Write the letter A 100 times and line them up. One leans slightly left. Another has a longer leg. Same letter, same hand, never quite the same result.

“We’re not computers, right?” said Woon Ju Park, a Georgia Tech neuroscientist and assistant professor of psychology.

Park uses the handwriting analogy to explain something about hearing. Listen to the same note again and again, and you may hear or judge it a little differently each time. The sound hasn't changed. Your response to it has.

In a study published in The Journal of Neuroscience, Park and her colleagues Jasmine Awad and Ione Fine tested nine adults who had been blind since birth or before age 2 and 18 sighted adults of similar ages and musical backgrounds.

Volunteers listened to pairs of notes and pressed a button to indicate whether the second was higher or lower. The adults who grew up blind were better at telling the notes apart. But in another test, the groups performed about the same when asked to identify which of two bursts of noise contained a faint tone.

Park suspected the pitch advantage came from more finely tuned hearing, allowing people to pick up smaller differences between notes. But when the researchers used computer models to test possible explanations, they found stronger support for something else: internal noise.

It isn't static or background sound. Internal noise refers to the small variations Park described with her handwritten A's. The models suggested that adults who grew up blind had less of that variation when hearing or judging pitch, which could explain why they were better at telling similar notes apart.

“We really didn't expect internal noise to play a role,” Park said. “We were surprised and wanted to make sure we weren't seeing something that wasn't really there.” The researchers drew that conclusion from the volunteers' answers, not measurements of their brains. They still don't know exactly where the variation arises.

Musical experience showed a similar pattern. Among both blind and sighted volunteers, people with more years of musical training tended to have lower internal noise in the models. But musical training alone couldn't explain the difference between the groups.

Park wants to know whether the same advantage appears in everyday listening, from following conversations in noisy places to recognizing what's happening through sound. She also wants to understand whether experience and practice can change how reliably people distinguish sounds.

“The brain is working really hard to tune its responses to the world that matters to us,” she said.

Park is affiliated with the Institute for Neuroscience, Neurotechnology, and Society. Her research was supported by the National Institutes of Health and Georgia Tech's Smithgall-Watts Early Career Award.