Regularity Analysis of the Magnetoencephalogram Background Activity in Alzheimer's Disease Patients Using Auto Mutual Information

Alzheimer’s disease (AD) is the most common form of dementia (Bird, 2001), a group of conditions that gradually destroys brain cells and leads to progressive decline in mental function. This irreversible brain disorder is characterized by neuronal loss and the appearance of neuritic plaques containing amyloid-β-peptide and neurofibrillary tangles (Cummings, Pike, Shankle, & Cotman, 1996). Approximately 50–60% of patients with dementia over 65 years are clinically related to AD and the number of patients is expected to increase continuously (Lahiri, Farlow, Greig, & Sambamurti, 2002). A differential diagnosis with other types of dementia and with major depression is used. It can include Mini-Mental Statues Examination (MMSE) (Folstein, Folstein, & McHugh, 1975), magnetic resonance imaging, computerized axial tomography, positron emission tomography, and verbal tests. Nevertheless, a definite diagnosis of AD is only possible by necropsy. In order to complete the diagnosis, nonlinear analysis of the brain recordings might be used. 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.