Functional segregation of movement-related rhythmic activity in the human brain.

Multiple synaptic interconnections in the human brain support concerted rhythmic activity of a large number of cortical neurons, typically close to 10 and 20 Hz. Our present neuromagnetic data provide evidence for distinct functional roles of these spectral components in the somatomotor cortex. The sites of suppression during movement and the subsequent rebound of the 20-Hz rhythm followed, along the motor cortex, the representation of fingers, toes, and mouth, as opposed to the stable origin of the 10-Hz rhythms close to the hand somatosensory cortex. The 20-Hz activity appears to be a signature of active immobilization following movement, whereas the reactive 10-Hz signals likely reflect lack of relevant sensory input from the important upper limbs.

Reactivity of magnetic parieto-occipital alpha rhythm during visual imagery.

Spontaneous MEG signals were recorded during visual imagery from 13 healthy adults with a whole-scalp neuromagnetometer. The parieto-occipital 7-14 Hz alpha activity was suppressed strongly while subjects visualized and evaluated letters. The act of forming a visual image caused a smaller suppression than did inspection of the imaged pattern for a named property. The maximum suppression depended on the baseline alpha level and, for the majority of the subjects, occurred close to the area with the strongest alpha, showing no systematic hemispheric asymmetry. Sources for the alpha activity, modeled with equivalent current dipoles, clustered in the parietal and occipital lobes. The strongest suppression of the activity occurred near the parieto-occipital sulcus.

Magnetoencephalographic evaluation of children and adolescents with intractable epilepsy.

Magnetoencephalographic (MEG) discharges were recorded with multichannel superconducting quantum interference device (SQUID) gradiometers in 13 young candidates for epilepsy surgery. The sources of epileptic activity were related to generators of somatosensory and auditory evoked cortical responses and projected on magnetic resonance imaging (MRI) scans. Seven subjects had restricted or regional MEG foci, located in the frontoopercular (1), sensorimotor (3), perisylvian (1), mesiotemporal (1), or temporooccipital cortex (1). The MEG foci in the 3 patients who underwent operation agreed with the intracranial findings. Findings in the other patients emphasize the need to collect further data to define the ultimate role of MEG in preoperative evaluation of epilepsy.

A comparison of the localization of spontaneous neuromagnetic activity in the frequency and time domains.

We investigated the localization of current sources for spontaneous magnetoencephalographic data in the frequency and time domains. The two analysis techniques yielded complementary information about the underlying neuronal generators. Phase-coherent sources for occipital 10 Hz alpha band activity were identified in the frequency domain from the dependence of the equivalent current dipole strengths on the phase of the Fourier transform. Source localization in both the frequency and time domains was used to analyze data containing interictal spike and slow-wave activity. Predominantly 2-6 Hz spectral components localized in the frequency domain were found within an 8 cm3 volume centered at the time-domain source location of the spike. In general, the characteristics of the noise sources in magnetoencephalographic systems favor the use of frequency-domain analysis for rhythmic spontaneous activity.

Parietal epileptic mirror focus detected with a whole-head neuromagnetometer.

Whole-head magnetoencephalographic recordings revealed two parietal epileptic foci in homotopic areas of the hemispheres. The discharges occurred 17-20 ms later on the left than on the right hemisphere, implying the existence of a left-sided mirror focus. The foci were about 1 cm posterior to the hand primary somatosensory area, identified by evoked response measurements, and thus suggested epileptic activity at the parietal association cortex, in agreement with the observed callosal conduction time.

Sampling theory for neuromagnetic detector arrays.

The sampling theorem for wave-number-limited multivariable functions is applied to the problem of neuromagnetic field mapping. The wave-number spectrum and other relevant properties of these fields are estimated. A theory is derived for reconstructing neuromagnetic fields from measurements using sensor arrays which sample either the field component Bz perpendicular to the planar grid of measurement points, or the two components delta Bz/delta x and delta Bz/delta y of its gradient in the xy plane. The maximum sensor spacing consistent with a unique reconstruction is determined for both cases. It is shown that, when two orthogonal components of the gradient are measured at every site of the measurement grid, the density of these sensor-pair units can be reduced, without risk of aliasing, to half of what is necessary for single-channel sensors in an array sampling Bz alone. Thus the planar and axial gradiometer arrays are equivalent in the sampling sense provided that the number of independent measurements per unit area is equal.

Magnetoencephalographic localization of epileptic cortex--impact on surgical treatment.

A 24-channel, planar, superconducting quantum interference device gradiometer, sampling a fourth of the head surface over brain tissue, was used to determine the site of an epileptic focus in a 36-year-old woman with intractable complex partial epilepsy. The other presurgical findings appeared divergent: a large arachnoid cyst over the right parietal convexity, dissimilar interictal electroencephalographic patterns, and several neuropsychological dysfunctions. The equivalent current sources of magnetoencephalographic spikes were in the right posterior temporal region of the cortex, 4 cm apart from the cyst. Surgical exploration of the area pinpointed by magnetoencephalography revealed a pachygyric patch of cortex displaying focal discharges on the electrocorticogram. After resection, a dramatic reduction of seizures occurred. The good agreement between electrocorticography and magnetoencephalography warrants future investigation of multichannel magnetoencephalography as a potential alternative to invasive presurgical recordings.

Development of multichannel neuromagnetic instrumentation in Finland.

In this paper we describe the instrumentation for biomagnetic measurements available in our laboratory. The focus is on our 24-channel planar gradiometer system. In addition, a 122-channel system under construction will be discussed.