BioEd Online

Humans can learn skills without remembering what they have done, according to a study of patients with severe amnesia. Such learning is seen in monkeys, but experts were unsure whether humans retained this ability, because of our tendency to think consciously about whatever we are learning.

Most people gather information and abilities through a process of 'declarative' learning, in which they remember the act of learning as well as the new skill or knowledge itself. Hopefully, you'll recall reading this article, as well as remembering the nuggets of information it contains.

This process of explicit thought offers a fast route to learning, but requires sophisticated mental machinery. Declarative learning is centred on a brain region called the medial temporal lobe, which is thought to coordinate the storage of memories in the brain.

Trial and error

Other animals, which lack our cognitive powers, use a slower, more primitive method called habit learning. If one item in a pair of objects is designated the 'correct' item, monkeys can learn to select this particular item by simple trial and error. Without realizing they are doing it, they gradually acquire the habit of picking the right option.

Neuroscientists had been unsure whether humans share this primitive ability, or whether our capacity for conscious learning has rendered it obsolete. To find out, researchers at the University of California, San Diego, gave the task of identifying the correct items in eight pairs to patients with severe damage to their medial temporal lobe.

Healthy humans can master the task in no time, because of declarative learning. The study's two volunteers, both of whom had permanent amnesia as a result of encephalitis, did not remember the objects or the task at the start of each session. But they were still able to master the task, albeit over several weeks.

How'd I do that?

Despite ultimately achieving success rates of 85% and 92.5%, the two patients could not explain why they thought a given item was the correct one to choose, says Larry Squire, who led the study. "The patients were surprised," he recalls, "and one asked 'How am I doing this?'"

To prove that the subjects had learned the task purely by rote, the researchers gave them all of the objects together and asked them to sort them into 'correct' and 'incorrect' groupings. They were unable to do this, despite having mastered the task when presented with the items in pairs, the team reports in Nature¹.

The fact that humans can learn in the absence of conscious memory is a surprise, says Squire. "We thought it unlikely that such successful habit learning would be revealed, because we expected that performance would be dominated by efforts to engage in declarative memory," he says.

The exact function of habit learning in humans remains unclear. But Squire is now convinced that it has a fundamental role in shaping healthy minds as well as those of amnesiacs. "Here arise our preferences, dispositions, skills and myriad ways of interacting with the world," he says.