In support of an effect, Thomas et al. (1986) and Liboff et al. (1989) reported a temporary loss of stable baseline performance on a component of the multiple fixed-ratio-differential low-rate schedule dealing with differential reinforcement of low rates of responding. This loss followed a 30-min exposure to a combination of a static magnetic field at 26.1 µT and a 60-Hz linearly polarized magnetic field at 0.139 µT. Once again a decrease in performance accuracy on this task does not imply a deleterious effect of magnetic-field exposure and might be more in line with a detection or perception of the field.

To assess the potential aversion quality of 60-Hz magnetic fields, Lovely et al. (1992) tested the preference or aversion to 60-Hz magnetic fields at 3.03 mT in a shuttle box. In two sequential studies using the appropriate control and sham conditions, animals did not prefer or avoid the exposed chamber. The authors discussed their results in relation to significant responses observed with large 60-Hz electric fields. They suggested that the lack of aversion in these magnetic-field experiments indicates that aversive behavior produced by electric fields might be associated with body-surface interactions rather than internal-body currents resulting from electric-field exposure.

Summary of Neurobehavioral Effects

Mammals clearly can detect 60-Hz electric fields at relatively modest field strengths (above a few kilovolts per meter). However, the effect of electric fields, even at field strengths an order of magnitude higher, is not perceived as aversive. Further, the action of the field appears to be mediated primarily through the stimulation of the receptors and the skin through hair movement or vibration rather than through the direct interaction with neuronal membranes.

Even though little evidence exists showing that 60-Hz magnetic fields can be detected by animals, at the highest field strengths where rats appear to detect such fields (3 mT at 60 Hz), they do not produce an avoidance behavior. In addition, no general adverse health effects are detectable for field exposures, as measured behaviorally, chemically, or pathologically. However, repeated studies have reported behavioral, chemical, and electrophysiologic effects of long-term and short-term exposure to 60-Hz magnetic fields. These effects include a decrease in stable baseline performance on multiple-operant schedules dealing with reinforced behavior, on the one hand, and a suppression or decrease in induced-seizure duration, on the other hand. Both of those effects could be linked hypothetically by reports that 60-Hz magnetic fields inhibit endogenous opiate activity. A decrease in opiate activity could decrease the reinforcing properties of stimuli and exogenous opiates are known to enhance seizures. Thus, a decrease in endogenous opiates might inhibit seizures.

The underlying biologic mechanisms that mediate these effects are not known, but the results present interesting biologic questions that might or might not be construed as being health related. No link has been made between the

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