are now professors in the Neuroscience and Psychology Departments at Brown University, respectively. Simmons developed methods for conducting psychophysical studies of sonar processing by bats, and researchers around the world have adopted these methods to address a wide range of research questions. Simmons was the first to use electronically delayed playbacks of the bat's echolocation signals to simulate target echoes for the study of perception in bats. He used such "phantom" target echoes to measure, for the first time, target range difference discrimination thresholds of echolocating bats. Simmons studied sonar ranging performance in bats by determining the minimum difference in echo delay that an echolocating animal can discriminate. With this paradigm, Simmons estimated that bats can discriminate a range difference of approximately 1 centimeter, corresponding to an echo delay difference of approximately 60 microseconds. Simmons continued to study sonar ranging performance in echolocating bats, and in the late 1970s, he introduced a new behavioral task, requiring the bat to discriminate a sonar target returning echoes at a fixed delay from one returning echoes that alternated between two delays. In this experiment, Simmons found that the echolocating bat can discriminate a jitter in echo delay in the submicrosecond range, corresponding to a change in target distance of less than 0.1 mm. This result, originally published in 1979 in Science, "Perception of echo phase information in bat sonar," demonstrated astonishing ranging accuracy by the echolocating bat. Many researchers in the field challenged the report, because they asserted it was not biologically possible for the bat's sonar system to discriminate such small time differences at ultrasonic frequencies. Simmons continues to work on this problem to explore biological processes that could support sensitivity to small changes in echo delay. Through behavioral experiments, Simmons demonstrated time-varying gain in the sonar receiver of echolocating bats. The hearing sensitivity of the big brown bat decreases before each sonar pulse is emitted and then recovers in a logarithmic fashion to compensate for the two-way transmission loss of sonar returns, thereby maintaining a constant echo sensation level over a distance of about 1.5 meters. This is functionally important to the bat, as it stabilizes the bat's estimate of echo arrival time, its cue for target distance. In addition to Simmons's contributions to our understanding of perception by sonar, he has conducted neurophysiological experiments in echolocating bats. One of his manuscripts published with co-authors Albert Feng and Shelley Kick in