. Tuning curves, spontaneous activity, and rate-intensity (RI) functions were obtained from units in the chick cochlear nerve. The characteristic frequency (CF) was determined from each tuning curve. The shape of each RI function was subjectively evaluated and assigned to one of four RI types. The breakpoint, discharge rate at the highest SPLs, and slopes of the primary and secondary segments were quantified for each function. The CF and RI type were then related to these variables. A new RI function was observed in which the discharge activity in the secondary segment diminished as stimulus level increased above the breakpoint. This function was called a "slopingdown" type. In 959 units, saturating, sloping-up, sloping-down, and straight RI types were identified in 39.2, 35.5, 12.6, and 12.7% of the sample, respectively. The slope of the primary segment was nearly the same in each of the four types and averaged 5.48 S ⅐ s Ϫ1 ⅐ dB Ϫ1 across all units. The slopes of the secondary segments formed four groupings when segregated by RI type based on the subjective assignments and averaged 0.03, 1.22, Ϫ0.90, and 3.95 S ⅐ s Ϫ1 ⅐ dB Ϫ1 in the saturating, sloping-up, sloping-down, and straight types, respectively. The data describing the secondary segments of all units were fit with a multicompartment polynomial and showed a continuous distribution that segregated, with some overlap, into the different RI categories. The proportion of RI types, as well as the secondary and primary slopes were approximately constant across CFs. In addition, it would appear that the other parameters that define the four types were, for the most part, homogeneously distributed across the frequency axis of the chick inner ear. Finally, a comparison of RI functions having a common CF suggested that the compressive nonlinearity that determines RI type may be a phenomenon localized to individual hair cells in the bird ear.

No abstract

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.

Lateral-line sensitivity in blind cavefish (Anoptichthys jordani) was determined by an avoidance-conditioning method. The frequency range of lateral-line sensitivity extends from 10-200 Hz. Maximum sensitivity occurs at 35 Hz. at a level of 8 db. below one dyne per square centimeter. Cavefish were considerably more sensitive than sighted goldfish indicating the possibility that the lateral line is used to navigate by detecting the displacement waves created by their own oscillatory swimming motion and reflected by targets.