Keita Tanaka

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.

Magnetocardiography Signal Reconstruction With Reduced Source Space Based on Current Source Variance

In this study we use magnetocardiogram (MCG) signals to perform the current source reconstruction of the signal generators. The source model used is a distributed source model that allows visualizing the current flow in the human heart. The heart volume is divided into voxels, each one being the site of one current dipole that has varying orientation and magnitude. We then perform a variance analysis of the dipole components waveforms to detect which dipoles carry the most significant information. Since every dipole has a fixed position, the localization of the ones that mainly contribute to the field generation allows identifying in what portion of the source space region the activation wavefront is mainly localized. After excluding the dipoles with the less information content, the forward field computation is performed to check whether the magnetic signals differ significantly from the measured data. The method is applied on the MCG of a patient affected by Wolff-Parkinson-White syndrome to localize the accessory conduction pathway in terms of current density. The comparison of the results obtained between vector and normal field components data allows improving the results when using vector data.

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.

Effects of Task Difficulty and Training of Visuospatial Working Memory Task on Brain Activity

Visuospatial working memory is one of the important cognitive processes and makes it possible to maintain and manipulate the visual information stored in brain systems. In this study, effects of task difficulty and two-week training of visuospatial working memory task on brain activity and its neural mechanisms were investigated.

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.

Brain Activation during Visuo-Spatial Memory Task: an fMRI Study

Effect of information capacity and one week training of visuo-spatial memory task on brain activity were investigated. Brain activity during a visuo-spatial memory task with 10 and 30 visual targets were measured with functional MRI before and after one week training of the task. Statistical probability mapping revealed that activity in prefrontal cortex depended on visual information capacity in visuo- spatial task, and the activated area decreased after one-week training of the task.

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.