The electroencephalogram (EEG) makes a scalp recording of electrical activity, or brain waves, emitted by nerve cells from the cortex of the brain. The EEG has different "bands", defined by the frequency of the waves; delta (slow) waves are less than 4 Hz; the theta bands are 4-8 Hz, the alpha from 8 to 12 Hz, the beta from about 14-30 Hz and the gamma from 30-80 Hz. The alpha bands are best seen in the parieto-occipital area, and the beta bands are usually more prominent in the frontal and central regions. These bands, when simultaneously recorded, differ from each other and reflect different cognitive processes. The alpha rhythm is best seen when the subject is awake and relaxed, with eyes closed (Emerson, 1995), and beta waves during the REM stage of sleep (see later in the section on EEG studies and sleep). Brain electrical activity is also characterized by the amplitude or power of the oscillations. An increase is called synchronization whereas a decrease in amplitude is called desynchronization. Event related desynchronization/ synchronization (ERD/ERS) stands for a technique in which the power of a specific EEG frequency band is expressed as the relative change in power between two experimental conditions. It is a within-subject measure of relative changes in power between two experimental conditions and is expressed as a percentage (Krause et al., 2004).
Cook and colleagues (2006) comment that EEG and similar methods can be more easily applied to volunteers than brain imaging methods, since there is no ionizing radiation and no strong magnetic fields. However, interference can arise from applied ELF and RF fields. They state: "The EEG electrodes and leads can act as antennas that can a) inject current into the subject's scalp and b) induce potentials on the EEG leads which have significantly greater amplitude than the brain signals being measured. Hence reliable measurements during exposure are almost impossible".
Cook and colleagues (2006) comment that EEG and similar methods can be more easily applied to volunteers than brain imaging methods, since there is no ionizing radiation and no strong magnetic fields. However, interference can arise from applied ELF and RF fields. They state: "The EEG electrodes and leads can act as antennas that can a) inject current into the subject's scalp and b) induce potentials on the EEG leads which have significantly greater amplitude than the brain signals being measured. Hence reliable measurements during exposure are almost impossible".
Another method is the magnetoencephalogram (MEG), which offers better spatial resolution than the EEG, but disadvantages are that the brain magnetic filed activity is very weak and the MEG is extremely sensitive to external noise.
Some EEG studies have been done while the subjects are awake and resting (Table 1). Reiser (1995) reported a change in EEG tracings on exposure to 900 MHz radiation, but others have stated that similar changes can be seen when the level of awareness is altered. Roschke and Mann (1996) found no changes in healthy male volunteers exposed to 900 MHz, and Hietanen and colleagues (2000) found no effects on EEGs from exposure to different cell phones, using both 900 and 1800 MHz. Huber (2002) found changes in the alpha range during pulse-modulated exposure, but not with continuous wave exposure. Regel (2007) had similar findings, 30 minutes after pulse-modulated exposure, but not with continuous exposure. Croft (2002) found that EMF exposure decreased 1-4 Hz activity in right hemisphere sites, and was associated with increasing 8-12 Hz activity as a function of exposure duration in the midline posterior sites. Cook (2004) suggested that 30% of the variation in alpha activity seen in their study were due to the pulsed magnetic field exposure. Kramareko (2003) used a telemetric EEG, and found that within 20-40 seconds of exposure to a 900 MHz phone signal subjects showed slow-wave activity in the contralateral frontal and temporal areas. They lasted for one second and repeated every 15-20 seconds. When the signal was stopped the slow waves progressively disappeared in the next 10 minutes
Some EEG studies have been done while the subjects are awake and resting (Table 1). Reiser (1995) reported a change in EEG tracings on exposure to 900 MHz radiation, but others have stated that similar changes can be seen when the level of awareness is altered. Roschke and Mann (1996) found no changes in healthy male volunteers exposed to 900 MHz, and Hietanen and colleagues (2000) found no effects on EEGs from exposure to different cell phones, using both 900 and 1800 MHz. Huber (2002) found changes in the alpha range during pulse-modulated exposure, but not with continuous wave exposure. Regel (2007) had similar findings, 30 minutes after pulse-modulated exposure, but not with continuous exposure. Croft (2002) found that EMF exposure decreased 1-4 Hz activity in right hemisphere sites, and was associated with increasing 8-12 Hz activity as a function of exposure duration in the midline posterior sites. Cook (2004) suggested that 30% of the variation in alpha activity seen in their study were due to the pulsed magnetic field exposure. Kramareko (2003) used a telemetric EEG, and found that within 20-40 seconds of exposure to a 900 MHz phone signal subjects showed slow-wave activity in the contralateral frontal and temporal areas. They lasted for one second and repeated every 15-20 seconds. When the signal was stopped the slow waves progressively disappeared in the next 10 minutes
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