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Ecologically relevant stimuli are rarely used in scientific studies because they are difficult to control. Instead, researchers employ simple stimuli with sharp boundaries (in space and time). Here, we explore how the rhythmogram can be used to provide much needed rigorous control of natural continuous stimuli like music and speech. The analysis correlates important features in the time course of stimuli with corresponding features in brain activations elicited by the same stimuli. Correlating the identified regularities of the stimulus time course with the features extracted from the activations of each voxel of a tomographic analysis of brain activity provides a powerful view of how different brain regions are influenced by the stimulus at different times and over different (user-selected) timescales. The application of the analysis to tomographic solutions extracted from magnetoencephalographic data recorded while subjects listen to music reveals a surprising and aesthetically pleasing aspect of brain function: an area believed to be specialized for visual processing is recruited to analyze the music after the acoustic signal is transformed to a feature map. The methodology is ideal for exploring processing of complex stimuli, e.g., linguistic structure and meaning and how it fails, for example, in developmental dyslexia., (© 2012 IEEE)

Reconstruction of neural current sources from magnetoencephalography (MEG) data provides two independent estimates of the instantaneous current modulus and its direction. Here, we explore how different information on the modulus and direction affects the inter-hemisphere connectivity of the human medial temporal complex (hMT+). Connectivity was quantified by mutual information values of paired time series of current moduli or directions, with the joint probability distribution estimated with an optimized Gaussian kernel. These time series were obtained from tomographic analysis of single-trial MEG responses to a visual motion stimulus. With a high-contrast stimulus, connectivity measures based on the modulus were relatively strong in the prestimulus period, continuing until 100 ms after stimulus onset. The strongest modulus connectivity was produced with a long lag (19 ms) of the right hMT+ after the left hMT+. On the other hand, connectivity measures based on direction were relatively strong after 100 ms, with a short delay of less than 6 ms. These results suggest that nonspecific and probably indirect communication between the homologous areas is turned, by the stimulus arrival, into more precise and direct communication through the corpus callosum. The orientation of the estimated current vector for the strong connectivity can be explained by the curvature of the active cortical sheet. The temporal patterns of modulus and directional connectivity were different at low contrast, but similar to those at high contrast. We conclude that the modulus and direction indicate distinct functional connectivity modes., (Copyright 2010 Elsevier B.V. All rights reserved.)

Purpose: The aim of present magnetoencephalography study was conducted to investigate how the cerebellum is involved in intracerebral activity immediately before conscious and voluntary teeth-clenching., Methods: We recorded the neuromagnetic signal immediately before teeth-clenching in five healthy subjects. A single trial consisted of self-paced teeth-clenching for 2s followed by a semi-randomized interval of 2-4s, and 25 trials were performed in a single session. Each subject performed a total of 125 trials over 5 sessions. The subjects also performed five control sessions without the teeth-clenching task. The subjects were asked to observe a visual cue providing task information. We used magnetic field tomography to obtain a tomographic reconstruction of brain activity for each time slice of the average signal for each subject and session. Activation of regions of interest by cerebellar neuron activity was computed from the magnetic field tomography result., Results: Cerebellar activation before the teeth-clenching task was identified in all five subjects: bilaterally in three, and unilaterally in two. The peak amplitudes for the left and right cerebellar hemispheres in the main session were significantly greater than those in the control session (P<0.01). Left and right cerebellar hemisphere activities were detected approximately 150 ms after visual cue., Conclusion: Our findings suggest that the cerebellum is involved in the signal pathway immediately before teeth-clenching.

Magnetoencephalography (MEG) encompasses a family of non-contact, non-invasive techniques for detecting the magnetic field generated by the electrical activity of the brain, for analyzing this MEG signal and for using the results to study brain function. The overall purpose of MEG is to extract estimates of the spatiotemporal patterns of electrical activity in the brain from the measured magnetic field outside the head. The electrical activity in the brain is a manifestation of collective neuronal activity and, to a large extent, the currency of brain function. The estimates of brain activity derived from MEG can therefore be used to study mechanisms and processes that support normal brain function in humans and help us understand why, when and how they fail.

In normal viewing conditions, many objects are often hidden or occluded by others, therefore restricting the information that enters the eye. One ability that the human visual system has developed to compensate for this visual limitation is to relate the surrounding elements to globally interpret the whole scene. The appearance of illusory figures (IF) based on surrounding elements also relies on this similar function. In the present study, we hypothesized that different mechanisms may be used by the brain to process IF from the center and periphery of the visual field. We compared magnetoencephalographic responses to IFs presented at different parts of the visual field under three task loads. For central presentation, IF specific responses peaked first in V1/V2 (96-101 ms), and then in the lateral occipital complex (LOC; 132-141 ms), independent of task. For peripheral presentation, the relative modulation towards IF was markedly reduced in V1/V2 and LOC while prominent activation peaks now shifted to the Fusiform Gyrus (from 200 ms onwards). Additionally, the type of task influenced processing at early stages beginning in V1/V2 (87 ms). Our results show that retinal eccentricity plays a crucial role in IF processing: figural completion at the center of the visual field is achieved in an 'automatic' and seemingly effortless fashion whereas peripheral stimulus locations necessitate higher-order object completion stages which rely more heavily on attentional demands.

The cerebellum has traditionally been associated with motor movements but recent studies suggest its involvement with fine timing, sensory analysis and cognition. Much of the new data comes from neuroimaging techniques such as fMRI and PET, which have high spatial resolution and show that for even simple stimuli many cerebellar and cortical areas are involved. We use examples from recent studies to demonstrate that magnetic field tomography (MFT) offers a new and powerful tool for studying cerebellar function through real time localization of cortical, brainstem and cerebellar activations over timescales ranging from a fraction of a millisecond to seconds, minutes and hours. The examples include demonstration of cerebellar activations along well-established anatomical pathways during saccades and the visualization of the ascending medullar volley after median nerve stimulation. MFT analysis of single trial MEG signals elicited by the presentation of faces in emotion and object recognition tasks, show changes in cerebellar activation between schizophrenics and normal subjects in agreement with proposals for disturbed cerebellar function in schizophrenia. The ability of MFT to identify cerebellar, brainstem and cortical activations in real time can add new insights about dynamics of brain activity to the recent findings about cerebellar function from PET and fMRI.