Abstract:
Auditory-nerve fiber responses were modeled to estimate neural representations of frequency changes in the second formant ((Delta)F[inf 2]) of the steady-state vowel /eh. Simulations were performed at vowel levels of 33--84 dB, both in quiet and in continuous noise at a constant 3-dB signal-to-noise ratio (S/N). Signal detection analysis of model outputs suggested that formant changes at higher stimulus levels and in background noise were better represented by rate responses of fibers with low spontaneous rates (SR). Psychophysical tests were then performed in cats to measure behavioral thresholds for the detection of formant frequency changes under stimulus conditions similar to those of the neural simulations. Behavioral (Delta)F[inf 2]'s were obtained at vowel levels of 10--70 dB, in quiet and in continuous background noise at S/Ns of 3, 13, and 23 dB. (Delta)F[inf 2] decreased with increasing vowel level and increased with decreasing S/N. These trends in psychophysical performance paralleled changes in the quality of vowel representations, particularly those based on discharge rates of low SR auditory-nerve fibers. [Work supported by NIDCD Grant Nos. 5R01DC01388-04 and 2R01DC00109-22.]