Multi-modal imaging of the auditory-larynx motor network for voicing perception.

Voicing is one of the most important characteristics of phonetic speech sounds. Despite its importance, voicing perception mechanisms remain largely unknown. To explore auditory-motor networks associated with voicing perception, we firstly examined the brain regions that showed common activities for voicing production and perception using functional magnetic resonance imaging. Results indicated that the auditory and speech motor areas were activated with the operculum parietale 4 (OP4) during both voicing production and perception. Secondly, we used a magnetoencephalography and examined the dynamical functional connectivity of the auditory-motor networks during a perceptual categorization task of /da/-/ta/ continuum stimuli varying in voice onset time (VOT) from 0 to 40 ms in 10 ms steps. Significant functional connectivities from the auditory cortical regions to the larynx motor area via OP4 were observed only when perceiving the stimulus with VOT 30 ms. In addition, regional activity analysis showed that the neural representation of VOT in the auditory cortical regions was mostly correlated with categorical perception of voicing but did not reflect the perception of stimulus with VOT 30 ms. We suggest that the larynx motor area, which is considered to play a crucial role in voicing production, contributes to categorical perception of voicing by complementing the temporal processing in the auditory cortical regions.

Monaural 40-Hz auditory steady-state magnetic responses can be useful for identifying epileptic focus in mesial temporal lobe epilepsy.

OBJECTIVE: Patients with mesial temporal lobe epilepsy (mTLE) often exhibit central auditory processing (CAP) dysfunction. Monaural 40-Hz auditory steady-state magnetic responses (ASSRs) were recorded to explore the pathophysiology of mTLE. METHODS: Eighteen left mTLE patients, 11 right mTLE patients and 16 healthy controls (HCs) were examined. Monaural clicks were presented at a rate of 40 Hz. Phase-locking factor (PLF) and power values were analyzed within bilateral Heschl's gyri. RESULTS: Monaural 40-Hz ASSR demonstrated temporal frequency dynamics in both PLF and power data. Symmetrical hemispheric contralaterality was revealed in HCs. However, predominant contralaterality was absent in mTLE patients. Specifically, right mTLE patients exhibited a lack of contralaterality in response to left ear but not right ear stimulation, and vice versa in left mTLE patients. CONCLUSION: This is the first study to use monaural 40-Hz ASSR with unilateral mTLE patients to clarify the relationship between CAP and epileptic focus. CAP dysfunction was characterized by a lack of contralaterality corresponding to epileptic focus. SIGNIFICANCE: Monaural 40-Hz ASSR can provide useful information for localizing epileptic focus in mTLE patients.

Altered neural synchronization to pure tone stimulation in patients with mesial temporal lobe epilepsy: An MEG study.

OBJECTIVE: Our previous study of monaural auditory evoked magnetic fields (AEFs) demonstrated that hippocampal sclerosis significantly modulated auditory processing in patients with mesial temporal lobe epilepsy (mTLE). However, the small sample size (n = 17) and focus on the M100 response were insufficient to elucidate the lateralization of the epileptic focus. Therefore, we increased the number of patients with mTLE (n = 39) to examine whether neural synchronization induced by monaural pure tone stimulation provides useful diagnostic information about epileptic foci in patients with unilateral mTLE. METHODS: Twenty-five patients with left mTLE, 14 patients with right mTLE, and 32 healthy controls (HCs) were recruited. Auditory stimuli of 500-Hz tone burst were monaurally presented to subjects. The AEF data were analyzed with source estimation of M100 responses in bilateral auditory cortices (ACs). Neural synchronization within ACs and between ACs was evaluated with phase-locking factor (PLF) and phase-locking value (PLV), respectively. Linear discriminant analysis was performed for diagnosis and lateralization of epileptic focus. RESULTS: The M100 amplitude revealed that patients with right mTLE exhibited smaller M100 amplitude than patients with left mTLE and HCs. Interestingly, PLF was able to differentiate the groups with mTLE, with decreased PLFs in the alpha band observed in patients with right mTLE compared with those (PLFs) in patients with left mTLE. Right hemispheric predominance was confirmed in both HCs and patients with left mTLE while patients with right mTLE showed a lack of right hemispheric predominance. Functional connectivity between bilateral ACs (PLV) was reduced in both patients with right and left mTLE compared with that of HCs. The accuracy of diagnosis and lateralization was 80%-90%. CONCLUSION: Auditory cortex subnormal function was more pronounced in patients with right mTLE compared with that in patients with left mTLE as well as HCs. Monaural AEFs can be used to reveal the pathophysiology of mTLE. Overall, our results indicate that altered neural synchronization may provide useful information about possible functional deterioration in patients with unilateral mTLE.

Spatiotemporal brain dynamics of auditory temporal assimilation.

Time is a fundamental dimension, but millisecond-level judgments sometimes lead to perceptual illusions. We previously introduced a "time-shrinking illusion" using a psychological paradigm that induces auditory temporal assimilation (ATA). In ATA, the duration of two successive intervals (T1 and T2), marked by three auditory stimuli, can be perceived as equal when they are not. Here, we investigate the spatiotemporal profile of human temporal judgments using magnetoencephalography (MEG). Behavioural results showed typical ATA: participants judged T1 and T2 as equal when T2 - T1 ≤ +80 ms. MEG source-localisation analysis demonstrated that regional activity differences between judgment and no-judgment conditions emerged in the temporoparietal junction (TPJ) during T2. This observation in the TPJ may indicate its involvement in the encoding process when T1 ≠ T2. Activation in the inferior frontal gyrus (IFG) was enhanced irrespective of the stimulus patterns when participants engaged in temporal judgment. Furthermore, just after the final marker, activity in the IFG was enhanced specifically for the time-shrinking pattern. This indicates that activity in the IFG is also related to the illusory perception of time-interval equality. Based on these observations, we propose neural signatures for judgments of temporal equality in the human brain.

EEG investigations of duration discrimination: the intermodal effect is induced by an attentional bias.

Previous studies indicated that empty time intervals are better discriminated in the auditory than in the visual modality, and when delimited by signals delivered from the same (intramodal intervals) rather than from different sensory modalities (intermodal intervals). The present electrophysiological study was conducted to determine the mechanisms which modulated the performances in inter- and intramodal conditions. Participants were asked to categorise as short or long empty intervals marked by auditory (A) and/or visual (V) signals (intramodal intervals: AA, VV; intermodal intervals: AV, VA). Behavioural data revealed that the performances were higher for the AA intervals than for the three other intervals and lower for inter- compared to intramodal intervals. Electrophysiological results indicated that the CNV amplitude recorded at fronto-central electrodes increased significantly until the end of the presentation of the long intervals in the AA conditions, while no significant change in the time course of this component was observed for the other three modalities of presentation. They also indicated that the N1 and P2 amplitudes recorded after the presentation of the signals which delimited the beginning of the intervals were higher for the inter- (AV/VA) compared to the intramodal intervals (AA/VV). The time course of the CNV revealed that the high performances observed with AA intervals would be related to the effectiveness of the neural mechanisms underlying the processing of the ongoing interval. The greater amplitude of the N1 and P2 components during the intermodal intervals suggests that the weak performances observed in these conditions would be caused by an attentional bias induced by the cognitive load and the necessity to switch between modalities.