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Marine Biological Laboratory, Woods Hole, Massachusetts 02543 and Loyola University Chicago, Chicago, Illinois 60626
Both the grunt and the boatwhistle sounds of toadfish are a series of rapidly produced sound pulses. These sounds can therefore be described, not only by their spectral components, but also by the pulse repetition rate (pulses/s) or interval between the beginning of consecutive pulses (interpulse interval, IPI). Indeed, some of the information conveyed in these communication sounds may be encoded in the pulse repetition rate or in the IPI (1). As part of our investigation on auditory processing in the toadfish (e.g., 2), we have evaluated the sensitivity of the auditory midbrain (torus semicircularis, TS) to sound pulses produced at several periodic rates. The stimuli for our previous experiments have been various frequencies of sinusoidal particle motion (described in 3). Here, we have used sound pulses to determine whether cells in the TS are selective for IPI or pulse rate.
Fish were anesthetized with 3 aminobenzoic acid (1:1000, Sigma), immobilized with an injection of pancuronium bromide (0.05 mg/kg); and lidocaine was applied to the skin prior to surgery. After its left midbrain region was exposed, the fish was secured to a seawater-filled dish in the stimulus shaker system (3). Glass micropipettes were broken to obtain low impedance electrodes (5–20 megohms) when filled with 4% neurobiotin in 2 M NaCl for determining electrode tip locations. All recordings were made in the left torus semicircularis.
The stimulus pulse was a 150-Hz sinusoid multiplied by the first half-cycle (10 ms duration) of a 50-Hz cosine. This produced a pulse as brief as possible, but still having frequency components low enough to stimulate the toadfish auditory system. Pulse trains were produced at 13 repetition rates between 10 and 83 pulses/s, or IPIs of 12–100 ms. The shortest IPI was dictated by the pulse duration (10 ms), because higher rates would have resulted in pulse coincidence. The pulse trains (500-ms duration) were presented 8 times each, in random order, at several levels (peak-peak displacement in dB with respect to 1 nm) within a cell’s dynamic range. The total number of spikes produced for each set of 8 pulse-train repetitions was measured at each level.
Responses were classified as selective or nonselective. A selective unit was defined as one for which the number of spikes evoked by the most effective IPI was at least 2.5 standard deviation units above the average number of spikes for all pulsed stimuli, and for which the most effective IPI was independent of sound intensity. Of 71 units evaluated, 43 (61%) were selective and the rest non-selective. Of the selective units, 56% responded best at the shortest IPI (12 ms, or 84 pulses/s). The remaining selective units responded best to IPIs between 16 and 50 ms, with a secondary mode at 20 ms.
Figure 1 shows the pulse response functions for three selective units (panels A–C). Panel A shows the data for one unit that responded best to the shortest IPI (12 ms). Panel B shows a unit that is clearly most selective to a 20-ms IPI, and panel C shows a unit responding best to a 40-ms IPI.
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Research funded by an NIH, NIDCD R01 grant to R. Fay (1 R01 DC 03666-4), and by an NIH, NIDCD Program Project Grant (1 P01 DC 00293-11) to the Parmly Hearing Institute.
Literature Cited
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