Final Report from Lewis Lab to NIDCD

Grant DC-00112
Overall project period-- July 1, 1975 to June 30, 2001
Final project period-- July 1, 1996 to June 30, 2001

Overview

The project "Morphology and Physiology of the Inner Ear," supported by grant DC-00112 began in 1975 as grant NS-12359 from the National Institute of Neurological and Communicative Disorders and Stroke. It was transferred to the National Institute of Deafness and Communicative Disorders when that institute was founded. Pilot studies for the project were begun in 1970, supported in part by a grant from NSF (GK-3845) and in part by a program project grant (GM-17523) from the National Institute of General Medical Science. Since 1961 I had been attempting, through modeling, to understand the operation of small invertebrate neural networks known as central pattern generators. In 1967 I was invited to continue those studies as a member of the Berkeley faculty. The NSF grant supported that continuation. As a neural modeler, I had been keenly interested in the roles and limitations of models in science. I had been thinking especially in terms of formal circuit theory-- a metamodel that is astoundingly broadly applicable. By 1969, it was clear to me that the metamodel imposed profound limitations on my own work-- the sort of reductionist understanding of neural networks that I had been hoping to achieve was not possible. I presented my findings to the very best of the neural modeling community-- which was unable to refute them, and turned my attention to finding a new area of research. The NIGMS program project was very helpful in that regard. It supported new research by three faculty members (Everhart, Lewis, Sakrison) on development of biomedical applications for the scanning electron microscope. My role was to develop techniques for SEM studies of neural tissue-- including techniques for mapping small invertebrate networks. Because of their accessibility, however, the ciliated sensory receptors were especially attractive for this work, and I began to focus on them. For the next ten years my work in neurobiology was largely morphological and largely aimed at the receptors of the amphibian inner ear.

In our pilot studies for NS12359/DC00112, my students (P.S. Nemanic and C.W. Li) and I had noted an amazing array of hair bundle types, distributed largely in discrete populations over the various inner-ear sensory surfaces (Lewis and Nemanic, 1972; Lewis and Li, 1975). It seemed reasonable to suspect corresponding distribution of function (e.g., see Lewis, 1972). We also had noticed that considerable hair-cell proliferation occurred along specific edges of the various sensory surfaces and that it continued in adult frogs (Lewis and Li, 1973). We found a full range of intermediate forms between the ciliated supporting cell and the juvenile hair-cell form that was common to all of the sensory surfaces. The morphogenetic sequence implied by these studies (see Lewis and Li, 1973) was verified subsequently by the Corwin group and the Baird group. Although these observations were new at the time, and I was fascinated by their implications, I was even more fascinated by what the frog's fully-formed hair bundles might be sensing. I decided to focus on the hypothetical relationships between hair-bundle form and hair-cell function.

In my application for the initial funding of "Morphology and Physiology of the Inner Ear," I proposed to penetrate individual eighth-nerve afferent neurons with glass-micropipette electrodes, identify key features of their receptive fields, then inject an intracellular marker that would allow me to trace each afferent axon to its origin. This way, I proposed to develop precise functional overlays for the detailed maps of hair-bundle distributions that we already had produced. With considerable help from two doctoral students (R.A. Baird and E.L. Leverenz) and two postdocs (H. Koyama and S.F. Myers), this goal was achieved (e.g., see Lewis 1979; Lewis, Baird and Leverenz, 1980; Lewis, Leverenz and Koyama, 1980). Although two other laboratories (Liberman, Honrubia) were soon to follow, we were the first to do this, and the results were satisfying indeed. Whereas, for example, Fernandez and Goldberg had strong circumstantial evidence for the distributions of phasic and tonic sensitivities over vertebrate vestibular maculae, we now had definitive evidence of those distributions-- and it was the first. We were working in an amphibian, Fernandez and Goldberg had worked in mammals. Subsequently, R.A. Baird joined the Fernandez and Goldberg laboratory as a postdoc and transferred the techniques to the mammalian preparation. ***We had obtained definitive functional maps for the frog's acoustic sensors as well as its vestibular sensors, and we suddenly were confronted with questions at two levels-- the system level and the cellular level. The former concerned the sensory images conveyed by the eighth nerve to the brainstem and the signal-processing strategies sculpted by evolution for acoustic and vestibular sensors; the latter concerned the cellular devices and mechanisms giving rise to the functional variations we had observed. My experience with the circuit-theory metamodel made the decision easy. The system level was full of promise. At the cellular level, on the other hand, I would be confronted by the same fundamental limitations that had brought an end to my neural modeling career.

In the application for "Morphology and Physiology of the Inner Ear," I had noted those limitations, and I had also noted the possibility of bi-directional interaction between the cellular and acellular components of the inner-ear sensory structures. With respect to the sort of physical understanding that I would seek, that interaction (i.e., bi-directional transduction) would combine the cellular and acellular components into a single, irreducible entity. Therefore, my stated intention was to leave the otic capsule intact, so as not to alter the milieu of the acellular components, and to leave physiological or biophysical exploration of the components inside the otic capsule to others. My laboratory research would focus on the physiological properties of the ear as observed in eighth-nerve afferent axons emerging from the intact otic capsule. My collaborators and I have remained faithful to that strategy throughout the 25-year history of this project.***return to Bullfrog Sacculus

By the late 1980s, however, we had expanded our physiological studies to include the gerbil cochlea and the turtle basilar papilla as well as the various acoustic and vestibular sensors of the frog inner ear. Work on the gerbil cochlea was carried out in collaboration with Prof. K.R. Henry of UC Davis. Work on the turtle was carried out in collaboration with Dr. M.G. Sneary (now on the biology faculty at San Jose State University). In the early 1980s, we also began a series of field studies that have continued to the present. These were behavioral studies of the use, by various vertebrates, of seismic signals. This was motivated by our discovery of exquisite sensitivity of the frog saccule to seismic (vibratory) stimuli (Lewis and Baird, 1980; Koyama et al., 1982). The subjects of the behavioral studies were the white-lipped frog of Puerto Rico (studies carried out in collaboration with Prof. P.M. Narins, UCLA), the banner-tailed kangaroo rat of the Chihuahuan Desert (with Prof. J.A. Randall, California State University San Francisco), the Namib golden mole and Cape mole rat, both of Southern Africa (with P.M. Narins and Prof. J. J.U.M. Jarvis, Capetown University). Instrumentation for the physiological studies of the seismic sense was provided by a grant from the National Science Foundation. The early field studies of the white-lipped frog were supported by another NSF grant. Subsequent field studies were supported, in part, by grants from the UC Berkeley Committee on Research (State of California research funds). They also were supported, in part, out of pocket (i.e., all PI travel expenses). Some of the work on frog vestibular physiology (that of Dr. S.F. Meyers) was supported in part by a grant from NASA; and some of the work on turtle auditory physiology was supported in part by an NIH postdoctoral award to Dr. M.G. Sneary. Grant NS12359/DC00112 supported the rest of the work. This included data analysis for our continuing field studies, some student travel to Puerto Rico, all of our vestibular physiological studies of the 1990s, all of our studies of frog and gerbil auditory physiology (the vast bulk of our research throughout the 1980s and 1990s), and all of our recent re-analysis of turtle auditory physiology. It provided our core support throughout the 1980s and 1990s.

Among the publications attributable to the project's present funding period (07/01/96 - 06/30/01) are two that summarize and place in context much of what we have done over the past 25 years with regard to the three main acoustic sensors of the frog. One (Lewis and Narins, 1999) covers the morphology and physiology of the three sensors (frog saccule, basilar papilla and amphibian papilla). The other (Lewis et al, 2001) covers pretty much the entire history of our physiological and behavioral studies of seismic communication in the white-lipped frog. A third publication (Cortopassi and Lewis, 1998) pretty well summarizes our vestibular research since 1991 and our reasons for undertaking it; and a fourth (Lewis, Henry and Yamada, 2000), summarizes and places into context much of our work on the gerbil cochlea since 1994.

During the past two decades, single-unit physiology of the eighth nerve has nearly disappeared in hearing and vestibular research. Over those same decades, powerful new signal -processing and analytical tools have emerged. Our commitment to single-unit studies has allowed us often to be the first to apply those new tools to the eighth nerve. We have done so largely as biologists and electrical engineers, always trying to maintain the perspectives of both disciplines (for example, on the one hand asking what sort of functions a pair of ears has to perform in order to be selectively advantageous to its owner, and on the other hand asking what sort of signal-processing schemes would a master electrical engineer design in order to achieve those functions). This approach has led us to what I believe is a much more complete picture of what it is that the ear tells the brainstem, through the eighth nerve. We have focussed on what it is that the ear does-- not on how it does it. I firmly believe that it is there-- at the level of what the ear does, that our understanding of the ear must begin. It is there that the results of this project have contributed to biological science and to health science.