When U.S. congresswoman Gabrielle Giffords was shot last January in a parking lot in Tuscon, the bullet entered just over her left eye, travelled the length of the left hemisphere of her brain, and exited.

This week during Giffords's first public speaking appearance, her husband, former space shuttle commander Mark Kelly explained how the trajectory of the bullet saved her life.

"It's clear that any lower, it would've killed her, any further midline, it would've killed her. If it crossed hemispheres, it would've killed her. Any further outboard, she'd never be able to speak again. Any higher, she'd never be able to walk."

Gabrielle Giffords's wound was catastrophic, but not fatal. And though it robbed her for a time of her ability to speak, music — or more specifically something called melodic intonation therapy — was able to return her voice.

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Congresswoman Gabrielle Giffords hugs her husband, retired astronaut Mark Kelley as he received the Legion of Honour medal from Vice-President Joe Biden in October 2011. It was only Giffords second public appearance since she was shot in Tuscon on Jan. 8, 2011.

Less than a month after she was shot, Giffords met with music therapist Maegan Morrow at TIRR Memorial Hermann Rehabilitation Hospital in Houston.

As Morrow told me this week on CBC Radio's Day 6, she knew going into the meeting that Giffords wasn't able to vocalize at all. But she felt she might have a way to cut through the trauma Giffords's brain was dealing with by way of "a very familiar song."

'Happy birthday to you'

Morrow sang Happy Birthday to the mute congresswoman "and I left the end of the phrase out and tried to get Gabby to sing along with me."

Giffords didn't sing. Instead, she grabbed the hand of the therapist to physically but silently complete the missing word of the phrase "Happy Birthday to ...."

Music had become the cognitive link between patient and therapist.

"I knew that there was awareness there," Morrow said, "and she knew that she wanted to fill in that blank any way possible.

"So I knew there would be more breakthroughs after that."

An ancient vocabulary

But how does music find a pathway inside a damaged brain that regular speech can't negotiate? According to Morrow, it has to do with the parts of the brain where music comes from. And that there are so many of them.

Language and the left hemisphere

The left hemisphere of the brain is famously the side of the brain that controls handedness, motor control and, crucially, language in most people.

The interconnections of language — its associations, matchings and semantic decisions, as well as the hierarchical values of words — all of these complex organizations the brain undertakes when processing speech, seem to happen in the left hemisphere.

Two regions of the left hemisphere, Broca's area and Wernicke's area, are directly implicated in cases of language dysfunction or aphasia, the loss of speech. Writing about Wernicke's region in 1976, neurophysiologist Joe Bogen stated, "If a person has a cerebral lesion that produces a loss of language far out of proportion to the loss of intelligence, the odds are about 50 to 1 that the lesion is in the left hemisphere."

"Music centres are all over the brain," says Morrow. "I might be able to retrieve lyrics from the right side, from the middle, from the back of the brain. There are so many components to music that I can tap into … to reach words again and to reformulate them in the brain."

In terms of human evolution, speech is a relatively recent addition to our compartmentalized brains. Some believe music may precede it. There's no doubt that toddlers babble and vocalize long before they speak.

"It used to be thought that music was a superfluous thing, and no one understood why it developed from an evolutionary standpoint," says Michael De Georgia, director of the Centre for Music and Medicine at Case Western Reserve University's Medical Centre in Cleveland.

"In the last 10 years, we've just started to understand how broad and diffuse the effect of music is on all parts of the brain," he added. "We are just starting to understand how powerful music can be."

Melodic intonation therapy

One of the therapies that Morrow tried with Giffords is called melodic intonation therapy. It's an intensive program that was developed after researchers noticed that patients with severe aphasia (the loss of the ability to speak) could sing words and phrases they could no longer speak.

In the ABC TV show last week with Giffords, Morrow is seen working with the congresswoman. trying to get her to say the word "light."

Giffords can't vocalize it and she's clearly frustrated. Then, moments later, as Morrow sings the spiritual This Little Light of Mine, Giffords chimes in forcefully, perfectly forming the word.

It's astonishing to see, and you can feel Giffords's connection with words that moments earlier were completely lost.

Melodic intonation therapy requires long hours at first in daily sessions. But after a while it then allows patients to incorporate more of these relearned words and phrases in their everyday life.

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Giffords undergoing music therapy at TIRR Memorial Hermann Rehabilitation Hospital in Houston. The image is from her interview in November 2011 with Diane Sawyer on ABC's 20/20.

The theory here is that music engages areas of the brain in the right hemisphere that become useful in restoring articulation and speech in the damaged left hemisphere, where language is usually based.

As Morrow describes it on Day 6: "We call this concept neuroplasticity. The brain is kind of re-wiring itself.

"I actually have patients who don't have a left side of the brain anymore. But the whole other side of the brain took over and learned to talk again."

Our brains on music

Gabby Giffords's recovery has advanced to the point that she was able to release a recorded message to her constituents earlier this week.

Her voice sounds strong, but it's stilted. The message suggests fluency may still be an issue as the congresswoman weighs next year's election.

Meanwhile, as research continues into the therapeutic power of music, some neurologists, such as Gottfried Schlaug at Harvard, one of the early proponents of melodic therapy, wonder if we are underestimating the role music plays in our everyday lives.

"There's enough data out there," he says, "that would support a strong role for the arts in general and certainly for music making in our general education system."

From the therapeutic power of This Little Light of Mine to the soothing qualities of Bach, it seems we are only just beginning to understand the ways music lights up our brains.