Neural changes in the auditory cortex of awake guinea pigs after two tinnitus inducers: salicylate and acoustic trauma.

Tinnitus, also called phantom auditory perception, is a major health problem in western countries. As such, a significant amount of effort has been devoted to understanding its mechanisms, including studies in animals wherein a supposed "tinnitus state" can be induced. Here, we studied on the same awake animals the effects of a high-dose of salicylate and of an acoustic trauma both at levels known to induce tinnitus. Recordings of cortical activity (local field potentials) from chronically implanted electrodes in the same animals under each condition allowed direct comparison of the effects of salicylate and trauma (noise trauma was carried out several days after full recovery from salicylate administration). Salicylate induced a systematic and reversible increase in amplitude of cortical responses evoked by tone bursts over a wide range of frequencies and intensities. The effects of noise trauma, though much more variable than those of salicylate, resulted in both increases and decreases in the amplitude of cortical responses. These alterations of cortical response amplitudes likely reflect associated hypoacusis and hyperacusis. The effects of salicylate administration and noise trauma on spontaneous activity were also studied. Fourier analysis did not reveal any increase in power within any given frequency band; rather, both treatments induced a decrease of power spectrum over a relatively broad frequency band (approximately 10-30 Hz). Entropy rate of spontaneous activity, a measure of complexity (temporal correlations), was found to decrease after salicylate but not after acoustic trauma. The present data on evoked potentials confirm salicylate effects at the cortical level and partially extend such effects to acoustic trauma. While the present study showed that both salicylate and noise trauma induced some changes of spontaneous activity in auditory cortex, none of these changes are interpretable in terms of potential neural correlate of tinnitus.

Stress in hearing and balance in Meniere's disease.

Stress is an unavoidable every-day phenomenon. Physiological coping with stress depends on the appropriate release of stress hormones as well as their alleviation at the termination of the stress. Despite quite a body of research indicating that stress affects inner ear function, this concept has found little application in otolaryngology. Today's evidence clearly indicates that the inner ear is equipped to detect stress hormones and some of these hormones have been shown to affect the inner ear function. Major stress control pathways shown to affect the inner ear include several third order axes, the hypothalamus-pituitary-adrenal axis, the hypothalamus-pituitary-thyroid axis and the hypothalamus-pituitary-gonadal axis whose functioning are interactive and inter-dependent. Less well-studied are the second order hypothalamus-pituitary control axis and its interaction with other hormones. To explore these we carried out a retrospective study on a series of Ménière's patients who had undergone a neurotomy of the vestibular nerve in the dept of ORL at the Hopital Nord, Marseille. Ménière's patients were particularly appropriate for this study since stress has long been recognised as a factor associated with the triggering of the symptoms of this pathology. Patients with acoustic neuroma and facial spasm were taken as a control population. We investigated the level of a battery of stress hormones including prolactin beta-endorphin and growth hormone. The blood sample was taken on the morning before surgery. The most striking observation was the presence of hyperprolactinemia in 30% of the Ménière patients (more than 20 microg/l) with confirmation of prolactinoma in 6 patients. The level of beta-endorphin could also be elevated. Horner, K.C., Guieu, R., Magnan, J., Chays, A. and Cazal, Y. Neuropysychopharmacology, (2001) 26:135-138. These observations suggest that neuroendocrinological feedback pathways controlling stress can be disturbed in Ménière's patients and depression of hypothalamic dopaminergic inhibition of prolactin secretion might be implicated. A further study on non-operated Ménière's patients presenting hyperprolactinemia and on dopamine agonist treatment, is needed in order to assess the role of stress in Ménière's patients. Progress in this domain could open the door towards integration of the stress concept into clinical management of various inner ear disorders.

Onset overmasking of a brief amplitude increment in a pure tone and sensorineural hearing impairment.

HYPOTHESIS: The goal of this investigation was to determine, in patients with sensorineural hearing loss who may show an audiologic alteration in onset overmasking, whether different pathologic conditions differ in this respect, and whether patients with a vestibular neurotomy damaging the cochlear efferents will be affected. BACKGROUND: Auditory detection of brief signals, when presented at the beginning of a simultaneous long masking sound, may require a higher acoustic level than when presented after several hundred milliseconds. The proposed explanation, in terms of auditory nerve fibers adaptation has been based on the observation of a proportionally smaller increase of firing in response to an amplitude increment at the onset of a stimulus. However, this may not explain all the data, and other underlying processes are certainly involved. The degree or type of sensorineural pathologic condition may be a contributing factor. In addition, the cochlear efferent system, which exhibits a time course and a high-frequency predominance compatible with that of onset overmasking, could be involved. METHODS: Onset overmasking of a brief amplitude increment in one pure tone was examined in 6 normal subjects, 12 patients who had undergone vestibular neurotomy, 8 subjects with Meniere's-like symptoms, 5 subjects with presbyacusis, and 3 patients with a small neuroma. Both ears of all subjects were tested. Detection thresholds, amount of onset overmasking, and differences between the two ears were examined. RESULTS: All results from subjects with presbyacusis and neuroma were within the range observed in the group of normal subjects. In the group of eight Ménière's syndrome patients, four had results outside the normal range, three had deteriorated detection, and one had better detection. Among the 12 subjects who had undergone neurotomy, 2 had better detection in the unoperated ear. CONCLUSIONS: The results from Ménière's patients indicate that, in addition to the previously reported improved detection threshold for short onset delay, a deterioration of detection thresholds may occur in some subjects. Overall, the results from neurotomized ears do not provide evidence for an involvement of cochlear efferents in this tested psychoacoustic task.

Sympathectomy improves the ear's resistance to acoustic trauma--could stress render the ear more sensitive?

Emotional stress is a phenomenon experienced by many people at some time in their lives. Some of its early manifestations, such as unbearable loudness of ambient sounds and sensations of dizziness, might be linked to inner ear dysfunction. Although the inner ear is supplied with a substantial sympathetic innervation, previous studies have failed to demonstrate any significant functional impact. We show here that in the awake guinea pig and following unilateral ablation of the superior cervical ganglion, the temporary threshold shift induced by a 1-min exposure to 8 kHz pure tone at 96 dB sound pressure level was reduced by as much as 40 dB. Of interest, the protective effect was bilateral suggesting an intimate relationship between the sympathetic and the olivocochlear efferent systems. The data presented here provide new evidence for a key role for the sympathetic system in modulating temporary threshold shifts following exposure to moderate sound stimulation. This opens new perspectives for investigation of sympathetic control in noise-induced permanent hearing losses.

Auditory sensori-neural alterations induced by salicylate.

Early after the development of aspirin, almost 150 years ago, its auditory toxicity has been associated with high doses employed in the treatment of chronic inflammatory diseases. Tinnitus, loss of absolute acoustic sensitivity and alterations of perceived sounds are the three auditory alterations described by human subjects after ingestion of large doses of salicylate. They develop over the initials days of treatment but may then level off, fluctuate or decrease, and are reversible within a few days of cessation of treatment. They may also occur within hours of ingestion of an extremely large dose. Individual subjects vary notably as to their susceptibility to salicylate-induced auditory toxicity. Tinnitus may be the first subjective symptom, and is often described as a continuous high pitch sound of mild loudness. The hearing loss is slight to moderate, bilaterally symmetrical and affects all frequencies with often a predominance at the high frequencies. Alterations of perceived sounds include broadening of frequency filtering, alterations in temporal detection, deterioration of speech understanding and hypersensitivity to noise. Behavioral conditioning of animals provides evidence for mild and reversible hearing loss and tinnitus, similar to those observed in humans. Anatomical examinations revealed significant alterations only at outer hair cell lateral membrane. Electrophysiological investigations showed no change in endocochlear resting potential, and small changes in the compound sensory potentials, cochlear microphonic and summating potential, at low acoustic levels. Measures of cochlear mechanical responses to sounds indicated a clear loss of absolute sensitivity and an associated broadening of frequency filtering, both of a magnitude similar to audiometric alterations in humans, but at extremely high salicylate levels. Otoacoustic emissions demonstrated changes in the mechano-sensory functioning of the cochlea in the form of decrease of spontaneous emissions and reduced nonlinearities. In vitro measures of isolated outer hair cells showed reduction of their fast motile responses which are thought to be at the origin of cochlear absolute sensitivity and associated fine filtering. Acoustically evoked neural responses from the eighth nerve to the auditory cortex showed reversible and mild losses of absolute sensitivity and associated broadening of frequency filtering. There is no evidence of a direct alteration of cochlear efferent innervation. Evidence was obtained for decreases in cochlear blood supply under control of autonomous innervation. Spontaneous neural activity of the auditory nerve revealed increases in firings and/or in underlying temporal synchronies. Similar effects were found at the inferior colliculus, mostly at the external nucleus, and at the cortex, mostly at the anterior and less at the secondary auditory cortex but not at the primary auditory cortex. These changes in spontaneous activity might underlie tinnitus as they affect mostly neural elements coding high frequencies, can occur without a loss of sensitivity, are dose dependent, develop progressively, and are reversible. Biochemical cochlear alterations are poorly known. Modifications of oxydative phosphorylation does not seem to occur, involvement of inhibition of prostaglandin synthesis appears controversial but could underlie changes in blood supply. Other biochemical alterations certainly also occur at outer hair cells and at afferent nerve fibers but remain unknown.

Alterations in average spectrum of cochleoneural activity by long-term salicylate treatment in the guinea pig: a plausible index of tinnitus.

Salicylate, one of the most widely used drugs, produces at repetitive high doses reversible tinnitus and hearing loss. Neural correlates of hearing loss have long been established, whereas they remain elusive for tinnitus. The average spectrum of electrophysiological cochleoneural activity (ASECA), a measure of spontaneous auditory nerve activity, was monitored in guinea pigs over weeks of salicylate administration. Auditory nerve compound action potential (CAP) was also recorded to monitor acoustic sensitivity. In the first days of treatment, ASECA decreased acutely during hours after salicylate administration; after several days this decrease could be reduced. Over weeks of treatment the level of ASECA increased progressively. No change in CAP threshold was observed. The ASECA decrease induced by a contralateral broadband noise remained unchanged. At the end of treatment, acoustic tuning of ASECA showed a partially decreased sensitivity. After cessation of treatment the ASECA level returned progressively to initial values. In control animals delivery of an ipsilateral acoustic noise could reproduce the ASECA increase observed in long-term salicylate-treated animals. This white noise was of moderate sound pressure level and it elevated slightly CAP thresholds at high frequencies. These data provide evidence for salicylate-induced ASECA alterations without changes in CAP thresholds, in accord with clinical reports of tinnitus being the first subjective sign of salicylate ototoxicity. The similarities in occurrence, development, reversibility, frequency content, and acoustic level support the idea that ASECA changes, which indicates alterations of spontaneous eighth nerve activity and reflects the presence of salicylate-induced high-pitch tinnitus.

Efferent-mediated protection of the cochlear base from acoustic overexposure by low doses of lithium.

Many studies on anaesthetized animals and a few on awake animals have suggested that the cholinergic olivocochlear efferent feedback to outer hair cells can participate in the protection of the cochlea from acoustic overexposure. Lithium is known to stimulate acetylcholine synthesis and release in the brain and it is likely to act similarly at the level of the cochlear efferent synapses. We demonstrate here that, in the awake guinea-pig with a chronically implanted electrode on the round window of the cochlea, the temporary threshold shift induced by 1 minute exposure to different pure tones at around 90 dB sound pressure level (SPL) was reduced by as much as 40 dB, when exposure occurred after lithium treatment. The protection effect was not observed in anaesthetized animals. The effect was seen across the test frequency range of 6.4-12.5 kHz, suggesting that both 'fast' and 'slow' efferent effects are likely to be mediated by acetylcholine. Together our results provide new evidence that the olivocochlear efferents can provide a more efficient protection from acoustic overexposure when animals are awake.

Reversible hearing impairment induced by lithium in the guinea pig.

Lithium salts remain one of the most widely used treatments for depressive illness. The mechanisms involved probably include reduction in free inositol. Visual perceptive disturbances can be a side effect of the treatment. We report here for the first time that chronic lithium treatment in the guinea pig induces a predominantly low frequency hearing loss and, in the longer term, loss of sensitivity is observed across the whole audiogram. The changes are reversed when treatment is arrested. The observations could be accounted for, at least partially, by a lithium-induced perturbation of the phosphoinositide cascade within the inner ear.

Average spectrum of cochlear activity: a possible synchronized firing, its olivo-cochlear feedback and alterations under anesthesia.

Average spectrum of electrophysiological cochlear activity (ASECA) recorded from the cochlea or the eighth nerve is related to firing of auditory neurons and has been used recently in search of an objective measure of tinnitus both in animal models and in humans. Little is known about neuro-sensory processes underlying the spectral features of ASECA. The present study used awake and/or anesthetized animals and investigated effects of various sounds presented contralaterally and ipsilaterally. Contralateral stimulation with noise bands at frequencies above about 8 kHz and below acoustic interaural cross-talk decreased the amplitude of the 1 kHz peak of ASECA. When presented ipsilaterally noises produced either an increase or a decrease of this spectral peak when the acoustic bandwidth was respectively above or below 1.5 kHz. Pure tones when presented contralaterally had no detectable effect. When presented ipsilaterally pure tones with frequencies higher than about 4 kHz decreased the 1 kHz peak of ASECA. The detailed time course of sound-induced variations of the 1 kHz peak was measured by time averaging. The resulting response patterns resemble PST histograms of the auditory nerve. Sedation and anesthesia deepened the 500 Hz trough of ASECA and shifted it towards 400 Hz. Sedation induced a diminution and anesthesia an almost complete suppression of the decrease of the 1 kHz peak induced by contralateral noise. Overall these data indicate that ASECA would reflect synchronized firings and they provide evidence for an influence of olivo-cochlear feedback sensitive to the state of awakeness.

Effects of Ginkgo biloba extract (EGb 761) on cochlear vasculature in the guinea pig: morphometric measurements and laser Doppler flowmetry.

Ginkgo biloba extract (EGb 761) was administered orally for 4 or 6 weeks to healthy adult guinea pigs. Animals were then decapitated under deep ketamine anesthesia. Post-mortem morphometric measurements of cochlear vessels in the spiral lamina revealed a vasodilating effect of the extract in four of ten animals following 6 weeks of treatment. In vivo testing of the effect of 4 or 6 weeks of treatment with EGb 761 was monitored with laser Doppler flowmetry of the cochlear blood flow under pathological conditions. Results demonstrated that EGb 761 partly counteracted sodium salicylate-induced decreases in cochlear blood flow (CBF) and enhanced CBF increases induced by hypoxia. These findings indicate that EGb 761 may help to improve oxygenation in cochleas with compromised blood flow.

Occlusive silence duration of voiceless intervocalic plosives and voicing perception by normal and hearing-impaired subjects.

OBJECTIVE: The duration of occlusive silence of intervocalic unvoiced plosives is one of several acoustic cues contributing to the correct identification of voicelessness for normally hearing subjects. In a previous investigation on one velar plosive, some hearing-impaired subjects required an abnormally long duration of its occlusive silence in order to avoid voicing confusions. The present study investigated whether this would occur for the different plosives in various intervocalic contexts and for male and female voices. DESIGN: Speech signals from a male and a female voice pronouncing the three voiceless plosives (p,t,k) in three different vocalic contexts (a,i,u) were recorded. Each of these 18 vowel-consonant-vowel (VCV) utterances was edited to obtain a series of tokens with occlusive silence durations varying from 0 to 180 msec. Phonetic identification of these tokens was assessed for seven normally hearing persons and for seven subjects with a sensorineural hearing loss. RESULTS: For the various VCVs, about half of the hearing impaired subjects required abnormally long occlusive silence durations to avoid voicing confusions. For normal as well as hearing-impaired subjects duration thresholds varied considerably and not systematically for the different VCVs. The amount of voicing confusions tended to be related to audiogram loss. CONCLUSIONS: For hearing-impaired subjects, a normal occlusive silence duration can lead to voicing misperceptions for the various plosives in different vowel contexts and for male and female voices. The large variability in the results of each subject for the different VCVs indicates that even a strong anomaly in temporal perception, which certainly underlies a need for abnormally long occlusive silence duration, did not systematically overrule voicing perception.

Low-pass filtering in amplitude modulation detection associated with vowel and consonant identification in subjects with cochlear implants.

Temporal auditory analysis of acoustic events in various frequency channels is influenced by the ability to detect amplitude modulations which for normal hearing involves low-pass filtering with a cutoff frequency around 100 Hz and a rejection slope of about 10 dB per decade. These characteristics were established in previous studies measuring modulation transfer functions. For cochlear implant subjects, the delivery of detailed amplitude modulation information has been recently shown to result in very significant improvements in speech understanding. Several previous studies on cochlear implant subjects have reported capacities for temporal resolution rather equivalent to those of normally hearing subjects but with some notable individual differences. Recently two studies on some cochlear implant subjects indicated modulation transfer functions often quite similar to those of normal hearing but exhibiting marked individual differences in shape and absolute sensitivity. The present study compared amplitude modulation detection and phonetic recognition in a group of cochlear implant subjects to determine the extent to which the two tasks are correlated. Nine individuals who had been implanted with an Ineraid device and who demonstrated open speech understanding ranging from excellent to poor were chosen and tested in the present study. For each subject modulation transfer functions were measured at the most apical electrode and phonetic recognition of isolated vowels and intervocalic consonants was assessed. Results showed a strong correlation between the depth of high-frequency rejection in modulation transfer functions and success in vowel and consonant intelligibility. These results emphasize the importance of temporal speech features and offer perspectives for customizing signal processing in cochlear implants.

Periodicity of long-term context can influence gap detection.

Temporal gap detection in bands of noise is a basic paradigm to investigate auditory temporal resolution. This study further examined the role of the long-term temporal organization for gap detection. Our experiments aimed at determining whether a gap superimposed on an amplitude-modulated noise was easier to detect when the amplitude modulation was regular--or periodic--than when it was not. Care was taken to ensure that the stimulus portion, where the gap was inserted, was exactly the same for both periodic and aperiodic conditions. Results show that gap detection in periodically modulated white noise (125 Hz fundamental frequency, or 8 ms duration from peak to peak) is easier (with a threshold 4.7 ms lower) than in an aperiodically modulated noise (modulation randomly varied from 5 to 18 ms). This effect is observed for variations of the modulation frequency from 125 to 1000 Hz. These results indicate that the regularity of long-term temporal organization of amplitude modulations in noise can improve the detection of a gap.

Alterations of auditory nerve responses by hypoxia in normal and hydropic ears of awake guinea pigs.

Total interruption of blood or oxygen supply to the inner ear produces very rapid and drastic effects, whereas moderate decreases can be well tolerated by normal ears. In experimental endolymphatic hydrops some moderate alterations of cochlear vasculature have been described which might affect cochlear adaptation to moderate blood or oxygen deficiencies. In order to test this hypothesis an hypoxia at 5% oxygen was imposed for 30 min in normal and hydropic ears of awake guinea pigs and cochlear function was monitored with an electrode at the round window. Electrophysiological recordings used measures of compound action potential (CAP) amplitudes evoked by high-intensity tones, and of CAP thresholds. In normal ears hypoxia induced threshold elevations at all frequencies and decreases of CAP amplitude only for high frequencies. Hydropic ears presented similar or smaller threshold elevations but showed CAP amplitude decreases extending to lower frequencies and showed a much slower recovery both for CAP thresholds and amplitudes. The data indicate that hypoxia had different effects on auditory nerve responses evoked by high versus low intensity tones. The deleterious effects of hypoxia were increased in hydropic ears. Hypoxia-induced alterations were measured twice at one week intervals during which an anti-ischemic drug was administered to the animals; some beneficial effects of the drug treatment were observed on normal but not on hydropic ears.

[Effect of hypoxia at 5% oxygen on global electrophysiological responses from normal awake guinea pigs].

All experiments were conducted on awake guinea pigs warranting a normal physiological function of the circulatory system. Global electrophysiological responses from the round window of cochlea were monitored: cochlear microphonic (CM), summating potential (SP) and auditory compound action potential (AP). The results showed that hypoxia induced a threshold elevation at all frequencies thus affecting the whole cochlea, and also very large diminutions of AP at high intensity level exclusively in response to a high frequency, and very large increases of SP at 2 kHz and 8 kHz. After hypoxia, which lasted thirty minutes, a considerable recovery occurred, and at the end of the session recovery reached around eighty percent. Alterations of cochlea action potential in response to high intensity stimuli were dissociated from threshold changes. We speculate that threshold elevation could be associated with over-all energetic (ATP) decrease leading to diminution of endolymphatic resting potential. Action potential changes at high intensity and at cochlear base could reflect neurotransmission (afferent, efferent or sympathetic) disturbance.

Plastic changes in ipsi-contralateral differences of auditory cortex and inferior colliculus evoked potentials after injury to one ear in the adult guinea pig.

In normal adult guinea pigs, evoked potentials recorded at the ipsilateral auditory cortex to monaural high-frequency acoustic stimuli present higher thresholds and lower amplitudes than at the contralateral cortex; in the inferior colliculus, such ipsi-contralateral differences (ICDs) are smaller than in the auditory cortex. Changes in the ICDs were studied after opposite ear injury. Following quasi-complete hair cell destruction induced by sisomicin injection into the contralateral inner ear, threshold ICDs almost disappeared after about two to six days and ipsilateral amplitudes progressively increased in two to three weeks. The occurrence of ICDs at higher auditory centers revealed in this study, indicates peculiar processing of high frequency stimuli in normal guinea pigs. The alteration of ICDs after opposite ear impairment provides a new possibility to study the auditory plasticity in adult animals.

Axo-somatic contacts in the postnatal developing white rat spiral ganglion.

Development of the spiral ganglion in white rats was followed during the first 2 weeks after birth and morphological characteristics of the two neuronal types (I and II) were examined. In some neurons different stages of partial degeneration leading to formation of residual bodies were found without observation of degenerated cell, supporting the idea that differentiation at this time is not associated with cell death. Contacts between cell body of type II neurons and neuronal endings is reported for the first time. Such axo-somatic contacts previously observed only in monkey and man, also exists in lower mammals.

Differences in hair bundles associated with type I and type II vestibular hair cells of the guinea pig saccule.

Several studies have reported variations in shape and size of stereociliary bundles and in a limited number of observations have associated them to type I and type II hair cells. A systematic study has been undertaken for which a technique was developed in order to identify both cell types and their corresponding hair bundles. Numerous fissures were obtained in saccular epithelia and observed in scanning electron microscopy. Saccular type I and type II hair cells in the guinea pig were found to have distinctive hair bundles. The tallest stereocilia of almost all type I cells were longer than 6 microns, and were shorter in the striola compared to the periphery. In contrast, the tallest stereocilia of almost all type II cells were shorter than 6 microns and were not found to vary notably in size from the striola to the periphery. Hair bundles with stereocilia organized in straight or in staggered rows were found for both types of cells across the whole saccular epithelium, with no apparent particular distribution. Possible physiological significance of differences in hair bundles is discussed.

Indication of a relation between speech perception and temporal resolution for cochlear implantees.

In order to better understand the reasons for success or failure of a cochlear implant system for various patients, it appears necessary to analyze the patients' basic psychophysical capacities in relation to speech perception. Five patients with intracochlear multichannel Ineraid implants were studied in terms of their performance on temporal analysis in relation to their perception of consonants. For temporal analysis we measured the detection of a silent gap in noise and of an interval between two clicks. For consonant perception we established a confusion matrix based on 12 consonants presented in a vowel-consonant-vowel context using the vowel /a/. The results showed a correlation between temporal resolution for two successive clicks at the most basal cochlear electrode used, and the perception of place of articulation of consonants. This finding indicates that delivering fine temporal coding can be crucial for the success of an implant and that for a given subject, optimal capacity for temporal resolution may serve as a criterion for choosing a basal electrode.

Effect of silence duration in intervocalic velar plosive on voicing perception for normal and hearing-impaired subjects.

For normally hearing subjects shortening the silence duration of an intervocalic voiceless plosive induces a misperception of voicing. The time boundary for this effect is about 60 ms, which corresponds to a possible forward masking effect at the frequency of voicing. If recovery from masking is indeed involved, hearing-impaired subjects, who may have prolonged forward masking, can be expected to show abnormally long time boundary for voicing misperception. This study investigated the perception of voicing of an intervocalic plosive for a natural speech sample "aka" as a function of occlusive silence duration for normally hearing and hearing-impaired subjects. To investigate a correlation with forward masking, a second test was performed on the subjects. The same first a of the "aka" was selected and at its end was concatenated a voiced murmur taken from an "aga" elocution from the same speaker, and the minimum duration of the voiced murmur necessary for it to be perceived was measured. About half of the hearing-impaired subjects needed an abnormally long silence duration to avoid voicing misperception. The data indicate a significant correlation between the results of the two tests with a slope of regression line close to unity, and thus support the hypothesis of a voicing perception ruled by recovery from forward masking. Increase in silence duration of voiceless plosives might then be a beneficial acoustical processing for some hearing-impaired subjects.