Magnetic field tomography of coherent thalamocortical 40-Hz oscillations in humans.

Abstract

This paper introduces the use of magnetic field tomography (MFT), a noninvasive technique based on distributed source analysis of magnetoencephalography data, which makes possible the three-dimensional reconstruction of dynamic brain activity in humans. MFE has a temporal resolution better than 1 msec and a spatial accuracy of 2-5 mm at the cortical level, which deteriorates to 1-3 cm at depths of 6 cm or more. MFT is used here to visualize the origin of a spatiotemporally organized pattern of coherent 40-Hz electrical activity. This coherence, initially observed during auditory input, was proposed to be generated by recurrent corticothalamic oscillation. In support of this hypothesis, we illustrate well-defined 40-Hz coherence between corticalsubcortical sites with a time shift that is consistent with thalamocortical conduction times. Studies on Alzheimer patients indicate that, while a similar activity pattern is present, the cortical component is reduced in these subjects.In the past decade significant advances have been made in noninvasive technology capable of imaging brain activity with sufficient spatial resolution to complement the structural imaging analysis offered by magnetic resonance imaging (MRI) (1) and computerized tomography (2). However, all imaging techniques available to date fall short of the optimal temporal resolution.The utilization of electrical or magnetic signals has been known to afford the necessary temporal resolution but, until very recently, magnetoencephalography (MEG) analysis was based solely on the assumption of a single point source, the current dipole (3-7), which is only valid when the underlying activity is highly localized and, in general, cannot support imaging capabilities. The development of inverse problem algorithms with primary sources specified by continuous current densities confined to a well-defined region referred to as the source space (8-10) removes the limitations of point source models in the spatial domain without imposing restrictions on the temporal resolution. In earlier work, some of us used a two-dimensional surface as the source space (8,9). In this paper, we solve the inverse problem by using a three-dimensional source space (a cylinder). This threedimensional solution allows the generation of a set of twodimensional images that provide a sequence of slices through the source space. Each image shows the square of the magnitude of the current density at points in the appropriate slice, represented by a color map ranging from black (low activity) to yellow (high activity). By analogy with computerized tomography and positron emission tomography (11), we call this technique magnetic field tomography (MFT). The basic methodology utilized here has been extensively tested and the results have shown excellent reproducibility with computer-generated data (refs. 8 and 9; see Fig. 1C). The two-dimensional paradigm was initially tested on spontaneous and evoked activity in normal subjects and in patients (8,12).We describe here, as an example of the ap...

EEG synchrony during a perceptual-cognitive task: Widespread phase synchrony at all frequencies

“…(3) A separate but related possibility is that proposed by Llinas and Ribary (Ribary et al, 1991;Joliot et al, 1994;Llinas et al, 1998). These authors propose that a thalamic pacemaker, driven by the intrinsic properties of the ionic conductances in various kinds of thalamic cell and modified by sensory input, may be responsible for the coherence of 40-Hz cortical rhythms they observe.…”

Comparison of single current dipole and Magnetic Field Tomography analyses of the cortical response to auditory stimuli

“…A color or gray level scale is then used to represent the square of the modulus of the current density at each point in the image. This technique is known as Magnetic Field Tomography (MFT), and was first applied to real data in the 3D reconstruction of a possible 40Hz thalamo-cortical interaction (Ribary et al 1991). A range of appropriate graphical representations for displaying the spatial form and dynamic evolution of these MFT solutions have been developed (Singh 1991;Singh et al 1991).…”

“…Twodimensional source space models have been employed to study cortical activity for a number of evoked MEG signals (Ioannides et al 1990b). Three dimensional reconstructions have been performed for a variety of MEG signals, including 40 Hz steady state response from normal subjects and Alzheimer's patients (Ribary et al 1991), and interictal epileptic activity (Ioannides et al 1993).…”

Oscillatory Brain Dynamics, Wavelet Analysis, and Cognition

“…EP analysis based on systems theory rules and application of digital filters also showed that a marked 40-Hz component follows stimulation (Başar, Rosen, Başar-Eroglu, & Greitschus, 1987). For further studies of evoked electric and magnetic 40-Hz activity see, e.g., Galambos, Makeig, & Talmachoff (1981), Pantev et al (1991), and Ribary et al (1992). A large number of studies at the single-cell level complement these results (Llinás, 1988;Gray & Singer, 1987;Gray, König, Engel, & Singer, 1989;Eckhorn et al, 1988).…”