For tens of thousands of profoundly deaf adults and children worldwide, cochlear implants have provided a useful substitute for natural hearing. These devices electrically stimulate the auditory nerve within the cochlea, enabling many users to carry on a conversation without visual cues, such as over the telephone. But for patients whose nerve endings have degenerated or whose auditory nerves have been destroyed, the only hope for restoring hearing is to access later stages of the auditory system. Now California researchers are gearing up to do just that, going beyond cochlear implants with a device that will plug directly into the brain.

At the Huntington Medical Research Institutes (HMRI) in Pasadena, Calif., neurophysiologist Douglas McCreery shows off a cat that is already using the new device. Like a cochlear implant, it consists of an external speech processor and a receiver implanted under the scalp. But the wires from the receiver bypass the cochlea and instead travel all the way to the brain stem. They end in an array of six iridium microelectrodes that penetrate the ventral cochlear nucleus, one of the auditory centers that normally receive input from the cochlea. The implant isn't meant to enable McCreery's cat to hear--its natural hearing is in fact still intact. Rather McCreery records the neural signals the implant produces and finds that the signals convey the frequency-coded information appropriate for the comprehension of speech.

This article was originally published with the title Brain Invaders.

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