Northwestern University researchers have found that people can quickly learn to distinguish between almost-identical smells if choosing incorrectly nets them a nasty shock.

This in the first study that shows conditioning can hone the human nose and alter the brain.

An MRI scanned the smell centers of subjects’ brains as they distinguished between two pairs of subtly different smells, and as a slight electric shock accompanied one of the smells. They couldn’t distinguish any of the molecules before the shock, but almost all could by six jolts later. Their brains also responded differently to each smell in the pair that included the shock, while their sensitivity and brain response to the other pair didn’t change.

A version of this reaction could be partly responsible for the heightened sensitivities of patients with anxiety disorders, phobias, and post-traumatic stress disorder, which often develop following an accident, a violent attack, or other ordeal.