Effects of hippocampal lesions on spatial delayed responses in dog.

Thirteen dogs were trained to perform spatial delayed responses to auditory cues in a three-choice Nencki testing apparatus with a delay of 0 s and then 10 s with a criterion of 90% correct responses in 90 consecutive trials. Then six dogs received bilateral surgical removal of the hippocampus via the cortex of the suprasylvian gyrus (without additional injury to the entorhinal and parahippocampal cortex). Three dogs received control surgical ablation of the suprasylvian gyrus, which was damaged in ablation of the hippocampus, and four dogs served as intact controls. After the surgery or rest period, the dogs were tested for their retention (10-s delay), and then they were given additional tests with extended delays (30, 60, and 120 s) and with distractions during the 60-s delay period. In comparison with both control groups, dogs with hippocampal ablations had moderately impaired postoperative retention, as evidenced by the elevated numbers of errors on criterion. In subsequent stages of testing with extended delays, the impairment was greater and was significantly correlated with the extent of injury to the hippocampus. These data, together with an analysis of the animals' responses to the three-choice situation, indicate that in dogs lesions of the hippocampus impair spatial memory.