Ventral hippocampus lesions and allocentric spatial memory in the radial maze: Anterograde and retrograde deficits.

Although the dorsal hippocampus (DHip) has been clearly implicated in spatial learning and memory, there is currently debate as to whether the ventral hippocampus (VHip) is also necessary in allocentric-based navigation tasks. To differentiate between these two subregions of the hippocampal dorsoventral axis, we examined the effect of neurotoxic lesions to the DHip and VHip in different learning situations, using a four-arm plus-shaped maze. In experiment 1 a spatial reference memory task was used, with results showing an acquisition deficit in DHip-lesioned rats but perfect learning in VHip-lesioned rats. However, in experiment 2 an acquisition deficit was found in VHip-lesioned rats using a doubly marked training protocol. In this case the position of the goal arm during training was marked simultaneously by the extramaze constellation of stimuli around the maze and an intramaze cue. The main results indicated that DHip and VHip groups presented significantly more allocentric errors in the probe test than the control rats. In experiments 3 and 4, animals with their brains still intact learned, respectively, a spatial reference memory task or a purely cue-guided navigation task, and DHip and VHip lesions were made 2-3 days after reaching learning criterion. Results indicated a profound retrograde deficit in both lesioned groups but only with regard to allocentric information. So, depending on the training protocol used, our results point to increased integration and cooperation throughout the hippocampal dorsoventral axis when allocentric learning and memory is involved. These data support the existence of a functional continuum from the dorsal to the ventral hippocampus.