Masaki Kawakatsu

Neural Signatures of Phonetic Learning in Adulthood: A Magnetoencephalography Study

The present study used magnetoencephalography (MEG) to examine perceptual learning of American English /r/ and /l/ categories by Japanese adults who had limited English exposure. A training software program was developed based on the principles of infant phonetic learning, featuring systematic acoustic exaggeration, multi-talker variability, visible articulation, and adaptive listening. The program was designed to help Japanese listeners utilize an acoustic dimension relevant for phonemic categorization of /r-l/ in English. Although training did not produce native-like phonetic boundary along the /r-l/ synthetic continuum in the second language learners, success was seen in highly significant identification improvement over twelve training sessions and transfer of learning to novel stimuli. Consistent with behavioral results, pre-post MEG measures showed not only enhanced neural sensitivity to the /r-l/ distinction in the left-hemisphere mismatch field (MMF) response but also bilateral decreases in equivalent current dipole (ECD) cluster and duration measures for stimulus coding in the inferior parietal region. The learning-induced increases in neural sensitivity and efficiency were also found in distributed source analysis using Minimum Current Estimates (MCE). Furthermore, the pre-post changes exhibited significant brain-behavior correlations between speech discrimination scores and MMF amplitudes as well as between the behavioral scores and ECD measures of neural efficiency. Together, the data provide corroborating evidence that substantial neural plasticity for second-language learning in adulthood can be induced with adaptive and enriched linguistic exposure. Like the MMF, the ECD cluster and duration measures are sensitive neural markers of phonetic learning.

The silent period between sounds has a stronger effect than the interstimulus interval on auditory evoked magnetic fields

Auditory evoked cortical responses, electric. N1 and magnetic N1m, increase when the interstimulus interval (ISI) increases. We assumed that the response to a tone is mainly affected by the immediately preceding ISI, by the immediately preceding pause between stimuli (PBS) and by the previous stimulus duration (PSD). These 3 values are connected by the following expression: ISI = PBS + PSD. We examined the dependence of the auditory evoked brain magnetic responses on the ISI with the constant PSD (conventional paradigm), on the PBS with the constant ISI, and on the ISI with the constant PBS. Peak latencies and peak amplitudes of the 3 components, P1m, N1m and P2m, are recorded in one block using all possible combinations of 5 PSDs (0.05, 0.5, 1.0, 1.5 and 2.0 s) and 5 ISIs (0.5, 1.0, 1.5, 2.0 and 2.5 s). Peak latencies of these 3 components do not show any significant dependence either on the PBS or on the ISI. Neither the PBS nor the ISI brings a statistically significant effect on the P1m peak amplitude. On the other hand, the N1m peak amplitude increases as either the PBS (constant ISI) or the ISI (constant PSD) increases. The regression coefficient to the PBS is more than a double of that to the ISI. Moreover, the ISI does not show any significant effect on the N1m peak amplitude when the PBS is constant. This stronger PBS effect means that the N1m peak amplitude dependence on the ISI, which has been reported in several papers using the constant PSDs, includes more dependence on the PBS. The P2m peak amplitude shows the same tendency as the N1m because of the strong correlation in peak amplitude between them.

Stochastic resonance in auditory steady-state responses in a magnetoencephalogram

OBJECTIVE To see whether stochastic resonance can be triggered in the auditory steady-state responses (ASSRs) in a magnetoencephalogram (MEG). METHODS We measured ASSRs to 1kHz sinusoidal tone modulated at 40Hz with various intensities of white noise and obtained its power and degree of phase synchrony. RESULTS Group statistics showed a significant enhancement in phase synchrony of ASSR by the presence of white noise of appropriate intensity. Tests on individual subjects showed that the data of four out of nine subjects exhibited enhancements in power or phase synchrony. CONCLUSIONS The ASSRs exhibit stochastic resonance of the so-called I-type (I for information) shown in phase synchrony when responding to modulated sinusoidal sound superimposed with weak white noise. SIGNIFICANCE The gamma-band component and other oscillatory components in the brain activity have been recently ascribed by some researchers to the result of stochastic resonance caused by internal noise in the brain. Therefore the presence of stochastic resonance in ASSRs may be evidence to the hypothesis that ASSRs are related to the ongoing gamma-band component.

Stochastic Resonance in Auditory Steady State Responses in MEG Observed as Increase in Their Power and Phase Synchrony

We measured auditory steady state responses (ASSRs) in magnetoencephalogram to the sinusoidal amplitude modulated tone superimposed with ongoing white noise of various intensities. The power and phase coherence of the ASSR were investigated and stochastic resonance (SR) was indicated by increased synchrony to the stimulus and by increase in power of the ASSR when there was a certain amount of noise compared with those measured without the noise. We also measured the hearing threshold for the noise alone and found that the optimum noise intensity for SR was slightly larger than the threshold for most of the subjects. As the power and coherence curves plotted against the noise to signal ratio varied considerably among subjects, we used the maximum values of power and phase coherence as test statistics for testing the group data.

Perceptual Temporal Asymmetry Associated with Distinct ON and OFF Responses to Time-Varying Sounds with Rising versus Falling Intensity: A Magnetoencephalography Study

This magnetoencephalography (MEG) study investigated evoked ON and OFF responses to ramped and damped sounds in normal-hearing human adults. Two pairs of stimuli that differed in spectral complexity were used in a passive listening task; each pair contained identical acoustical properties except for the intensity envelope. Behavioral duration judgment was conducted in separate sessions, which replicated the perceptual bias in favour of the ramped sounds and the effect of spectral complexity on perceived duration asymmetry. MEG results showed similar cortical sites for the ON and OFF responses. There was a dominant ON response with stronger phase-locking factor (PLF) in the alpha (8–14 Hz) and theta (4–8 Hz) bands for the damped sounds. In contrast, the OFF response for sounds with rising intensity was associated with stronger PLF in the gamma band (30–70 Hz). Exploratory correlation analysis showed that the OFF response in the left auditory cortex was a good predictor of the perceived temporal asymmetry for the spectrally simpler pair. The results indicate distinct asymmetry in ON and OFF responses and neural oscillation patterns associated with the dynamic intensity changes, which provides important preliminary data for future studies to examine how the auditory system develops such an asymmetry as a function of age and learning experience and whether the absence of asymmetry or abnormal ON and OFF responses can be taken as a biomarker for certain neurological conditions associated with auditory processing deficits.

A new multiple-kernel-learning weighting method for localizing human brain magnetic activity

This paper shows that pattern classification based on machine learning is a powerful tool to analyze human brain activity data obtained by magnetoencephalography (MEG). We propose a new weighting method using a multiple kernel learning (MKL) algorithm to localize the brain area contributing to the accurate vowel discrimination. Our MKL simultaneously estimates both the classification boundary and the weight of each MEG sensor; MEG amplitude obtained from each pair of sensors is an element of the feature vector. The estimated weight indicates how the corresponding sensor is useful for classifying the MEG response patterns. Our results show both the large-weight MEG sensors mainly in a language area of the brain and the high classification accuracy (73.0%) in the 100 ~ 200 ms latency range.

Stochastic resonance in brain activity elicited by auditory stimuli

We measured auditory steady state responses (ASSRs) in magnetoencephalogram to an ongoing sinusoidal amplitude modulated tone presented to the subject’s left ear while bursts of white noise of various intensities were presented to the right ear. Because the power and coherence as functions of the noise to signal ratio differed considerably among subjects, we used their maximum values as test statistics for testing the group data. The results showed a significant enhancement in the phase coherence of ASSRs obtained over the right temporal regions by the presence of white noise of appropriate intensity. The observed stochastic resonance (SR) most likely occurred within the central nervous system. Our finding may be quite important as mechanisms of SR in biological systems are mostly unknown.

Stochastic Resonance in Evoked Magnetoencephalogram Investigated by Analysis of Coherences

We examined the phase coherence and power of auditory steady state responses (ASSRs) in magnetoencephalogram to an ongoing sinusoially amplitude modulated tone presented to the subjects one ear while bursts of white noise of various intensities were presented to the opposite ear. We also examined the real and imaginary parts of coherence between the ASSRs of the right and left auditory temporal regions. Stochastic resonance (SR) was seen in the phase coherence and power of the ASSR from the contralateral temporal area and in the real part of the right-left coherence. The results showed that the SR phenomenon occurred not in the periphery but in the central nervous system and suggested that it was not probably due to the interaction between the left and right auditory cortex.

Magnetoencephalographic study of the components of event-related fields in object recognition.

Neuromagnetic fields were recorded from normal participants to study neural activation while they performed a delayed matching-to-sample task of visual stimuli. Each participant was asked to compare sequentially presented three-dimensional object images under same view and different view conditions. Minimum current estimates were calculated to characterize cortical activities for the object recognition. Activation was observed in the bilateral parieto-occipital regions at approximately 180 ms. Activation within the right parieto-occipital region was significantly enhanced under the different view conditions. This result indicates that the activity at the right parieto-occipital region is related to viewpoint and may play an important role in three-dimensional object recognition.

Stochastic Resonance Seen as Increase in Phase Synchrony or Power in Auditory Steady-state Responses in MEG

This report shows the first evidence that stochastic resonance can be triggered in the auditory steady state responses (ASSRs) in MEG (magnetoencephalogram) which was indicated either by increased synchrony to the stimulus or by increase in power of ASSR when there was certain amount of noise in addition to the sinusoidal AM sound compared with the ASSR when there was no noise. To obtain the above conclusion, we applied statistical analysis to each subjects data rather than to the whole set of the data of all subjects because individual differences smoothed out the effect of noise addition when the data were averaged over all the subjects.