Neural repetition suppression effects in the human hippocampus

Neurons in the temporal lobe cortex exhibit reduced responses when a stimulus or a stimulus feature is repeated. This phenomenon, termed "repetition suppression", is the basis for many functional imaging studies that have used Blood Oxygenation Level Dependent (BOLD) activity differences between novel and repeated items as an index of neural selectivity in hippocampal subfields. However, it is not clear how hippocampal neural activity changes across repeated exposure to a stimulus. Here, we used direct intracranial electroencephalography (iEEG) recordings of hippocampal activity to examine whether neural activity in the human hippocampus is modulated across successive repetitions of an item. Time-frequency analyses revealed that high-frequency activity, which is thought to include gamma oscillations and possible correlates of multi-unit activity, declined monotonically across successive presentations of an item. In contrast, low-frequency oscillations in the alpha and beta bands monotonically increased across successive presentations of an object. These results provide support for the assumption that, at least under some circumstances, repetition suppression (as measured by declines in high frequency activity) can be observed in the hippocampus, and these effects are accompanied by increases in low frequency oscillations as well.