Long-term habituation of the speech-elicited mismatch negativity.

A significant issue in the use of the mismatch negativity evoked potential (MMN) concerns its low signal-to-noise ratio (SNR). One can improve the noise level by increasing the number of samples included in the averaged response. However, improvement achieved in this way assumes that the signal, the MMN, remains stable for extended test times, an assumption which has not been tested. If the MMN is not stable, or exhibits habituation over the test session, then SNR would be adversely affected. MMN response magnitude was measured in 5-min intervals over the course of a test session in response to various speech syllable contrasts. Significant long-term habituation of MMN was observed for all three subject populations tested: young adults, school-age children, and guinea pigs. The time course of the habituation and the stimulus conditions under which it occurs have important implications for research and clinical applications of the MMN. Recording procedures that minimize habituation effects may be used to advantage to improve the signal-to-noise ratio of the MMN.

Consequences of neural asynchrony: a case of auditory neuropathy.

The neural representation of sensory events depends upon neural synchrony. Auditory neuropathy, a disorder of stimulus-timing-related neural synchrony, provides a model for studying the role of synchrony in auditory perception. This article presents electrophysiological and behavioral data from a rare case of auditory neuropathy in a woman with normal hearing thresholds, making it possible to separate audibility from neuropathy. The experimental results, which encompass a wide range of auditory perceptual abilities and neurophysiologic responses to sound, provide new information linking neural synchrony with auditory perception. Findings illustrate that optimal eighth nerve and auditory brainstem synchrony do not appear to be essential for understanding speech in quiet listening situations. However, synchrony is critical for understanding speech in the presence of noise.

Effects of lengthened formant transition duration on discrimination and neural representation of synthetic CV syllables by normal and learning-disabled children.

In order to investigate the precise acoustic features of stop consonants that pose perceptual difficulties for some children with learning problems, discrimination thresholds along two separate synthetic /da-ga/ continua were compared in a group of children with learning problems (LP) and a group of normal children. The continua differed only in the duration of the formant transitions. Results showed that simply lengthening the formant transition duration from 40 to 80 ms did not result in improved discrimination thresholds for the LP group relative to the normal group. Consistent with previous findings, an electrophysiologic response that is known to reflect the brain's representation of a change from one auditory stimulus to another--the mismatch negativity (MMN)--indicated diminished responses in the LP group relative to the normal group to /da/ versus /ga/ when the transition duration was 40 ms. In the lengthened transition duration condition the MMN responses from the LP group were more similar to those from the normal group, and were enhanced relative to the short transition duration condition. These data suggest that extending the duration of the critical portion of the acoustic stimulus can result in enhanced encoding at a preattentive neural level; however, this stimulus manipulation on its own is not a sufficient acoustic enhancement to facilitate increased perceptual discrimination of this place-of-articulation contrast.

Speech-sound discrimination in school-age children: psychophysical and neurophysiologic measures.

This study measured behavioral speech-sound discrimination and a neurophysiologic correlate of discrimination in normal school-age children (ages 6 to 15) to determine if developmental effects exist. Just noticeable differences (JNDs) and mismatch responses (MMNs) were assessed for synthetic syllables that differed in third-formant onset frequency (/da-ga/) and formant transition duration (/ba-wa/). These stimuli were selected because children with learning problems often find it difficult to discriminate rapid spectrotemporal changes like /da-ga/, whereas the ability to distinguish /ba-wa/ is relatively unimpaired. Results indicate that JNDs for /da-ga/ show no developmental effects and that JNDs for /ba-wa/ decrease slightly with age (although likely for task-related reasons). MMNs elicited by two /da-ga/ stimulus pairs (onset frequency differences = 20 Hz, 280 Hz) and three /ba-wa/ stimulus pairs (transition duration differences = 3, 5, 15 ms) showed no systematic or significant differences for onset latency, duration, or area as a function of age. Normative JND and MMN data are provided. These norms provide a metric against which children with suspected central auditory processing difficulties or auditory-based language disorders can be compared.

Acoustic-phonetic approach toward understanding neural processes and speech perception.

This review paper describes an "acoustic-phonetic" experimental approach aimed at understanding normal and abnormal speech perception processes from both a behavioral and an electrophysiologic perspective. First, we consider the relevant acoustic characteristics of speech and identify a set of acoustic-phonetic classes that represent the parameters most important for making an acoustic signal sound like speech. Second, we review what is known about the neurophysiologic representation of acoustic-phonetic speech parameters in animal and human subjects. Third, we describe how an acoustic-phonetic approach has been useful in understanding the biologic basis of some auditory learning problems in children and in characterizing the behavioral and neurophysiologic changes resulting from speech-sound training. Finally, we discuss these findings and how they may expand the diagnostic and rehabilitative repertoire of practicing audiologists.

Ozone differential absorption lidar algorithm intercomparison.

An intercomparison of ozone differential absorption lidar algorithms was performed in 1996 within the framework of the Network for the Detection of Stratospheric Changes (NDSC) lidar working group. The objective of this research was mainly to test the differentiating techniques used by the various lidar teams involved in the NDSC for the calculation of the ozone number density from the lidar signals. The exercise consisted of processing synthetic lidar signals computed from simple Rayleigh scattering and three initial ozone profiles. Two of these profiles contained perturbations in the low and the high stratosphere to test the vertical resolution of the various algorithms. For the unperturbed profiles the results of the simulations show the correct behavior of the lidar processing methods in the low and the middle stratosphere with biases of less than 1% with respect to the initial profile to as high as 30 km in most cases. In the upper stratosphere, significant biases reaching 10% at 45 km for most of the algorithms are obtained. This bias is due to the decrease in the signal-to-noise ratio with altitude, which makes it necessary to increase the number of points of the derivative low-pass filter used for data processing. As a consequence the response of the various retrieval algorithms to perturbations in the ozone profile is much better in the lower stratosphere than in the higher range. These results show the necessity of limiting the vertical smoothing in the ozone lidar retrieval algorithm and questions the ability of current lidar systems to detect long-term ozone trends above 40 km. Otherwise the simulations show in general a correct estimation of the ozone profile random error and, as shown by the tests involving the perturbed ozone profiles, some inconsistency in the estimation of the vertical resolution among the lidar teams involved in this experiment.

Speech sound representation, perception, and plasticity: a neurophysiologic perceptive.

Historically, auditory research has focused predominately upon how relatively simple acoustic signals are represented in the neuronal responses of the auditory periphery. However, in order to understand the neurophysiology underlying speech perception, the ultimate objective is to discover how speech sounds are represented in the central auditory system and to relate that representation to the perception of speech as a meaningful acoustic signal. This paper reviews three areas that pertain to the central auditory representation of speech: (1) the differences in neural representation of speech sounds at different levels of the auditory system; (2) the relation between the representation of sound in the auditory pathway and the perception/misperception of speech, and (3) the training-related plasticity of speech sound neural representation and speech perception.

Speech sound perception and learning: biologic bases.

Historically, auditory research has focused predominantly on how relatively simple acoustic signals are represented in the neuronal responses of the auditory periphery. However, in order to understand the neurophysiology underlying speech perception, the ultimate objective is to discover how speech sounds are represented in the central auditory system and to relate that representation to the perception of speech as a meaningful acoustic signal. This paper reviews three areas pertaining to the central auditory representation of speech: (1) the differences in neural representation of speech sounds at different levels of the auditory system, (2) the relation between the representation of sound in the auditory pathway and the perception/misperception of speech, and (3) the plasticity of speech-sound neural representation and speech perception.

Developmental changes in P1 and N1 central auditory responses elicited by consonant-vowel syllables.

Normal maturation and functioning of the central auditory system affects the development of speech perception and oral language capabilities. This study examined maturation of central auditory pathways as reflected by age-related changes in the P1/N1 components of the auditory evoked potential (AEP). A synthesized consonant-vowel syllable (ba) was used to elicit cortical AEPs in 86 normal children ranging in age from 6 to 15 years and ten normal adults. Distinct age-related changes were observed in the morphology of the AEP waveform. The adult response consists of a prominent negativity (N1) at about 100 ms, preceded by a smaller P1 component at about 50 ms. In contrast, the child response is characterized by a large P1 response at about 100 ms. This wave decreases significantly in latency and amplitude up to about 20 years of age. In children, P1 is followed by a broad negativity at about 200 ms which we term N1b. Many subjects (especially older children) also show an earlier negativity (N1a). Both N1a and N1b latencies decrease significantly with age. Amplitudes of N1a and N1b do not show significant age-related changes. All children have the N1b; however, the frequency of occurrence of N1a increases with age. Data indicate that the child P1 develops systematically into the adult response; however, the relationship of N1a and N1b to the adult N1 is unclear. These results indicate that maturational changes in the central auditory system are complex and extend well into the second decade of life.

Temperature measurements made with a combined Rayleigh -Mie and Raman lidar.

The NASA Goddard Space Flight Center stratospheric ozone lidar system has the capability of collecting both Rayleigh -Mie and Raman backscatter data simultaneously at a number of wavelengths. Here we report on an improved method by which temperature can be derived from a combination of the Rayleigh -Mie return at 351-nm lidar channels and the Raman nitrogen return at 382-nm lidar channels. We also examine some common techniques by which temperatures are retrieved from lidar data. Finally, we show results obtained in 1995 during two Network for the Detection of Stratospheric Change intercomparison campaigns at Lauder, New Zealand and Mauna Loa, Hawaii.

Auditory neurophysiologic responses and discrimination deficits in children with learning problems.

Children with learning problems often cannot discriminate rapid acoustic changes that occur in speech. In this study of normal children and children with learning problems, impaired behavioral discrimination of a rapid speech change (/dalpha/versus/galpha/) was correlated with diminished magnitude of an electrophysiologic measure that is not dependent on attention or a voluntary response. The ability of children with learning problems to discriminate another rapid speech change (/balpha/versus/walpha/) also was reflected in the neurophysiology. These results indicate that some children's discrimination deficits originate in the auditory pathway before conscious perception and have implications for differential diagnosis and targeted therapeutic strategies for children with learning disabilities and attention disorders.

Speech-evoked cognitive P300 potentials in cochlear implant recipients.

The cognitive P300 evoked potential was elicited by speech stimuli in successful cochlear implant recipients, and the resulting P300 morphology was remarkably similar to that of normal-hearing individuals. The P300 was elicited by the synthesized speech pair/da/and/di/ presented using an oddball paradigm to nine "good" Nucleus cochlear implant users and nine age-matched normal-hearing subjects (34-81 yr old). There were no significant differences in P300 amplitude and latency between the two groups. Moreover, the N1 and P2 potentials occurred at similar latencies in the two groups, although the N1 amplitude was significantly smaller in the cochlear implant users. The P300 was absent in one "poor" cochlear implant user. The results suggest that the P300 may serve as a useful tool for evaluating the cognitive aspects of auditory processing in cochlear implant recipients, and that it may aid in assessing the success of cochlear implantation.

Measurements of stratospheric aerosols with a combined elastic-Raman-backscatter lidar.

Improvements made to the NASA Goddard Space Flight Center Stratospheric Ozone Lidar system have extended its atmospheric-aerosol-measuring capabilities. The methods by which aerosol-scattering ratio, aerosol backscatter, and aerosol extinction are simultaneously derived from lidar data are reported, and results obtained during several intercomparison campaigns at worldwide locations are shown. The results track the evolution of the Mt. Pinatubo aerosol cloud from 1991 to 1994 and report wavelength-dependence information for aerosol backscatter between 308 and 351 nm. Two analysis techniques, a more common inversion method and a combined elastic-Raman-backscatter approach, are also compared.

Mismatch Negativity in the Assessment of Central Auditory Function.

The mismatch negativity (MMN) is an auditory evoked potential that is undergoing research both on its basic characteristics and its potential clinical applications. Current work indicates that the MMN is a passively elicited measure of the brain's response to stimulus change and is therefore a measure of auditory discrimination. If the MMN can be evoked by acoustic stimulus differences that are important in everyday communication, it may serve as a tool for evaluating central auditory processing in individuals whose hearing sensitivity is normal but whose history and behavior indicate difficulties in auditory perception.In this Short Course, we provide an overview of the MMN, how it is recorded and current thinking on what it reflects. We describe its characteristics in normal school-age children, particularly in response to speech stimuli. We also describe our early experience with populations for whom the assessment of central auditory processing is important for clinical management.

Measurement intercomparison of the JPL and GSFC stratospheric ozone lidar systems.

For approximately one month during October and November 1988 the NASA Goddard Space Flight Center mobile lidar system was brought to the Jet Propulsion Laboratory, Table Mountain Facility, to make side-byside measurements with the JPL lidar of stratospheric ozone concentration profiles. Measurements were made by both excimer laser DIAL systems on fifteen nights during this period. The results showed good agreement of the ozone profiles measured between 20- and 40-km altitude. This is the first (to the best of our knowledge) reported side-by-side measurement intercomparison of two stratospheric ozone lidar systems.

Experimental evidence against middle ear oxygen absorption.

The present theory of eustachian tube (ET) function and middle ear (ME) ventilation posits that oxygen absorbed by the ME mucosa causes negative ME pressure which is relieved by periodic opening of the ET during swallowing and yawning. After developing a method to cannulate the ET of mongrel dogs we connected the cannulas hermetically to manometers. This system excluded ET function and tested the oxygen absorption capacity of the ME. When we controlled respiration and maintained blood gas PO2 and PCO2 at normal levels, we were unable to find any manometric evidence of negative pressure of gas absorption in the dog ME. Lowering the PCO2 and raising the PO2 of the blood by hyperventilation caused negative ME pressure which could be measured manometrically. We confirmed these findings with the tympanometer. Raising the PCO2 and lowering the PO2 by hypoventilation caused positive pressure in the ME. There is no evidence in these experiments that O2 absorption occurs or causes negative ME pressure in the dog. To the contrary there is evidence that elevated blood levels of the more diffusible CO2 cause an increase in the ME pressure and lowered CO2 level causes a negative ME pressure.

Auditory middle latency responses in the guinea pig.

Auditory middle latency responses (latencies 6 to 50 msec in guinea pigs) were recorded from eight awake, restrained guinea pigs. Before recording, screw electrodes were implanted in the skull in a coronal plane in line with the bregma. Another electrode, which served to monitor auditory brainstem responses, was placed 1 cm posterior to the bregma. All electrodes were referenced to a lead positioned 2 cm anterior to the bregma. During the recording session, click stimuli of various repetition rates and intensity levels were delivered monaurally in a closed sound system. Auditory brainstem responses were monitored to ensure normal functioning of the peripheral auditory system. Responses from electrodes at the midline and over the temporal area ipsilateral to the stimulus ear were greatly attenuated or absent. From an electrode over the temporal area contralateral to the stimulus ear, two positive peaks occurred at latencies of approximately 12 and 27 msec. A negative trough was identified at approximately 17 msec. Latency and amplitude functions for this waveform were determined for various stimulus levels. Response amplitude increased as stimulus repetition rate was decreased. Anesthesia greatly altered waveform structure and prolonged peak latencies. These effects were more marked at stimulus repetition rates faster than 10/sec than at slower rates. Properties of the guinea pig middle latency response are compared with those previously reported for cats and humans.

Effects of conductive hearing loss on auditory brainstem response.

The purpose of this paper is not to propose that auditory brainstem response (ABR) be utilized for the assessment of conductive losses, but to define the effects of conductive hearing loss on the ABR when such a complication occurs. Conductive losses attenuate cochlear stimulation. Since wave V latency is inversely related to stimulus intensity, the magnitude of the conductive loss should be a predictor of the wave V latency delay. In this study, ABR wave V latencies from patients with known conductive losses due to canal occlusion, middle ear effusion, ossicular fixation and chain interruption were compared with latency values calculated from the magnitude of the loss. In those patients with occlusion of the external auditory canal and middle ear effusion, the shift of the wave V latency-intensity function correlated well with the air-bone gap. This correlation was poor for patients with ossicular chain disorders. In mixed hearing losses, the increased wave V latency due to the conductive component may totally mask an increase in latency caused by a retrocochlear component.

Stratospheric ozone and hydroxyl radical measurements by balloon-borne lidar.

A balloon-borne lidar system for the measurement of ozone and hydroxyl radical in the stratosphere has been constructed and flown by Goddard Space Flight Center. On this flight ozone concentration at altitudes between 20 and 37 km were determined with vertical resolution of approximately 0.5 km. In addition, horizontally resolved ozone measurements with 0.15-km resolution were obtained over a 2-km range. The temporal variation of the hydroxyl radical concentration was measured in the 34-37-km altitude region, ranging from 40 parts/trillion shortly after noon to approximately 5 parts/trillion 2 h after sunset. Improvements in the system are proposed to increase the sensitivity of the instrument below the parts/trillion level, permitting hydroxyl determinations in the 20-30-km altitude range.