Biological Effects - RF/Microwave Interaction with Biological Tissues - page 103

only brain tissue and cerebral blood flow but also blood vessels. The recorded
brain tissue impedance changes of vascular pulsation are explained as an effect
not only on the cerebral blood flow and the wall of blood vessels but also on
the electrical conductivity of the tissue and the rheological characteristics of
the blood [53].
A nonlinear effect of modulated waves on chicken cerebral tissue has been
demonstrated. The samples were impregnated with radioactive 45Ca 2+ and
were exposed to 0.8 mW/cm -2 at 147 MHz, amplitude modulated by a sinusoidal
signal of 0.5-35 Hz. A statistically significant increase in net 45Ca 2+ transport
was observed for modulating frequencies of 6-16 Hz followed by a fall in the
range 20-35 Hz [54]. The existence of frequency windows has been confirmed
[55]: For an incident power flux of 1 mW cm -2 , there was a positive response
when the modulation lay between 6 and 12 Hz and little response at 0.5 and
20 Hz. A power window was also shown to exist at constant frequency: When
the chicken cerebral tissue was submitted to a 450-MHz carrier wave modu-
lated at 16 Hz there was a significant increase in 45Ca 2+ transport for power
levels of 0.1 and 1 mW cm -2 , while no effect was observed for power levels of
0.05 and 5 mW cm -2 . The limits of frequency and power windows seem to be
6-20 Hz and 0.1-1mW cm -2 , respectively [29]. The carrier frequency should
be less than 1 GHz, but itself has little effect. The optimum frequency appears
to lie between 150 and 450 MHz. For example, for a 450-MHz carrier modulated
by a 16-Hz sine wave with an average power density of 0.5 mW cm -2 , the ampli-
tude of the phenomenon reaches 38% of its value at rest in 10 min [13].
Low-level effects are thought to be due to the direct interaction between
neuron membranes and the local electric field. In the vicinity of a membrane,
the local field is not very different from its value in free space, that is, 61 and
194 V m -1 for 0.1 and 1 mW cm -2 , respectively. This field is negligible in com-
parison with the value of 2 ¥ 10 7 Vm -1 characterizing the static transmembrane
potential (90 mV across 4 nm) but is nevertheless large in comparison
with slow brain waves (1 V m -1 ) or with terrestrial ELF fields (from 10 -3 to
10 -6 Vm -1 ) picked up by fish, birds, and mammals and used by them for navi-
gation, detection of prey, and regulation of the circadian rhythm. One model
has examined the possibility of induced transmembrane potential on the order
of 10-100 mV [56]. Other studies [57] have shown that, because of the spike
or edge effect, the interface field may locally exceed the macroscopic field by
up to two orders of magnitude, which is sufficient to reduce by a factor of 10 2
the threshold of sensitivity to the EM flux. This phenomenon is confined to a
very narrow ELF modulation band and is believed unlikely to cause any
damage. The American National Standards Institute (ANSI) subcommittee in
charge of the revision of the American safety standards did not judge it nec-
essary to take this effect into account [13].
Frequency and intensity windows of Ca 2+ have been observed in the presence
of weak fields below 100 Hz, similar to the ELF-modulated RF fields [58, 59].
These findings suggest further interactions, resulting in specific EEG changes
that are affected by modulated RF fields. Furthermore, different effects of