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

Effects of Interaural Decorrelation on Neural and Behavioral Detection of Spatial Cues

Binaural signal detection in an NoSπ task relies on interaural disparities introduced by adding an antiphasic signal to diotic noise. What metric of interaural disparity best predicts performance? Some models use interaural correlation; others differentiate between dynamic interaural time differences (ITDs) and interaural level differences (ILDs) of the effective stimulus. To examine the relative contributions of ITDs and ILDs in binaural detection, we developed a novel signal processing technique that selectively degrades different aspects (potential cues) of binaural stimuli (e.g., only ITDs are scrambled). Degrading a particular cue will affect performance only if that cue is relevant to the binaural processing underlying detection. This selective scrambling technique was applied to the stimuli of a classic N0Sπ task in which the listener had to detect an antiphasic 500-Hz signal in the presence of a diotic wideband noise masker. Data obtained from five listeners showed that (1) selective scrambling of ILDs had little effect on binaural detection, (2) selective scrambling of ITDs significantly degraded detection, and (3) combined scrambling of ILDs and ITDs had the same effect as exclusive scrambling of ITDs. Regarding the question which stimulus properties determine detection, we conclude that for this binaural task (1) dynamic ITDs dominate detection performance, (2) ILDs are largely irrelevant, and (3) interaural correlation of the stimulus is a poor predictor of detection. Two simple stimulus-based models that each reproduce all binaural aspects of the data quite well are described: (1) a single-parameter detection model using ITD variance as detection criterion and (2) a compressive transformation followed by a crosscorrelation analysis. The success of both of these contrasting models shows that our data alone cannot reveal the mechanisms underlying the dominance of ITD cues. The physiological implications of our findings are discussed.

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.

Sensitivity to changes in interaural time difference and interaural correlation in the inferior colliculus

Interaural correlation processing is critical for grouping and segregating auditory streams under reverberant environments with multiple sources. Although detection of dynamic changes in interaural correlation has been extensively studied in the field of psychoacoustics, the underlying neural mechanism remains largely unknown. In this study, frequency-following responses (FFRs) to narrow-band noises were measured at various levels of the auditory system in rats, including the inferior colliculus (IC), ventral division of the medial geniculate body (MGB), and primary auditory cortex (A1). The results of Experiment 1 show that FFRs recorded in the IC were affected by both interaural correlation and the interaural time difference (ITD). Moreover, results of Experiment 2 show that a break in interaural correlation (BIC) could elicit marked FFRs in each of the three central auditory structures, and the BIC-induced FFRs were significantly affected by the ITD. The results suggest that the rat's central auditory system is able to resolve and compare fast changes in fine-structure details of arbitrary noises presented at the two ears.

Changing the interaural time difference (ITD) of a continuous binaural noise causes a shift in the perceived lateralization of the noise and evokes a late auditory evoked potential with negative peak at 130 ms and a positive peak at 220 ms. The response is mainly evoked by stimulus frequencies below 2,000 Hz and is mediated through the middle and apical regions of the cochlea. The threshold for perceiving the lateralization reversal and for eliciting a clear evoked potential is approximately 15 dB higher than the intensity required to perceive the onset of the noise. Increasing the ITD up to 1.0 ms increases the amplitude of the evoked potential and the perceived lateralization of the noise. Further increases in the ITD decrease the amplitude of the evoked potential and make the perception of the sound less 'compact'. Decreasing the intensity of the sound in one ear decreases the response to a change in ITD, but recognizable responses occur with interaural intensity differences up to 30 dB.

Effects of interaural time differences on the responses of chinchilla inferior colliculus neurons to consonant-vowel syllables

Syntactic and auditory spatial processing in the human temporal cortex: An MEG study

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.

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.

The binaural system is exquisitely sensitive in its ability to resolve very small interaural differences of arrival time for signals presented to the two ears. It is at first surprising, then, that the system is relatively poor at processing temporal changes in these interaural time differences. In recent years there has been an increased interest in the perception of binaural stimuli with time varying interaural time delays (ITDs), interaural intensity differences (IIDs), and interaural correlation. In this paper we review some of the results of psychoacoustical experiments using stimuli with time-varying interaural differences, and we present some new theoretical predictions to describe some of these data.KeywordsInteraural Time DifferenceTonal TargetInteraural Level DifferenceInteraural Intensity DifferenceInteraural DifferenceThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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.

Speech reception thresholds (SRTs) decrease as target and maskers are spatially separated (spatial release from masking, SRM). The current study systematically assessed how SRTs and SRM for a frontal target in a spatially symmetric masker configuration depend on spectro-temporal masker properties, the availability of short-time interaural level difference (ILD) and interaural time difference (ITD), and informational masking. Maskers ranged from stationary noise to single, interfering talkers and were modified by head-related transfer functions to provide: (i) different binaural cues (ILD, ITD, or both) and (ii) independent maskers in each ear ("infinite ILD"). Additionally, a condition was tested in which only information from short-time spectro-temporal segments of the ear with a favorable signal-to-noise ratio (better-ear glimpses) was presented. For noise-based maskers, ILD, ITD, and spectral changes related to masker location contributed similarly to SRM, while ILD cues played a larger role if temporal modulation was introduced. For speech maskers, glimpsing and perceived location contributed roughly equally and ITD contributed less. The "infinite ILD" condition might suggest better-ear glimpsing limitations resulting in a maximal SRM of 12 dB for maskers with low or absent informational masking. Comparison to binaural model predictions highlighted the importance of short-time processing and helped to clarify the contribution of the different binaural cues and mechanisms.

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.

The ability to detect amplitude modulation (AM) of a tonal probe can be disrupted by the presence of modulated masking tones. Two experiments examined whether a disparity in the interaural parameters of the probe and masker can reduce the amount of interference. In the first experiment, the effects of interaural time and intensity differences were studied in separate sets of conditions. With low-frequency carriers, the detection of 10-Hz probe modulation in the presence of 10-Hz masker modulation was not significantly affected by interaural time differences. With higher-frequency carriers, dichotic stimuli were generated through combinations of diotic, dichotic, or monotic probe and masker presentations in which the probe and masker did not share a common interaural intensity difference. In these conditions, the amount of interference was affected by the interaural configuration. However, monotic level differences between the probe and masker may have contributed to the effect of interaural configuration. In the second experiment, the probe and masker were presented through separate speakers in an enclosed listening environment. Spatial separation between the sources for the probe and masker led to a small reduction in the amount of interference. When the masker modulation rate was varied with the probe AM rate fixed at 10 Hz, the extent of tuning in the modulation domain in the sound-field conditions was similar to that obtained with diotic stimulus presentation over headphones.