RESPONSE-RELATED AND STIMULUS-RELATED ERP ASYMMETRIES IN A TONAL ODDBALL TASK - A LAPLACIAN ANALYSIS

Previous studies have found greater P3 amplitude over right than left hemisphere sites in a tonal oddball task with a reaction time (RT) res ponse. This asymmetry had a central topography, and interacted with re sponse hand. identification of the processes underlying these asymmetr ies requires the use of additional methods for separating response-and stimulus-related contributions. We applied local Hjorth and spherical spline algorithms to compute surface Laplacian topographies of ERP da ta recorded from 30 scalp electrodes in a pooled sample of 46 right-ha nded healthy adults. For both methods, the current sources underlying the late positive complex were largest at medial parietal regions, but were asymmetric at central and frontocentral sites. Although a fronto central sink contralateral to the response hand contributed to the asy mmetry of the classic P3 peak, the source asymmetry was most robust af ter the sink had resolved. The late source was largest at electrode C4 for right hand responses, and was further enhanced in subjects showin g a dichotic left ear advantage, but was unrelated to response speed. We conclude that the right hemisphere source reflects an interaction o f response-related asymmetries with right hemisphere processes respons ible for pitch discrimination.