Background
To assess the effects of stimulus context on the P300 component, an eight-orientation Landolt ring task was introduced. As the stimulus context of this task differs from the traditional two-stimulus oddball paradigm, the purpose here was to apply EEG/fMRI integration to investigate the localization and activities of the P300 generators involved with this task.
Material and Method
Ten healthy subjects performed the visual P300 task while fMRI and 64-channel ERP data were acquired. The voltage topographical maps of the P300 component were calculated and analyzed for the main activation foci. Furthermore, constraints from fMRI were used to identify the source activities of visual P300 ERP.
Results
Analysis of the hemodynamic response to the visual target stimuli revealed a distributed network of neural sources in the bilateral parietal lobules, middle and inferior frontal gyrus, precentral and postcentral cortex, and anterior cingulate gyrus. The analysis particularly showed preponderant activations of the bilateral superior parietal lobules. In this target detection design, two distinct P300 peaks were observed in the dipole waveforms, the bilateral prefrontal and the right inferior parietal dipole waveforms displayed the higher peak at short latency, while the four parietal, the anterior cingulate, and the temporal dipole waveforms had the higher peak at long latency.
Conclusions
Compared with the classical oddball paradigm, the amplitude decreased in this study, which might be related to its particular stimulus context. The source technique was utilized to yield a realistic 11-dipole model and distinguish the anatomical generators of early and late components of the P300 response.

Keywords: Photic Stimulation, Magnetic Resonance Imaging, Event-Related Potentials, P300 - physiology, Adolescent