John P. Madden
Dept. of Speech and Hear., Cleveland State Univ., Cleveland, OH 44115
Normal-hearing subjects were asked to discriminate between two sinusoidal signals. One, a 50-ms glide, moved from its initial frequency over a linear path to its final frequency. The other, the step signal, was the same except that its trajectory followed a series of discrete steps in frequency. The smallest frequency increase per step (FIS) at which the step signal could be distinguished from the glide was determined. These FIS thresholds (FISTs) were obtained for signals with 2 to 11 steps at center frequencies of 0.25 0.5, 1.0, 2.0, 4.0, and 6.0 kHz. At center frequencies from 0.5 to 2.0 kHz, the FIST was roughly constant for signals with up to about 8 steps. Beginning at about 9 steps, the FIST rose sharply, eventually reaching the limits of the possible FIS. It is inferred that beginning at this point, the duration of the individual steps (5--6 ms) limits discrimination. At center frequencies above 2000 Hz, performance deteriorated markedly, unlike the results of gap detection studies. The implications of these results for mechanisms tracking FM signals are discussed. [Work supported by NIH.]