Animal Subjects

In its quest to construct strong inferences about inner-ear function, the Lewis Lab took a comparative approach, focusing on one species from each of three terrestrial vertebrate classes—one amphibian, one reptile, and one mammal. Each of the three species had a long history as a standard animal subject for inner-ear research, giving us a well-established database from which to start.

The American bullfrog, our amphibian of choice, had been the subject of extensive research in many laboratories. Its ear appears to have evolved recently relative to that of many other amphibians. It has what one might call a relatively modern ear. Like all frogs, however, the bullfrog has eight distinct inner-ear sensory organs— four orientation and motion sensors, three acoustic sensors, and one organ that is subdivided into an orientation/motion sensing region and an acoustic region. Being keenly interested in the functional distinctions between acoustic sensors and orientation/motion sensors, the Lewis Lab studied all eight organs—rather thoroughly, and applied reverse engineering in one form or another to all eight.

The red-eared turtle, our reptile, had a special place in the history of auditory research. It was the first animal in which electrical resonances were discovered in inner-ear sensory cells. It is now widely accepted that these resonances—in the turtle and in many other lower vertebrates, play major roles in allowing the auditory sensors in the ear to produce dynamic spectrographic images of the sound. These motion-picture-like images are sent to the brain by way of the auditory nerve. For the past twenty years, the Lewis Lab was especially interested in the signal-processing strategies underlying these images. The turtle and its ear appear to have arisen very early in the evolution of reptiles. It has what one might call a relatively ancient ear—a place to look for a surviving snap-shot of early signal-processing strategies in the evolving ears of terrestrial vertebrates. The turtle ear was introduced to the lab by Dr. Michael Sneary, now Professor of Biology at San Jose State University. While a member of the Lewis Lab, he carried out the physiological studies on which we subsequently based our reverse engineering.

The Mongolian gerbil, our mammal, had been used widely in hearing research. Among the laboratories using it was that of Professor Kenneth R. Henry at the University of California at Davis. Prof. Henry had been studying stimulus-evoked responses in the auditory nerve and auditory brainstem of the gerbil. These responses were analogous to brain waves, involving simultaneous activity over large populations of neurons. His studies led to a fundamental question that could be resolved only by reverse engineering at the level of the individual neuron. In 1986 he asked me to join him in trying to answer the question. Together we would learn to observe electrical action potentials in individual cochlear-nerve neurons, and then we would proceed with the reverse engineering. I accepted, and the Lewis Lab joined the Henry Lab in a partnership that lasted until both of us retired. Nigel K. Woolf and Allen F. Ryan, both of UC San Diego, taught us the techniques of observing action potentials in single axons of the gerbil cochlear nerve. Then Prof. Henry and I spent many months learning to put what we had been taught into practice. Once we had done that, we quickly found the answer to Prof. Henry’s question, and then moved on to one new question after another—for nearly twenty years. The gerbil became the mammalian representative for our reverse-engineering studies of signal-processing strategies in hearing.