Impaired remote spatial memory after hippocampal lesions despite extensive training beginning early in life

Damage to the hippocampus typically produces temporally graded retrograde amnesia, whereby memories acquired recently are impaired more than memories acquired remotely. This phenomenon has been demonstrated repeatedly in a variety of species and tasks, and it has figured prominently in theoretical treatments of memory and hippocampal function. A striking exception to the finding of temporally graded retrograde amnesia comes from studies with rodents using spatial tasks like the water maze. In these studies, recent and remote memory were similarly impaired following hippocampal lesions. In contrast to work with rodents, studies of patients with medial temporal lobe lesions, including complete hippocampal lesions, indicate that remote spatial memory can be intact. One difference between studies in humans and studies in rodents is that spatial memory in animal studies is acquired during a limited period of time when the animals are adults. In contrast, the spatial memory studied in humans was acquired beginning at an early age and learning continued for a considerable period of time. We initiated training in a standard water maze immediately after rats had been weaned at 21 days of age and continued training until the rats were young adults (90 days old). Large hippocampal lesions were made 100 days after the completion of training. After recovery from surgery, control rats exhibited good retention on the first retention probe trial, but rats with hippocampal lesions performed at chance. Thus, even after extended training beginning early in life, and with a prolonged training-surgery interval, hippocampal lesions impair performance in the water maze task. Possible reasons for these findings are discussed in the context of the specific performance requirements of the water maze task.