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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↗
PubMed2001

Between sound and perception: reviewing the search for a neural code.

This review investigates the roles of representation, transformation and coding as part of a hierarchical process between sound and perception. This is followed by a survey of how speech sounds and elements thereof are represented in the activity patterns along the auditory pathway. Then the evidence for a place representation of texture features of sound, comprising frequency, periodicity pitch, harmonicity in vowels, and direction and speed of frequency modulation, and for a temporal and synchrony representation of sound contours, comprising onsets, offsets, voice onset time, and low rate amplitude modulation, in auditory cortex is reviewed. Contours mark changes and transitions in sound and auditory cortex appears particularly sensitive to these dynamic aspects of sound. Texture determines which neurons, both cortical and subcortical, are activated by the sound whereas the contours modulate the activity of those neurons. Because contours are temporally represented in the majority of neurons activated by the texture aspects of sound, each of these neurons is part of an ensemble formed by the combination of contour and texture sensitivity. A multiplexed coding of complex sound is proposed whereby the contours set up widespread synchrony across those neurons in all auditory cortical areas that are activated by the texture of sound.

Animals↗
PubMed2002

Mapping lateralization of click trains in younger and older populations.

The main purpose of this study was to describe and compare lateralization of earphone-presented stimuli in younger and older individuals. Lateralization functions, relating perceived location to either interaural time differences (ITDs) or interaural level differences (ILDs) were determined for 78 subjects, aged 21-88 years, who responded by pressing one of nine keys to indicate the perceived location of the stimulus. All subjects were healthy, without any history of hearing loss or ear surgery and within the normal pure tone audiometric range for their age group. Interaural pure tone and click thresholds did not differ by more than 5 dB across ears. The ILD lateralization functions, ranging from 10 dB favoring the left ear to 10 dB favoring the right ear were linear. In contrast, the ITD lateralization functions were S-shaped with a clear linear component ranging from 750 micros favoring one ear to 750 micros favoring the other ear and with an asymptote from 750 micros to 1 ms. The same general shape of the ITD and ILD lateralization functions was found at all ages, but the linear slope of the ITD lateralization function became shallower with age. The ability to discriminate midline-located click trains (ITD and ILD=0) from ITD-lateralized click trains deteriorated with age, while the comparable ability to discriminate ILD-lateralized click trains did not change significantly with age. The data support two general conclusions. First there seems to be an overall reduction in the range of ITD-based lateralization due to aging. Second, there is a greater reduction in sensitivity due to aging in changes from the perceived midline position (ITD and ILD=0) when ITD is manipulated than when ILD is manipulated.

Acoustic Stimulation↗
PubMed2002

Origin of the binaural interaction component in wave P4 of the short-latency auditory evoked potentials in the cat: evaluation of serial depth recordings from the brainstem.

There is no general agreement on the origin of the binaural interaction (BI) component in auditory brainstem responses (ABRs). To study this issue the ABRs to monaural and binaural clicks with various interaural time differences (ITDs) were simultaneously recorded from the vertex and from a recording electrode aiming at the superior olive (SO) in cats. Electrode path was along the fibers of the lateral lemniscus (LL). Binaural difference potentials (BDPs), which were computed by subtracting the sum of the two monaural responses from the binaural response, were obtained at systematic depths and across a range of ITD values. It was observed that only a specific BDP deflection recorded at the level at which lemniscal fibers terminate in the nuclei of LL coincided in time with the most prominent BDP in the cat's vertex-recorded ABRs, the BDP in their wave P4. As ITD was increased, the latency shifts and amplitude decrements of the scalp-recorded far-field BDP wave exactly followed those recorded at this lemniscal near-field BDP locus. The data support our hypothesis that the BI component in wave P4 results from a binaural reduction in dischargings of axons ascending in the LL, with this reduction due to contralateral inhibition of the discharge activity of the inhibitory-excitatory units in the lateral nucleus of the SO. Furthermore, at the level of the SO, the BDP in the responses to contra-leading binaural clicks always had larger magnitudes than those evoked by ipsi-leading ones. This bilateral asymmetry is consistent with the view that the BDP in scalp-recorded ABRs is related to the function of sound lateralization.

Acoustic Stimulation↗
PubMed2002

Concurrent motion detection based on dynamic changes in interaural delay.

The ability to detect a dynamic change in the interaural delay of a pure tone in the presence of a distracter tone of a different frequency was investigated in four conditions: (1) a control condition in which no distracter tone was present, (2) the distracter tone was stationary (fixed interaural delay), (3) the distracter had an interaural delay that changed in the same direction as that of the target tone, i.e., concurrent auditory motion in the same direction, and (4) the distracter had an interaural delay that changed in a direction opposite to that of the target tone, i.e., concurrent auditory motion in opposite directions. In a cued single-interval two-alternative forced-choice design, the observer had to determine if the target tone had a constant or dynamic interaural delay. The target was a 500-Hz tone and the distracter was a tone with a frequency of 300, 510, 550, 600, 800, or 1000 Hz. Detection was also examined for a range of stimulus durations, rates of change in interaural delay (i.e., velocity), and extent of change in interaural time difference (i.e., 'distance'). Results showed that the best performance (highest d') was associated with the no-distracter condition, followed by the stationary-distracter, opposite-direction, and same-direction conditions, respectively. Detection improved with increasing frequency difference between distracter and target tones, but was nonetheless lower than that associated with the no-distracter condition, even when the distracter frequency was several critical bands removed from the target frequency.

Auditory Perception↗
PubMed2002

Monaural middle ear destruction in juvenile and adult mice: effects on responses to sound direction in the inferior colliculus ipsilateral to the intact ear.

This study examined the effect of monaural middle ear destruction on auditory responses to sound direction in the inferior colliculus (IC) of the laboratory mice, Mus musculus. Monaural middle ear destruction was performed on juvenile and adult mice (the experimental mice). Auditory response properties of neurons to ipsilateral and contralateral sounds (I-40 degrees and C-40 degrees ) were examined in the IC ipsilateral to the intact ear 4 weeks later. IC neurons of control mice had higher minimum thresholds (MTs), larger Q(n) (Q(10), Q(30)) values but smaller dynamic ranges at I-40 degrees than at C-40 degrees. These direction-dependent response properties were not observed for IC neurons of experimental juvenile and adult mice. However, Q(n) values of IC neurons were significantly smaller in experimental juvenile than in control and experimental adult mice. Normal tonotopic organization in terms of positive correlation between recording depth and best frequency (BF) was observed in the IC of control and experimental adult mice at both sound directions but not in the IC of experimental juvenile mice. A positive correlation of increasing MT with BF was only observed for IC neurons in control mice but not in both experimental mice. Possible mechanisms for these different response properties are discussed.

Aging↗
PubMed2003

Spectral directionality of the external ear of the lesser spear-nosed bat, Phyllostomus discolor.

The directional dependence of sound pressure transformation of head and pinna has been measured in the phyllostomid bat Phyllostomus discolor for the frontal hemisphere using a maximum length sequence method. The azimuthal position of the axis of highest pinna gain came closer to the midsagital plane with increasing frequency. The acoustic axis of highest pinna gain was further characterized by an increase of the elevation angle with increasing frequency and a specific decrease at 55 kHz. Additionally, a spectral notch separated two regions of high and low frequency hearing at specific elevation and frequency combinations. The special influence of the tragus on the position of the pinna gain axis and the spectral notches is demonstrated. The functional implications of the spectral notch for hearing in P. discolor are discussed.

Animals↗
PubMed1996

Reissner's membrane vibrations in the apical turn of a living guinea pig cochlea.

Mechanical tuning curves were recorded at several radial locations on the Reissner's membrane, over a wide range of frequencies, and sound pressure levels. The position coordinates of each location were also recorded. The shape of the tuning curves changed dramatically with the radial location. Near the outer edge of the cochlea the response was broadly tuned, with a maxima near 300 Hz, while near the inner edge the response showed at least three maxima and minima. Responses were also measured at the reticular lamina. The shapes of the frequency responses at the Reissner's membrane are quite different from those measured at the reticular lamina below it.

Acoustic Stimulation↗
PubMed1996

Temporal coding of low-frequency amplitude modulation in the torus semicircularis of the grass frog.

Single neuron responses to sinusoidal 20 Hz amplitude modulated tone bursts (612.5 ms stimulus-on time at the rate of once per 2.2 s) were studied in the auditory midbrain (torus semicircularis) of the immobilized grass frog (Rana temporaria temporaria). The characteristic frequency stimuli at 30 dB above the minimum threshold included 12 full modulation periods with fixed initial phase. Neurons generally showed good phase-locking to the envelope waveform. 160 of the 186 investigated neurons responded to 80% amplitude modulated stimuli with discharges synchronized to the modulation cycle. For this modulation depth the best phase-locking capability was observed for certain phasic and build-up units. The synchronous response to 10% modulated stimuli was observed in 104 units. Though a few (2 of 29) phasic units were capable of reproducing this modulation with very high fidelity, the general tendency was the increasing of phase-locking capacity for units with a substantial sustained activity. In this condition for 66 units (63% of the units displaying the synchronous response) we observed a significant improvement of phase-locking from the initial to the terminal periods of modulation. This effect could be interpreted as an initial stage of the enhancement of small amplitude changes in the course of the long-term adaptation.

Acoustic Stimulation↗
PubMed1996

Temporal integration in the echolocating bat, Megaderma lyra.

Temporal integration is a crucial feature of auditory temporal processing. We measured the psychophysical temporal integration of acoustic intensity in the echolocating bat Megaderma lyra using a two-alternative forced-choice procedure. A measuring paradigm was chosen in which the absolute threshold for pairs of short tone pips was determined as a function of the temporal separation between the pips. The time constants determined with this paradigm are a crucial characteristic of the sonar system of M. lyra, a species orientating in its environment by very short broadband sonar calls emitted at high rates. Two different carrier frequencies for the tone pips were used to obtain data from the lower and the higher half of the hearing area of M. lyra. Both in the lower and in the higher frequency range, M. lyra showed very short time constants of about 220 microseconds. Our results are comparable to data from the echolocating dolphin, Tursiops truncatus, showing click integration times of about 260 microseconds and to estimates of auditory temporal integration in the context of echo clutter interference in the big brown bat.

Acoustic Stimulation↗
PubMed1996

Sound lateralization in patients with lesions including the auditory cortex: comparison of interaural time difference (ITD) discrimination and interaural intensity difference (IID) discrimination.

We examined sound lateralization using dichotic presentation of noises in 15 patients with left unilateral (12 patients) or bilateral (3 patients) temporal lobe lesions, that included the auditory cortex, and evaluated their abilities to discriminate interaural time and intensity difference (ITD, IID) separately. On the ITD discrimination test, discrimination thresholds in patients with left unilateral lesions were significantly higher than those in normal subjects, but all patients with left unilateral lesions could detect ITD. However, none of 3 patients with bilateral lesions could detect ITD. On the IID discrimination test, all patients with either unilateral or bilateral lesions could detect IID. IID discrimination thresholds in these patients were significantly higher than those in normal subjects. The auditory cortex plays an important role in discriminating both cues, but appears to be necessary for discriminating ITD.

Adolescent↗
PubMed1996

Electrically evoked cubic distortion product otoacoustic emissions from gerbil cochlea.

It has been demonstrated that electrical stimulation of the cochlear partition results in basilar membrane vibration and otoacoustic emissions. Electromotility of stimulated outer hair cells (OHCs) elicits the electrically evoked otoacoustic emissions (EEOAEs). Although electrically evoked upper and lower sideband distortion products (DPs) have been reported, electrically evoked cubic DP has not been investigated. Since the acoustically evoked cubic DP is the most commonly used otoacoustic measure of cochlear nonlinearity, this study tested whether electrical stimuli evoke a cubic DP otoacoustic emission. An electrical current containing the frequency component f1 and f2 (f1 < f2) was delivered to the round window niche of the gerbil, and electrically induced sound pressure change in the external ear canal was measured with a microphone. It was found that, in addition to f1 and f2 EEOAEs, cubic DP (2f1-f2) and other emissions at 3f1-2f2, 2f2-f1 and f2-f1 frequencies are electrically evoked. The electrically evoked cubic DP growth is similar to that of an acoustically evoked cubic DP. An electrical stimulus at f1 or f2 and an acoustic stimulus at f2 or f1 produce an identical cubic DP to that evoked by two electrical stimuli and/or two acoustic stimuli at f1 and f2 frequencies. An acoustic suppressor at a frequency near f2 can completely suppress an electrically evoked cubic DP emission. These data demonstrate that DPs can be provoked by a complex two frequency electrical current delivered to the round window niche. These stimuli elicit mechanical vibrations, from stimulated OHCs near the round window, which propagate apically toward their characteristic frequency places on the basilar membrane, and produce combination DPs. Electrically evoked cubic DPs appear to be produced by the same nonlinear mechanism that generates acoustically evoked DPs.

Acoustic Stimulation↗
PubMed1997

Excitatory and inhibitory response adaptation in the superior olive complex affects binaural acoustic processing.

Short-term adaptation was examined in single unit recordings from 113 superior olive neurons of anaesthetized 3- to 6-month-old Long-Evans rats. Responses to an equal intensity BF probe tone presented 1 ms after an 'adapting' BF tone were adapted by 56.3 +/- 2.6% (mean +/- S.E.) compared to responses at a 512 ms delay. The rapid decrease in discharge rate during adapting tones often approximated exponential time courses with time constants of less than 20 ms. The recovery from adaptation was exponential with time constants of 106 +/- 20.0 ms. The magnitude of adaptation and time course of recovery following monaural stimulation of binaurally excited (EE) neurons were not significantly different in both input pathways. Additionally, in 60% of EE neurons, an 'adapting' tone presented to one ear reduced subsequent responses to probe tones presented to the opposite ear. Binaural stimulation resulted in equal or greater adaptation of responses than monaural stimulation of either ear. The recovery of binaural excitatory responses generally followed a time course between recovery functions for ipsilateral and contralateral monaural stimuli. Lateral Superior Olive (LSO) neurons encode sound source location through the interaction of ipsilateral excitation and contralateral inhibition (IE). Ipsilaterally driven excitatory responses in LSO neurons exhibited the greatest magnitude of adaptation (68.5 +/- 21.1%). Adaptation of inhibition was observed in over half of IE neurons. Responses of LSO neurons to binaural BF probe stimuli were greatest immediately after a 200 ms BF 'inhibitory adapting' stimulus to the contralateral ear, and decreased with greater interstimulus delays. Responses to binaural stimulation were constant after prior binaural adaptation, when the magnitude and recovery of adaptation to monaural stimuli were similar for excitation and inhibition (8/25 IE cells). The functional significance and possible sites of adaptation processes are discussed.

Acoustic Stimulation↗
PubMed1997

Response map properties of units in the dorsal cochlear nucleus of barbiturate-anesthetized gerbil (Meriones unguiculatus).

The response map scheme introduced by Evans and Nelson (1973) and modified by others, including Davis et al. (1996) for use with gerbils, has been used primarily for classifying units recorded in the cochlear nucleus of unanesthetized decerebrate preparations. Units lacking spontaneous activity (SpAc) have been classified as either type I/III or type II units based on the relative strength of their responses to broad-band noise compared to their responses to best-frequency (BF) tones. The relative noise index (rho), a ratio of these responses after SpAc is subtracted out, provides a convenient measure of this relative strength. In this paper, responses of 320 units recorded in the dorsal cochlear nucleus (DCN) of barbiturate-anesthetized gerbils to short-duration BF tones and broad-band noise were recorded. Since 87.5% of these units lacked SpAc, their response maps resembled those of type II and type I/III units. Units were characterized by rho and the normalized slope (m) of a best line fit to the BF rate versus level plot starting from the sound level corresponding to the first inflection point of the rate curve (typically its maximum value or the start of its sloping saturation). The distributions of rho and m values do not form distinct clusters as they do for units in the decerebrate preparation. Thus, the criteria developed for classifying DCN units in the decerebrate preparation do not appear appropriate for units in the barbiturate-anesthetized preparation. Deposits of horseradish peroxidase were used to locate 52 units. Most of the low SpAc units, 56% with poor noise responses (5/9) and nearly 70% with strong noise responses (25/36), and nearly all of the high SpAc units (6/7), were located either within or below the fusiform cell layer.

Acoustic Stimulation↗
PubMed1997

Effects of pars flaccida on sound conduction in ears of Mongolian gerbil: acoustic and anatomical measurements.

This paper presents evidence on how the pars flaccida of the tympanic membrane affects the acoustic input to the middle and inner ear. Measurements of middle-ear acoustic input admittance and sound-pressure levels in both the ear canal and the middle-ear cavity were made in ears of eight gerbils before and after manipulations of the middle ear and tympanic membrane. The results are interpreted in terms of a model proposed by Kohllöffel [Hear, Res. 13(1984) 83-88]. The input-admittance measurements show that the pars flaccida of gerbil acts as a resonator with a resonance frequency of approx. 500 Hz. The admittance of this resonator appears in parallel with the input admittance of the pars tensa and its ossicular and cochlear load. At frequencies below the resonance, the pars flaccida admittance is compliance-like and its magnitude is comparable to that of the pars tensa and its load; consequently, the presence of pars flaccida increases the overall middle-ear input admittance and decreases the pressure difference across the tympanic membrane. At higher frequencies, the admittance of pars flaccida is mass-like and small in magnitude, and it has negligible influence on the overall middle-ear input admittance and the pressure difference across the tympanic membrane. These results suggest that the presence of pars flaccida reduces low-frequency acoustic input to the middle- and inner-ear and consequently decreases hearing sensitivity in this frequency range. Our measurements suggest that with a constant sound pressure stimulus, stiffening the gerbil pars flaccida would increase the motion of the pars tensa by 3-10 dB at frequencies below the normal flaccida resonance.

Acoustic Stimulation↗
PubMed1997

Evidence for separate processing in the human brainstem of interaural intensity and temporal disparities for sound lateralization.

Sound lateralization can be induced by interaural intensity disparities (IIDs) or by interaural temporal disparities (ITDs). The purpose of this study was to indicate whether IIDs and ITDs are processed by the same central units that detect interaural disparity in timing of afferent activity. If sound lateralization to intensity and time cues was determined by the same afferent latency disparity detectors in the brainstem, lateralization would be the same, regardless of whether latency disparity was induced by IIDs or ITDs. Moreover, the disparity detectors, and thus their dipole equivalents, would be the same for equal lateralizations, whether induced by IIDs or ITDs. Auditory brainstem evoked potentials (ABEPs) were recorded in response to monaural and binaural clicks, with a variety of IIDs and ITDs. Peak II (proximal auditory nerve activity), peak III (input to the superior olivary complex), and binaural interaction components (BICs) BeI and BeII (binaurally activated upper pons) were identified and their latencies measured. The psychophysical lateralization of the clicks (in cm from vertex) was also measured in response to the same binaural stimuli. The correlations between interaural afferent latency disparities (difference in corresponding peak latencies originating in each ear) and psychophysical click lateralization were calculated. Similarly, the correlations with click lateralization of the BICs equivalent dipole latency as well as orientation change (relative to symmetrical clicks) were determined. A strong correlation with lateralization was found for peaks II and III latency disparities, with steeper slopes for IIDs than for ITDs. Moreover, binaural activity across the same lateralizations differed between IIDs and ITDs. These results, therefore, indicate that interaural time and intensity cues are processed by separate systems in the brainstem, both at the afferent convergence level and after interaural disparities are determined.

Acoustic Stimulation↗
PubMed1997

Inhibition sensitive to interaural time difference in the barn owl's inferior colliculus.

In spontaneously active neurons in the external nucleus of the inferior colliculus of the barn owl, a stimulus-driven discharge was followed by a quiescent period lasting tens of milliseconds before the spontaneous activity resumed. The more favorable the interaural time difference, the longer the quiet period. The duration of the quiescent period also depended on stimulus frequency. Frequencies different from the neuron's best frequency induced shorter quiescent periods, although they could elicit similar rates of impulses. Also, the duration of the quiescent period was independent of interaural intensity difference. Thus, the quiet period is not due to an after-hyperpolarization but was an inhibitory effect that depended on the activity of other neurons. In some neurons, discharge continued after the stimulus without a quiescent period and gradually decayed over a period of 50-100 ms past the stimulus offset. The similarity between the quiescent period of the neurons mentioned above and the time course of the poststimulus discharge in these neurons suggests that these neurons serve as inhibitory interneurons.

Acoustic Stimulation↗
PubMed1997

Cochlear mechanisms of frequency and intensity coding. I. The place code for pitch.

In the past, several researchers have reported a substantial shift in the peak of the tone-evoked excitation pattern toward the base of the cochlea following an increase in the SPL of the stimulating tone. Evidence for such peak shifts has been found in the responses of auditory nerve fibers, cochlear microphonics, and the responses of outer hair cells and supporting cells in the cochlea, as well as in basilar membrane vibration measurements, and indirectly, in psychophysical data. However, direct evidence for such a peak shift in inner hair cell (IHC) responses has been relatively sparse. If the peak shift is preserved in the information conveyed to the auditory nerve fibers by the IHCs, the classical 'place theory' for frequency coding in the cochlea requires modification. In this study, the nature and extent of the SPL-dependent peak shift is examined with the help of recordings in the IHCs and other cells of the organ of Corti in the 0.5-2.5 kHz region of the Mongolian gerbil cochlea. It is demonstrated that the peak shift is a universal phenomenon in the diverse cell types in this region of the cochlea. Most importantly, a large SPL-dependent peak shift is demonstrated in IHC responses. On the other hand, the recordings indicate that the apical cutoff of the spatial excitation pattern is SPL-independent. We conclude, therefore, that the place theory of pitch perception must be abandoned or at least modified.

Acoustic Stimulation↗