Hiding in Plain View: Lesions of the Medial Temporal Lobe Impair Online Representation

The hippocampus is necessary for the normal formation of enduring declarative memories, but its role in cognitive processes spanning short intervals is less well understood. Within the last decade, several reports have described modest behavioral deficits in medial temporal lobe (MTL)-lesion patients when they perform tasks that do not seem likely to rely on enduring memory. An intriguing but sparsely-tested implication of such results is that the MTL is involved in the online representation of information, possibly of an associative/relational nature, irrespective of delay. We administered several tests that simultaneously presented all information necessary for accurate responses to a group of MTL-lesion patients with severe declarative memory deficits but otherwise normal cognition, and to matched brain-damaged and healthy comparison participants. MTL-lesion patients performed less well than either comparison group in the Hooper Visual Organization Test, and several patients performed outside the normal range on the Overlapping Figures Test. A novel follow-up borrowing characteristics of the Overlapping Figures Test revealed impaired identification of novel items by MTL-lesion patients when target items were obscured by distracters, and two additional novel tests of fragmented object identification further implicated the hippocampus/MTL in the integration of information across very brief intervals. These findings suggest that MTL structures including the hippocampus contribute similarly to cognition irrespective of timescale.