Integration of cognitive allocentric information in visuospatial short-term memory through the hippocampus

Visuospatial short-term memory relies on a widely distributed neocortical network: some areas support the encoding process of the visually acquired spatial information, whereas other ares are more involved in the active maintenance of the encoded information. Recently, in a pointing to remembered targets task, it has been shown in healthy subjects that, for memory delays of 5 s, spatial errors are affected also by cognitive allocentric information, i.e., covert spatial information derived from a pure mental representation. We tested the effect of a lesion of the hippocampus on the accuracy of pointing movements toward remembered targets, with memory delays falling in the 0.5-30 s range. The spatial distributions of the two target sets we used (line and left-right) allowed the exploitation of cognitive allocentric spatial information: both sets were in the frontal plane, the line one being composed by eleven points distributed uniformly along a virtual line tilted 45 degrees away from the vertical, whereas the left-right set was composed by two workspaces symmetrically distributed at the extremes of a horizontal virtual line. We have found a significant difference between the performance of three hippocampal amnesic subjects and a group of normal controls for delays equal to or longer than 15 s, the difference being along the allocentric axis, i.e., the direction of the virtual line defined by the target set. On this basis we suggest that the hippocampal formation may enhance the spatial information processed within short-term memory with cognitive allocentric information. The association that may be operated through the neocortical-hippocampal loop of the newly acquired spatial information with well established spatial cognitive items could affect the precision of the short-term memory storage for memory delays exceeding about 15 s and might be the result of a modulation of the span of the spatial memory buffer along context-specific directions.