Kenneth K. Jensen

Songbirds use pulse tone register in two voices to generate low-frequency sound.

The principal physical mechanism of sound generation is similar in songbirds and humans, despite large differences in their vocal organs. Whereas vocal fold dynamics in the human larynx are well characterized, the vibratory behaviour of the sound-generating labia in the songbird vocal organ, the syrinx, is unknown. We present the first high-speed video records of the intact syrinx during induced phonation. The syrinx of anaesthetized crows shows a vibration pattern of the labia similar to that of the human vocal fry register. Acoustic pulses result from short opening of the labia, and pulse generation alternates between the left and right sound sources. Spontaneously calling crows can also generate similar pulse characteristics with only one sound generator. Airflow recordings in zebra finches and starlings show that pulse tone sounds can be generated unilaterally, synchronously or by alternating between the two sides. Vocal fry-like dynamics therefore represent a common production mechanism for low-frequency sounds in songbirds. These results also illustrate that complex vibration patterns can emerge from the mechanical properties of the coupled sound generators in the syrinx. The use of vocal fry-like dynamics in the songbird syrinx extends the similarity to this unusual vocal register with mammalian sound production mechanisms.

Role of intracranial cavities in avian directional hearing

Whereas it is clear from anatomical studies that all birds have complex interaural canals connecting their middle ears, the effect of interaural coupling on directional hearing has been disputed. A reason for conflicting results in earlier studies may have been that the function of the tympanic ear and hence of the interaural coupling is sensitive to variations in the intracranial air pressure. In awake birds, the middle ears and connected cavities are vented actively through the pharyngotympanic tube. This venting reflex seems to be suppressed in anesthetized birds, leading to increasingly lower pressure in the interaural cavities, stiffening the eardrums, and displacing them medially. This causes the sensitivity, as well as the interaural coupling, to drop. Conversely, when the middle ears are properly vented, robust directional eardrum responses, most likely caused by internal coupling, have been reported. The anatomical basis of this coupling is the ‘interaural canal,’ which turns out to be a highly complex canal and cavity system, which we describe for the zebra finch. Surprisingly, given the complexity of the interaural canals, simple models of pipe-coupled middle ears fit the eardrum directionality data quite well, but future models taking the complex anatomy into consideration should be developed.

Interaural Time-Difference Discrimination as a Measure of Place of Stimulation for Cochlear-Implant Users With Single-Sided Deafness

Current clinical practice in programming a cochlear implant (CI) for individuals with single-sided deafness (SSD) is to maximize the transmission of speech information via the implant, with the implicit assumption that this will also result in improved spatial-hearing abilities. However, binaural sensitivity is reduced by interaural place-of-stimulation mismatch, a likely occurrence with a standard CI frequency-to-electrode allocation table (FAT). As a step toward reducing interaural mismatch, this study investigated whether a test of interaural-time-difference (ITD) discrimination could be used to estimate the acoustic frequency yielding the best place match for a given CI electrode. ITD-discrimination performance was measured by presenting 300-ms bursts of 100-pulses-per-second electrical pulse trains to a single CI electrode and band-limited pulse trains with variable carrier frequencies to the acoustic ear. Listeners discriminated between two reference intervals (four bursts each with constant ITD) and a moving target interval (four bursts with variable ITD). For 17 out of the 26 electrodes tested across eight listeners, the function describing the relationship between ITD-discrimination performance and carrier frequency had a discernable peak where listeners achieved 70% to 100% performance. On average, this peak occurred 1.15 octaves above the CI manufacturer’s default FAT. ITD discrimination shows promise as a method of estimating the cochlear place of stimulation for a given electrode, thereby providing information to optimize the FAT for SSD-CI listeners.

Does electrode placement affect the interaural-time-difference acuity in bilateral cochlear-implant listeners?

Postlingually deafened bilateral cochlear-implant (CI) listeners can show limited interaural-time-difference (ITD) sensitivity, even when tested using highly controlled time-synchronized research processors. It is assumed that ITD discrimination requires interaural frequency-matched inputs. However, current bilateral CI programming procedures do not account for potential interaural place-of-stimulation mismatch. This study investigated the magnitude of interaural place-of-stimulation mismatch and its effects on ITD sensitivity in bilateral CI listeners. Ten bilateral CI listeners were tested on a two-interval left-right ITD discrimination task. Loudness balanced, 300-ms, 100 or 200 pulse-per-second, constant-amplitude, monopolar pulse trains were delivered to single-electrode pairs using time-synchronized research processors. ITD just noticeable differences (JNDs) were measured for five reference electrodes evenly distributed along the array in one ear, and for at least five comparison electrodes, generating ITD tuning curves. The interaural mismatch was estimated using both the ITD tuning curves and differences in the angular insertion depth from computed-tomography (CT) scans. Data showed that ITD tuning curves were relatively broad when compared to the amount of physical interaural place-of-stimulation mismatch from the CT scans. This suggests that most bilateral CI listeners do not experience appreciably reduced binaural sensitivity due to differences in electrode placement.

Estimating the relative interaural place mismatch for bilateral cochlear-implant listeners

Frequency-matched bilateral input is important for the optimum encoding of binaural cues to facilitate sound localization, auditory scene analysis, and speech understanding in noise. Current bilateral cochlear-implant (CI) programming procedures do not account for potential interaural place-of-stimulation mismatch. This study investigated whether a perceptual test of interaural-time-difference (ITD) sensitivity can effectively estimate relative interaural mismatch for bilateral CI listeners. Ten bilateral CI listeners were tested on a two-interval left-right ITD discrimination task. Loudness balanced 300-ms, 100 pulse-per-second constant-amplitude pulse trains were delivered to single-electrode pairs using direct stimulation. Measurements were made for five reference electrodes evenly distributed along the array in one ear, and for at least five closely spaced comparison electrodes in the other ear for each reference electrode. The pair with the smallest thresholds was assumed to be the most closely place...

Measuring spectral asymmetry for cochlear-implant listeners with single-sided deafness

Cochlear implants (CI) for individuals with single-sided deafness (SSD) are generally programmed with a standard frequency-allocation table (FAT) to maximize the transmission of speech information through the CI. However, a standard FAT is likely to produce mismatch in interaural place of stimulation, which could limit the spatial-hearing benefits that SSD-CIs provide. This study compared three methods for estimating interaural mismatch for SSD-CI listeners: interaural-time-difference (ITD) discrimination, pitch discrimination, and computed-tomography (CT) scan estimates of electrode intracochlear position. For all three measures, the estimated matched acoustic frequency increased for subsequently more basal electrodes. The ITD-based and CT-based estimates were generally consistent with one another, and were on average about one octave above the standard FAT. The pitch-based estimates were considerably lower, averaging about a half-octave above the standard FAT. These results suggest that pitch-discrimina...

Elaborate network of avian intracranial air-filled cavities and its potential role in hearing

Many avian species possess an intracranial air-filled passage, directly connecting the medial surfaces of the tympanic membranes, called the interaural canal. It is known to greatly improve directional hearing by passive acoustics in small animals where the external interaural delay is too minute to allow temporal neural coding. For long, the avian interaural canal was assumed to be a simple cylindrical cavity. Contrary to this, we discovered through CT scans and other techniques that many birds (e.g., zebra finches and pigeons) do in fact have a rather elaborate system of interconnected air-filled cavities throughout the entire skull. The cavities communicate directly or indirectly with the tympanic membranes. How does this network affect the directional hearing in birds? On one hand, it may simply be an adaptation to flight and play little or no role in hearing. On the other hand, theoretical considerations suggest that the directional response may be optimized through frequency dependent “tuning” of at...

The fluctuating-masker benefit for normal-hearing and hearing-impaired listeners with equal audibility at a fixed signal-to-noise ratio

While normal-hearing (NH) listeners demonstrate better speech intelligibility for fluctuating-masker than for stationary-noise conditions, hearing-impaired (HI) listeners generally show little or no fluctuating-masker benefit (FMB). This result has been interpreted in terms of suprathreshold deficits (e.g., reduced spectral or temporal resolution or distorted stream-segregation cues) that limit ?dip-listening?. However, reduced FMB for HI listeners might instead be attributable to audibility limitations or to differences between the signal-to-noise ratios (SNRs) at which NH and HI listeners are tested. This study examined this issue by equalizing stationary-noise performance to allow measurements at a common SNR, equalizing audibility, and presenting identical signals to pairs of NH and HI listeners. Audibility was equalized using linear gain, low-pass filtering (4 kHz) and intensity filtering to remove speech-signal elements below the HI audiometric threshold. Nonsense-syllable identification performance in stationary noise was equalized by adjusting the response set size. Stationary-noise trials (adapting set size) were interleaved with fluctuating-masker trials (adapting SNR), ensuring stable stationary-noise performance throughout the test. Fluctuating maskers included lowand high-rate modulated noise, speech-modulated noise and an interfering-talker condition. Results determine whether and under which conditions the HI listeners demonstrate reduced FMB not attributable to SNR or audibility effects.