurons and that such modulation improves the resolution of binaural coincidence detection (Fischl et al. 2012). GABAB receptors are G-protein-coupled receptors that, when expressed presynaptically, act through a variety of potential mechanisms to inhibit release probability. How might this contribute to ITD detection? An important clue comes from an insightful study by Brenowitz et al. (1998), who asked how GABAB activation affects the reliability of high-frequency synaptic transmission. Recording from neurons in the chick nucleus magnocellularis, avian analogs of the input neurons to the mammalian MSO, the authors stimulated auditory nerve afferents to evoke trains of EPSCs. They observed that while GABAB receptor activation reduced EPSC amplitudes generally, at frequencies above 100 Hz short-term depression was dramatically reduced, rendering later EPSC amplitudes greater than un-modulated controls. Thus, the strength and reliability of high-frequency synaptic transmission was improved, presumably by preserving vesicles for later release. A subsequent study by Kuba et al. (2002) in the avian equivalent of the MSO showed that artificial reductions in release probability could sharpen ITD detection by narrowing the time window for suprathreshold summation of bilateral EPSPs. Do such mechanisms reside in mammalian MSO?Hassfurth et al. (2010) found functional and anatomical evidence that excitatory and inhibitory inputs express GABAB receptors in MSO neurons. This expression appeared to be developmentally regulated for excitatory inputs, as GABAB-mediated depression of EPSC amplitudes dropped to smaller, but still significant, levels in the 3 weeks following the onset of hearing at around postnatal day 12. In contrast, GABAB-mediated depression of IPSC amplitudes remained consistent across development. In this issue, Fischl and colleagues provide an elegant examination of the functional implications of GABAB receptor modulation in the MSO (Fischl et al. 2012). Recording from MSO neurons in slices, they confirmed the earlier finding that activation of GABAB receptors depresses individual EPSCs and IPSCs. In contrast to the Hassfurth study, they found that IPSC decay kinetics were slowed by GABAB activation at mature ages and that EPSC depression remained strong across development. Next, in a result reminiscent of the Brenowitz findings, the authors demonstrated that GABAB activation reduced the amount of depression within 50–200 Hz trains of PSCs, yielding trains with more stable amplitudes. Fischl and colleagues then simulated binaural ITDs in the slice using trains of bilateral synaptic stimuli to evoke EPSPs. The firing of MSO neurons was highly sensitive to these simulated ITDs. Upon addition of the GABAB agonist baclofen to the bath, ITD functions narrowed significantly. Conversely, when a GABAB antagonist was added, ITD functions widened. This latter result provides important confirmation that GABA can be released under physiological conditions (albeit in a brain slice), and even more importantly, GABA can achieve concentrations sufficient to modulate ITD detection. As conventional stimulation techniques do not permit separate stimulation of excitatory and inhibitory inputs to the MSO, ionotropic inhibitory receptors were blocked in these experiments. To circumvent this, the authors developed a model incorporating excitatory and inhibitory inputs driven by in vivo-like patterns. Simulations revealed that both the amplitude modulation of EPSCs and IPSCs and the kinetic slowing of IPSCs by GABAB modulation significantly sharpened ITD tuning in MSO neurons. Moreover, this sharpening persisted over a range of simulated sound intensities. Together, these results provide compelling evidence that GABAB receptors on presynaptic terminals in the MSO afford a mechanism for rapidly and reversibly sharpening the receptive fields of MSO neurons. When this mechanism comes into play is an exciting question for future studies. At present, the source of GABAergic input to the post-hearing MSO remains a mystery (the primary inhibitory inputs to MSO are glycinergic), but two likely possibilities present themselves. GABA release from an auditory source might be linked to sound intensity levels and provide a way to dynamically dampen highly active synapses. Alternatively, GABAergic inputs from a non-auditory centre might link receptive field modulation with attention or other behavioural states, possibly enabling the system to select between detecting weak auditory stimuli and precisely localizing strong ones. Getting to the bottom of this will probably require a combination of careful anatomy and electrophysiology. More broadly, the present results highlight the growing evidence that computations in the auditory brainstem are subject to modulation (e.g. Kotak et al. 2001; Magnusson et al. 2008). Given the highly dynamic nature of auditory stimuli, this is perhaps not as surprising as it once seemed. It does hint, however, that a rich repertoire of modulatory mechanisms awaits discovery.

The binaural interaction component (BIC) of the auditory brainstem response (ABR) is obtained by subtracting the sum of the monaural right and left ear ABRs from the binaurally evoked ABR. The result is a small but prominent negative peak (herein called "DN1"), indicating a smaller binaural than summed ABR, which occurs around the latency of wave V or its roll-off slope. The BIC has been proposed to have diagnostic value as a biomarker of binaural processing abilities; however, there have been conflicting reports regarding the reliability of BIC measures in human subjects. The objectives of the current study were to: (1) examine prevalence of BIC across a large group of normal-hearing young adults; (2) determine effects of interaural time differences (ITDs) on BIC; and (3) examine any relationship between BIC and behavioral ITD discrimination acuity. Subjects were 40 normal-hearing adults (20 males and 20 females), aged 21 to 48 years, with no history of otologic or neurologic disorders. Midline ABRs were recorded from electrodes at high forehead (Fz) referenced to the nape of the neck (near the seventh cervical vertebra), with Fpz (low forehead) as the ground. ABRs were also recorded with a conventional earlobe reference for comparison to midline results. Stimuli were 90 dB peSPL biphasic clicks. For BIC measurements, stimuli were presented in a block as interleaved right monaural, left monaural, and binaural stimuli with 2000+ presentations per condition. Four measurements were averaged for a total of 8000+ stimuli per analyzed waveform. BIC was measured for ITD = 0 (simultaneous bilateral) and for ITDs of ±500 and ±750 µs. Subjects separately performed a lateralization task, using the same stimuli, to determine ITD discrimination thresholds. An identifiable BIC DN1 was obtained in 39 of 40 subjects at ITD = 0 µs in at least one of two measurement sessions, but was seen in lesser numbers of subjects in a single session or as ITD increased. BIC was most often seen when a subject was relaxed or sleeping, and less often when they fidgeted or reported neck tension, suggesting myogenic activity as a possible factor in disrupting BIC measurements. Mean BIC latencies systematically increased with increasing ITD, and mean BIC amplitudes tended to decrease. However, across subjects, there was no significant relationship between the amplitude or latency of the BIC and behavioral ITD thresholds. Consistent with previous studies, measurement of the BIC was time consuming and a BIC was sometimes difficult to obtain in awake normal-hearing subjects. The BIC will thus continue to be of limited clinical utility unless stimulus parameters and measurement techniques can be identified that produce a more robust response. Nonetheless, modulation of BIC characteristics by ITD supports the concept that the ABR BIC indexes aspects of binaural brainstem processing and thus may prove useful in selected research applications, e.g. in the examination of populations expected to have aberrant binaural signal processing ability.

Klumpp and Eady (1956, J Acoust Soc Am 28, p.859-860) reported preliminary data on human sensitivity to interaural time differences (ITD) with various stimuli. At 10 μs ITD the best discrimination of 79% correct was reported for band-pass filtered (150-1700 Hz) noise. Despite the preliminary nature, and presentation methods different from todays, the above is still the best available reference for optimal ITD discrimination. The goal of the current study is to systematically determine the stimulus and the experimental paradigm that results in the smallest threshold ITD and to provide an accurate reference value. We varied seven stimulus and procedure parameters: stimulus waveform, stimulation level, stimulus duration, adaptive versus constant stimulus procedure, alternative-forced-choice (AFC) procedure, inter-stimulus pause duration, and complete waveform versus ongoing ITD. The condition yielding the lowest threshold ITD was Gaussian noise band-pass filtered from 20 to 1400 Hz, presented at 70 dB SPL, with a short inter-stimulus pause of 50 ms, and an interval duration of 0.5 s. Averaged across 8 trained subjects, the threshold ITD for this condition at the 79% correct level was 7 μs. The influence of each parameter will be discussed together with the obstacles of accurately determining this value.

The discrimination and lateralization of interaural time differences (ITD) in rapidly modulated high-frequency sounds is dominated by cues present in the initial portion of the sound (i.e., at sound onset). The importance of initial ITD at low frequencies is, however, less clear. Here, ITD discrimination thresholds were measured in 500 Hz pure tones with diotic envelopes and static or dynamic fine-structure ITD. Static-ITD thresholds improved as tone duration increased from 40 to 640 ms but by an amount less than expected from uniform temporal weighting of binaural information. Dynamic conditions eliminated ITD from either the beginning or end of the sound by presenting slightly different frequencies to the two ears. While overall thresholds were lower when ITD was available at sound onset than when it was not, listeners differed appreciably in that regard. The results demonstrate that weighting of ITD is not temporally uniform. Instead, for many listeners, ITD discrimination at 500 Hz appears dominated by ITD cues present in the initial part of the sound. To a variable degree, other listeners rely more equally on ITD cues occurring near sound onsets and offsets, although no listeners appear to utilize such cues uniformly throughout the sound's duration.

Background/Objectives: Speech understanding in noise relies on both temporal fine structure (TFS) and temporal envelope (ENV) cues. While TFS primarily conveys interaural time differences (ITDs) at low frequencies, ENV cues can also support ITD processing, especially when TFS is unavailable or degraded. Expanding the ENV by increasing modulation depth has been proposed to improve speech perception, but its effects on spatial release from masking (SRM) and binaural temporal processing in normal-hearing listeners remain unclear. The goal of this study was to evaluate the effect of ENV enhancement on SRM in young adults with normal hearing and its influence on ITD sensitivity and interaural coherence (IC). Method: Thirty normal-hearing native Kannada speakers (19-34 years) participated. Speech stimuli consisted of Kannada sentences embedded in four-talker babble at -5, 0, and +5 dB signal to noise ratio (SNR). Target and masker were spatialized using head-related transfer functions at 0°, 15°, and 37.5° azimuths. Stimuli were presented with and without ENV enhancement (compression-expansion algorithm). Speech recognition scores were analyzed using generalized linear mixed models, and SRM was calculated as performance differences between co-located and spatially separated conditions. Cross-correlation analyses were performed to estimate ITDs and IC across SNRs. Result: ENV enhancement yielded significantly higher SRM values across all SNRs and spatial separations. Benefits were greatest at lower SNRs and wider target-masker separations. Cross-correlation analysis showed enhanced IC and more reliable ITD estimates under the expanded condition, particularly at moderate SNRs. Conclusions: Temporal ENV enhancement strengthens spatial unmasking and binaural timing cues in normal-hearing adults, especially under adverse listening conditions. These findings highlight its potential application in auditory rehabilitation and hearing technologies where ENV cues are critical.