Impaired retention of preoperatively acquired spatial reference memory in homing pigeons following hippocampal ablation.

Hippocampal-parahippocampal-ablated homing pigeons have been shown to suffer a retrograde loss of information used in the recognition of their home loft. Here we report that the range of retrograde deficits includes spatial reference memory in the form of information gained from repeated training sites that can be used to direct a homeward orientation response. Following 8 training releases from each of two sites, 28 of 42 homing pigeons underwent hippocampal-parahippocampal ablation. In the subsequent test releases from the two sites, untreated controls whose navigational map was rendered temporarily dysfunctional by an anosmic procedure showed no impairment in determining the home direction, indicating the successful retention and utilization of directionally useful information around the release sites. Hippocampal-ablated controls who were not rendered anosmic and thus had access to their navigational map also showed no impairment in determining the home direction, indicating no general impairment in initial orientation as a result of hippocampal ablation. In contrast, hippocampal-ablated pigeons whose navigational map was rendered temporarily dysfunctional failed to successfully orient homeward from the training sites, indicating impairment in the retention and/or implementation of directional information gathered at the release sites during training. The data reveal a spatial reference memory deficit involving pre-ablation acquired directional information in homing pigeons following hippocampal ablation.