Biomedical subjects

C W Ponton

Publications and source records attributed to C W Ponton.

At least 19 recordsLinked to original sources

PubMed2001

Plasticity in the adult human central auditory system: evidence from late-onset profound unilateral deafness.

Experience-related changes in central nervous system (CNS) activity have been observed in the adult brain of many mammalian species, including humans. In humans, late-onset profound unilateral deafness creates an opportunity to study plasticity in the adult CNS consequent to monaural auditory deprivation. CNS activity was assessed by measuring long-latency auditory evoked potentials (AEPs) recorded from teens and adults with late-onset (post-childhood) profound unilateral deafness. Compared to monaurally stimulated normal-hearing subjects, the AEPs recorded from central electrode sites located over auditory cortical areas showed significant increases in inter-hemispheric waveform cross-correlation coefficients, and in inter-hemispheric AEP peak amplitude correlations. These increases provide evidence of substantial changes from the normal pattern of asymmetrical (contralateral > ipsilateral amplitude) and asynchronous (contralateral earlier than ipsilateral) central auditory system activation in the normal-hearing population to a much more symmetrical and synchronous activation in the unilaterally deaf. These cross-sectional analyses of AEP data recorded from the unilaterally deaf also suggest that the changes in cortical activity occur gradually and continue for at least 2 years after the onset of hearing loss. Analyses of peak amplitude correlations suggest that the increased inter-hemispheric symmetry may be a consequence of changes in the generators producing the N (approximately 100 ms peak latency) potential. These experience-related changes in central auditory system activity following late-onset profound unilateral deafness thus provide evidence of the presence and the time course of auditory system plasticity in the adult brain.

Adolescent↗

PubMed2000

Maturation of human central auditory system activity: evidence from multi-channel evoked potentials.

OBJECTIVE: The purpose of this study was to evaluate central auditory system maturation based on detailed data from multi-electrode recordings of long-latency auditory evoked potentials (AEPs). METHODS: AEPs were measured at 30 scalp-electrode locations from 118 subjects between 5 and 20 years of age. Analyses focused on age-related latency and amplitude changes in the P1, N1b, P2, and N2 peaks of the AEPs generated by a brief train of clicks presented to the left ear. RESULTS: Substantial and unexpected changes that extend well into adolescence were found for both the amplitude and latency of the AEP components. While the maturational changes in latency followed a pattern of gradual change, amplitude changes tended to be more abrupt and step-like. Age-related latency decreases were largest for the P1 and N1b peaks. In contrast, P2 latency did not change significantly and the N2 peak increased in latency as a function of age. Abrupt changes in P1, P1-N1b, and N2 peak amplitude (also RMS amplitude) were observed around age 10 at the lateral electrode locations C3 and C4, but not at the midline electrodes Cz and Fz. These changes in amplitude coincided with a sharp increase and plateau in AEP peak and RMS amplitude variability from 9 to 11 years of age. CONCLUSIONS: These analyses demonstrated that the observed pattern of AEP maturation depends on the scalp location at which the responses are recorded. The distinct maturational time courses observed for individual AEP peaks support a model of AEP generation in which activity originates from two or more at least partly independent central nervous system pathways. A striking parallel was observed between previously reported maturational changes in auditory cortex synaptic density and, in particular, the age-related changes in P1 amplitude. The results indicate that some areas of the brain activated by sound stimulation have a maturational time course that extends into adolescence. Maturation of certain auditory processing skills such as speech recognition in noise also has a prolonged time course. This raises the possibility that the emergence of adult-like auditory processing skills may be governed by the same maturing neural processes that affect AEP latency and amplitude.

Adolescent↗

PubMed1999

Activating separate ascending auditory pathways produces different human thalamic/cortical responses.

When auditory nerve function is lost due to surgical removal of bilateral acoustic tumors in cases of neurofibromatosis type 2, a sense of hearing may be restored by means of an auditory brainstem implant (ABI), which electrically stimulates the cochlear nucleus. Electrically evoked auditory brainstem responses recorded from ABI subjects exhibit a variety of waveforms due to the presence or absence of different components. Evidently, ABI stimulation activates different ascending auditory pathways in different individuals. This study examined whether such differences at the brainstem level are associated with corresponding differences at higher levels. Multichannel recordings of electrically evoked middle-latency and late auditory responses were obtained from two ABI subjects whose very different electrically evoked auditory brainstem responses represent distinct categories of waveform morphology. The waveforms of both types of response were qualitatively similar in that for each condition tested there were corresponding main peaks and troughs. Quantitatively, however, there were differences in the scalp distributions and magnitudes of all components present. One subject had distributions suggesting bilateral activation and an N1-P2 complex of large amplitude, whereas the other subject had distributions suggesting unilateral activation contralateral to the side of stimulation and an N1-P2 complex of small amplitude. The differences suggest that activation of different ascending pathways in the auditory system results in different spatial and temporal patterns of neural activity in the thalamic and/or cortical auditory areas.

Adult↗

PubMed1999

Prolonged deafness limits auditory system developmental plasticity: evidence from an evoked potentials study in children with cochlear implants.

The use of cochlear implants to restore hearing in profoundly deaf children is increasing, with a trend toward earlier implantation. However, little is known about how auditory deprivation and subsequent implant use affects the maturing central auditory system. Previously reported results indicate that stimulation of the auditory system by a cochlear implant is sufficient to restore at least some aspects of central auditory pathway maturation, as reflected by age-related changes in the auditory evoked potentials. We review animal and human studies on sensory deprivation and report new results based on longitudinal evoked potentials data recorded from two individuals. Analyses show that age-related changes in the EPs may asymptote at levels very different from those found in the adult normal-hearing population. These results suggest that maturation of at least some aspects of central auditory system activity is limited by the onset and duration of the period of deafness prior to implantation.

Adult↗

PubMed1998

The effects of sensory hearing loss on cochlear filter times estimated from auditory brainstem response latencies.

Derived-band auditory brainstem responses (ABRs) were obtained in 43 normal-hearing and 80 cochlear hearing-impaired individuals using clicks and high-pass noise masking. The response times across the cochlea [the latency difference between wave V's of the 5.7- and 1.4-kHz center frequency (CF) derived bands] were calculated for five levels of click stimulation ranging from 53 to 93 dB p.-p.e. SPL (23 to 63 dB nHL) in 10-dB steps. Cochlear response times appeared to shorten significantly with hearing loss, especially when the average pure tone (1 to 8 kHz) hearing loss exceeded 30 dB. Examination of derived-band latencies indicates that this shortening is due to a dramatic decrease of wave V latency in the lower CF derived band. Estimates of cochlear filter times in terms of the number of periods to maximum response (Nmax) were calculated from derived-band latencies corrected for gender-dependent cochlear transport and neural conduction times. Nmax decreased as a function of hearing loss, especially for the low CF derived bands. The functions were similar for both males and females. These results are consistent with broader cochlear tuning due to peripheral hearing loss. Estimating filter response times from ABR latencies enhances objective noninvasive diagnosis and allows delineation of the differential effects of pathology on the underlying cochlear mechanisms involved in cochlear transport and filter build-up times.

Acoustic Stimulation↗

PubMed1997

Integrated mismatch negativity (MMNi): a noise-free representation of evoked responses allowing single-point distribution-free statistical tests.

If the repeated presentation of a single (standard) auditory stimulus is randomly interspersed with a second acoustically different (deviant) stimulus, the cortical activity evoked by the deviant stimulus can contain a negative component known as the mismatch negativity (MMN). The MMN is derived by subtracting the averaged response evoked by the standard stimulus from that evoked by the deviant stimulus. When the magnitude of the response is small or the signal-to-noise ratio is poor, it is difficult to judge the presence or absence of the MMN simply by visual inspection, and statistical detection techniques become necessary. A method of analysis is proposed to quantify the magnitude and statistically evaluate the presence of the MMN based on time-integrated evoked responses. This paper demonstrates the use of this integrated mismatch negativity (MMNi) analysis to detect the MMN evoked by stimulus contrasts near the perceptual threshold of two subjects. The MMNi, by virtue of being equivalent to a low-pass filtered response, presents an almost noise-free estimate of MMN magnitude. A single measure of the integrated evoked response at a fixed time point is used in a distribution-free statistic that compares the magnitude of the averaged response evoked by the deviant stimulus with a magnitude distribution derived from 200 subaveraged responses to the standard stimulus (with the number of sweeps per average equal to that of the deviant stimulus). This allows a calculation of the exact probability for the null hypothesis that the negative magnitude of the response evoked by the deviant stimulus is drawn from the magnitude distribution of responses evoked by the standard stimulus. Rejection of this hypothesis provides objective evidence of the presence of the MMN.

Acoustic Stimulation↗

PubMed1997

Maturational delays in cortical evoked potentials in cochlear implant users.

We studied the effects of prolonged auditory deprivation in children in whom auditory stimulation was restored by a cochlear implant. The latency of the P1 component of the late cortical potential was used as the indicator of auditory system maturation. For normal-hearing children there is a gradual evolution of evoked potential features that extends through adolescence with P1 latency becoming adult-like at about age 15. It appears that maturation of P1 latency in normal and implanted children occurs at the same rate, but the time to maturity in implanted subjects is delayed by an amount approximately equal to the duration of deafness.

Adolescent↗

PubMed1996

Auditory system plasticity in children after long periods of complete deafness.

Deaf children fitted with a cochlear implant provide a unique opportunity to examine the effects of auditory deprivation on the maturation of the human auditory system. We compared cortical evoked potentials recorded in implanted and normal-hearing children and found that age-dependent latency changes for the P1 component, fitted to a decaying exponential curve, showed the same rate of maturation. For implanted children, however, maturational delays for P1 latency approximated the period of auditory deprivation prior to implantation. This indicates the auditory system does not mature without stimulation. Nonetheless, the auditory system retains its plasticity during the period of deafness since the re-introduction of stimulation by the cochlear implant resumes the normal maturational sequence.

Acoustic Stimulation↗

PubMed1996

Comparison of distortion product otoacoustic emission (DPOAE) and auditory brain stem response (ABR) traveling wave delay measurements suggests frequency-specific synapse maturation.

OBJECTIVE: To determine whether the source of age-dependent latency changes for ABR wave I results from cochlear mechanics or the haircell-neuron synapse. DESIGN: Cochlear traveling wave delays were estimated on the basis of derived ABR response latencies and DPOAE phase delays. The difference in travel time between adjacent one octave-separated frequencies was calculated for four age groups: 30 to 33 wk old, 34 to 37 wk old, 38 to 42 wk old (term), and young adults. RESULTS: We found that there were essentially no travel time differences between newborns in the 34 to 37 and the 38 to 42 wk conceptional age (CA) groups as estimated from DPOAE phase delays. For the 30- to 33-wk-olds, DPOAE travel times were increased at all frequencies, likely due to mild (about 10 to 15 dB) conductive hearing losses. Differences in travel times between adjacent bands, however, were not different from the other neonatal groups. Estimates on basis of wave I latency showed delays for the high-frequency region, 6 to 11 kHz, that were still immature at term. CONCLUSIONS: A comparison of frequency-dependent travel times calculated for wave I and DPOAE data in comparable age groups suggests mature cochlear functioning at 35 wk CA and a delayed maturation for the haircell-auditory nerve synapses relative to the preneural components for the basal turn with center frequencies above 6 kHz.

Acoustic Stimulation↗

PubMed1996

Auditory brain stem response generation by parallel pathways: differential maturation of axonal conduction time and synaptic transmission.

In attempting to correlate developmental anatomical data with electrophysiological data on maturation of the auditory brain stem response (ABR), a model of ABR generation was necessary to match neuroanatomical structures to ABR components. This model has been developed by reviewing quantitative studies of human brain stem nuclei, results of intrasurgical recordings, studies of correlation of pathology with ABR waveform alterations, and findings from direct stimulation of the human cochlear nuclei through a brain stem implant device. Based on this material, it was assumed that waves I and II are generated peripherally in the auditory nerve and that waves III, IV, and V are generated centrally, i.e., by brain stem structures. It was further assumed that wave III is generated by axons emerging from the cochlear nuclei in the ventral acoustic stria and that waves IV and V reflect activity in parallel subpopulations of these ascending axons at a higher brain stem level. Beyond the cochlear nucleus, the largest component of the brain stem auditory pathway consists of axons projecting without interruption from the cochlear nuclei to the contralateral lateral lemniscus and inferior colliculus. In the proposed model of ABR generation, the III-IV interwave interval is assumed to reflect only axonal conduction in this asynaptic pathway. Electrophysiological data from infants indicate that the III-IV interwave interval becomes adult-like by the time of term birth. The second largest component of the brain stem auditory pathway is the bilateral projection through the medial olivary nucleus. The model assumes that activity in this monosynaptic pathway, consisting of axonal conduction time plus one synaptic delay, is reflected in the III-V interwave interval. If both of the preceding assumptions are true, the IV-V interwave interval represents the difference between the two pathways, i.e., the time of transmission across one synapse. The electrophysiological ABR data indicates that the IV-V interval does not mature until one year of age. It is also possible to apply this model to the peripherally generated portion of the ABR. The I-II interwave interval, assumed to solely represent conduction in VIIIth nerve axons, is adult-like before the time of term birth. The II-III interval, presumed to contain a synapse in the cochlear nuclear complex, does not reach an adult level until between 1 and 2 yr postnatal age.

Axons↗

PubMed1996

Perinatal maturation of the auditory brain stem response: changes in path length and conduction velocity.

OBJECTIVE: The goal of this study was to correlate developmental data on brain stem auditory path length with data on auditory brain stem response (ABR) conduction time. This was done to estimate changing axonal conduction velocity during the perinatal period. DESIGN: Pathway length was determined by three-dimensional reconstruction of postmortem fetal and infant brain stems in an AutoCAD system. Brain stem conduction time was obtained from previous ABR studies of premature, term, and post-term infants. The process of correlation of path length and conduction time was based on a model of ABR generation (Ponton, Moore, & Eggermont, this issue) that assumes that the III-IV interpeak interval represents activity in an asynaptic pathway and, thus, consists of only axonal conduction time. RESULTS: Brain stem conduction time is adult-like by the time of term birth. However, the brain stem auditory pathway continues to lengthen postnatally, with portions of the pathway not reaching adult dimensions until 3 yr of age. We determined lengths at various perinatal ages for three different segments of the auditory pathway. Each segment began at the cochlear nucleus (site of wave III generation) and ended at a more rostral location that is a possible site of wave IV generation. Conduction velocity was estimated by dividing path length by axonal conduction time (III-IV interpeak interval). All three assumed sites of generation of wave IV gave estimates of a threefold increase in conduction velocity between 29 wk CA and adulthood. However, three highly discrepant measures of absolute conduction velocity were obtained for the different path segments. The most reasonable conduction velocity estimates, from 5 m/sec at 29 wk conceptional age to 20 m/sec in adults, were produced by assuming a site of generation for wave IV near the contralateral medial superior olivary nucleus. CONCLUSIONS: Prenatally, increasing conduction velocity more than compensates for increasing path length, causing ABR conduction time to decrease. Postnatally, increasing conduction velocity exactly compensates for increasing path length while ABR conduction time remains stable. Different aspects of myelin development may underlie these two phenomena.

Evoked Potentials, Auditory, Brain Stem↗

PubMed1996

Maturation of human cortical auditory function: differences between normal-hearing children and children with cochlear implants.

OBJECTIVE: We investigated maturation of cortical auditory function in normal-hearing children and in children who receive stimulation of their auditory system through a cochlear implant. DESIGN: As a measure of cortical auditory function, auditory evoked responses (AERs) were recorded from normal-hearing children and adults as well as from children and adults fitted with a cochlear implant. Morphological and latency changes for evoked responses recorded at electrode Cz are reported. RESULTS: For normal-hearing children, there is a gradual evolution of AER features that extends through adolescence, with P1 latency becoming adult-like in the late teens. Latency changes for P1 occur at the same rate for implanted children, but the overall maturation sequence is delayed. By extrapolation from the existing data, the age at which P1 latency becomes adult-like is delayed by approximately 5 yr for the implanted population. Other typical features of the AER, namely N1 and P2, are either delayed in developing or absent in the implanted children. CONCLUSIONS: These preliminary findings suggest both similarities and differences in cortical auditory maturation for normal-hearing and implanted children. For implanted children, the 5 yr delay for maturation of P1 latency roughly corresponds to the average 4.5 yr interval between the onset of deafness and the time of implantation. These findings suggest that during the period of deafness, maturation of cortical auditory function does not progress. However, some, if not all, maturational processes resume after stimulation is reintroduced.

Adult↗

PubMed1996

Possible application of functional imaging of the human auditory system in the study of acclimatization and late onset deprivation.

After some period of experience with a single hearing aid, speech recognition performance may increase for material presented to the aided ear. Conversely, performance may decline for material presented to the unaided ear. Improved performance for the normally aided ear beyond that observed at the initial fitting of the hearing aid has been described as the acclimatization effect. The decline in speech recognition for material presented to the unaided ear has been described as the late onset auditory deprivation effect. For both the acclimatization and deprivation effects, the observed changes in performance are not considered to be a consequence of a change in the functional status of the cochlea. Rather, the benefits and decrements in speech recognition performance presumably reflect functional changes or reorganization in the central auditory pathway. In nonhuman species, changes in central auditory function can be examined by physiological recordings directly from various structures along the auditory pathway. However, these techniques are invasive and inappropriate for studying possible changes in central function for the human auditory system. The purpose of this review is to describe noninvasive "imaging" techniques appropriate for use with human subjects and the ways they could be applied to objectively identify physiological changes that might be associated with either acclimatization or late onset deprivation effects. Currently, few of these techniques have been applied to the study of acclimatization and late onset auditory deprivation. Possible application of these techniques to assess the differential performance changes for material presented to the normally aided and normally unaided ear will be discussed.

Acoustic Stimulation↗

PubMed1996

Variable effects of click polarity on auditory brain-stem response latencies: analyses of narrow-band ABRs suggest possible explanations.

The auditory brain-stem responses (ABRs) to rarefaction and condensation clicks were obtained for 12 normal-hearing subjects in quiet, and high-pass masking at 8, 4, 2, 1, and 0.5 kHz. Derived narrow-band wave V latency differences were analyzed with respect to (1) stimulus polarity, (2) absolute differences irrespective of polarity. The analyses revealed no significant stimulus polarity effects on latency for the derived bands. Absolute latency differences regardless of polarity tended to be greater for those derived bands having lower characteristic frequencies (CFs). However, these differences were smaller than the expected half-period of the theoretical CF. Further analyses in three additional subjects using repeated runs of the same polarity indicate that this increase in absolute latency difference with lower derived band CF does not reflect a simple half-period change owing to polarity, but rather to the increase variability in measuring the peak latency of the lower CF derived bands. The variability is consistent with variability of eighth nerve PST histograms behavior observed in animal work [Kiang et al., "Discharge patterns of single fibers in the cat's auditory nerve," Research Monograph No. 35 (MIT, Cambridge, MA, 1965)]. Thus claimed polarity effects observed in other ABR work using absolute values may have been affected by this variability. It appears from these current data that half-period latency shifts of wave V owing to stimulus polarity differences are not observed in derived bands responses initiated from frequency specific regions of the cochlea.

Adolescent↗

PubMed1995

The mismatch negativity in cochlear implant users.

For individuals with severe or profound hearing loss, electrical stimulation of surviving neural elements by a cochlear implant may partly restore a sensation of hearing. Determining the extent of restoration based on behavioral measures may be difficult, particularly when evaluating young children or individuals who have little or no experience with normal hearing. In normal-hearing individuals, an objective measure of sound discrimination may be obtained by studying the mismatch negativity (MMN) component of the auditory evoked potential. The MMN may be evoked by a number of physical differences in acoustic stimuli including duration and pitch. For cochlear implant users, analogous stimulus differences may be produced by changing the length of a stimulus pulse train or by changing the pair of activated electrodes along a multi-electrode implant array. This paper will provide an overview of our current results, comparing evoked response data recorded from both normal-hearing individuals and cochlear implant users. In both normal-hearing individuals and cochlear implant users, MMNs were evoked by differences in stimulus train duration and pitch (or electrode pair activation in cochlear implant users). These findings suggest that the MMN may be a useful method for assessing the discriminability of electrical stimulation patterns produced by a cochlear implant. Eventually, information gained by MMN testing may yield important information for developing rehabilitation programs for the individual user.

Adult↗

PubMed1994

Auditory brainstem response (ABR) peak amplitude variability reflects individual differences in cochlear response times.

Previously, it was shown [Don et al., J. Acoust. Soc. Am. 94, 2135-2148 (1993)] that cochlear response times are gender specific and about 13% shorter in females than in males. It is also suggested that one of the possible reasons click-evoked auditory brainstem response (ABR) waveforms recorded from females are better defined and have larger amplitudes than those of males is due to a sex difference in cochlear response times leading to better synchronization of the cochlear output across the frequency regions. Variability in cochlear response times would also lead to variability in click evoked ABR amplitudes. The high-pass noise masking derived ABR technique was used to investigate the effect of normalizing the individual temporal variability at the neural and cochlear levels. This involved adjusting for differences in neural conduction time (I-V delay) by a compression or expansion of the derived ABR waveforms and by adjusting for differences in cochlear response times by a shift of the derived ABR waveforms. A summation of the compressed and shifted ABRs results in a normalized unmasked ABR waveform that can then be compared for amplitude variability with the unprocessed unmasked ABRs. Compensation for the neutral I-V variability had little effect while compensation for cochlear response times, particularly the delay between the 5.7- and 2.8-kHz regions, greatly affected the amplitude of wave V of the compounded ABR. This work provides a better understanding of the significant relationship between cochlear response times and variability of the ABR peak amplitudes.

Adolescent↗

PubMed1993

Gender differences in cochlear response time: an explanation for gender amplitude differences in the unmasked auditory brain-stem response.

Derived narrow-band auditory brain-stem responses (ABRs) in young normal-hearing subjects revealed a significant gender difference in response time between frequency regions of the cochlea. Females showed shorter delays than males between derived bands. This differential has not been previously reported. As in many early studies, the unmasked amplitude of the wave V complex was significantly larger (30%) in females than males. However, differences in amplitudes of the narrow-band responses were too small to account for the differential in the unmasked response. It is hypothesized that the larger amplitude of the unmasked wave V complex in females occurs because of a faster response time across the cochlea leading to better neural synchrony and, therefore, larger amplitudes. Furthermore, results can be explained by assuming that the stiffness gradient in the cochlea is 13% larger in females than in males. If males and females have the same cochlear tonotopic mapping, the female cochlea should be 13% shorter. This prediction is highly consistent with recent anatomical studies of cochlear length and gender. The results of the present study indicated possibly important cochlear mechanisms that influence the main parameters of ABRs. An understanding of these cochlear mechanisms may improve the diagnostic capabilities of ABRs in patients with peripheral hearing loss.

Acoustic Stimulation↗

PubMed1993

The relation between head size and auditory brain-stem response interpeak latency maturation.

In developmental populations, duration of auditory brain-stem response (ABR) I-III, III-V, and I-V vary substantially across individuals, particularly among preterm infants. Adult ABR interpeak latency has a strong correlation with brain-stem size and weaker correlation with head size. To determine if head size might contribute to this increased interpeak latency variability among infants, ABR data were normalized based on head circumference. Normalization by head circumference did not reduce interpeak variability. Further analyses revealed a negative correlation between interpeak latency and head circumference that varied as a function of age. Before 42 weeks conceptional age (CA), a significant relation exists between increased head circumference and decreased duration of the III-V and I-V intervals, but not the I-III interval. For infants older than 42 weeks CA, there was a significant relation between increased head circumference and decreased duration for the I-III intervals but not the III-V and I-V intervals. An age-dependent correlation between decreasing interpeak latency and increasing head circumference suggests that improved neural transmission through the auditory nerve and brain-stem pathway offset or even overcompensate for developmental lengthening of the sensory pathway. Also, developmental time constants obtained from nonlinear curve fit analyses were shorter for normalized than non-normalized data, particularly for the I-V interval. Therefore, correction of ABR data for the length of the sensory pathway may be important to estimate accurately maturation rate for developmental populations.

Acoustic Stimulation↗