Tests of an improved electronic implant that can simulate hearing have begun in California, and researchers are hopeful that it will allow some deaf people to understand speech.

The device, a four-channel cochlear implant, directly stimulates the auditory nerve, bypassing damaged hair cells that usually respond to vibrations.

Until now, a one-channel version has been used, translating sounds into clicks and buzzes. The four-channel device is like "the difference between hitting all the keys of the piano at once or playing one key at a time," says Dr. Robert Schindler of the University of California at San Francisco.

Schindler and his colleagues hope to implant up to 250 devices over the next three years. They also plan to increase the number of sound channels to eight.

The cochlear implant can be used only on people whose auditory nerves are still functioning. A hearing-aid-like device, worn outside the ear, converts sound into electronic impulses and transmits them to the implant.

Earl Trottier, 67, who has received a four-channel implant, is able to understand about a third of what's said with no visual cues and 80 percent if he can see the lips, the university reports. The speech he hears, Trottier says, has a "Louis Armstrong quality."