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Dick set out to see if this was true. To do so required a way to change the direction of otolith motion within the inner ear. He could combine this with his expertise in measuring the neural responses of the auditory nerves connected to the hair cells and he had a way of knowing which inner ear organ a nerve fiber came from. Thus, he would be able to show if and how the sensitivity of auditory nerves depended on the directional interaction of otolith motion and hair cell orientation. That is, he assumed that the sensitivity of the neural response would depend on the relative orientation of otolith motion to the orientation of the hair cell. The relative orientation that produced the largest neural response might provide a directional cue the brain could use to help in its deter- mination of a sound source location.
But, how does one produce otolith motion in different directions? And, how does one do so on the microscopic scale of displacements of hair cell stereocilia (100 pm to 1 μm)? The answer was the “Shaker Table” (see Figure 3) that was described in Dick’s Science (1984) paper. Dick designed and then supervised the building of a specially milled stimulating “dish” to which the fish’s head would be attached. The dish could be vibrated in all three direc- tions by two pairs of shakers oriented at 90° to each other and a large vibrator to move the dish vertically. Then, three sensitive accelerometers were attached to the dish to measure its motion. Finally, an optical displacement transducer was used to confirm that the dish and the fish’s head attached to the dish were moving in phase and at the required frequency (140 Hz).
It was no mean feat to get the system machined to the spec- ifications Dick required (which was done with the help of his brother at Grumman Aircraft Company in New York) and then to perform the meticulous measurements and calculations to ensure the accuracy of the fish’s head move- ments. Dick kept careful notes on all of the steps in the process in case another shaker table had to be built, which happened later when others wanted one for their studies (e.g., see Sisneros, 2016). Interestingly, before Dick was developing his shaker table, a device with similar shaker functions had also been developed by European scientists (Enger et al., 1973; Hawkins and Horner 1981).
The shaker table became almost a metaphor for Dick’s incredible contributions. Several other studies have used
this table (e.g., Lu et al., 1996; Meyer et al., 2011). Having the shaker table built was not an easy task for Dick, but Dick is not the type who is easily deterred by such mat- ters, as is evident by the success of the shaker table and Dick’s research productivity.
The results of Dick’s experiment (Fay, 1984) clearly showed that the auditory nerve responses of the gold- fish were directionally sensitive, with the pattern of sensitivity differing among the three inner ear receptor organs. And, as Dick concluded, “These are the essential features of a detection system that, after appropriate cen- tral processing, is capable of directional hearing, range determination, and impedance characterization.” A key part of his conclusion was “after appropriate central pro- cessing.” Little was known at the time about the central (brainstem and cortical areas) processing of sound by fish. So, in a typical Fay manner, Dick set out to know more.
A New Species
From the mid-1980s to the end of his career, a main topic of Dick’s research was better understanding of the fish’s auditory central nervous system. Over time, the bulk of this work was done during the summers in Dick’s lab at the MBL. One motivation for this work was to under- stand how the central nervous system might process the directional information he had measured in the auditory
Figure 3. Dick’s original shaker table with four vibrators (black) moving the dish in which the fish is placed in the horizontal direction and a large vibrator under the table (not visible) moving the dish in the vertical direction. The table is still in use in the lab of Joe Sisneros (University of Washington, Seattle). Picture courtesy of Joe Sisneros.
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