The fast detection of novel or deviant stimuli is a striking property of the auditory processing which reflects basic organizational principles of the auditory system and at the same time is of high practical significance. In human electrophysiology, deviance detection has been related to the occurrence of the mismatch negativity (MMN) — a component of the event-related potential (ERP) evoked 100 to 250 ms after the occurrence of a rare irregular sound. Recently, it has been shown in animal studies that a considerable portion of neurons in the auditory pathway exhibits a property called stimulus-specific adaptation enabling them to encode inter-sound relationships and to discharge at higher rates to rare changes in the acoustic stimulation. These neural responses have been linked to the deviant-evoked potential measured at the human scalp, but such responses occur at lower levels anatomically (e.g. the primary auditory cortex as well as the inferior colliculi) and are elicited earlier (20–30 ms after sound onset) in comparison to MMN. Further, they are not considerable enough in size to be interpreted as a direct neural correlate of the MMN. We review here a series of recent findings that provides a first step toward filling this gap between animal and human recordings by showing that comparably early modulations due to a sound’s deviancy can be observed in humans, particularly in the middle-latency portion of the ERP within the first 50 ms after sound onset. The existence of those early indices of deviance detection preceding the well-studied MMN component strongly supports the idea that the encoding of regularities and the detection of violations is a basic principle of human auditory processing acting on multiple levels. This sustains the notion of a hierarchically organized novelty and deviance detection system in the human auditory system.
The auditory system faithfully represents sufficient details from sound sources such that downstream cognitive processes are capable of acting upon this information effectively even in the face of signal uncertainty, degradation or interference. This robust sound source representation leads to an invariance in perception vital for animals to interact effectively with their environment. Due to unique nonlinearities in the cochlea, sound representations early in the auditory system exhibit a large amount of variability as a function of stimulus intensity. In other words, changes in stimulus intensity, such as for sound sources at differing distances, create a unique challenge for the auditory system to encode sounds invariantly across the intensity dimension. This challenge and some strategies available to sensory systems to eliminate intensity as an encoding variable are discussed, with a special emphasis upon sound encoding.
Occurrence of Mismatch Negativity in Response to Changes in the Durations of Short Pauses during Continuous Auditory Stimulation
Mismatch negativity (MMN) is a component of the auditory evoked potential which is widely used in research and clinical investigations. Using the oddball paradigm, which is traditional for obtaining mismatch negativity, we recorded MMN-like responses to changes in the duration of short pauses (standard = 25 msec; deviant = 50 msec, and vice versa) on the background of continuous auditory stimulation (1 kHz, 50 dB). Our data show that this is the first description of this type of mismatch negativity. We compared the responses to changes in pause duration with “classical” mismatch negativity to changes in pause duration with the same time characteristics. Mismatch negativity to changes in pauses was characterized by longer latent periods and the absence of significant inversion in the mastoid leads. Both responses, to changes in sound duration and to changes in pause duration, demonstrated shorter latent periods when the deviants were longer than the standards as compared with the reverse situation. This new type of mismatch negativity may widen the potential for its use in clinical and research studies.
Perceptual and neurophysiological enhancements in linguistic processing in musicians suggest that domain specific experience may enhance neural resources recruited for language specific behaviors. In everyday situations, listeners are faced with extracting speech signals in degraded listening conditions. Here, we examine whether musical training provides resilience to the degradative effects of reverberation on subcortical representations of pitch and formant-related harmonic information of speech. Brainstem frequency-following responses (FFRs) were recorded from musicians and non-musician controls in response to the vowel /i/ in four different levels of reverberation and analyzed based on their spectro-temporal composition. For both groups, reverberation had little effect on the neural encoding of pitch but significantly degraded neural encoding of formant-related harmonics (i.e., vowel quality) suggesting a differential impact on the source-filter components of speech. However, in quiet and across nearly all reverberation conditions, musicians showed more robust responses than non-musicians. Neurophysiologic results were confirmed behaviorally by comparing brainstem spectral magnitudes with perceptual measures of fundamental (F0) and first formant (F1) frequency difference limens (DLs). For both types of discrimination, musicians obtained DLs which were 2-4 times better than non-musicians. Results suggest that musicians’ enhanced neural encoding of acoustic features, an experience-dependent effect, is more resistant to reverberation degradation which may explain their enhanced perceptual ability on behaviorally relevant speech and/or music tasks in adverse listening conditions.
from Brain Research
Normal hearing listeners exploit the formant transition (FT) detection to identify place of articulation for stop consonants. Neuro-imaging studies revealed that short FT induced less cortical activation than long FT. To determine the ability of hearing impaired listeners to distinguish short and long formant transitions (FT) from vowels of the same duration, 84 mild to severe hearing impaired listeners and 5 normal hearing listeners were asked to detect 10 synthesized stimuli with long (200 ms) or short (40 ms) FT among 30 stimuli of the same duration without FT. Hearing impaired listeners were tested with and without hearing aids. The effect of the difficulty of the task (short/long FT) was analysed as a function of the hearing loss with and without hearing aids. Normal hearing listeners were able to detect every FT (short and long). For hearing impaired listeners, the detection of long FT was better than that of short ones irrespective of their degree of hearing loss. The use of hearing aids improved detection of both kinds of FT; however, the detection of long FT remained much better than the detection of the short ones. The length of FT modified the ability of hearing impaired patients to detect FT. Short FT had access to less cortical processing than long FT and cochlea damages enhanced this specific deficit in short FT brain processing. These findings help to understand the limit of deafness rehabilitation in the time domain and should be taken into account in future devices development.
Pre-attentive registration of aberrations in predictable sound patterns is attributed to the temporal cortex. However, electrophysiology suggests that frontal areas become more important when deviance complexity increases. To play an instrument in an ensemble, professional musicians have to rely on the ability to detect even slight deviances from expected musical patterns and therefore have highly trained aural skills. Here, we aimed to identify the neural correlates of experience-driven plasticity related to the processing of complex sound features. We used functional magnetic resonance imaging in combination with an event-related oddball paradigm and compared brain activity in professional musicians and non-musicians during pre-attentive processing of melodic contour variations. The melodic pattern consisted of a sequence of five tones each lasting 50 ms interrupted by silent interstimulus intervals of 50 ms. Compared to non-musicians, the professional musicians showed enhanced activity in the left middle and superior temporal gyri, the left inferior frontal gyrus and in the right ventromedial prefrontal cortex in response to pattern deviation. This differential brain activity pattern was correlated with behaviorally tested musical aptitude. Our results thus support an experience-related role of the left temporal cortex in fast melodic contour processing and suggest involvement of the prefrontal cortex. Hum Brain Mapp, 2009. © 2009 Wiley-Liss, Inc.
from Human Brain Mapping
Independent component analysis for robust assessment of auditory system maturation in children with cochlear implants
The long latency auditory evoked potential (LLAEP) has been used for tracking changes in latency and morphology of the P1 peak in order to evaluate the maturation of the auditory system in children with cochlear implants (CIs). Cochlear implants can induce an artefact in the recordings when sounds are presented, which makes the analysis of LLAEPs much harder. Independent component analysis (ICA) has been used to remove this artefact. In this paper we apply a procedure based on ICA to reduce the CI artefact, to detect the LLAEPs and to use the changes in the spatial projections of their independent components (ICs) for a robust evaluation of the maturation of the auditory system in children with CIs. Copyright © 2009 John Wiley & Sons, Ltd.
Cross-Modal Interactions of Auditory and Somatic Inputs in the Brainstem and Midbrain and Their Imbalance in Tinnitus and Deafness
Purpose: This review outlines the anatomical and functional bases of somatosensory influences on auditory processing in the normal brainstem and midbrain. It then explores how interactions between the auditory and somatosensory system are modified through deafness, and their impact on tinnitus is discussed.
Method: Literature review, tract tracing, immunohistochemistry, and in vivo electrophysiological recordings were used.
Results: Somatosensory input originates in the dorsal root ganglia and trigeminal ganglia, and is transmitted directly and indirectly through 2nd-order nuclei to the ventral cochlear nucleus, dorsal cochlear nucleus (DCN), and inferior colliculus. The glutamatergic somatosensory afferents can be segregated from auditory nerve inputs by the type of vesicular glutamate transporters present in their terminals. Electrical stimulation of the somatosensory input results in a complex combination of excitation and inhibition, and alters the rate and timing of responses to acoustic stimulation. Deafness increases the spontaneous rates of those neurons that receive excitatory somatosensory input and results in a greater sensitivity of DCN neurons to trigeminal stimulation.
Conclusions: Auditory-somatosensory bimodal integration is already present in 1st-order auditory nuclei. The balance of excitation and inhibition elicited by somatosensory input is altered following deafness. The increase in somatosensory influence on auditory neurons when their auditory input is diminished could be due to cross-modal reinnervation or increased synaptic strength, and may contribute to mechanisms underlying somatic tinnitus.
from the American Journal of Audiology
Objectives: To establish the efficacy of low-level laser therapy for tinnitus. Methods: We performed a prospective, randomized double-blind study on 60 outpatients with tinnitus presenting sensorineural hearing loss in the affected ear. They were randomly divided into two groups, the first performing active laser therapy 20 min a day for 3 months with a 650-nm, 5-mW soft laser (group L), the second using a dummy device which duplicated all aspects of active laser therapy except for the activation of the laser beam (group C). One subject in both groups dropped out due to an increase in tinnitus loudness. Two more patients in each group ceased to comply with the protocol due to familiar problems. Results: The Tinnitus Handicap Inventory (THI) was considered the main outcome measure; no statistical difference was detected between the 2 groups in the THI total score (p = 0.97), and its functional (p = 0.89), emotional (p = 0.89) and catastrophic (p = 0.89) subscales. Moreover, a visual analog scale for self-perceived loudness of the tinnitus showed no difference between the groups (p = 0.69). Regarding psychoacoustic parameters, the minimum masking level showed no difference (p = 0.42), while loudness expressed in sensation level exhibited lower values in group L (p = 0.0127). Group L subjects also presented a decreased rate of hyperacusis (p = 0.02). No changes were detected in the audiometric threshold in both groups. Conclusions: Soft laser therapy demonstrated no efficacy as a therapeutic measure for tinnitus.