Masashi Kawasaki was killed in an auto accident in Madison County, Virginia, United States on May 18, 2025Footnote 1—just a few days before his 70th birthday. His wife Yasuko and her sister, Ryoko Saito, were also involved in the accident and sustained serious but non-life-threatening injuries. His sudden passing was a profound loss to the international community of neuroethology and to the University of Virginia, Charlottesville. Kawasaki is best known for his research on neural mechanisms underlying echolocation and electroreception. His close association with the Journal of Comparative Physiology A is reflected by the publication of nearly half of his papers in this journal, including many highly-cited and influential papers. In addition, he served on its Advisory Board from 2012 until the time of his death.

Family and upbringing

Masashi Kawasaki was born in Tokyo, Japan, on June 1, 1955, but he grew up mostly in Chigasaki, a seaside town about 60 km southeast of Tokyo. Both of his parents were professional musiciansFootnote 2, which sparked in him a lifelong interest in music. He also developed an early fascination for biology, likely triggered by the dogs, cats, and other animals that were part of the family at his parents’ home. When he was in middle school, he set up his first tank with fish. His desire to become a biologist and study the behavior of animals was strongly reinforced when Konrad Lorenz, Niko Tinbergen, and Karl von Frisch were awarded the Nobel Prize in Physiology or Medicine in 1973, for their work on how behavior is organized and elicited.

Undergraduate and graduate education

In 1975, Masashi enrolled as an undergraduate student in the biology degree program at Waseda University in Shinjuku, Tokyo. After graduating with a bachelor’s degree in 1979, he was admitted into the doctoral program at Sophia University, a private Jesuit research university in Tokyo. During his graduate study, in 1982, he married Yasuko, whom he had known since high school. One year later, their first child, Tomomi, was born.

For his thesis research, Masashi joined the laboratory of Kiyoshi Aoki. In his first project, he examined, through electrophysiology, the responses of single units in the optic tectum of the Japanese dace (Tribolodon hakonensis) to stationary and moving visual stimuli. The results of this study were published in the Journal of Comparative Physiology A (Kawasaki and Aoki 1983).

For the second project of his thesis research, Masashi continued studying the visual system of fish. However, these experiments were carried out in the laboratory of Ken-Ichi Naka at the National Institute for Basic Biology in Okazaki. Employing again electrophysiological methods, he examined how background illumination affects the sensitivity of horizontal cells in the retina of channel catfish (Ictalurus punctatus). The assumption at that time in sensory physiology and psychophysics was that the presence of background illumination depresses the sensitivity of retinal neurons proportionally to the intensity of illumination, an effect predicted by the Weber-Fechner law. However, this ‘law’ does not apply universally, as some experimental evidence had suggested that background illumination can have the opposite effect by enhancing the response of retinal neurons to test flashes (Burkhardt 1974; Hartline and Lange 1974). In human psychophysics, such a sensitizing effect of background illumination is referred to as the Westheimer effect (Westheimer 1970).

Masashi found in I. punctatus that, contrary to the Weber-Fechner law but consistent with the Westheimer effect, background illumination enhances the response of horizontal cells to both increments and decrements in light pulses or to white-noise modulation (Kawasaki et al. 1984). However, such an enhancement was not evident in cones. Masashi and his co-authors hypothesized that the enhancement of the sensitivity of horizontal cells by background illumination serves as a mechanism to increase the retina’s ability for discriminating changes in visual stimuli against ambient illumination.

Postdoctoral research on bat echolocation in the laboratory of Nobuo Suga

After Masashi had received his PhD in 1983, he left Japan for the United States, to pursue doctoral research —first with Nobuo Suga and later with Walter Heiligenberg. He had met each of the two scholars when they visited the lab of Kiyoshi Aoki while Masashi worked there as a graduate student.

Masashi joined Suga’s lab at Washington University in St. Louis, Missouri, in 1984, staying for 2 years. The focus of Suga’s research was on the neurophysiology of echolocation in bats, especially the neural processing of distance information and multiple harmonic components in echoes. Bats use echoes of self-generated sound to extract various pieces of information about the complex environment, including distance to a target. James Simmons had shown earlier that echolocating bats perceive distance by measuring the time delay between the emitted pulse (in most bat species a frequency-modulated sweep) and the returning echo (Simmons 1971, 1973). Subsequently, Feng et al. (1978) and Neill and Suga (1979) discovered specialized neurons in the central nervous system of echolocating bats that respond preferentially to specific time delays between the pulse and the echo. These neural cells are commonly referred to as ‘delay-tuned neurons.’

At the time of Masashi’s arrival in Suga’s lab, one limitation of all previous studies on delay-tuned neurons was that they had been conducted using only synthesized sounds and echoes delivered through loudspeakers as substitutes for the sounds and echoes occurring during active vocalization of bats. Addressing this limitation, Masashi and his co-authors, Daniel Margoliash and Nobuo Suga, were the first to examine systematically the properties of delay-tuned neurons in the auditory cortex of actively vocalizing bats. They showed that cortical neurons in the mustached bat (Pteronotus parnelli) exhibit harmonic combination sensitivity and delay tuning similar to that observed in silent bats stimulated with synthetic sounds. In particular, each harmonic in the echo produces its own delay estimate. This seminal study stimulated further work on the perceptual and computational effects of the individual harmonics in complex echoes. One important technical contribution of the study by Kawasaki et al. (1988a) was that the authors conducted the experiments in an echo-free environment — they put the bat on a platform hung outside the laboratory window, so that extraneous echoes were eliminated.

Postdoctoral research on the neuroethology of electric fish in the laboratory of Walter Heiligenberg

In 1986, Masashi moved from St. Louis to La Jolla, California, to join the lab of Walter Heiligenberg. In retrospect, this decision propelled Masashi’s research career tremendously, as Heiligenberg’s laboratory at the Scripps Institution of Oceanography (part of the University of California, San Diego) entered at that time its most productive phase. Masashi contributed to this success story significantly, with a total of 10 papers resulting from the research work he carried out over the following 4 years. It was also during that time (in 1988) that Masashi’s and Yasuko’s second child, a son named Daigo, was born.

A key factor in Masashi’s success was that he and Walter matched very well. Each of them not only was an excellent electrophysiologist but also an exceptionally gifted craftsman who liked to design and build devices needed for experimental setups and to write complex codes for data registration and analysis — at a time when commercial software programs for carrying out these tasks were still in their infancy. Beyond the lab, they shared a love for classical music. Walter played the piano very well, and his wife Zsuzsa was a concert pianist and private music instructor. Walter had a deep appreciation for Japanese culture, language, and food. Both Walter and Masashi enjoyed having found a partner for discussing, for example, the etymology of Japanese names. Tragically, they also shared with each other the fact that their lives were taken by accidents that were beyond their controls. Walter Heiligenberg was killed when a U.S. Air Boeing 737 crashed near Pittsburgh, Pennsylvania in 1994 (Zupanc and Lamprecht 1994; Zupanc and Bullock 2006).

One of the areas in which Masashi’s work in the Heiligenberg lab left a lasting impact on other researchers concerned the identification of neural mechanisms of communication behavior in weakly electric fish. Heiligenberg and his students, as well as scientists in other labs, had already shown that weakly electric fish of the genera Eigenmannia and Apteronotus produce during courtship and aggressive encounters certain frequency-amplitude modulations of the otherwise highly regular electric organ discharges. These modulations were, therefore, believed to function as communication signals. Neuroanatomical evidence had suggested that these signals are controlled by a nucleus in the dorsal thalamus — the prepacemaker nucleus — which is the only brain site that provides input to the pacemaker nucleus. The neural oscillations of the latter nucleus, located in the brainstem, drive the discharges of the electric organ.

In their first joint study, Kawasaki and Heiligenberg (1988) demonstrated that electrical stimulation of the prepacemaker nucleus elicits modulations of the electric organ discharge indistinguishable from those produced by the natural communication signals, thus corroborating the idea that it is, indeed, the prepacemaker nucleus that controls these behaviors. In a follow-up study carried out in collaboration with Walter Heiligenberg, Leonard Maler (a visiting professor from the University of Ottawa) and Gary Rose (another postdoc in the Heiligenberg lab), Masashi combined neurophysiological stimulation techniques with neuroanatomical labeling of cells in the prepacemaker nucleus by neural-tract-tracing from the pacemaker nucleus (Kawasaki et al. 1988b). They showed that iontophoresis of L-glutamate evokes different types of communication signals from different subnuclei of the prepacemaker nucleus, characterized by morphologically different neurons. In two subsequent investigations, Masashi, in collaboration with other members of the Heiligenberg lab, provided experimental evidence that the generation of distinct types of electric communication signals is mediated by different classes of glutamate receptors (Dye et al. 1989; Kawasaki and Heiligenberg 1990). Taken together, these four studies inspired many other investigations aimed at identifying neural mechanisms that underlie the premotor and motor control of communication behaviors in vertebrate organisms.

A second area in which Masashi made lasting contributions relates to the fundamental question of how temporal acuity at the behavioral level is encoded neurally. Since behavioral acuity is superior to that of individual sensory receptors, central neural integrative mechanisms must be involved. Masashi, Rose, and Heiligenberg addressed this issue by taking advantage of the jamming avoidance response in the weakly electric fish Eigenmannia. This reflex-like behavior is performed when two fish meet with similar frequency of their electric organ discharges. Then, each of the two fish shifts the frequency of its discharges away from the frequency of the neighboring fish. Thus, to correctly perform this behavior, it is necessary that each fish determines the sign of the frequency difference between the two electric signals. To achieve this task, the fish analyzes modulations in both phase differences and amplitude that result from the interference of the two electric signals. Phase differences are detected as differences in the firing of zero-crossings of signals received at different parts of the fish’s body surface. Behavioral experiments have shown that Eigenmannia can detect modulations in phase difference, i.e., temporal disparities, smaller than 1 µs (Rose and Heiligenberg 1985).

Using a sophisticated experimental design, Kawasaki et al. (1988a, b, c) created temporal disparities between two parts of the body of varying size, while recording the neural responses of ‘sign-selective’ neurons (neurons that detect the sign of the frequency difference between the fish’s signal and the neighbor’s signal) in the prepacemaker nucleus. They found that single sign-selective neurons are sensitive to such temporal disparities as small as 1 µs — the highest temporal sensitivity ever observed at the single-cell level at the time their study was published!