Abstract.-The auditory sensitivity of 4 specimens of the bullhead catfish (Ictalurus nebulosis) was determined by shock-avoidance training in an aquatic shuttle box. The range of hearing extended from 100 to 4000 cycles per second, with the maximum sensitivity around 600 to 700 cycles per second.Previous work by Stetter1 indicated that bullhead catfish, then called Ameiurus nebulosus, had a hearing range extending to 13,139 cycles per second (Hz). Poggendorf2 also tested the auditory range of a single specimen of bullhead catfish but did not find sensitivity beyond the 5000 Hz limit commonly reported in the general literature for fishes. Because the structure of the fish ear, which involves macular organs weighted with large otoliths, does not appear to be suitable for high frequency response,3 the auditory range of the bullhead catfish needs careful reassessment.Procedure. The unconditioned shock stimulus was provided by the 110 v a-c line stepped down through an isolation transformer and a variable autotransformer and switch-operated to give 0-24 v a-c pulses between wire-grid electrodes on the sides of the tank. The shock level was found to be most effective at 5 v a-c.The intertrial interval, the trial duration, and conditioned-unconditioned stimulus interval were monitored by two Standard Electric Time Company model S-1 clocks. A Digital Electronics Company Digiac 3010 computer controlled the intertrial interval, the conditioned-unconditioned stimulus interval, and the presentation or termination of both the tonal and shock stimuli. The computer was programmed to vary the intertrial interval between 47-, 60-, and 73-second periods to 1)r'Vellt, time conditioning. Six Clairex type CL707 HI, photocells, placed in a vertical array three on each side of the barrier opposite 12 v bulbs, detected the movement of the subject across the barrier and provided the d-c pulse required to trigger the computer. The lamps and photocells were powered by three Electro Products model EC-2 power supplies.
A conditioned suppression technique has been applied to five agoutis and behavioral thresholds measured. Each animal was water deprived and allowed to drink only in the experimental situation. Training was effected by pairing audible tone pulses with shock until the subject learned to cease drinking reliably in the presence of a signal. Auditory thresholds were then assessed at octave intervals. Maximum sensitivity occurred at 8 kHz (0 dB re 2 × 10−4 dyn/cm2), a high frequency cutoff at 64 kHz, and a low-frequency cutoff at 500 Hz. Low-frequency sensitivity may have been impaired by masking or TTS resulting from the loud licking noises produced by this species, although control studies have not verified this hypothesis. It is the authors' opinion that hearing in the Agouti is oriented toward the higher frequencies despite the presence of certain low-frequency structures such as the large bulla. The observed high-frequency bias supports previous findings regarding this animal's basilar membrane.
The auditory range of the bullhead catfish was found to be from 100 to 5000 Hz. Peak sensitivity was at 700 Hz at 35 dB below 1 dyn/cm2. The upper limit of sensitivity was much below that found by previous investigators and seems to correspond with hearing functions explained by frequency and volley theory without a place mechanism that is anatomically lacking. [Research supported by NINDB and ONR.]
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