Equivalent current dipole of word repetition effects in patients with obsessive-compulsive disorder

We investigated cortical source localization of word repetition effects in patients with obsessive-compulsive disorder (OCD) by employing the equivalent current dipole (ECD) model with high-density 128 channels EEG and individual MRI as a realistic head model. Twelve OCD patients and 13 healthy control subjects performed a word/nonword discrimination task, in which the words and nonwords were visually presented, and some of the words appeared twice with a lag of one or five items. During the 200-500 ms post-stimulus period, the control group showed more positivity in the ERPs elicited by old words than in those elicited by new words, whereas the OCD patients did not. Furthermore, the OCD patients showed prolonged response times to the old words, as compared to the controls. We calculated the location and the power of the ECD sources at approximately 400 ms post-stimulus with the peak mean global field potentials. In both groups, the sources of word repetition effects were determined to be located in the inferior frontal gyrus. The right ECD powers of the ERP generators elicited by the new words were significantly higher in the OCD patients than in the control subjects. The OCD patients also exhibited significant alterations in the hemispheric asymmetry of ECD power during the processing of new words. These results suggest that OCD patients suffer from the encoding deficits in word processing, particularly in the left hemisphere.