IDENTIFICATION OF SPATIALLY-FILTERED VISUAL OBJECTS: THE HIPPOCAMPUS DISTINGUISHES BETWEEN IDENTIFIED AND UNIDENTIFIED OBJECTS

Recent evidence suggests that the medial temporal lobe does not only mediate explicit episodic memory formation but may also participate in visual object processing. Here, we recorded intracranial event-related potentials directly from within the hippocampus proper in eight patients with pharmacoresistant epilepsy undergoing invasive presurgical evaluation. During a visual object identification task, patients were asked to identify spatially-filtered versions of visual objects, each presented at 9 different levels of blurring (from completely blurred to completely clear). We found that within the hippocampus, a “dual-time” pattern of response distinguished between identified and unidentified objects: While correctly identified objects elicited a small negative component peaking around 300 ms (hippocampal-N300) and a high amplitude positive component, around 650 msec (hippocampal-P600), the N300 was increased and the P600 was reduced in amplitude in response to unidentified objects. The same pattern of hippocampal activity was elicited by objects identified at different levels of spatial filtering, suggesting that hippocampal responses were not affected by the amount of physical information comprised in the visual stimulus. These results provide new insights into the neural bases of visual object processing and suggest that the hippocampus proper may contribute to this task during the recognition of complex visual objects.