Jiuk Jung

An automatic identification and removal method for eye-blink artifacts in event-related magnetoencephalographic measurements.

While measuring event-related magnetoencephalographic (MEG) signals using visual stimuli, eye blinks were inevitable and generated large magnetic artifacts. Since trials containing eye blinks were excluded from the analyses, the signal-to-noise ratio of the event-related signals was decreased. In this study, we propose a method to identify the eye blink magnetic artifacts and remove them automatically using independent component analysis preprocessed by principal component analysis. The method evaluates the spatiotemporal similarity between independent components and both MEG and electro-oculogram data based on a newly devised cost function. Testing of the method on event-related MEG signals measured by a 306-channel whole-head system in a visual perception task led to the successful identification and removal of eye-blink artifacts in all trials containing eye blinks from all the seven subjects.

Event-related potentials during a target discrimination task based on texture cue

In the current research, to elucidate cerebral processes having to do with target-ground discrimination based on a texture cue in visual stimuli, the authors investigated the relationship between the target presentation area in the visual field and the reaction time required until discrimination. Also, they simultaneously measured the event-related potentials (ERPs) during a target discrimination task at 63 locations on the scalp and then performed a spline Laplacian analysis to examine brain activities. The results clearly showed that a significant difference occurred in the reaction time depending on the area in which the target was presented. In addition, four distinct ERP components with different latencies accompanying execution of the discrimination task were observed. The spline Laplacian analysis of the ERPs indicated that the sources of the signals of these components were in the bilateral hemispheres of the frontal, parietal, and occipital lobes. Considering the reaction time, the frontal activity observed after the latency of about 270 ms may mainly be involved in the target recognition.

A study on multiple source separation using spline Laplacian: simulation and ERP measurements

Abstract The spline Laplacian (SL) method has been developed to increase topographic information of EEG or event-related potential (ERP). To determine the number of electrodes needed and the minimum distance of two cerebral cortical sources to be separated by the spline Laplacian, topographical mapping on the surface of a three-concentric sphere head model with multiple dipoles in various configurations was simulated. ERPs during cognition of visual stimulus, which may activate bilateral visual areas, were also measured to confirm the simulation results. It was found that the spline Laplacian distribution clearly separated two current dipoles adjacent to each other when their spacing was 3 cm even if the two dipoles had arbitrary orientations when 63 electrodes were used. The results of our simulation and ERP measurement indicate that it is necessary to place more than 30 electrodes over the entire head to estimate different cortical sources activated simultaneously even when spline Laplacian mapping is used.

15. Awareness and somatosensory event-related potentials following near threshold stimuli

The effectiveness of CPAP therapy on central sleep apnea (CSA) remains controversial and factors associated with such effectiveness are still unclear. We investigated effectiveness of CPAP on CSA and related factors, focusing on body position during sleep and presence or absence of underlying cardiac dysfunction. Among 7268 subjects who underwent polysomnography, 115 patients who had central apnea hypopnea index (CAHI) P15 were analyzed. We excluded 47 patients who had rarely lateral position during sleep (<20%). We found 34 subjects (50%) showed significant reductions in AHI on CPAP (i.e. effective group, defined AHI 65 on CPAP or >80% reduction from the diagnostic AHI) whereas residual 34 subjects did not show such reductions (i.e. ineffective group). Ineffective group showed significantly greater AHI on lateral position (L-AHI) and lower % of slow wave sleep (%SWS) on diagnostic study, and more frequently had cardiac dysfunction as compared with effective group. Multivariate logistic regression analysis showed greater LAHI, lower %SWS and presence of cardiac dysfunction are independently associated with the CPAP-ineffectiveness. Our findings suggest that in addition to the underlying cardiac dysfunction, CSA on lateral position may play a role to determine CPAP effectiveness on CSA.

fMRI-MEG Integrative Neuroimaging in an Apparent Visual Motion Perception Task

We have developed an fMRI-MEG integrative neuroimaging method that is capable of analyzing spatiotemporal multiple cortical activities. The method determines the center of gravity in each fMRI activated cluster as the location of the linear constraints. First, activated multiple clusters were determined by statistically analyzing fMRI data. Secondly, using the fMRI activated clusters as spatial constraints, the orientations of equivalent current dipoles (ECDs) placed at the center of gravity in each voxel of all activated clusters were estimated by a procedure maximizing inner product of lead field and measured field vectors. Thirdly, the activities of all ECDs were estimated, and dynamic activities of them were visualized. The linear constraints for the voxels in a cluster are defined to suppress the power from the source at the center of gravity of the other cluster. The spatial distributions and the time courses of activities for all ECDs were successfully reconstructed. We applied the newly developed method to data obtained during an apparent visual motion perception task and could confirm its availability. Although there were individual differences, the present fMRI-MEG integrative neuroimaging method successfully detected dynamic cortical activities in multiple visual areas, such as V1, V2, V5 and IPS.

The analyses of brain functions based on wavelet coherence of EEG gamma-band activities: An application to a perceptual grouping task

The wavelet coherence analysis of induced EEG gamma-band activities was applied to investigate brain mechanisms of perceptual grouping. EEGs were measured from 63 scalp positions on the whole head during a perceptual grouping task. It was found that induced EEG gamma-band activities depended on the display position of a visual target presented in the task. The induced gamma-band activities were mainly observed at the frontal, parietal and occipital areas. Furthermore, it was found that the duration of induced EEG gamma-band activities reflected the difficulty of the task. The wavelet coherence between the frontal and parietal/occipital areas showed the significant decrease. The wavelet coherence at pre-stimulus periods was found to be dependent on the distance between two activated areas. In addition, the duration of the significant decrease of wavelet coherence increased corresponding with the difficulty of the task. Thus it was inferred that the decrease of wavelet coherence between two activated areas was caused by the finite transmission time of information from one area to the other. These results demonstrate that wavelet coherence could clearly detect short, significant episodes of synchronized cortical activities among multiple cortical areas associated with perceptual grouping.

Brain activity associated with multistable perceptions: a functional MRI study

A potentially promising experimental paradigm in the understanding of visual awareness, which is a particular kind of consciousness, is multistable perceptions wherein images given to the eyes remain the same while the percept changes. Controversies remain as to precisely what region of the brain is involved in multistable perceptions. To address this question, we used functional magnetic resonance imaging and compared brain activities during multistable and monostable perceptions presenting a diamond changing its width. Although there were individual differences, significant differential activations were commonly observed in the bilateral parietal, prefrontal and middle temporal areas in a multistable condition compared to those in a monostable control condition. These findings suggest that activity in a distributed network of the parietal and prefrontal areas might be associated with multistable perceptions.

Event-related potentials in a similarity grouping task

Abstract In order to investigate the neural activities related to perceptual grouping, we measured the event-related potentials (ERPs) at 63 scalp positions during the performance of a similarity grouping task. Subjects were instructed to press a button when they could identify the location of a disparate quadrant in a circular pattern consisting of two different micropatterns. The time required to locate the quadrant was measured as the reaction time (RT). It was found that the reaction time depends on the displayed position of the disparate quadrant. Four distinct ERP components were observed before the reaction time. The peak latency of only the last positive ERP component, which was about 200 ms faster than the reaction time, varied with the reaction time. These findings suggest that the last positive ERP component observed in the parietal area reflects the neural activities related to similarity grouping.