Robust habit learning in the absence of awareness and independent of the medial temporal lobe : Nature:
In contrast to the findings from humans, monkeys learned the same concurrent discrimination task gradually in several hundred trials, and after hippocampal lesions they learned this task (or a related version) at a normal rate7, 12, 13. A standard interpretation of the monkey data is that monkeys acquire the concurrent discrimination task by trial-and-error (habit) learning with the support of the basal ganglia1, 13, 14. That is, they acquire a disposition to respond appropriately to each object pair. Habit memory is proposed to involve slowly acquired associations between stimuli and responses that develop outside awareness and are rigidly organized, with the result that what is learned is not readily expressed except when the task is presented just as it was during training.
One possibility is that habit learning is only weakly developed in humans, and some amount of declarative memory must be available to guide the learning. Indeed, successful learning of habit-like tasks has been reported only for moderately impaired amnesic patients who retain a considerable capacity for declarative memory15, 16. Moreover, some amnesic patients do not learn such tasks17. A related view is that the concept of an independent habit system is unnecessary, because habit learning and other kinds of learning depend on similar mechanisms18. Alternatively, if the capacity for habit learning is as well developed in humans as it appears to be, for example, in the monkey, then patients with large medial temporal lobe lesions and no capacity for declarative memory should be able to acquire the concurrent learning task gradually and to a high level of proficiency in the same manner that monkeys learn the task.
Evolutionary hypothesis tested.