Detection of magnetic evoked fields associated with synchronous population activities in the transverse CA1 slice of the guinea pig.

1. Magnetic evoked fields (MEFs) associated with synchronous population activities in the transverse CA1 of guinea pig (approximately 1.2 mm3) were characterized with a high-resolution superconducting magnetic field detector. 2. Electrical stimulations of the stratum radiatum with a pair of bipolar electrodes produced synchronous population spikes in the field potential and a strong MEF (as much as 3.5 picoteslas 3 mm above tissue) in the presence of 0.05 mM picrotoxin. The MEF's spatial pattern and direction indicated currents in pyramidal cells as its source. Bath application of kynurenic acid extinguished most of the signals, indicating a strong contribution of post-synaptic currents to the field. The kynurenic acid-insensitive component was abolished by tetrodotoxin, indicating the remaining component was neuronal in origin as well. 3. It may be possible to refine our technique to study the genesis of MEG signals--i.e., the relationship between evoked fields and the underlying neuronal currents--in a mammalian CNS structure, since the electrophysiology of the hippocampus is well understood.