(CNN) -- The surprise on 3-year-old Grayson Clamp's face in the video is priceless. His mouth opens wide as he points to the person in front of him speaking.
It was the first time Grayson had heard sound. He was born without a cochlear nerve, which connects the brain stem to audio waves in the outside world. His parents had him fitted for a cochlear implant at a young age, but the device didn't help.
Last month, Grayson became the first child in the United States to receive an auditory brain stem implant.
Len and Nicole Clamp were Grayson's foster parents when he was a newborn. They adopted him as soon as they could, believing God had called them to care for the medically challenged boy.
Auditory brain stem implants already have been available for adults. When the Clamps heard about a Food and Drug Administration-approved trial for children at the UNC Hospitals in Chapel Hill, North Carolina, they jumped at the chance to participate.
Grayson now has an external speech processor containing a microphone. The processor breaks up sounds into their frequency components and sends that information across Grayson's skin to an implanted device, said his surgeon, Dr. Craig Buchman. The device then stimulates electrodes on his brain's cochlear nucleus. The electrodes are placed where his cochlear nerve would be, if he had one.
"He's sound aware, but we don't know what exactly he hears," Buchman said. "We're relying on the plasticity of brain to start to sort this out."
The implant is powered by a battery that's outside Grayson's body.
The procedure has been done mostly in Europe up to this point. The FDA approved the implant in the United States for only 10 children -- five who, like Grayson, were born without nerves and five who suffered traumatic damage.
These cases will be used to determine whether a bigger trial that includes more children is safe and whether the device may one day be available across the country.
The biggest risk occurs during surgery, Buchman said; the brain can be bruised when doctors place the device.
Len Clamp said Grayson is discovering all kinds of new things about his world now. He's drawn to music, Clamp said, often running over to turn on the radio.
Clamp said he hopes other children with hearing loss get the same opportunity as Grayson.
"It was the most amazing thing I've ever seen," he said. "(There are) no words to describe how excited we were, how validated we were, for all of the challenges. ... He's got a long way to go, but he's going to get there."