Exposure to a theta-burst patterned magnetic field impairs memory acquisition and consolidation for contextual but not discrete conditioned fear in rats

Preceding or immediately following fear-conditioning rats were exposed for 30 min to either a sham field, one of two symmetrical (sine-wave 7, 20 Hz) magnetic fields or to one of two complex magnetic fields whose waveforms we re modeled after salient electrophysiological patterns within either the hi ppocampal formation (theta-burst) or the amygdaloid complex (burst-firing). The magnetic fields were presented in successive 2 s intervals through eac h of the th ree spatial planes and then simultaneously within all three pla nes. Field strengths ranged between 0.5 and 1 microTesla. Only the group ex posed after the conditioning to the theta-burst (hippocampal) magnetic fiel ds displayed evidence of forgetting, as inferred by their marked attenuatio n of freezing behavior, during contextual extinction 24 h later. This power ful treatment explained 75% of the variance in the extinction scores. Behav ioral responses to the discrete conditioned stimulus were not affected. The se findings are consistent with the involvement of the hippocampus in learn ed fear to contextual stimuli but not to discrete auditory stimuli and sugg est that physiologically relevant stimuli may be delivered to the brain by weak, complex magnetic fields whose intensities are ubiquitous within moder n environments. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.