Auditory N1m Response Research Articles

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.

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.

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.

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

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.

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.

A magnetoencephalographic study of Japanese vowel processing

Magnetic brain responses were recorded to clarify the cortical representation of vowel processing in Japanese. We investigated the peak latencies and equivalent current dipoles of the auditory N1m responses to the Japanese vowels [a], [i], [o], and [u]. In intraindividual analyses for a single participant, well-replicated results for the dipole parameters supported the existence of phoneme-specific cortical maps for vowels. In the interindividual analyses for the eight participants, [a] and [i] elicited significantly earlier N1m responses than [u], and the dipole for [i] was more posteriorly oriented than [a] in the left hemisphere. The results of the current study suggest left hemispheric predominance in vowel processing and that factors associated with a different language system may modify the cortical map.

Disentangling the effects of phonation and articulation: Hemispheric asymmetries in the auditory N1m response of the human brain

BackgroundThe cortical activity underlying the perception of vowel identity has typically been addressed by manipulating the first and second formant frequency (F1 & F2) of the speech stimuli. These two values, originating from articulation, are already sufficient for the phonetic characterization of vowel category. In the present study, we investigated how the spectral cues caused by articulation are reflected in cortical speech processing when combined with phonation, the other major part of speech production manifested as the fundamental frequency (F0) and its harmonic integer multiples. To study the combined effects of articulation and phonation we presented vowels with either high (/a/) or low (/u/) formant frequencies which were driven by three different types of excitation: a natural periodic pulseform reflecting the vibration of the vocal folds, an aperiodic noise excitation, or a tonal waveform. The auditory N1m response was recorded with whole-head magnetoencephalography (MEG) from ten human subjects in order to resolve whether brain events reflecting articulation and phonation are specific to the left or right hemisphere of the human brain.ResultsThe N1m responses for the six stimulus types displayed a considerable dynamic range of 115–135 ms, and were elicited faster (~10 ms) by the high-formant /a/ than by the low-formant /u/, indicating an effect of articulation. While excitation type had no effect on the latency of the right-hemispheric N1m, the left-hemispheric N1m elicited by the tonally excited /a/ was some 10 ms earlier than that elicited by the periodic and the aperiodic excitation. The amplitude of the N1m in both hemispheres was systematically stronger to stimulation with natural periodic excitation. Also, stimulus type had a marked (up to 7 mm) effect on the source location of the N1m, with periodic excitation resulting in more anterior sources than aperiodic and tonal excitation.ConclusionThe auditory brain areas of the two hemispheres exhibit differential tuning to natural speech signals, observable already in the passive recording condition. The variations in the latency and strength of the auditory N1m response can be traced back to the spectral structure of the stimuli. More specifically, the combined effects of the harmonic comb structure originating from the natural voice excitation caused by the fluctuating vocal folds and the location of the formant frequencies originating from the vocal tract leads to asymmetric behaviour of the left and right hemisphere.

Glides in speech fundamental frequency are reflected in the auditory N1m response.

The cortical dynamics underlying the perception of constant and gliding speech fundamental frequency (F0) was investigated in 10 subjects using magnetoencephalography (MEG). The stimuli comprised vowels having either constant, ascending or descending F0s and tones of corresponding frequencies, matched with the vowels in intensity or loudness. The amplitude of the N1m response was highly sensitive to F0 variation embedded in vowels and insensitive to corresponding variation in tones. The latency of the N1m elicited by the tones with respect to vowels was significantly delayed. Thus, the speech-specific behavior of the N1m arises out of cortical sensitivity to the acoustic structure of voiced speech, that is to the F0 and its harmonics, which underlie the perception of pitch and intonation in speech.

The auditory N1m reveals the left-hemispheric representation of vowel identity in humans

The cortical correlates of the perception of the sustained vowels /a/, /o/ and /u/ were studied by using whole-head magnetoencephalography (MEG). The three vowels which were located on a line in the space spanned by the first (F1) and second (F2) formants and having equal F2−F1 differences evoked equally strong auditory N1m responses at 120 ms after stimulus onset. The left-hemispheric distribution of the source locations, estimated by equivalent current dipoles, reflected the acoustic similarity of the vowels: the growing distance of the vowels in the F2,F1-space was accompanied by a growing distance between the centres of gravity of activation elicited by each vowel. Thus, direct evidence for the orderly left-hemispheric representation of phonemes in human auditory cortex was found.

The auditory N1m reveals the left-hemispheric representation of vowel identity in humans

The cortical correlates of the perception of the sustained vowels /a/, /o/ and /u/ were studied by using whole-head magnetoencephalography (MEG). The three vowels which were located on a line in the space spanned by the first (F1) and second (F2) formants and having equal F2−F1 differences evoked equally strong auditory N1m responses at 120 ms after stimulus onset. The left-hemispheric distribution of the source locations, estimated by equivalent current dipoles, reflected the acoustic similarity of the vowels: the growing distance of the vowels in the F2,F1-space was accompanied by a growing distance between the centres of gravity of activation elicited by each vowel. Thus, direct evidence for the orderly left-hemispheric representation of phonemes in human auditory cortex was found.

Visual short-term memory load affects sensory processing of irrelevant sounds in human auditory cortex

We used whole-head magnetoencephalography (MEG) to investigate neural activity in human auditory cortex elicited by irrelevant tones while the subjects were engaged in a short-term memory task presented in the visual modality. As compared to a no-memory-task condition, memory load enhanced the amplitude of the auditory N1m response. In addition, the N1m amplitude depended on the phase of the memory task, with larger response amplitudes observed during encoding than retention. Further, these amplitude modulations were accompanied by anterior–posterior shifts in N1m source locations. The results show that a memory task for visually presented stimuli alters sensory processing in human auditory cortex, even when subjects are explicitly instructed to ignore any auditory stimuli. Thus, it appears that task demands requiring attentional allocation and short-term memory result in interaction across visual and auditory brain areas carrying out the processing of stimulus features.