Physiological Studies of Directional Hearing

The contribution of high frequencies to human brain activity underlying horizontal localization of natural spatial sounds

Background

In the field of auditory neuroscience, much research has focused on the neural processes underlying human sound localization. A recent magnetoencephalography (MEG) study investigated localization-related brain activity by measuring the N1m event-related response originating in the auditory cortex. It was found that the dynamic range of the right-hemispheric N1m response, defined as the mean difference in response magnitude between contralateral and ipsilateral stimulation, reflects cortical activity related to the discrimination of horizontal sound direction. Interestingly, the results also suggested that the presence of realistic spectral information within horizontally located spatial sounds resulted in a larger right-hemispheric N1m dynamic range. Spectral cues being predominant at high frequencies, the present study further investigated the issue by removing frequencies from the spatial stimuli with low-pass filtering. This resulted in a stepwise elimination of direction-specific spectral information. Interaural time and level differences were kept constant. The original, unfiltered stimuli were broadband noise signals presented from five frontal horizontal directions and binaurally recorded for eight human subjects with miniature microphones placed in each subject's ear canals. Stimuli were presented to the subjects during MEG registration and in a behavioral listening experiment.

Results

The dynamic range of the right-hemispheric N1m amplitude was not significantly affected even when all frequencies above 600 Hz were removed. The dynamic range of the left-hemispheric N1m response was significantly diminished by the removal of frequencies over 7.5 kHz. The subjects' behavioral sound direction discrimination was only affected by the removal of frequencies over 600 Hz.

Conclusion

In accord with previous psychophysical findings, the current results indicate that frontal horizontal sound localization and related right-hemispheric cortical processes are insensitive to the presence of high-frequency spectral information. The previously described changes in localization-related brain activity, reflected in the enlarged N1m dynamic range elicited by natural spatial stimuli, can most likely be attributed to the processing of individualized spatial cues present already at relatively low frequencies. The left-hemispheric effect could be an indication of left-hemispheric processing of high-frequency sound information unrelated to sound localization. Taken together, these results provide converging evidence for a hemispheric asymmetry in sound localization.

Sound source localization in real sound fields based on empirical statistics of interaural parameters

The role of temporal fluctuations and systematic variations of interaural parameters in localization of sound sources in spatially distributed, nonstationary noise conditions was investigated. For this, Bayesian estimation was applied to interaural parameters calculated with physiologically plausible time and frequency resolution. Probability density functions (PDFs) of the interaural level differences (ILDs) and phase differences (IPDs) were estimated by measuring histograms for a directional sound source perturbed by several types of interfering noise at signal-to-noise ratios (SNRs) between -5 and +30 dB. A moment analysis of the PDFs reveals that the expected values shift and the standard deviations increase considerably with decreasing SNR, and that the PDFs have non-Gaussian shape at medium SNRs. A d' analysis of the PDFs indicates that elevation discrimination is possible even at low SNRs in the median plane by integrating information across frequency. Absolute sound localization was simulated by a Bayesian maximum a posteriori (MAP) procedure. The simulation is based on frequency integration of broadly tuned "detectors." Confusion patterns of real and estimated sound source directions are similar to those of human listeners. The results indicate that robust processing strategies are needed to exploit interaural parameters successfully in noise conditions due to their strong temporal fluctuations.

Auditory processing of interaural timing information: new insights

Differences in the time-of-arrival of sounds at the two ears, or interaural temporal disparities (ITDs), constitute one of the major binaural cues that underlie our ability to localize sounds in space. In addition, ITDs contribute to our ability to detect and to discriminate sounds, such as speech, in noisy environments. For low-frequency signals, ITDs are conveyed primarily by "cycle-by-cycle" disparities present in the fine-structure of the waveform. For high-frequency signals, ITDs are conveyed by disparities within the time-varying amplitude, or envelope, of the waveform. The results of laboratory studies conducted over the past few decades indicate that ITDs within the envelopes of high-frequency are less potent than those within the fine-structure of low-frequency stimuli. This is true for both measures of sensitivity to changes in ITD and for measures of the extent of the perceived lateral displacement of sounds containing ITDs. Colburn and Esquissaud (1976) hypothesized that it is differences in the specific aspects of the waveform that are coded neurally within each monaural (single ear) channel that account for the greater potency of ITDs at low frequencies rather than any differences in the more central binaural mechanisms that serve these different frequency regions. In this review, the results of new studies are reported that employed special high-frequency "transposed" stimuli that were designed to provide the high-frequency channels of the binaural processor with envelope-based information that mimics waveform-based information normally available only in low-frequency channels. The results demonstrate that these high-frequency transposed stimuli (1) yield sensitivity to ITDs that approaches, or is equivalent to, that obtained with "conventional" low-frequency stimuli and (2) yield large extents of laterality that are similar to those measured with conventional low-frequency stimuli. These findings suggest that by providing the high-frequency channels of the binaural processor with information that mimics that normally available only at low frequencies, the potency of ITDs in the two frequency regions can be made to be similar, if not identical. These outcomes provide strong support for Colburn and Esquissaud's (1976) hypothesis. The use of high-frequency transposed stimuli, in both behavioral and physiological investigations offers the promise of new and important insights into the nature of binaural processing.

Timing in the auditory system of the bat

Echolocating bats use audition to guide much of their behavior. As in all vertebrates, their lower brainstem contains a number of parallel auditory pathways that provide excitatory or inhibitory outputs differing in their temporal discharge patterns and latencies. These pathways converge in the auditory midbrain, where many neurons are tuned to biologically important parameters of sound, including signal duration, frequency-modulated sweep direction, and the rate of periodic frequency or amplitude modulations. This tuning to biologically relevant temporal patterns of sound is created through the interplay of the time-delayed excitatory and inhibitory inputs to midbrain neurons. Because the tuning process requires integration over a relatively long time period, the rate at which midbrain auditory neurons respond corresponds to the cadence of sounds rather than their fine structure and may provide an output that is closely matched to the rate at which motor systems operate.

Nonlinear feedback models for the tuning of auditory nerve fibers

The tuning of auditory nerve (AN) fibers is generally characterized by an increase in bandwidth and, for mid- to high-frequency fibers, a downward shift in the center frequency as sound level increases. Changes in bandwidth are accompanied by changes in the phase properties of the fibers; thus the timing of neural discharges also changes as a function of sound level. This study focuses on the magnitude and phase properties of models designed to reproduce the nonlinear properties of AN fibers that were studied electrophysiologically. The forward path of each model consisted of a linear second-order resonance, and each feedback path contained a saturating nonlinearity. In model 1, the feedback path was a simple memoryless, saturating nonlinearity. In model 2, a low-pass filter was added after the feedback nonlinearity. The ability of each model to simulate aspects of the nonlinear tuning of AN fibers is discussed. Model 2 was able to simulate a wider range of nonlinear behavior for different AN fibers and thus has promise for use in simulations of populations of fibers tuned to different frequencies.

Convergence and divergence of ascending binaural and monaural pathways from the superior olives of the mustached bat

The lateral superior olive and medial superior olive give rise to pathways that terminate in the dorsal nucleus of the lateral lemniscus and central nucleus of the inferior colliculus. In most mammals, neurons in both the medial and lateral superior olives are binaural, but in the mustached bat most neurons in the medial superior olive are monaural. The aims of this study were to determine how the inputs to the medial superior olive contribute to its monaurality and to determine whether the ascending projections from the lateral and medial superior olives overlap or remain segregated at their targets. Injections of two different tracers were placed in tonotopically matched areas of the lateral and medial superior olives in the same animal. Retrograde transport from injections in the medial superior olive labeled spherical cells in the contralateral anteroventral cochlear nucleus and principal cells in the ipsilateral medial nucleus of the trapezoid body. Few cells were labeled in ipsilateral cochlear nucleus. Anterograde transport resulted in tonotopically specific distributions of label with the same laterality as in nonecholocating mammals. In the dorsal nucleus of the lateral lemniscus, label from the lateral and medial superior olives largely overlapped. In the inferior colliculus, label from the lateral and medial superior olives largely overlapped. In the inferior colliculus, label from the two sources overlapped in the high and low frequency ranges, but in the frequency range around 60 kHz, label from the medial superior olive extended more dorsally than that from the lateral superior olive. These results indicate that projections of the lateral and medial superior olives overlap extensively at their targets.

Connections of the superior olivary complex in the rufous horseshoe bat Rhinolophus rouxi

In the rufous horseshoe bat (Rhinolophus rouxi), the superior olivary complex contains four main divisions. In comparison with other species, the most lateral division is clearly homologous to the lateral superior olive (LSO); the most medial division is homologous to the medial nucleus of the trapezoid body (MNTB). Lying between these landmarks, in approximately the position of the medial superior olive (MSO) of other mammals, are two additional divisions that are cytoarchitecturally distinct from one another yet do not greatly resemble the MSO of nonecholocating mammals such as the cat. We refer to these nuclei as the dorsal medial superior olive (DMSO) and the ventral medial superior olive (VMSO). We examined the afferent and efferent connections of all of these cell groups with retrograde and anterograde transport of WGA-HRP from the superior olivary complex. In the same animals we recorded the binaural response properties of single units in the superior olivary complex. Virtually all units recorded in LSO were excitatory to the ipsilateral ear and inhibitory to the contralateral ear (EI); all of the units sampled in the MNTB and most of those sampled in the VMSO responded only to the contralateral ear (OE). In DMSO the binaural properties of units were varied: the number of units that were inhibitory to the ipsilateral ear and excitatory to the contralateral ear (IE) was about equal to the number of units excitatory to both ears (EE); a few units had OE responses; no units had EI responses. Connectional correlates for these binaural response properties are seen in the patterns of retrograde transport from WGA-HRP injections in the divisions of the superior olive. The LSO receives projections from the ipsilateral cochlear nucleus and MNTB; MNTB receives projections from the contralateral cochlear nucleus. The DMSO and VMSO both receive bilateral projections from the cochlear nuclei. The results of retrograde and anterograde transport suggest that VMSO, in addition, receives projections from the ipsilateral MNTB. The LSO, DMSO, and VMSO all project to the ventral two-thirds of the central nucleus of the inferior colliculus, and their targets are approximately coextensive. However, the LSO projects bilaterally to the inferior colliculus, whereas the medial cell groups project mainly ipsilaterally.