Experiment 1 derived mathematical models for estimating the neural rate representation of changes in the second formant (F2) frequency of the vowel /ε/. Models were based on linear fits to response patterns of auditory-nerve fibers with high, medium and low spontaneous rates (SRs), as characterized in previous electrophysiological studies of anesthetized cats (Le Prell et al., 1996). Simulations were run at several vowel levels in quiet and in the presence of continuous background noise. Noise levels were adjusted to produce a constant signal-to-noise ratio (S/N) of 3 dB at each vowel level. A signal detection analysis of model outputs suggested that auditory-nerve fibers with low SR provided the best rate representation of changes in F2 frequency at higher vowel levels and in background noise. Experiment 2 examined the predictions of the auditory nerve model by measuring psychophysical thresholds for F2 frequency changes (ΔF2) in cats. Behavioral tests were performed at vowel levels of 31, 51, and 71 dB in continuous background noise at S/Ns of 3, 13, and 23 dB. ΔF2 increased with decreasing S/N at each of these three vowel levels. Trends in behavioral performance corresponded well with the quality of vowel representations that are provided by high SR auditory-nerve fibers at low vowel levels and low SR fibers at moderate-to-high levels.
Keywords: frequency discrimination; signal detection; speech encoding; speech perception.