Slow magnetic flux from human frontal cortex.

Slow magnetic fields concurrent with two successive contingent negative variations (CNVs) were elicited in 8 subjects during visual recognition tasks involving pattern versus place discrimination. All stimuli were presented as a rectangular array of lights with various patterns of 6 lights at the center and, simultaneously, with places indicated by missing lights at the periphery. One of two possible stimuli (warning) started each trial, indicating whether pattern or place recognition should be performed on the following two stimuli. The purposes of the experiment were to localize the sources of the slow magnetic fields equivalent to the CNVs and to address the issue of regional specialization of prefrontal cortical function. Results indicated that the equivalent current dipoles (ECDs) found as solutions for the measured slow fields were indeed localized in the prefrontal cortex of each hemisphere. Also, in the right hemisphere, the source location of the CNVs was dependent on task, which supported the hypothesis of specialization of prefrontal function. The place recognition task was associated with more anterior and inferior CNV sources than the pattern recognition task. Finally, it was observed that ECDs for the warning period CNVs were indistinguishable from those for the test period of the tasks.