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We tested the effects of electromagnetic radio frequency (RF) signals having a carrier frequency of 900 MHz, unmodulated and pulse modulated at 217 Hz with a duty cycle of 12.5% and a power density of 0.1 mW/cm2 because this signal is similar to that used by the GSM (global system for mobile communication) telephone system. The test subjects were 34 adult zebra finches (Taenopygia guttata), anesthetized with a mixture of ketamine (0.05 mg/g) and xylazine (0.01 mg/g) injected i.m. into the pectoralis major. The anesthetized bird was mounted in a nonconducting plastic holder. The bird and the holder were placed inside a tuned RF cavity (23.5 cm diameter, 100.5 cm long) made of perforated metal. The cavity was fitted with two tuned RF stubs (each 23.5 cm from opposite ends): one for emitting the signal and one for monitoring the frequency and power of the signal within the cavity. To record from neurons in the brain of the bird, a small hole (4 mm diameter) was made through the skull. A glass microelectrode (tip diameter 1–2 µm) filled with a conducting solution physiological saline was slowly advanced into the brain through this hole until a spontaneously active nerve cell was detected. A silver reference electrode was inserted beneath the skin along the back of the head directly behind the glass microelectrode to complete the circuit. Arranging the electrodes along the long axis of the cavity prevented them from acting as an antenna and electrically stimulating the cells. Once a spontaneously active cell was located, it was tested with the stimulus. The protocol for all the testing procedures was a 10 min prestimulus period, a 10 min stimulus period, and a 10 min poststimulus period. The rates of the cell's activity during these three time intervals were compared to detect any effect of the stimulation.
We recorded 133 spontaneously active units
from 34 anesthetized adult zebra finches; 91 units (69%) showed some
response to the stimulation: 69 (52%) responded with excitation (Fig. 1A)
and 22 (17%) responded with inhibition (Fig. 1B). The remaining 42 (31%)
cells showed no discernible response. The cells showing excitation responded
with increases in their rate of firing to the stimulation (mean rate during
stimulation = 3.5 ± 0.30 [SE] times prestimulus rate). Most of the
inhibitory responses were small (mean rate during stimulation = 0.4 ± 0.07
times prestimulus rate), in part because the cells were firing slowly before
the stimulation. Two of the cells showing inhibition exhibited marked
depression in their rates of spontaneous activity (Fig. 1B). All responses
we recorded were to power densities of 0.1 mW/cm2 and stronger (up to 0.5 mW/cm2).
The mean latency from the initiation of the stimulus to the start of the
response was 104 (± 197) sec, with the response lasting beyond the end of
the stimulus period in half of the responding cells. The mean persistence
beyond the end of stimulation was 308 (± 68) sec, but there was no
correlation (r = 0.489, P > 0.05) between the latency of the response and
how long the cell continued responding beyond the end of the stimulus.
One concern is that the electrodes themselves were acting as an antenna and stimulating the cells electrically. The arrangement of the active and reference electrode along the long axis of the waveguide chamber prevented them from serving as a loop antenna. In preliminary experiments we varied the positions of the electrodes to determine whether they could, in fact, act as an antenna. When the electrodes were not aligned, the stimulus artifact was detected directly and observed on the oscilloscope display. Whether such a stimulus was strong enough to stimulate the cells is unknown. A second factor that supports the idea that the cells were not stimulated electrically is that not all cells responded to the stimulus, even those in the close neighbourhood of a responding cell. This speaks clearly against an artifact.
These high frequency RF fields produced a response in many types of neurons in the avian Central Nervous System (in both cerebellum and cerebrum) and did not appear to be limited to any specialized receptor. Similar responses (long latency and ongoing higher activity after cessation of the fields) also were reported to a 52 GHz carriar, 16Hz modulated signal (Semm er al., unpubl. data). Thus, the effect does not appear to be limited to magnetic sensory cells , but may occur in any part of the brain. The stimulus might mimic a natural mechanism involved in cell communication, producing responses from many different types of neurons. It is unlikely that the effects we observed are the result of thermal excitation caused by the RF radiation because the power densities we applied were 2 to 3 orders of magnitude below what is required (10 mW/cm2) to produce heating of even 0.5° C (Bernhardt 1992). Consequently, we conclude that the effects we observed are not the result of thermal agitation but at this point we cannot offer an athermal mechanism to account for the observations.
Although individual neurons in the zebra finch brain responded to the pulsed RF stimulus, we do not know whether these responses by the nervous system are manifested in the bird's behavior or its health. Bruderer and coworkers [4, 5] reported no behavioral responses of birds to pulsed or continuous RF microwave signals, but their studies involved different frequencies and lower power densities of the stimulus. Whether similar neuronal responses occur in mammals, including humans, requires further investigation. Borbély and coworkers  reported that exposure to a RF signal similar to the one we used influenced sleep and sleep electroencephalogram in humans. Their results and the responses we recorded clearly indicate the potential for effects on the human nervous system.
We gratefully acknowledge financial
support of the Deutsche Telekom and the Geneseo Foundation. Technical
assistance and the loan of equipment were provided by the Deutche Telekom.