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PubMed2003

Cortical auditory disorder caused by bilateral strategic cerebral bleedings. Analysis of two cases.

The authors present the anatomical and clinical features of cortical auditory dysfunction in two patients, in whom hypertensive bleedings destroyed the cortical auditory centres in both hemispheres. The second haemorrhage occurred four years after the first bleeding in both cases. The bleedings isolated the cortical hearing centres due to the destruction of the right and left temporal white matter. During the clinical course the symptomatology has changed in both patients: pure word deafness has transformed to cortical deafness and generalised auditory agnosia. Normal pattern of brain stem auditory evoked potentials suggested intact auditory pathways. Authors conclude that transformation of clinical forms of cortical auditory disorder can be explained by the tissue reaction to the subcortical bleeding in the cortical auditory centres.

Auditory Cortex↗
PubMed1995

A common neural code for frequency- and amplitude-modulated sounds.

Most naturally occurring sounds are modulated in amplitude or frequency; important examples include animal vocalizations and species-specific communication signals in mammals, insects, reptiles, birds and amphibians. Deciphering the information from amplitude-modulated (AM) sounds is a well-understood process, requiring a phase locking of primary auditory afferents to the modulation envelopes. The mechanism for decoding frequency modulation (FM) is not as clear because the FM envelope is flat (Fig. 1). One biological solution is to monitor amplitude fluctuations in frequency-tuned cochlear filters as the instantaneous frequency of the FM sweeps through the passband of these filters. This view postulates an FM-to-AM transduction whereby a change in frequency is transmitted as a change in amplitude. This is an appealing idea because, if such transduction occurs early in the auditory pathway, it provides a neurally economical solution to how the auditory system encodes these important sounds. Here we illustrate that an FM and AM sound must be transformed into a common neural code in the brain stem. Observers can accurately determine if the phase of an FM presented to one ear is leading or lagging, by only a fraction of a millisecond, the phase of an AM presented to the other ear. A single intracranial image is perceived, the spatial position of which is a function of this phase difference.

Brain Stem↗
PubMed2004

Serotonin reuptake inhibitors in auditory processing disorders in elderly patients: preliminary results.

OBJECTIVE/HYPOTHESIS: One mechanism associated with degeneration in the elderly is the decrease of neurotransmitters. In the central auditory pathway serotonin, can be found from cochlear nucleus to the auditory cortex, and it constitutes one of the most important neuromodulatory circuits in hearing processing. The present study analyzed the action of citalopram, a selective inhibitor of serotonin reuptake, in aged patients with normal to moderate sensorineural hearing loss (HL) and low performance on auditory processing. STUDY DESIGN/METHOD: Prospective, double-blind, randomized, placebo-controlled study. Thirty-eight selected patients were randomly divided into two groups. Nineteen patients made up group A and received placebo for 60 days. Nineteen patients of Group B received 20 mg per day of citalopram for 60 days. Hearing evaluation was performed initially and after 60 days and included pure-tone audiometry, speech discrimination test (SDT), emittanciometry (acoustic impedance audiometry), identification of synthetic sentences with an ipsilateral competitive message (SSI/ICM), tests of pitch-pattern sequences (PPS), and the staggered spondaic words test (SSW). RESULTS: Comparisons of tests of auditory processing pre- and posttreatment in each group showed a statistical improvement in performance on all tests in group B after 2 months of therapy. Comparisons pre- and posttreatment between groups showed that patients who received citalopram presented statistically significantly better results in the SSI/ICM test (P < .0001) after treatment. The same comparison in results for the PPS test and the SSW test revealed a tendency (P = .09 and 0.058, respectively) toward better performance in the group receiving citalopram. CONCLUSION: These preliminary results suggest that the use of citalopram can have a positive impact on auditory processes in elderly patients with low performance in auditory process.

Aged↗
PubMed1993

Auditory evoked potentials recorded from different skull locations in the rat.

Auditory evoked potentials were recorded from different skull sites in the rat. From over the cerebellum, the inferior colliculus, the thalamus and the auditory cortex, a series of discrete slow potentials of positive polarity were identified. The timing of these waveforms was compared with each other and with the high frequency components of the brainstem auditory evoked potential. Potentials recorded from over the inferior colliculus and the thalamus consisted of two possible responses, one of which predominated in an individual animal. Tentative origins for these slow potentials include the cerebellar cortex, the termination of the lateral lemniscus, the brachium of the inferior colliculus, the medial geniculate body, the auditory radiations and primary auditory cortex. It is concluded that as the afferent volley ascends the central auditory pathways, it generates initially the fast waves of the brainstem auditory potential followed by a series of slow positivities.

Acoustic Stimulation↗
PubMed1993

A survey of auditory brainstem nuclei in the chicken (Gallus domesticus) with cytochrome oxidase histochemistry.

The chicken's auditory brainstem nuclei from the cochlear nuclei to the nuclei of the lateral leminiscus were studied with cytochrome oxidase histochemistry. A strong reactivity in the cochlear and laminar nuclei was confirmed. Additional structures displaying high activity levels include the superior olive and both partitions of the nucleus intermedius lemnisci lateralis. Unilateral cochlea removal led to a strong reduction of activity in the cochlear nuclei and the nucleus laminaris, whereas there was no remarkable effect in higher brainstem centers. After bilateral cochlea extirpation auditory structures still displayed higher enzyme levels than most other nuclei. These observations point to the extraordinary metabolic activity in the ascending auditory pathway which is largely independent of sensory input from the auditory nerve.

Animals↗
PubMed1998

Representation of binaural spatial cues in field L of the barn owl forebrain.

This study examined the representation of spatial information in the barn owl Field L, the first telencephalic processing stage of the classical auditory pathway. Field L units were recorded extracellularly, and their responses to dichotically presented interaural time differences (ITD) and interaural level differences (ILD) were tested. We observed a variety of tuning profiles in Field L. Some sites were not sensitive to ITD or ILD. Other sites, especially those in the high-frequency region, were highly selective for values of ITD and ILD. These sites had multipeaked (commonly called "phase ambiguous") ITD tuning profiles and were tuned for a single value of ILD. The tuning properties of these sites are similar to those seen in the lateral shell of the central nucleus of the inferior colliculus. Although the tuning properties of Field L sites were similar to those observed in the inferior colliculus, the functional organization of this spatial information was fundamentally different. Whereas in the inferior colliculus spatial information is organized into global topographics maps, in Field L spatial information is organized into local clusters, with sites having similar binaural tuning properties grouped together. The representation of binaural cues in Field L suggests that it is involved in auditory space processing but at a lower level of information processing than the auditory archistriatum, a forebrain area that is specialized for processing spatial information, and that the levels of information processing in the forebrain space processing pathway are remarkably similar to those in the well-known midbrain space processing pathway.

Animals↗
PubMed2003

Diversity of axonal ramifications belonging to single lateral and medial olivocochlear neurons.

A classification of olivocochlear (OC) neurons based on their location in either the lateral or medial region of the superior olivary complex provides a powerful tool for predicting their terminations beneath the inner (IHC) or outer hair cells (OHC) of the cochlea, respectively. Yet the morphology of axonal terminations belonging to single lateral OC (LOC) and medial OC (MOC) neurons, which can provide clues about the functional capabilities of individual efferent neurons, has been relatively unexplored. Following injections of biotinylated dextran amine into regions containing OC neurons in the rat, we reconstructed 19 LOC and 15 MOC axons in surface preparations of the cochlea. Confirming previous studies, LOC axons could be classified as either intrinsic or shell based on the length (short versus long) of their terminal ramifications beneath the IHC. However, intrinsic LOC axons were of two types, those that traveled to the organ of Corti without branching (simple intrinsic) and those that had three or more branches that converged on the same discrete patch of IHCs (converging intrinsic). Regarding shell neurons, we found that they may have as many as four intraganglionic branches that could innervate as much as 41% of cochlear length. Lastly, we found that MOC neurons were extremely diverse, not only in the number of their tunnel-crossing fibers (1-15), but also in both the number of boutons they formed (1-48) and in their basal-apical spans (1-45%). Analysis revealed that the number of tunnel-crossing fibers formed by a given axon was closely related to the total number of its terminal boutons, but not to its cochlear span. Analysis further suggested the existence of two distinct subtypes of MOC neurons on the basis of the number of tunnel-crossing fibers and boutons each possessed: the more common sparsely-branched and the quite rare highly-branched MOC neurons. In conclusion, the variety of axonal ramifications of individual LOC and MOC neurons has functional implications and raises the question of whether the various types of efferent neurons might be subject to selective control by ascending and descending central auditory and possibly non-auditory pathways.

Animals↗
PubMed1993

Auditory magnetic fields in cochlear-implant patients.

Neuromagnetic recordings were used to check if electrical stimulation of the auditory nerve in cochlear-implant patients activates cortical auditory areas in a similar manner as acoustic stimulation in normal hearing subjects or whether the excitation processes are different. The waveform of the evoked magnetic field complex elicited by electrical stimulation of the auditory nerve as well as the distribution of the magnetic responses over the head resemble that of auditory-evoked fields elicited by acoustic stimulation of normal subjects. The location and direction of the equivalent dipole are consistent with activation of the auditory cortex. Neuromagnetic recordings could be used in the future (i) to check activation of the auditory cortices in cochlear-implant patients; (ii) to gather information about the functional properties of this activity, and (iii) to study the integrity of central auditory pathways in totally deaf patients.

Auditory Cortex↗
PubMed1986

Follow-up of auditory-evoked potentials and temporary threshold shift in subjects developing noise-induced permanent hearing loss.

The effects of auditory fatigue, using a temporary threshold shift (TTS) paradigm, on cochlear microphonics (CM) and on auditory brainstem-evoked potentials (ABEP), were studied in normal-hearing subjects during the development of permanent threshold shift (PTS). Behavioral threshold shifts were accompanied by different effects on CM and on ABEP, as PTS was gradually induced by occupational noise. Measures of the effect of increasing stimulus rate (ISR) on ABEP revealed decreased latency shifts during auditory fatigue. ABEP proved useful in early detection of changes in the auditory system, resulting from exposure to noise. In addition, this study further supports the suggestion that TTS acts as a peripheral attenuator of the effect of ISR on the central auditory pathway.

Adolescent↗
PubMed1989

Transcutaneous nerve stimulation (TNS) in tinnitus.

Low-frequency (2 Hz) TNS applied distally to peripheral nerves of the upper extremity is known to induce a wide-spread, non-segmental and prolonged relief of pain and an increased microcirculation due to sympatho-inhibition in a number of vascular beds. Such stimulation was administered in 29 tinnitus patients of various etiology. Reduction of tinnitus was encountered in 9 subjects in response to a 45-min TNS-session. The improvement was mainly seen in tinnitus characterized by lower frequencies (125-500 Hz). In 7 of the 9 patients, the tinnitus reduction was associated with improvement of hearing, predominantly in the low-frequency band. The effects were still present after one week following daily stimulation at home. On continued treatment, the effects were found to be transitory in 4 of the patients, whereas the remaining 5 patients are still using the stimulator after 2 to 5 years. It is suggested that the mechanism behind the beneficial effects is increased microcirculation in part of the auditory pathways.

Auditory Threshold↗
PubMed1995

Acoustic features and acoustic changes are represented by different central pathways.

The central processing of acoustic stimulus changes can be observed neurophysiologically in the mismatch negativity auditory evoked potential (MMN). Stimuli differing in interaural phase were used to investigate the contributions of the primary and non-primary auditory pathways to the encoding of binaural stimuli and to investigate passively elicited measures of binaural processing in experimental animals. In guinea pigs, the MMN was obtained in response to 1000 Hz tones embedded in white noise (S:N = 2 dB). Using a modified oddball paradigm (that is, two stimuli presented in a series, each with a different probability of occurrence), stimuli were presented binaurally with both the tone and noise in-phase to the two ears (S0N0) as the standard stimulus ans the tone 180 degrees out-of-phase (S(PI)N0) as the deviant stimulus. The MMN, by definition, should occur only in response to a change, or 'mismatch,' between the standard and deviant stimuli. The response to the deviant stimulus in the oddball paradigm was compared to the response to the same stimulus when presented in a series alone. The responses to S0N0 and S(PI)N0 collected in a series alone, termed the intrinsic responses, were also compared. Responses were recorded from two surface epidural electrodes - one at the posterior midline and one over the left temporal lobe. AEPs from these locations have been shown to reflect the activity of primary and non-primary thalamo-cortical pathways respectively. A significant MMN was observed at the midline electrode, but no MMN was observed over the temporal lobe.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1990

Effects of passive tactile and auditory stimuli on left visual neglect.

Patients with left-sided visual neglect fail to copy the left part of drawings or the drawings on the left side of a sheet of paper. Our aim was to study the variations in copying drawings induced by passive stimulation in patients with left-sided visual neglect. No stimulation at all, tactile unilateral and bilateral, binaural auditory verbal, and nonverbal stimuli were randomly applied to 14 patients with right-hemisphere strokes. Only nonverbal stimuli decreased the neglect. As nonverbal stimuli mainly activate the right hemisphere, the decrease in neglect suggests right-hemispheric hypoactivity at rest in these patients. The absence of modification of neglect during verbal stimulation suggests a bilateral hemispheric activation and the persistence of interhemispheric imbalance. Our results showed that auditory pathways take part in the network involved with neglect. Passive nonverbal auditory stimuli may be of interest in the rehabilitation of patients with left visual neglect.

Acoustic Stimulation↗
PubMed2004

Learning impaired children exhibit timing deficits and training-related improvements in auditory cortical responses to speech in noise.

The physiological mechanisms that contribute to abnormal encoding of speech in children with learning problems are yet to be well understood. Furthermore, speech perception problems appear to be particularly exacerbated by background noise in this population. This study compared speech-evoked cortical responses recorded in a noisy background to those recorded in quiet in normal children (NL) and children with learning problems (LP). Timing differences between responses recorded in quiet and in background noise were assessed by cross-correlating the responses with each other. Overall response magnitude was measured with root-mean-square (RMS) amplitude. Cross-correlation scores indicated that 23% of LP children exhibited cortical neural timing abnormalities such that their neurophysiological representation of speech sounds became distorted in the presence of background noise. The latency of the N2 response in noise was isolated as being the root of this distortion. RMS amplitudes in these children did not differ from NL children, indicating that this result was not due to a difference in response magnitude. LP children who participated in a commercial auditory training program and exhibited improved cortical timing also showed improvements in phonological perception. Consequently, auditory pathway timing deficits can be objectively observed in LP children, and auditory training can diminish these deficits.

Acoustic Stimulation↗
PubMed2005

Development of the human fetal cochlear nerve: a morphometric study.

Ontogenesis of the human peripheral auditory pathway is relatively less explored. While the distal part of the auditory perception apparatus (i.e. the cochlea) received attention, studies on the neural element carrying information to the brainstem (i.e. the cochlear nerve) are scarce. In the present study, axonal differentiation, maturation and myelination of the distal end of the human cochlear nerve (CN) were assessed using light and electron microscopy. Seven human fetuses of 12, 15, 18, 20, 22, 28 and 38 weeks' gestation (WG) were analyzed. Light microscopy revealed nerve fascicles as early as 12 WG, initially arranged loosely but later compacted by 18 WG. Myelinated fibers were clearly detected at 28 WG. Ultrastructurally, at 12 WG developing Schwann cells were present between the thin unmyelinated axons. At 15 WG, the fascicular arrangement was distinct with blood vessels in the perineurium. The maximum number of axons was found at 20 WG, which subsequently reduced to reach the adult level at 22 WG. The myelinated axons in the CN were first observed on the left side at 20 WG, following which the number and proportion of myelinated axons increased until term, incorporating both small and large axons. The right CN lagged behind in maturation. Axon size also increased with age. Thus, the maturation of the human CN commences during the mid-gestation period and produces exuberant axons that are eventually pruned at a time when axons start to myelinate. During this developmental period the human CN maintains maturational asymmetry, the functional consequences of which remain to be elucidated.

Axons↗
PubMed2003

Subjective tinnitus, temporomandibular joint dysfunction, and serotonin modulation of neural plasticity: causal or casual triad?

Tinnitus and temporomandibular joint dysfunction (TMJD) are among the most common complaints encountered by physicians. Though the relationship between tinnitus and TMJD has attracted great interest during the past several years, theories attempting to explain this association are still few and inconsistent. Conceivably, TMJD could irritate auricolo-temporal nerve (ATN), triggering a somatosensory pathway-induced disinhibition of dorsal cochlear nucleus (DCN) activity in the auditory pathway. In genetically-predisposed TMJD patients, signals from cronically stimulated DCNs activating specific cortical neuronal networks, could yield plastic neural changes resulting in tinnitus. Based on current evidence of serotoninergic modulation of neural activity and plasticity in sensory pathways, reduced serotoninergic tone could promote plastic changes underlying tinnitus through diminished filtering of incoming signals. Therefore, the early establishment of specific treatments aimed at improving TMJD and/or boosting serotoninergic activity may be required to prevent the creation of 'tinnitus memory circuits'.

Acoustic Stimulation↗
PubMed2002

Central auditory processing disorders.

Central auditory processing is essential for the perception of speech, environmental sounds and music, and may be deranged in two ways. Lesions of the ascending auditory pathway or cortex can produce deficits. Abnormal activity of the central auditory system is becoming increasingly recognized in disorders such as tinnitus. Recent work has investigated sound processing by the unconscious brain; such investigations may provide a 'window' into residual brain function and prognosis.

Agnosia↗
PubMed1999

Automated auditory brainstem response in neonatal hearing screening.

Severe congenital hearing impairment is an important handicap affecting 0.1% of apparently healthy liveborn infants and 1-2% of graduates of neonatal intensive care units. The prognosis for intellectual, emotional, language and speech development in the hearing-impaired child is improved when the diagnosis is made early and intervention is begun before the age of 6 mo. Universal screening is preferable, since about 50% of infants with hearing loss are not discovered if neonatal hearing screening is restricted to high-risk groups. The automated auditory brainstem response (AABR) screener is a dedicated hearing screening device which provides information not only about the outer/middle ear and cochlea but also about the auditory pathway up to the brainstem. AABR has an agreement with conventional auditory brainstem response up to 98%. It uses a 35 dB near hearing level click. No operator interpretation is needed and it can be used on the ward and during oxygen therapy without disturbance from ambient noise. Reported referral rates in a hospital-based screening programme at the time of discharge vary, with an average of 4%. AABR has also been used in a home-based setting, with the same results. The time necessary for screening varies with the setting, but ranges from 4 to 15 min. Initial costs range from $15 to $25 per test, which is similar to neonatal screening for metabolic diseases. In addition to individual healthcare savings, early diagnosis may lead to savings on costs of intensive speech-language intervention and educational facilities.

Audiometry↗
PubMed1961

Auditory information from subcortical electrical stimulation in cats.

Animals trained to respond to sound stimuli were found to perform the learned response when they were electrically stimulated through electrodes chronically implanted in subcortical structures of the auditory pathway. Other animals trained to respond to electrical stimulation of subcortical auditory structures showed differential transfer effects depending on the positions of the stimulating electrodes.

Acoustic Stimulation↗