Read this aloud and your inner ear, by itself, will be carrying out at least the equivalent of a billion floating-point operations per second, about the workload of a typical game console. The inner ear together with the brain can distinguish sounds that have intensities ranging over 120 decibels, from the roar of a jet engine to the rustle of a leaf, and it can pick out one conversation from among dozens in a crowded room. It is a feat no artificial system comes close to matching.
But what's truly amazing is the neural system's efficiency. Consuming about 50 watts, that game console throws off enough heat to bake a cookie, whereas the inner ear uses just 14 microwatts and could run for 15 years on one AA battery. If engineers could borrow nature's tricks, maybe they could build faster, better, and smaller devices that don't literally burn holes in our pockets. The idea, called neuromorphic engineering, has been around for 20 years, and its first fruits are finally approaching the market.
The likely first application is bionics--the use of devices implanted into the nervous system to help the deaf, blind, paralyzed, and others. There are two reasons for this choice: the biological inspiration crosses over to the application, and the premium on energy efficiency is particularly important.
Bionic ears are a case in point. Today's device, called a cochlear implant, consists of an implanted electrode array; a bulky, power-hungry digital-signal processor worn outside the ear; and a wireless link that conveys data and power to the implanted electrodes. In the near future, these devices will be fully implanted inside the body so that deaf people will be indistinguishable from everyone else in both appearance and, we hope, ability to hear. In the past year, my lab at the Massachusetts Institute of Technology has completed work on a bionic-ear processor that does the job of the digital-signal processor, is small enough to be implanted, and could run on a 2-gram battery needing a wireless recharge only every two weeks [see illustration, "Mimicking the Ear"]. As the best batteries currently available can be recharged about 1000 times, this device is the first to permit
30-year operation without surgery to replace the battery. Last year, a deaf woman replaced her conventional processor with ours, though it was not implanted, and afterward she could understand speech easily and well.
Neuromorphic engineering and, more generally, biologically inspired electronics are still in their infancy, but practitioners have already accomplished amazing things [see table, "Leading Labs"]. These include the attempt to understand biological systems, such as the retina of the human eye and the sonar systems of bats, by modeling them in microchips. Some of the lessons learned have been turned to practical purposes--for instance, applying the principles of vision in the housefly to the control of robotic motion and designing radio-frequency spectrum analyzers that mimic the architecture of the human inner ear. Some devices now measure oxygen saturation in the blood with sensors and processors inspired by the photoreceptors in our eyes; others employ pattern-recognition circuits that rely on the mix of analog and digital features found in the brain.
One of biology's big power-saving secrets is that it relies on the physics of special-purpose structures, such as ears and eyes, to do a lot of analog computing. Ears, for example, are complex structures that by their inherent physics alone perform filtering, frequency-spectrum analysis, and signal compression--all before the signals are transmitted to the brain. Many of the initial insights into biology's computing efficiency originated with Carver Mead, professor emeritus at the California Institute of Technology, in Pasadena--the founding father of neuromorphic engineering.
But ears, eyes, and even individual brain cells also have a digital aspect. Brain cells, or neurons, can be viewed as special-purpose analog-to-digital converters. They recognize particular patterns of voltage inputs from other neurons, integrate these signals in an analog manner, and then output a digital-like signal, a voltage spike (1) or its absence (0). Output spikes from one neuron act as inputs to the next neuron. And this simple process, amplified and repeated by billions of interconnected neurons, leads to movement, hearing, thought, and everything else under our brain's control [see the sidebar, " " which accompanies the online version of this article].
Analog devices in the ear, such as the eardrum and the cochlea, process sound. The ear then digitizes the processed sound signal by encoding it as spikes of voltage that travel down the auditory nerve to the brain, which interprets the spikes to distinguish a jazz tune from an oncoming train or a whisper. Because the ear has already done a great deal of analog computation on the sound, the information it provides the brain is more compact and far better suited than raw sound to human tasks, such as understanding what a child is whispering in a crowded movie theater. This scheme of low-power analog processing followed by digitization is one of the most important lessons biology has to teach designers of electronics.