MAGNETIC-FIELDS FROM HUMAN PREFRONTAL CORTEX DIFFER DURING 2 RECOGNITION TASKS

The present study represents our second successful use of magnetoencep halography to identify different sources of human prefrontal activity corresponding to subjects' engagement in different tasks. We used two visual recognition tasks: a familiar person recognition and an abstrac t pattern recognition task in the context of a design suitable for eli citing Contingent Negative Variations (CNVs) and their concurrent slow magnetic fields in this preliminary study of 5 subjects. Each trial o f either task was started by one of two specific warning symbols (SI), indicating whether a person's picture or an abstract pattern should b e attended during the presentation of a second stimulus (S2), and comp ared to the corresponding person's picture or pattern contained in the third stimulus, (S3) that followed. The S2 and S3 stimuli were common to both tasks, and were composed of patterns made with four line trac es superimposed on photographs of persons familiar to each subject. Su bjects responded with a right hand button press, following S3, indicat ing their judgments regarding the identity of the patterns or persons' pictures contained in the S2 and the S3 stimuli, for the two tasks, r espectively. Results showed that the sources of the CNV equivalent mag netic fields were localized in different cortical regions depending on the task and that this difference was consistent across all subjects. The sources were localized in the right hemisphere, in medial areas o f the prefrontal cortex for the person recognition task and in the dor solateral prefrontal cortex for the pattern recognition task. The same degree of consistency was not found for the left hemisphere sources. Moreover, as in our previous study, we found no difference between the sources active during the first and the second CNV periods (occurring during the S1-S2 and the S2-S3 intervals, respectively), within each task condition. (C) 1997 Elsevier Science B.V.