Ablations of the mammillary nuclei in monkeys: effects on postoperative memory

A new procedure was developed for ablating the mammillary nuclei in nonhuman primates via direct visual exposure. Using this technique, monkeys receiving lesions of the mammillary nuclei were compared to control animals after surgery to assess the retention of preoperatively acquired visuospatial discriminations and subsequent postoperative ability to attain a demanding spatial memory task. Although the lesions proved to be accurate and complete, no changes in gross behavior or deficits in preoperatively acquired visual and spatial behaviors were noted. The monkeys with mammillary body lesions were, however, impaired in their ability to acquire postoperatively the demanding spatial memory task. These findings are consistent with previous investigations suggesting that the mammillary bodies are involved in the acquisition of spatial discriminations and skills.