Most everyone can perform tasks that, once learned, seem to happen without thinking, like going on "autopilot" for the drive home from work or tying a shoelace. For the first time, researchers now have evidence that humans, like other mammals, can form these habits without using regions of the brain typically associated with long-term, conscious memory, instead probably using a primitive part of the brain called the basal ganglia.

Conscious memory, like memorizing a list of words, occurs in regions of the brain called the medial temporal lobes and the hippocampus. Researchers in California studied two men who sustained heavy damage to the temporal lobes and hippocampus after becoming infected with herpes simplex encephalitis.

Because of this brain damage, the two men suffer from amnesia and can no longer remember people, places, or events that they witness in their daily lives for any extended period. (The two men could retain short-term memories.)

"They can't form any new memories at all," explains coauthor Larry Squire, a professor at the University of California–San Diego School of Medicine and a scientist with the San Diego Veterans Affairs Health System.

"They don't remember us when we come to the door," says Squire, even after 200 visits. One man couldn't remember the floor plan of a small house that he'd lived in for 10 years.

But the men, whose basal ganglia remained intact, could form habits. After a long period of training, Squire and his colleagues found that the two men retained the ability to distinguish between pairs of obscure objects (miscellaneous pieces of twisted metal or plastic) when one of the two objects had "correct" written underneath, even though the patients had no idea how they could perform this task.

During the trial, when examiners asked, "Why are you selecting that one?" one patient responded, "It's just jumping out at me, 'I'm the one, I'm the one.' I just keep wanting to pick it up." Over the course of 17 days, both men could distinguish the correct object between eight pairs of objects over 90 percent of the time. However, when all 16 objects were placed together on a table, the patients completely failed to sort the "correct" objects from the "incorrect" objects.

"It is unusual to find patients like this where the damage is restricted to this region [the hippocampus and the temporal lobe] of the brain," says Squire. "These other forms of memory are well developed in invertebrate animals like fruit flies and sea slugs. This finding reminds us of our close connections to our evolutionary ancestors, including rats and monkeys."

Mortimer Mishkin, a researcher in the Laboratory of Neuropsychology at the National Institute of Mental Health, who was not involved in the study, says that Squire's finding is extremely important.

"Until now, we've not had any evidence in humans of what we think is going on in the monkey in terms of habit formation," Mishkin explains. "This is a first peek at this process, and it seems as though it was made possible by the complete absence, in these two cases, of conscious or declarative memory."

Mishkin says that scientists do know about other forms of learning that are independent from the medial temporal lobes and the hippocampus, such as fear conditioning in the amygdala and eye-blink conditioning after a puff of air that occurs in the cerebellum. But habit formation, until now, remained obscure.