Chip mimics hearing.

Chip mimics hearing

Confirming a 40-year-old theory of how the brain perceives pitch, two scientists have built a computer chip that duplicates the human ability to "hear" a fundamental note missing from a harmony.

The analog chip, which contains 125,000 transistors arranged in patterns that follow known structures in the brain, analyzes a "sound wave"--actually a varying electrical signal--as fast as a human ear can, report John Lazarro and Carver Mead of the California Institute of Technology in Pasadena. They describe the device in the December PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol. 86, No. 23).

The chip splits a complex signal into 62 sine waves, each sent to a processor that delays the wave and compares its signal with that of an undelayed wave. If the two signals line up, a "correlation neuron" fires. And if all the frequencies in the input signal are multiples of a missing fundamental frequency, the circuit's output conveys only the missing element.

Although the chips require a great deal of expertise to build, they provide a powerful tool for testing theories about the nervous system, says vision researcher Terrence J. Sejnowski of the Salk Institute for biological Studies in La Jolla, Calif. "It's a harbinger of the types of models that we need to develop," he says.

Sejnowski predicts, however, that "it's going to be a while before experimental biologists will be able to fully appreciate the power of this particular technique, because it's so far removed from the techniques they're familiar with."