Preserved learning in monkeys with medial temporal lesions: sparing of motor and cognitive skills

In an effort to bring into correspondence the findings from human amnesic patients and the findings from monkeys with surgical lesions of those brain regions thought to be affected in the human cases, we have addressed in three experiments the implication of findings that human amnesia spares motor and cognitive skills. In the first experiment, monkeys with conjoint lesions of hippocampus and amygdala (H-A), which reproduced the surgical removal sustained by the noted amnesic case H.M., were only mildly impaired in learning relatively difficult pattern discrimination tasks. Monkeys with lesions of temporal stem matter (TS) were severely impaired on the same tasks, due to an apparent deficiency in visual information processing. In the second experiment, monkeys with H-A lesions were severely impaired at learning relatively easy discrimination tasks that could be acquired rapidly by normal monkeys. Monkeys with TS lesions were not impaired. In the third experiment, monkeys with H-A lesions exhibited normal acquisition of two motor skill tasks. These data can be understood in the light of a distinction between kinds of memory, founded in recent studies of the neuropsychology of human amnesia. These studies have led to a distinction between the learning of skills or procedures, which is spared in human amnesia, and the learning of facts and episodes, which is impaired. Monkeys with H-A lesions are normal at skill learning like human amnesic patients with similar lesions. This conclusion depends in part on the argument developed here that pattern discrimination learning, as accomplished by monkeys, has a large skill-like component. These results bring into correspondence the behavioral data from human amnesic patients and operated monkeys and set the stage for identifying precisely what brain structures must be damaged to produce amnesia.