Adults who’ve had a lifelong irrational fear of spiders were able to hold a tarantula after just two hours of treatment that altered their brains' response to fear, according to a new study.

Psychologists, reporting in the Proceedings of the National Academy of Sciences, have for the first time documented the immediate and long-term brain changes that significantly reduced fear as a result of exposure therapy.

Researchers found that a single exposure therapy session was so successful that adults who once had debilitating spider phobia were able to touch and hold a spider in their bare hands even six months after the treatment.

Scientists say that the latest findings of the lasting effectiveness of short exposure therapy offer new directions for treating other phobias and anxiety disorders.

"Before treatment, some of these participants wouldn't walk on grass for fear of spiders or would stay out of their home or dorm room for days if they thought a spider was present," lead author Katherina Hauner, a post-doctoral fellow in neurology at Northwestern University, said in a university news release.

"But after a two or three-hour treatment, they were able to walk right up and touch or hold a tarantula. And they could still touch it after six months. They were thrilled by what they accomplished," Hauner added.

Hauner and her team studied 12 adults who had arachnophobia, a type of specific phobia that is also one of the most common anxiety disorders that affects about 7 percent of the population.

Other common specific phobias are the fear of blood, needles, snakes, flying and enclosed spaces.

Researchers reported that before the therapy sessions, participants were not able to get closer than 10 feet on average to a spider, and many were even too afraid to look at photographs of spiders.

The team reported that when participants were shown pictures of spiders regions of their brains associated with fear response, like the amygdala, insula, and cingulate cortex, had all became activated, according to fMRI scans.

In the therapy sessions, psychologists taught participants about tarantulas and participants learned that their terrible thoughts about spiders were not true.

"They thought the tarantula might be capable of jumping out of the cage and on to them," Hauner said. "Some thought the tarantula was capable of planning something evil to purposefully hurt them. I would teach them the tarantula is fragile and more interested in trying to hide herself."

Psychologists asked participants to gradually approach the tarantula in slow steps until they were able to touch the outside of the spider terrarium. Afterwards researchers instructed participants to gently touch the tarantula with a paintbrush, then a glove and eventually to pet or to hold the spider with their bare hands.

"They would see how soft it was and that its movements were very predictable and controllable," Hauner said. "Most tarantulas aren't aggressive, they just have a bad reputation."

Immediately after treatment, Hauner and her team reported that fMRI scans of participants showed that their brain regions associated with fear had decreased significantly in activity, a reduction that persisted even six months after the brief exposure therapy session.

Even after six months participants were able to walk right up to the tarantula and touch it.

"It was amazing to see because I remembered how terrified they were initially and so much time had passed since the therapy," Hauner said.

Researchers said they were able to predict which one of the participants would respond best to treatment based on their brain activity immediately after the therapy. 

Participants with higher levels of activity in brain regions associated with visual perception of fearful stimuli immediately after the treatment responded significantly better and had the lowest fear of spiders six months later, compared to others.

However, participants with higher levels in brain regions associated with inhibiting fear only showed changed immediately after the exposure therapy and not after six months.

Researchers said the findings suggest that differing brain mechanisms may be responsible for immediate versus long-term fear reduction.

"This suggests that observations of brain activity immediately after therapy may be a useful future tool in predicting an individual's long-term outcome," Hauner said.