TASK-SPECIFIC MAGNETIC-FIELDS FROM THE LEFT HUMAN FRONTAL-CORTEX

In this study we attempted to extend our previous results on regional specialization of frontal cortical function in humans,by means of magn etoencephalography (MEG). We used a verbal task and predicted that som e part of the left frontal lobe would be active during engagement in t hat task, since the left hemisphere is known to be implicated in langu age. We did not require a motor response because in previous experimen ts we observed bilateral frontal magnetic activity, and we suspected t hat it was due to the addition of movement-related fields to our recor dings. Six right handed subjects (three males and three females) parti cipated in the study. The task consisted in silently counting the numb er of word pairs that matched with respect to semantic category. Exper imental runs were composed by series of 120 trials or word pairs. All six subjects presented dipolar magnetic field distributions on the lef t fronto-temporal area of the scalp, but not on the right, during diff erent portions of the trial duration. These fields were successfully m odeled as equivalent current dipoles (ECDs). The spatial ECD coordinat es were translated onto magnetic resonance image (MRI) coordinates for each subject. The dipole positions were typically near the cortical s urface corresponding to areas 6 and 44 of Brodmann No dipole-like sour ces were observed in the right frontal lobe.