Christopher J. Smalt

Relationship between brainstem, cortical and behavioral measures relevant to pitch salience in humans

Neural representation of pitch-relevant information at both the brainstem and cortical levels of processing is influenced by language or music experience. However, the functional roles of brainstem and cortical neural mechanisms in the hierarchical network for language processing, and how they drive and maintain experience-dependent reorganization are not known. In an effort to evaluate the possible interplay between these two levels of pitch processing, we introduce a novel electrophysiological approach to evaluate pitch-relevant neural activity at the brainstem and auditory cortex concurrently. Brainstem frequency-following responses and cortical pitch responses were recorded from participants in response to iterated rippled noise stimuli that varied in stimulus periodicity (pitch salience). A control condition using iterated rippled noise devoid of pitch was employed to ensure pitch specificity of the cortical pitch response. Neural data were compared with behavioral pitch discrimination thresholds. Results showed that magnitudes of neural responses increase systematically and that behavioral pitch discrimination improves with increasing stimulus periodicity, indicating more robust encoding for salient pitch. Absence of cortical pitch response in the control condition confirms that the cortical pitch response is specific to pitch. Behavioral pitch discrimination was better predicted by brainstem and cortical responses together as compared to each separately. The close correspondence between neural and behavioral data suggest that neural correlates of pitch salience that emerge in early, preattentive stages of processing in the brainstem may drive and maintain with high fidelity the early cortical representations of pitch. These neural representations together contain adequate information for the development of perceptual pitch salience.

Language-dependent pitch encoding advantage in the brainstem is not limited to acceleration rates that occur in natural speech

Experience-dependent enhancement of neural encoding of pitch in the auditory brainstem has been observed for only specific portions of native pitch contours exhibiting high rates of pitch acceleration, irrespective of speech or nonspeech contexts. This experiment allows us to determine whether this language-dependent advantage transfers to acceleration rates that extend beyond the pitch range of natural speech. Brainstem frequency-following responses (FFRs) were recorded from Chinese and English participants in response to four, 250-ms dynamic click-train stimuli with different rates of pitch acceleration. The maximum pitch acceleration rates in a given stimulus ranged from low (0.3Hz/ms; Mandarin Tone 2) to high (2.7Hz/ms; 2 octaves). Pitch strength measurements were computed from the FFRs using autocorrelation algorithms with an analysis window centered at the point of maximum pitch acceleration in each stimulus. Between-group comparisons of pitch strength revealed that Chinese exhibit more robust pitch representation than English across all four acceleration rates. Regardless of language group, pitch strength was greater in response to acceleration rates within or proximal to natural speech relative to those beyond its range. Though both groups showed decreasing pitch strength with increasing acceleration rates, pitch representations of the Chinese group were more resistant to degradation. FFR spectral data were complementary across acceleration rates. These findings demonstrate that perceptually salient pitch cues associated with lexical tone influence brainstem pitch extraction not only in the speech domain, but also in auditory signals that clearly fall outside the range of dynamic pitch that a native listener is exposed to.

Neural correlates of adaptation in freely-moving normal hearing subjects under cochlear implant acoustic simulations

Neurobiological correlates of adaptation to spectrally degraded speech were investigated with fMRI before and after exposure to a portable real-time speech processor that implements an acoustic simulation model of a cochlear implant (CI). The speech processor, in conjunction with isolating insert earphones and a microphone to capture environment sounds, was worn by participants over a two week chronic exposure period. fMRI and behavioral speech comprehension testing were conducted before and after this two week period. After using the simulator each day for 2h, participants significantly improved in word and sentence recognition scores. fMRI shows that these improvements came accompanied by changes in patterns of neuronal activation. In particular, we found additional recruitment of visual, motor, and working memory areas after the perceptual training period. These findings suggest that the human brain is able to adapt in a short period of time to a degraded auditory signal under a natural learning environment, and gives insight on how a CI might interact with the central nervous system. This paradigm can be furthered to investigate neural correlates of new rehabilitation, training, and signal processing strategies non-invasively in normal hearing listeners to improve CI patient outcomes.

Modeling the Time-Varying and Level-Dependent Effects of the Medial Olivocochlear Reflex in Auditory Nerve Responses

The medial olivocochlear reflex (MOCR) has been hypothesized to provide benefit for listening in noisy environments. This advantage can be attributed to a feedback mechanism that suppresses auditory nerve (AN) firing in continuous background noise, resulting in increased sensitivity to a tone or speech. MOC neurons synapse on outer hair cells (OHCs), and their activity effectively reduces cochlear gain. The computational model developed in this study implements the time-varying, characteristic frequency (CF) and level-dependent effects of the MOCR within the framework of a well-established model for normal and hearing-impaired AN responses. A second-order linear system was used to model the time-course of the MOCR using physiological data in humans. The stimulus-level-dependent parameters of the efferent pathway were estimated by fitting AN sensitivity derived from responses in decerebrate cats using a tone-in-noise paradigm. The resulting model uses a binaural, time-varying, CF-dependent, level-dependent OHC gain reduction for both ipsilateral and contralateral stimuli that improves detection of a tone in noise, similarly to recorded AN responses. The MOCR may be important for speech recognition in continuous background noise as well as for protection from acoustic trauma. Further study of this model and its efferent feedback loop may improve our understanding of the effects of sensorineural hearing loss in noisy situations, a condition in which hearing aids currently struggle to restore normal speech perception.

Neural adaptation and perceptual learning using a portable real-time cochlear implant simulator in natural environments

A portable real-time speech processor that implements an acoustic simulation model of a cochlear implant (CI) has been developed on the Apple iPhone / iPod Touch to permit testing and experimentation under extended exposure in real-world environments. This simulator allows for both a variable number of noise band channels and electrode insertion depth. Utilizing this portable CI simulator, we tested perceptual learning in normal hearing listeners by measuring word and sentence comprehension behaviorally before and after 2 weeks of exposure. To evaluate changes in neural activation related to adaptation to transformed speech, fMRI was also conducted. Differences in brain activation after training occurred in the inferior frontal gyrus and areas related to language processing. A 15–20% improvement in word and sentence comprehension of cochlear implant simulated speech was also observed. These results demonstrate the effectiveness of a portable CI simulator as a research tool and provide new information about the physiological changes that accompany perceptual learning of degraded auditory input.

Linguistic status of timbre influences pitch encoding in the brainstem

The aim of this experiment is to assess the effects of the linguistic status of timbre on pitch processing in the brainstem. Brainstem frequency following responses were evoked by the Mandarin high-rising lexical tone superimposed on a native vowel quality ([i]), nonnative vowel quality ([œ]), and iterated rippled noise (nonspeech). Results revealed that voice fundamental frequency magnitudes were larger when concomitant with a native vowel quality compared with either nonnative vowel quality or nonspeech timbre. Such experience-dependent effects suggest that subcortical sensory encoding of pitch interacts with timbre in the human brainstem. As a consequence, responses of the perceptual system can be differentially shaped to pitch patterns in relation to the linguistic status of their concomitant timbre.

A portable, real‐time vocoder: Technology and preliminary perceptual learning findings.

Transformed or vocoded acoustic signals paralleling the processing of cochlear implants have proven extremely useful for simulating aural perception in deaf CI users with normal hearing participants. The benefits of current perceptual studies with vocoded stimuli are limited, however, in their ecological validity and direct applicability to the everyday challenges faced by CI users. Normal hearing subjects listen for relatively short periods of time to isolated, prerecorded words or sentences; therefore, their perceptual learning occurs without the semantic context, visual support, and conversational interplay that normally accompanies spoken language use. In this paper, the development of a new device, which enables vocoded speech research to overcome many of these limitations, is described. This technology carries out vocoding signal transformations in real‐time, with very short delays, and is small enough to fit in a participant’s pocket. The portable, real‐time vocoder (PRTV) therefore allows subjects...

Fit testing of a hearing protection device with integrated in-ear noise exposure monitoring

In-the-ear hearing protectors are often used in high-noise environments, such as in military operations and during weapons training. The fit of such a hearing protection device in the ear canal can cause significant variability in the noise dose experienced, an important factor in characterizing the auditory health risk. Our approach for improved dose estimates under these conditions involves a portable noise recorder used to capture in-the-ear noise behind a hearing protector, and on-body noise, while assessing hearing protection fit throughout recording. In this presentation we describe evaluation of this system using ANSI S12.42 testing using a shock tube and an acoustic test fixture, to evaluate impulse peak reduction and for measurement validation. Also explored were angle dependent effects on the peak insertion loss and measurement accuracy of the on-body recorder. An exploratory study was conducted with a small sample of experimenters during a recent Navy-sponsored noise survey conducted at Marine ...

Noise exposure in berthing rooms of Naval ships

Noise on aircraft carriers is known to exceed hazardous noise levels as jets launch and land on the flight deck and loud machinery operates below deck. Crew members often reach their daily noise allowance while performing work duties but the conditions for auditory recovery onboard are not well understood. To address this gap, we assisted the Navy in recording 24h persistent noise measurements in several berthing rooms on the USS Nimitz (CVN-68). During flight operations, the 8h time-weighted average (TWA) noise levels in these below-deck living spaces ranged between 75 and 81 dBA. While the levels fall below the Department of Defense TWA limit of 85 dBA, these conditions may not support auditory recovery from temporary threshold shifts that occurred during work hours. Another potential noise hazard in these rooms is impulse noise from flight deck catapults and arresting wires, with peak levels as high as 143 dB. In this presentation, we describe an analysis of the 24h noise exposure from aircraft-carrier...

Predicting sound-localization performance with hearing-protection devices using computational auditory models

Evaluation of the effect of hearing-protection devices (HPDs) on auditory tasks such as detection, localization, and speech intelligibility typically is done with human-subject testing. However, such data collections can be impractical due to the time-consuming processes of subject recruitment and the testing itself, particularly when multiple tasks and HPDs are included. An alternative, objective testing protocol involves the use of a binaural mannequin (a.k.a an acoustic test fixture) and computational models of the auditory system. For example, data collected at the eardrums of such a mannequin outfitted with an HPD can be fed into a binaural localization model. If the performance of the model with such input can be shown to be similar to that of human subjects, the model-based assessment may be sufficient to characterize the hearing protector and inform further design decisions. In this presentation we will describe the preliminary results of an effort to replicate human-subject localization performan...