Attenuated Boundary Extension Produces a Paradoxical Memory Advantage in Amnesic Patients

Background: When we view a scene, we construct an internal representation of the scene that extends beyond its given borders. This cognitive phenomenon is revealed by a subsequent memory error when we confidently misremember the extended scene instead of the original. This effect is known as boundary extension and is apparent in adults, children, and babies. Results: Here we show that seven patients with selective bilateral hippocampal damage and amnesia, who cannot imagine spatially coherent scenes, displayed attenuated levels of boundary extension on three separate measures. Paradoxically, this reduced boundary extension resulted in better memory for the stimuli compared with matched control participants, because the patients' recall was less encumbered by the boundary extension error. A further test revealed that although patients could generate appropriate semantic, conceptual, and contextual information about what might be beyond the view in a scene, their representation of the specifically spatial aspect of extended scenes was markedly impoverished. Conclusions: The patients' superior memory performance betrayed a fundamental deficit in scene processing. Our findings indicate that the hippocampus supports the internal representation of scenes and extended scenes when they are not physically in view, and this may involve providing a spatial framework in scenes. We suggest that interference with the ability to internally represent space may prevent the construction of spatially coherent scenes, with possible consequences for navigation, recollection of the past, and imagination of the future, which depend on this function.