Neuro-rehabilitation Approach for Sudden Sensorineural Hearing Loss.

Sudden sensorineural hearing loss (SSHL) is characterized by acute, idiopathic hearing loss. The estimated incidence rate is 5-30 cases per 100,000 people per year. The causes of SSHL and the mechanisms underlying SSHL currently remain unknown. Based on several hypotheses such as a circulatory disturbance to the cochlea, viral infection, and autoimmune disease, pharmaco-therapeutic approaches have been applied to treat SSHL patients; however, the efficacy of the standard treatment, corticosteroid therapy, is still under debate. Exposure to intense sounds has been shown to cause permanent damage to the auditory system; however, exposure to a moderate level enriched acoustic environment after noise trauma may reduce hearing impairments. Several neuroimaging studies recently suggested that the onset of SSHL induced maladaptive cortical reorganization in the human auditory cortex, and that the degree of cortical reorganization in the acute SSHL phase negatively correlated with the recovery rate from hearing loss. This article reports the development of a novel neuro-rehabilitation approach for SSHL, "constraint-induced sound therapy (CIST)". The aim of the CIST protocol is to prevent or reduce maladaptive cortical reorganization by using an enriched acoustic environment. The canal of the intact ear of SSHL patients is plugged in order to motivate them to actively use the affected ear and thereby prevent progress of maladaptive cortical reorganization. The affected ear is also exposed to music via a headphone for 6 hr per day during hospitalization. The CIST protocol appears to be a safe, easy, inexpensive, and effective treatment for SSHL.

Constraint-induced sound therapy for sudden sensorineural hearing loss--behavioral and neurophysiological outcomes.

Sudden sensorineural hearing loss is characterized by acute, idiopathic hearing deterioration. We report here the development and evaluation of "constraint-induced sound therapy", which is based on a well-established neuro-rehabilitation approach, and which is characterized by the plugging of the intact ear ("constraint") and the simultaneous, extensive stimulation of the affected ear with music. The sudden sensorineural hearing loss patients who received the constraint-induced sound therapy in addition to the standard corticosteroid therapy showed significantly better recovery of hearing function compared to those who had only received corticosteroid treatments. Additionally, the brain activity obtained in a subgroup of patients suggested that the constraint-induced sound therapy could have prevented maladaptive auditory cortex reorganization. Constraint-induced sound therapy thus appears to be an effective, practical, and safe treatment option for sudden sensorineural hearing loss.

Involuntary monitoring of sound signals in noise is reflected in the human auditory evoked N1m response.

Constant sound sequencing as operationalized by repeated stimulation with tones of the same frequency has multiple effects. On the one hand, it activates mechanisms of habituation and refractoriness, which are reflected in the decrease of response amplitude of evoked responses. On the other hand, the constant sequencing acts as spectral cueing, resulting in tones being detected faster and more accurately. With the present study, by means of magnetoencephalography, we investigated the impact of repeated tone stimulation on the N1m auditory evoked fields, while listeners were distracted from the test sounds. We stimulated subjects with trains of either four tones of the same frequency, or with trains of randomly assigned frequencies. The trains were presented either in a silent or in a noisy background. In silence, the patterns of source strength decline originating from repeated stimulation suggested both, refractoriness as well as habituation as underlying mechanisms. In noise, in contrast, there was no indication of source strength decline. Furthermore, we found facilitating effects of constant sequencing regarding the detection of the single tones as indexed by a shortening of N1m latency. We interpret our findings as a correlate of a bottom-up mechanism that is constantly monitoring the incoming auditory information, even when voluntary attention is directed to a different modality.

Bottom-up driven involuntary auditory evoked field change: constant sound sequencing amplifies but does not sharpen neural activity.

The capability of involuntarily tracking certain sound signals during the simultaneous presence of noise is essential in human daily life. Previous studies have demonstrated that top-down auditory focused attention can enhance excitatory and inhibitory neural activity, resulting in sharpening of frequency tuning of auditory neurons. In the present study, we investigated bottom-up driven involuntary neural processing of sound signals in noisy environments by means of magnetoencephalography. We contrasted two sound signal sequencing conditions: "constant sequencing" versus "random sequencing." Based on a pool of 16 different frequencies, either identical (constant sequencing) or pseudorandomly chosen (random sequencing) test frequencies were presented blockwise together with band-eliminated noises to nonattending subjects. The results demonstrated that the auditory evoked fields elicited in the constant sequencing condition were significantly enhanced compared with the random sequencing condition. However, the enhancement was not significantly different between different band-eliminated noise conditions. Thus the present study confirms that by constant sound signal sequencing under nonattentive listening the neural activity in human auditory cortex can be enhanced, but not sharpened. Our results indicate that bottom-up driven involuntary neural processing may mainly amplify excitatory neural networks, but may not effectively enhance inhibitory neural circuits.

Bottom-up driven involuntary attention modulates auditory signal in noise processing.

BACKGROUND: Auditory evoked responses can be modulated by both the sequencing and the signal-to-noise ratio of auditory stimuli. Constant sequencing as well as intense masking sounds basically lead to N1m response amplitude reduction. However, the interaction between these two factors has not been investigated so far. Here, we presented subjects tone stimuli of different frequencies, which were either concatenated in blocks of constant frequency or in blocks of randomly changing frequencies. The tones were presented either in silence or together with broad-band noises of varying levels. RESULTS: In silence, tones presented with random sequencing elicited a larger N1m response than tones presented with constant sequencing. With increasing noise level, this difference decreased and even vanished in the condition where noise intensity exceeded the tone intensity by 10 dB. Furthermore, under noisy conditions, the N1m latency was shorter in the constant sequencing condition compared to the random sequencing condition. CONCLUSIONS: Besides the well-known neural habituation mechanisms, bottom-up driven attention plays an important role during auditory processing in noisy environments. This bottom-up driven attention would allow us to track a certain auditory signal in noisy situations without voluntarily paying attention to the auditory modality.

Non-native phonemes in adult word learning: evidence from the N400m.

Newborns are equipped with a large phonemic inventory that becomes tuned to one's native language early in life. We review and add new data about how learning of a non-native phoneme can be accomplished in adults and how the efficiency of word learning can be assessed by neurophysiological measures. For this purpose, we studied the acquisition of the voiceless, bilabial fricative /Phi/ via a statistical-learning paradigm. Phonemes were embedded in minimal pairs of pseudowords, differing only with respect to the fricative (/aPhio/ versus /afo/). During learning, pseudowords were combined with pictures of objects with some combinations of pseudowords and pictures occurring more frequently than others. Behavioural data and the N400m component, as an index of lexical activation/semantic access, showed that participants had learned to associate the pseudowords with the pictures. However, they could not discriminate within the minimal pairs. Importantly, before learning, the novel words with the sound /Phi/ showed smaller N400 amplitudes than those with native phonemes, evidencing their non-word status. Learning abolished this difference indicating that /Phi/ had become integrated into the native category /f/, instead of establishing a novel category. Our data and review demonstrate that native phonemic categories are powerful attractors hampering the mastery of non-native contrasts.

Implicit and explicit categorization of speech sounds--dissociating behavioural and neurophysiological data.

During speech perception, sound is mapped onto abstract phonological categories. Assimilation of place or manner of articulation in connected speech challenges this categorization. Does assimilation result in categorizations that need to be corrected later on, or does the system get it right immediately? Participants were presented with isolated nasals (/m/ labial, /n/ alveolar, and /n'/ assimilated towards labial place of articulation), extracted from naturally produced German utterances. Behavioural two-alternative forced-choice tasks showed that participants could correctly categorize the /n/s and /m/s. The assimilated nasals were predominantly categorized as /m/, indicative of a perceived change in place. A pitch variation additively influenced the categorizations. Using magnetoencephalography (MEG), we analysed the N100m elicited by the same stimuli without a categorization task. In sharp contrast to the behavioural data, this early, automatic brain response ignored the assimilation in the surface form and reflected the underlying category. As shown by distributed source modelling, phonemic differences were processed exclusively left-laterally (temporally and parietally), whereas the pitch variation was processed in temporal regions bilaterally. In conclusion, explicit categorization draws attention to the surface form - to the changed place and acoustic information. The N100m reflects automatic categorization, which exploits any hint of an underlying feature.