Auditory N1m Research Articles

Localising the auditory N1m with event-related beamformers: localisation accuracy following bilateral and unilateral stimulation.

The auditory evoked N1m-P2m response complex presents a challenging case for MEG source-modelling, because symmetrical, phase-locked activity occurs in the hemispheres both contralateral and ipsilateral to stimulation. Beamformer methods, in particular, can be susceptible to localisation bias and spurious sources under these conditions. This study explored the accuracy and efficiency of event-related beamformer source models for auditory MEG data under typical experimental conditions: monaural and diotic stimulation; and whole-head beamformer analysis compared to a half-head analysis using only sensors from the hemisphere contralateral to stimulation. Event-related beamformer localisations were also compared with more traditional single-dipole models. At the group level, the event-related beamformer performed equally well as the single-dipole models in terms of accuracy for both the N1m and the P2m, and in terms of efficiency (number of successful source models) for the N1m. The results yielded by the half-head analysis did not differ significantly from those produced by the traditional whole-head analysis. Any localisation bias caused by the presence of correlated sources is minimal in the context of the inter-individual variability in source localisations. In conclusion, event-related beamformers provide a useful alternative to equivalent-current dipole models in localisation of auditory evoked responses.

Habituation deficit of auditory N100m in patients with fibromyalgia.

Habituation refers to the brain's inhibitory mechanism against sensory overload and its brain correlate has been investigated in the form of a well-defined event-related potential, N100 (N1). Fibromyalgia is an extensively described chronic pain syndrome with concurrent manifestations of reduced tolerance and enhanced sensation of painful and non-painful stimulation, suggesting an association with central amplification of all sensory domains. Among diverse sensory modalities, we utilized repetitive auditory stimulation to explore the anomalous sensory information processing in fibromyalgia as evidenced by N1 habituation. Auditory N1 was assessed in 19 fibromyalgia patients and age-, education- and gender-matched 21 healthy control subjects under the duration-deviant passive oddball paradigm and magnetoencephalography recording. The brain signal of the first standard stimulus (following each deviant) and last standard stimulus (preceding each deviant) were analysed to identify N1 responses. N1 amplitude difference and adjusted amplitude ratio were computed as habituation indices. Fibromyalgia patients showed lower N1 amplitude difference (left hemisphere: p=0.004; right hemisphere: p=0.034) and adjusted N1 amplitude ratio (left hemisphere: p=0.001; right hemisphere: p=0.052) than healthy control subjects, indicating deficient auditory habituation. Further, augmented N1 amplitude pattern (p=0.029) during the stimulus repetition was observed in fibromyalgia patients. Fibromyalgia patients failed to demonstrate auditory N1 habituation to repetitively presenting stimuli, which indicates their compromised early auditory information processing. Our findings provide neurophysiological evidence of inhibitory failure and cortical augmentation in fibromyalgia. WHAT'S ALREADY KNOWN ABOUT THIS TOPIC?: Fibromyalgia has been associated with altered filtering of irrelevant somatosensory input. However, whether this abnormality can extend to the auditory sensory system remains controversial. N!00, an event-related potential, has been widely utilized to assess the brain's habituation capacity against sensory overload. WHAT DOES THIS STUDY ADD?: Fibromyalgia patients showed defect in N100 habituation to repetitive auditory stimuli, indicating compromised early auditory functioning. This study identified deficient inhibitory control over irrelevant auditory stimuli in fibromyalgia.

Computational modelling suggests that temporal integration results from synaptic adaptation in auditory cortex.

Incoming sounds are represented in the context of preceding events, and this requires a memory mechanism that integrates information over time. Here, it was demonstrated that response adaptation, the suppression of neural responses due to stimulus repetition, might reflect a computational solution that auditory cortex uses for temporal integration. Adaptation is observed in single-unit measurements as two-tone forward masking effects and as stimulus-specific adaptation (SSA). In non-invasive observations, the amplitude of the auditory N1m response adapts strongly with stimulus repetition, and it is followed by response recovery (the so-called mismatch response) to rare deviant events. The current computational simulations described the serial core-belt-parabelt structure of auditory cortex, and included synaptic adaptation, the short-term, activity-dependent depression of excitatory corticocortical connections. It was found that synaptic adaptation is sufficient for columns to respond selectively to tone pairs and complex tone sequences. These responses were defined as combination sensitive, thus reflecting temporal integration, when a strong response to a stimulus sequence was coupled with weaker responses both to the time-reversed sequence and to the isolated sequence elements. The temporal complexity of the stimulus seemed to be reflected in the proportion of combination-sensitive columns across the different regions of the model. Our results suggest that while synaptic adaptation produces facilitation and suppression effects, including SSA and the modulation of the N1m response, its functional significance may actually be in its contribution to temporal integration. This integration seems to benefit from the serial structure of auditory cortex.

Human cortical sensitivity to interaural time difference in high-frequency sounds

Human sound source localization relies on various acoustical cues one of the most important being the interaural time difference (ITD). ITD is best detected in the fine structure of low-frequency sounds but it may also contribute to spatial hearing at higher frequencies if extracted from the sound envelope. The human brain mechanisms related to this envelope ITD cue remain unexplored. Here, we tested the sensitivity of the human auditory cortex to envelope ITD in magnetoencephalography (MEG) recordings. We found two types of sensitivity to envelope ITD. First, the amplitude of the auditory cortical N1m response was smaller for zero envelope ITD than for long envelope ITDs corresponding to the sound being in opposite phase in the two ears. Second, the N1m response amplitude showed ITD-specific adaptation for both fine-structure and for envelope ITD. The auditory cortical sensitivity was weaker for envelope ITD in high-frequency sounds than for fine-structure ITD in low-frequency sounds but occurred within a range of ITDs that are encountered in natural conditions. Finally, the participants were briefly tested for their behavioral ability to detect envelope ITD. Interestingly, we found a correlation between the behavioral performance and the neural sensitivity to envelope ITD. In conclusion, our findings show that the human auditory cortex is sensitive to ITD in the envelope of high-frequency sounds and this sensitivity may have behavioral relevance.

Abnormal, affect-specific modulatory effects on early auditory processing in schizophrenia: Magnetoencephalographic evidence

Abnormalities in the perception and identification of emotions have frequently been reported in schizophrenia. Hemodynamic neuroimaging studies found functional abnormalities in cortical and subcortical brain circuits that are involved in normal affective processing, but the temporal dynamics of abnormal emotion processing in schizophrenia remain largely elusive. To investigate this issue, we recorded early auditory evoked field components by means of whole-head magnetoencephalography that were in response to emotion-associated tones in seventeen patients with schizophrenia and in seventeen healthy, matched controls. Forty-two click-like tones (conditioned stimuli; CS) acquired differential emotional meaning through an affective associative learning procedure by pairing each CS three times with either pleasant, unpleasant or neutral auditory scenes. As expected, differential affect-specific modulation in patients vs. controls was evident, starting at the auditory N1m onset latency of approximately 70ms, extending to 230ms. While controls showed the expected enhanced processing of emotion associated CS, patients revealed an inverted pattern with reduced processing of arousal, when compared to neutral stimuli, in the right prefrontal cortex. The present finding suggests impairments in the prioritization of emotionally salient vs. non-salient stimuli in patients with schizophrenia. Dysfunction in higher cognitive processes and behavior in schizophrenia may therefore reflect dysfunction in fundamental, early emotion processing stages.

Transcutaneous vagus nerve stimulation in tinnitus: a pilot study

Conclusions: This pilot study shows that transcutaneous vagus nerve stimulation (tVNS), if combined with sound therapy (ST), reduces the severity of tinnitus and tinnitus-associated distress. Our magnetoencephalography (MEG) results show that auditory cortical activation can be modulated by the application of tVNS. Thus, tVNS might offer a new avenue to treat tinnitus and tinnitus-associated distress. Objectives: Recent studies suggest that tinnitus can be improved by tailored ST or by VNS plus ST. Our aims were to study whether tVNS has therapeutic effects on patients with tinnitus and, additionally, if tVNS has effects on acoustically evoked neuronal activity of the auditory cortex. Methods: The clinical efficacy was studied by a short-term tVNS plus ST trial in 10 patients with tinnitus using disease-specific and general well-being questionnaires. tVNS was delivered to the left tragus. The acute effects of tVNS were evaluated in eight patients in the MEG study in which the N1m response was analyzed in terms of source level amplitude and latency in the presence or absence of tVNS. Results: The treatment with tVNS plus ST produced improved mood and decreased tinnitus handicap scores, indicating reduced tinnitus severity. The application of tVNS decreased the amplitude of auditory N1m responses in both hemispheres.

Transient and sustained cortical activity elicited by connected speech of varying intelligibility

BackgroundThe robustness of speech perception in the face of acoustic variation is founded on the ability of the auditory system to integrate the acoustic features of speech and to segregate them from background noise. This auditory scene analysis process is facilitated by top-down mechanisms, such as recognition memory for speech content. However, the cortical processes underlying these facilitatory mechanisms remain unclear. The present magnetoencephalography (MEG) study examined how the activity of auditory cortical areas is modulated by acoustic degradation and intelligibility of connected speech. The experimental design allowed for the comparison of cortical activity patterns elicited by acoustically identical stimuli which were perceived as either intelligible or unintelligible.ResultsIn the experiment, a set of sentences was presented to the subject in distorted, undistorted, and again in distorted form. The intervening exposure to undistorted versions of sentences rendered the initially unintelligible, distorted sentences intelligible, as evidenced by an increase from 30% to 80% in the proportion of sentences reported as intelligible. These perceptual changes were reflected in the activity of the auditory cortex, with the auditory N1m response (~100 ms) being more prominent for the distorted stimuli than for the intact ones. In the time range of auditory P2m response (>200 ms), auditory cortex as well as regions anterior and posterior to this area generated a stronger response to sentences which were intelligible than unintelligible. During the sustained field (>300 ms), stronger activity was elicited by degraded stimuli in auditory cortex and by intelligible sentences in areas posterior to auditory cortex.ConclusionsThe current findings suggest that the auditory system comprises bottom-up and top-down processes which are reflected in transient and sustained brain activity. It appears that analysis of acoustic features occurs during the first 100 ms, and sensitivity to speech intelligibility emerges in auditory cortex and surrounding areas from 200 ms onwards. The two processes are intertwined, with the activity of auditory cortical areas being modulated by top-down processes related to memory traces of speech and supporting speech intelligibility.

Affect‐specific modulation of the N1m to shock‐conditioned tones: magnetoencephalographic correlates

Despite its fundamental relevance for representing the emotional world surrounding us, human affective neuroscience research has widely neglected the auditory system, at least in comparison to the visual domain. Here, we have investigated the spatiotemporal dynamics of human affective auditory processing using time-sensitive whole-head magnetoencephalography. A novel and highly challenging affective associative learning procedure, 'MultiCS conditioning', involving multiple conditioned stimuli (CS) per affective category, was adopted to test whether previous findings from intramodal conditioning of multiple click-tones with an equal number of auditory emotional scenes (Bröckelmann et al., 2011 J. Neurosci., 31, 7801) would generalise to crossmodal conditioning of multiple click-tones with an electric shock as single aversive somatosensory unconditioned stimulus (UCS). Event-related magnetic fields were recorded in response to 40 click-tones before and after four contingent pairings of 20 CS with a shock and the other half remaining unpaired. In line with previous findings from intramodal MultiCS conditioning we found an affect-specific modulation of the auditory N1m component 100-150 ms post-stimulus within a distributed frontal-temporal-parietal neural network. Increased activation for shock-associated tones was lateralised to right-hemispheric regions, whereas unpaired safety-signalling tones were preferentially processed in the left hemisphere. Participants did not show explicit awareness of the contingent CS-UCS relationship, yet behavioural conditioning effects were indicated on an indirect measure of stimulus valence. Our findings imply converging evidence for a rapid and highly differentiating affect-specific modulation of the auditory N1m after intramodal as well crossmodal MultiCS conditioning and a correspondence of the modulating impact of emotional attention on early affective processing in vision and audition.

Cortical processing of degraded speech sounds: Effects of distortion type and continuity

Human speech perception is highly resilient to acoustic distortions. In addition to distortions from external sound sources, degradation of the acoustic structure of the sound itself can substantially reduce the intelligibility of speech. The degradation of the internal structure of speech happens, for example, when the digital representation of the signal is impoverished by reducing its amplitude resolution. Further, the perception of speech is also influenced by whether the distortion is transient, coinciding with speech, or is heard continuously in the background. However, the complex effects of the acoustic structure and continuity of the distortion on the cortical processing of degraded speech are unclear. In the present magnetoencephalography study, we investigated how the cortical processing of degraded speech sounds as measured through the auditory N1m response is affected by variation of both the distortion type (internal, external) and the continuity of distortion (transient, continuous). We found that when the distortion was continuous, the N1m was significantly delayed, regardless of the type of distortion. The N1m amplitude, in turn, was affected only when speech sounds were degraded with transient internal distortion, which resulted in larger response amplitudes. The results suggest that external and internal distortions of speech result in divergent patterns of activity in the auditory cortex, and that the effects are modulated by the temporal continuity of the distortion.

Sound Processing Hierarchy within Human Auditory Cortex

Both attention and masking sounds can alter auditory neural processes and affect auditory signal perception. In the present study, we investigated the complex effects of auditory-focused attention and the signal-to-noise ratio of sound stimuli on three different auditory evoked field components (auditory steady-state response, N1m, and sustained field) by means of magnetoencephalography. The results indicate that the auditory steady-state response originating in primary auditory cortex reflects the signal-to-noise ratio of physical sound inputs (bottom-up process) rather than the listener's attentional state (top-down process), whereas the sustained field, originating in nonprimary auditory cortex, reflects the attentional state rather than the signal-to-noise ratio. The N1m was substantially influenced by both bottom-up and top-down neural processes. The differential sensitivity of the components to bottom-up and top-down neural processes, contingent on their level in the processing pathway, suggests a stream from bottom-up driven sensory neural processing to top-down driven auditory perception within human auditory cortex.

Responsiveness of the human auditory cortex to degraded speech sounds: Reduction of amplitude resolution vs. additive noise

The cortical mechanisms underlying human speech perception in acoustically adverse conditions remain largely unknown. Besides distortions from external sources, degradation of the acoustic structure of the sound itself poses further demands on perceptual mechanisms. We conducted a magnetoencephalography (MEG) study to reveal whether the perceptual differences between these distortions are reflected in cortically generated auditory evoked fields (AEFs). To mimic the degradation of the internal structure of sound and external distortion, we degraded speech sounds by reducing the amplitude resolution of the signal waveform and by using additive noise, respectively. Since both distortion types increase the relative strength of high frequencies in the signal spectrum, we also used versions of the stimuli which were low-pass filtered to match the tilted spectral envelope of the undistorted speech sound. This enabled us to examine whether the changes in the overall spectral shape of the stimuli affect the AEFs. We found that the auditory N1m response was substantially enhanced as the amplitude resolution was reduced. In contrast, the N1m was insensitive to distorted speech with additive noise. Changing the spectral envelope had no effect on the N1m. We propose that the observed amplitude enhancements are due to an increase in noisy spectral harmonics produced by the reduction of the amplitude resolution, which activates the periodicity-sensitive neuronal populations participating in pitch extraction processes. The current findings suggest that the auditory cortex processes speech sounds in a differential manner when the internal structure of sound is degraded compared with the speech distorted by external noise.

P33-14 Effect of temporal prediction on auditory N1m responses

P33-14 Effect of temporal prediction on auditory N1m responses

Effects of DBS on auditory and somatosensory processing in Parkinson's disease

Motor symptoms of Parkinson's disease (PD) can be relieved by deep brain stimulation (DBS). The mechanism of action of DBS is largely unclear. Magnetoencephalography (MEG) studies on DBS patients have been unfeasible because of strong magnetic artifacts. An artifact suppression method known as spatiotemporal signal space separation (tSSS) has mainly overcome these difficulties. We wanted to clarify whether tSSS enables noninvasive measurement of the modulation of cortical activity caused by DBS. We have studied auditory and somatosensory-evoked fields (AEFs and SEFs) of advanced PD patients with bilateral subthalamic nucleus (STN) DBS using MEG. AEFs were elicited by 1-kHz tones and SEFs by electrical pulses to the median nerve with DBS on and off. Data could be successfully acquired and analyzed from 12 out of 16 measured patients. The motor symptoms were significantly relieved by DBS, which clearly enhanced the ipsilateral auditory N100m responses in the right hemisphere. Contralateral N100m responses and somatosensory P60m responses also had a tendency to increase when bilateral DBS was on. MEG with tSSS offers a novel and powerful tool to investigate DBS modulation of the evoked cortical activity in PD with high temporal and spatial resolution. The results suggest that STN-DBS modulates auditory processing in advanced PD. Hum Brain Mapp, 2011. © 2010 Wiley-Liss, Inc.

Sensitivity of the human auditory cortex to acoustic degradation of speech and non-speech sounds

BackgroundRecent studies have shown that the human right-hemispheric auditory cortex is particularly sensitive to reduction in sound quality, with an increase in distortion resulting in an amplification of the auditory N1m response measured in the magnetoencephalography (MEG). Here, we examined whether this sensitivity is specific to the processing of acoustic properties of speech or whether it can be observed also in the processing of sounds with a simple spectral structure. We degraded speech stimuli (vowel /a/), complex non-speech stimuli (a composite of five sinusoidals), and sinusoidal tones by decreasing the amplitude resolution of the signal waveform. The amplitude resolution was impoverished by reducing the number of bits to represent the signal samples. Auditory evoked magnetic fields (AEFs) were measured in the left and right hemisphere of sixteen healthy subjects.ResultsWe found that the AEF amplitudes increased significantly with stimulus distortion for all stimulus types, which indicates that the right-hemispheric N1m sensitivity is not related exclusively to degradation of acoustic properties of speech. In addition, the P1m and P2m responses were amplified with increasing distortion similarly in both hemispheres. The AEF latencies were not systematically affected by the distortion.ConclusionsWe propose that the increased activity of AEFs reflects cortical processing of acoustic properties common to both speech and non-speech stimuli. More specifically, the enhancement is most likely caused by spectral changes brought about by the decrease of amplitude resolution, in particular the introduction of periodic, signal-dependent distortion to the original sound. Converging evidence suggests that the observed AEF amplification could reflect cortical sensitivity to periodic sounds.

Cortical Abnormalities in Creutzfeldt-Jakob Disease Measured by MEG, a Case Report

Event Abstract Back to Event Cortical Abnormalities in Creutzfeldt-Jakob Disease Measured by MEG, a Case Report Eero Pekkonen1*, Juha Wilenius2, Jukka Lyytinen1, Anders Paetau3, Maarit Palomäki4 and Jyrki Mäkelä5 1 University of Helsinki, Department of Neurology, Finland 2 University of Helsinki, BioMag laboratory, Finland 3 University of Helsinki, Department of Pathology, Finland 4 University of Helsinki, Department of Radiology, Finland 5 Helsinki University Central Hospital, BioMag laboratory, Finland Creutzfeldt-Jakob Disease (CJD) is a rare neurodegenerative disease, which belongs to a group of spongiform encephalopathies termed prion diseases.Main symptoms of CJD are progressive dementia, py-ramidal and extrapyramidal symptoms, and myoclonus. Currently there is no effective therapy for CJD. Definite diagnosis is based on autopsy findings.Here we report a male patient with a rapidly progres-sive memory disorder, ataxia and myoclonus. EEG showed slow wave activity, and head MRI revealed in-creased basal ganglia and left-dominant increase in cor-tical signal intensity. In cerebrospinal fluid (CSF) 14-3-3 test was positive. The autopsy confirmed the sporadic CJD.A 306-channel MEG measurement was performed to the patient. Spontaneous brain activity and auditory evoked fields (AEF) were recorded.Both hemispheres had slow wave activity and periodic sharp waves, activity being more pronounced in the left, more damaged, hemisphere. The source of auditory N100m were more lateral in the left than in the right hemisphere. P200m was larger and more posterior in the left hemisphere compared with the right hemisphere. This is the first MEG-recording of a patient with spo-radic CJD. The more damaged left hemisphere in MRI had more pronounced slow and sharp waves in MEG. The sources of auditory responses and amplitude of later 200m deflection were different between the more and less damaged hemispheres. MEG appears to be suitable to investigate and locate brain activity in prion diseases. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: MEG: Clinical applications Citation: Pekkonen E, Wilenius J, Lyytinen J, Paetau A, Palomäki M and Mäkelä J (2010). Cortical Abnormalities in Creutzfeldt-Jakob Disease Measured by MEG, a Case Report. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00286 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 05 Apr 2010; Published Online: 05 Apr 2010. * Correspondence: Eero Pekkonen, University of Helsinki, Department of Neurology, Helsinki, Finland, eero_pekkonen@hotmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Eero Pekkonen Juha Wilenius Jukka Lyytinen Anders Paetau Maarit Palomäki Jyrki Mäkelä Google Eero Pekkonen Juha Wilenius Jukka Lyytinen Anders Paetau Maarit Palomäki Jyrki Mäkelä Google Scholar Eero Pekkonen Juha Wilenius Jukka Lyytinen Anders Paetau Maarit Palomäki Jyrki Mäkelä PubMed Eero Pekkonen Juha Wilenius Jukka Lyytinen Anders Paetau Maarit Palomäki Jyrki Mäkelä Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Representation of the vocal roughness of aperiodic speech sounds in the auditory cortex

Aperiodicity of speech alters voice quality. The current study investigated the relationship between vowel aperiodicity and human auditory cortical N1m and sustained field (SF) responses with magnetoencephalography. Behavioral estimates of vocal roughness perception were also collected. Stimulus aperiodicity was experimentally varied by increasing vocal jitter with techniques that model the mechanisms of natural speech production. N1m and SF responses for vowels with high vocal jitter were reduced in amplitude as compared to those elicited by vowels of normal vocal periodicity. Behavioral results indicated that the ratings of vocal roughness increased up to the highest jitter values. Based on these findings, the representation of vocal jitter in the auditory cortex is suggested to be formed on the basis of reduced activity in periodicity-sensitive neural populations.

Cortical sensitivity to periodicity of speech sounds

Previous non-invasive brain research has reported auditory cortical sensitivity to periodicity as reflected by larger and more anterior responses to periodic than to aperiodic vowels. The current study investigated whether there is a lower fundamental frequency (F0) limit for this effect. Auditory evoked fields (AEFs) elicited by natural-sounding 400 ms periodic and aperiodic vowel stimuli were measured with magnetoencephalography. Vowel F0 ranged from normal male speech (113 Hz) to exceptionally low values (9 Hz). Both the auditory N1m and sustained fields were larger in amplitude for periodic than for aperiodic vowels. The AEF sources for periodic vowels were also anterior to those for the aperiodic vowels. Importantly, the AEF amplitudes and locations were unaffected by the F0 decrement of the periodic vowels. However, the N1m latency increased monotonically as F0 was decreased down to 19 Hz, below which this trend broke down. Also, a cascade of transient N1m-like responses was observed in the lowest F0 condition. Thus, the auditory system seems capable of extracting the periodicity even from very low F0 vowels. The behavior of the N1m latency and the emergence of a response cascade at very low F0 values may reflect the lower limit of pitch perception.

Formant transition-specific adaptation by lipreading of left auditory cortex N1m

To test for the feature specificity of adaptation of auditory-cortex magnetoencephalographic N1m responses to phonemes during lipreading, we presented eight healthy volunteers with a simplified sine-wave first-formant (F1) transition shared by /ba/, /ga/, and /da/, and a continuum of second-formant (F2) transitions contained in /ba/ (ascending), /da/ (level), and /ga/ (descending), during lipreading of /ba/ vs. /ga/ vs. a still-face baseline. N1m responses to the F1 transition were suppressed during lipreading, further, visual /ga/ (vs. /ba/) significantly suppressed left-hemisphere N1m responses to the F2 transition contained in /ga/. This suggests that visual speech activates and adapts auditory cortex neural populations tuned to formant transitions, the basic sound-sweep constituents of phonemes, potentially explaining enhanced speech perception during lipreading.

Latency variation of auditory N1m responses to vocal and nonvocal sounds

To investigate the temporal aspect of timbre processing, we recorded auditory-evoked neuromagnetic responses to periodic complex sounds, which were matched in all acoustic parameters except for two fundamental frequencies (F0s) and 12 spectral envelopes of vocal and nonvocal categories. Only for nonvocal sounds, a significant difference in N1m latency for F0 was detected in both hemispheres. A significant difference among stimuli was detected in both hemispheres for vocal and linear sounds, whereas only in the right hemisphere for instrumental sounds. Moreover, the results of paired comparison among F0s revealed that not only the vocal sounds but also some of the nonvocal sounds were F0-independent. This latency independence may be attributed to the relatively high power in the higher frequency spectrum.

Adaptation of neuromagnetic N1 without shifts in dipolar orientation

306-channel magnetoencephalography, coregistered with high-resolution volumetric magnetic resonance imaging, was used with 10 healthy participants to test if repetition adapts subsequent processing of sounds in a sequence and whether this adaptation influenced the orientation of the dipolar sources in the auditory cortex. Auditory N1m responses to 1 kHz pure tones were indexed by clusters of sensors situated bilaterally over the temporal lobes. N1m was augmented in amplitude at an interstimulus interval of 16 s relative to 1 s. This neuromagnetic amplitude augment occurred in dipoles in the vicinity of the auditory cortex, without significant shifts in the dipolar orientation. Recent repetition thus adapts auditory cortical neurons, in a manner subject to recovery after a period of silence.