Biological Effects - RF/Microwave Interaction with Biological Tissues - page 102

when the whole-body average SAR was at a constant level (7300 W kg -1 ). A
single microwave pulse could evoke body movements. The lowest whole-body
specific absorption (SA) tested was 0.18 kJ kg -1 , and the corresponding brain
SA was 0.29 kJ kg -1 . Bulk heating potentials of these SAs were less than 0.1°C.
For doses higher than 0.9 kJ kg -1 , the response incidence was also proportional
to subcutaneous temperature increment and subcutaneous heating rate. The
extrapolated absolute thresholds (0% incidence) were 1.21°C temperature
increment and 0.24°C s -1 heating rate. Due to high subcutaneous heating rates,
the mouse must have perceived the radiation as an intense thermal sensation
but not a pain sensation. The temperature increment was indeed well below
the threshold for thermal pain. Results of the study could be considered in
promulgation of personnel protection guideline against high-peak-power but
low- to average-power microwaves.
It has been observed that rats acutely exposed (45 min) to pulsed 2.45 GHz
(2-ms pulses, 500 pps, power density 1 mW cm -2 , average whole-body SAR
0.6Wkg -1 ) showed retarded learning while performing in the radial-arm maze
to obtain food rewards [49]. Deficits in memory functions, even transient ones,
can lead to serious detrimental consequences. A new series of experiments was
carried out to better understand this behavioral effect of microwaves and,
especially, the underlying neural mechanisms involved [50]. It was shown
that this behavioral deficit was reversed by treatment, before exposure, with
the cholinergic agonist physostigmine or the opiate antagonist naltrexone,
whereas pretreatment with the peripheral opiate antagonist naloxone methio-
dide showed no reversal of effect. These data indicate that both cholinergic
and endogenous opioid neurotransmitter systems in the brain are involved in
the microwave-induced spatial memory deficit. This would imply that reversal
of cholinergic activity in the hippocampus alone is sufficient to reverse the
behavioral deficit.
The effects of absorption of EM energy, when modulated, on brain tissue
and cell membranes depend on the frequency and type of modulation. They
appear to be especially important at frequency and amplitude modulation
below 300 Hz , although few results are available which illustrate a definite
effect. These modulation frequencies are comparable to the EEG wave fre-
quency spectra. Electrical coupling of both frequencies is however unlikely
[51]. Whole-body irradiation at 30 mW cm -2 (SAR 25 W kg -1 ) of rats caused an
increase in EEG activity (sum of EEG wave frequency bands) immediately at
the first stage after exposure and a slight increase in delta waves (0.5-4 Hz) of
the EEG recordings [52]. Simultaneously, the rheoencephalogram increased,
but there was no significant change in heart rate. A 10 mW cm -2 exposure with
SAR = 8.4Wkg -1 , however, did not cause change in any frequency bands of
the EEG. It produced a slight temperature rise in the brain. The rheoen-
cephalogram amplitude increased after both exposure levels as a consequence
of the increase of cerebral blood flow. The ECG records and heart rate did
not show any change after the radiation exposure. Brain localized exposure
showed that microwave field interactions affect the electrical activity of not