Approved For Release 2000/08/08 : CIA-RDP96-00789R002200600001-1 measures can reduce the ambient noise by a factor of about 10°—equivalent to the internal SQUID noise. Because a MEG responds best to neuronal currents that are parallel to the skull (i.e., currents producing magnetic fields oriented tangentially to the skull), neuronal currents perpendicular to the skull may be missed. In reality, however, few neuronal electrical currents are exactly perpendicular to the skull, so some tangential component is almost always available to the SQUID. Searching for a closely packed group of neurons can be a slow and tedious process. Due to technological restraints, a maximum of seven sensors can be used simultaneously to gather MEG measurements. Sensors on a seven—channel MEG are located on a 2-cm equilateral triangular grid forming the center and vertices of a regular hexagon. A subject wears a spandex cap with grid marks lined up with his nasion, inion, and earlobes to serve as a head-centered coordinate system. To identify the location of a neuronal-equivalent current dipole, many measurements have to be taken. Isocontour maps of field strength are used to represent the amplitude and polarity distribution of the magnetic fields. A least-squares procedure is applied to the observed fields to estimate the location of neuronal sources and orientation of the equivalent current dipole.’ The estimated location of the neuronal source can then be identified anatomically with a magnetic resonance image scan of the head. Developments in technology may soon allow for enough channels to cover the whole head at once, thereby reducing data collection time and increasing precision. MEG technology is based on a cryogenic SQUID operating in liquid helium. Because the Dewar flask cannot exceed a 45-degree angle, subjects must lie prone beneath the apparatus. MEG sensors are not attached to the head, but are lowered into position over the skull; the subject cannot move his head during monitoring without disturbing the measurement. For these two reasons, MEG equipment is not suited for long~term monitoring of a subject. These problems may be solved in the near future as new technology, such as high-temperature SQUIDs, develops. A response from the MEG is a complex waveform consisting of a series of negative and positive peaks or components. Specific components of this waveform can be correlated with perceptual and cognitive processes. The most commonly observed response to a visual or auditory stimulus, for example, is a large component occurring approximately 100 ms after the onset of the stimulus. One hundred milliseconds appears to be the average latency period between stimulus and the first correlated neuronal activation in the brain.® The earlier EEG technology measures electric potential, or event-related potentials (ERPs) produced by the electrical activity of the brain. A MEG measures the magnetic fields, or event-related fields (ERFs) produced by the electrical activity of specific groups of active neurons in the cortex. An EEG and a MEG, therefore, reveal different aspects of the electrical activity of 3 Approved For Release 2000/08/08 : CIA-RDP96-00789R002200600001-1
Approved For Release 2000/08/08 : CIA-RDP96-00789R002200600001-1 the brain and are often used as complementary technologies. In some areas, however, the MEG technique has definite advantages over the EEG: (1) (2) (3) ERPs taken from the scalp provide little information regarding the precise three-dimensional distribution of the neuronal sites producing the electrical activity. Brain tissues of unknown electrical conductivity and thickness, individual variations in skull thickness and geometry, and proximity to openings in the skull all make obtaining such detailed information difficult. The same is not true when using a MEG. Neuronal magnetic fields can travel through brain tissues without being significantly altered; this property, coupled with the dipole model, results in high spatial resolution of the neuronal activity. EEG procedures are occasionally costly and can be invasive: EEG electrodes must be attached directly to the skull or to the brain of the subject, whereas MEG sensors are extracranial and are simply lowered into position against the skull. There is much controversy over the appropriate reference electrode in EEG work (a reference electrode is required with electric potential measurements, because only differences in electric potential are measured). There is no such problem with a MEG, because the measurement of magnetic fields is absolute. In a cooperative arrangement with Los Alamos National Laboratory (LANL), we have been provided access to a seven~channel MEG under the auspices of the Neuromagnetism Laboratory. Approved For Release 2000/08/08 : CIA-RDP96-00789R002200600001-1