Functions of the human hippocampal formation: episodic memory or more?

Grunwald, T.1,2, Kurthen, M.2, and Elger, C. E.2
1Swiss Epilepsy Center, Zurich, Switzerland; 2Clinic of Epileptology, Bonn University Medical Center, Germany

The importance of the human hippocampal formation for episodic and declarative memory has been known for a long time. Using ERP recordings from depth electrodes in epilepsy patients undergoing presurgical evaluation we could show that this medial temporal lobe structure contributes to the detection of associative (situational) novelty thus mediating encoding for declarative memory. This hippocampal function depends on NMDA-receptors and seems to be related to hippocampal synaptic plasticity (long-term potentiation). However, we also found that the anterior parahippocampal region is also sensitive for word repetitions that are not consciously recognized. Second, we found that syntactically incorrect sentences elicited pronounced negative ERP responses peaking between 500 and 600 ms within the hippocampus proper. And third, we recently recorded depths ERPs within the hippocampus proper (but not the anterior parahippocampal region) that reliably differentiated between meaningful and meaningless visual objects thus indicating that the human hippocampus also contributes to semantic visual object processing. In sum, we take our data to suggest that the human hippocampal formation is not only involved in encoding for and recalling from episodic memory but may also contribute to other cognitive processes that depend on implicit or explicit knowledge.