Surgery – Diagnostic testing – Detecting brain electric signal
Reexamination Certificate
1999-04-01
2001-07-03
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Detecting brain electric signal
Reexamination Certificate
active
06256531
ABSTRACT:
The present invention relates to a method for performing brain research of cortical connections and reactivity by virtue of stimulating selected points on the cerebral cortex and, using EEG techniques applied externally to the head, then measuring the distribution of electrical activation evoked by the stimulation.
The invention also concerns an apparatus for research of cortical connections and brain reactivity.
With the help of at least one coil placed on the head, the cerebral cortex can be stimulated without health risks and pain by applying a strong magnetic field, advantageously with a duration of 50-500 &mgr;s, that induces an electric current at a desired point. Next, the activated nerve cells transmit along their axons a signal burst to those areas of the brain and peripheral nervous system that have a connection with the stimulated areas. Under the activation, the cells of such areas in turn give rise to electric currents that can be monitored using EEG electrodes placed on the head.
Today, the structure and condition of the brain can be examined by means of CT and MRI imaging equipment, for instance. However, these methods can give information on the condition of cortical connections in very distinct cases only, e.g., when a brain tumor or cerebral infarct has caused damage in the tissue. While EEG and MEG are suited for examination of reactivity of sensory areas to sensory stimuli provided that the peripheral nerve tracts are still functional, the use of these methods in the determination of reactivity to sensory stimuli in other areas of the brain is difficult.
As known from conventional technology, biological tissue and other conductive media can be excited by applying thereon an electromagnetic field composed of an electric field E and a magnetic field B; this fact is utilized in the stimulation of a plurality of different tissues such as the brain, peripheral nervous system and the heart by means of an electric field. Also conventionally, a suitable electromagnetic field can be induced using a coil placed on the object, whereby an alternating electric current fed to the coil induces an alternating magnetic field which further gives rise to an electric field in the object. An alternative method of imposing an electric field on a tissue is to feed current into the organism via electrodes placed on the skin.
As known from conventional techniques, the brain can be stimulated by inducing with the help of a set of coils placed externally to the head, [Ilmoniemi and Grandori 1993, FI Pat. No. 934,511] a strong rapidly changing magnetic field in the brain, whereby the magnetic field induces an electric field in the brain. By virtue of known techniques, the overall effect of the coil set can be focused on an area as small as a few cm
2
, and using computerized multichannel equipment, the focus of the applied field can be shifted steplessly by altering the relative amplitudes of coil currents in relationship to each other.
Also known in the art are methods in which the electrical activity of the brain is measured in a single-channel system by means of electrodes attached externally to the head, whereby the method is denoted as EEG, or alternatively, directly on the cerebral cortex during an operation, whereby the recording is called an electrocorticogram. The measurement may also be implemented using multichannel equipment, typically comprising 32-128 channels. Then, the electrical activity of the brain can be localized with an accuracy as good as about 5-10 mm.
With modern methods, the electric field used for evoking a response can be inflicted on the brain by subjecting the target area to a magnetic field B(r,t) varying as a function of time t by means of placing a coiled conductor (refer to
FIGS. 1
a
and
2
a
) close to the target and then feeding the coil with a current, typically of a pulsed waveform, produced by discharging the energy of, e.g., a charged capacitor, whereby an electric field is induced in the tissue in accordance with Maxwell's equations. If the structure of the object and the conductivities of its different parts (such as the skull and the brain) are known, the electric field induced by the current passing in the coil can be computed as a function of the position coordinates of the object. In the art are also known other methods suitable for positioning one or more coils about the head and then feeding an electric current to the coil or coils so that an electric field with a precisely defined electric field is resultingly induced in the object.
One of the problems hampering prior-art methods and equipment in certain applications is that the effects of brain stimulation can be recorded without artefacts only peripherally by measuring, e.g., muscular responses or observing the behavioral response of the tested person. In fact, only qualitative results can be obtained from evoked-response EEG results measured and interpreted by conventional techniques in conjunction with electromagnetic stimulation.
A further problem of prior-art techniques and equipment is that the cortical activity evoked by electromagnetic stimulation cannot be localized.
Still another problem of certain methods and equipment of the prior art is that the effect of cortical stimulation can be measured by EEG techniques only after tens or hundreds of milliseconds after the stimulus pulse.
The present invention is based on
1) stimulating selected areas of the cerebral cortex by magnetic or electrical means,
2) measuring the electrical activation of the brain as a function of time using multichannel equipment such as EEG or MEG, and
3) localizing with the help of multichannel techniques the originating loci of signals detected as a result of the electrical activation of the brain, or alternatively, focusing the sensitivity maximum of said electrical multichannel measurement on a certain location or locations.
In the context of the present invention, the term localization refers to a mono- or multi-dipole localization or to the computation of minimum-norm estimates or other current distributions estimating the location of diffuse source current patterns.
If a localization listed in item 3) above indicates that the brain is activated in an area B after the stimulation of a brain area A, it is obvious that a nerve cell connection exists from area A to area B. The magnitude of the stimulated activity, which can be determined in an approximating manner during localization, is a measure of the reactivity of area B for the stimulation of area A. In the case that the areas A and B are separate (section of areas A and B being essentially zero), the phenomenon is called secondary reactivity, whereby the evoked response in area B occurs with a distinct delay after the onset of the stimulus, since the velocities of conduction and intersynaptic delays in the transmission from one nerve cell to another are relatively slow phenomena. Typically the delays between different cortical areas vary from a few milliseconds to tens of milliseconds depending on the interarea distances and the type of cortical connections whether direct or formed by a chain of several nerve cells. If the areas A and B are practically the same area or B is included in A, the method gives the reactivity of the stimulated area to the stimulus. Then, the measured variable is called the primary reactivity.
In addition to primary and secondary reactivities, other changes in the spontaneous activity of the brain due to stimulation may be detected. When a conventional technique is used based on applying a nervous stimulus such as a sound on the test person, the stimulated response is generally called event-dependent desynchronization, wherein the strong brain waves resulting from the synchronous excitations of nerve cells in a rest state of the brain are attenuated due to the applied stimulus. Also other changes can be seen different from those of desynchronization. For instance, the frequency of rhythmic activity in the brain may vary or the position distribution of the activity may change. In the context of the pres
Ilmoniemi Risto
Karhu Jari
Ruohonen Jarmo
Virtanen Juha
Nasser Robert L.
Smith-Hill and Bedell
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