Hippocampal involvement in detection of deviant auditory and visual stimuli
Recent models of hippocampal function have emphasized its role in processing sequences of events. In this study, we used an oddball task to investigate hippocampal responses to the detection of deviant "target" stimuli that were embedded in a sequence of repetitive "standard" stimuli. Evidence from intracranial event-related potential studies has suggested a critical role for the hippocampus in oddball tasks. However, functional neuroimaging experiments have failed to detect activation in the hippocampus in response to deviant stimuli. Our study aimed to resolve this discrepancy by using a novel functional magnetic resonance imaging (fMRI) technique that drastically improves signal detection in the hippocampus. Significant hippocampal activation was observed during both auditory and visual oddball tasks. Although there was no difference in the overall level of hippocampal activation in the two modalities, significant modality differences in the profile of activation along the long axis of the hippocampus were observed. In both left and right hippocampi, an anterior-to-posterior gradient in the activation (anterior to posterior) was observed during the auditory oddball task, whereas a posterior-to-anterior gradient (posterior to anterior) was observed during the visual oddball task. These results indicate that the hippocampus is involved in the detection of deviant stimuli regardless of stimulus modality, and that there are prominent modality differences along the long axis of the hippocampus. The implications of our findings for understanding hippocampal involvement in processing sequences of events are discussed.