Abstract
Magnetoencephalograpy (MEG) is a noninvasive technique that allows recording the magnetic fields produced by brain activity. SQUID (Superconducting QUantum Interference Device) sensors immersed in liquid helium at 4.2 K are used to detect the extremely weak brain magnetic signals. MEG provides an excellent temporal resolution, orders of magnitude better than in other methods for measuring cerebral activity, as magnetic resonance imaging, single-photon-emission computed tomography and positron-emission tomography (Hämäläinen, Hari, Ilmoniemi, Knuutila, & Lounasmaa, 1993). A good spatial resolution is also provided, although this depends on the source configuration (Hari & Forss, 1999). Moreover, this technique is independent of any reference point. In addition, magnetic fields are not distorted by the resistive properties of the skull (Hämäläinen et al., 1993). Nevertheless, the recordings are very sensitive to external artifacts. Thus, the signals must be acquired in a magnetically shielded room. In this preliminary study, we examined the MEG background activity in patients with probable AD, and in age-matched control subjects using auto mutual information (AMI). Our purpose is to test the hypothesis that an abnormal type of nonlinear dynamics is associated with AD.
Key Terms in this Chapter
Auto Mutual Information (AMI): Mutual information between a signal and its time-delayed version.
Alzheimer’s Disease (AD): Progressive, degenerative, and irreversible brain disorder characterized by the appearance of neuritic plaques and neurofibrillary tangles.
Electroencephalogram (EEG): Neurophysiologic recording of the brain electrical activity by electrodes placed on the scalp.
Mutual Information (MI): Amount of information gained about one signal from the measurement of another.
Signal Processing: Extraction of information from complex signals, generally by conversion of the signals into digital form followed by analysis using different algorithms.
Nonlinear Analysis: Broad, interdisciplinary field characterized by a mixture of analysis, topology, and geometry.
Magnetoencephalogram (MEG): Neurophysiologic recording of the magnetic fields produced by brain activity using extremely sensitive devices called SQUIDs.