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The discriminability of small changes in amplitude and frequency of a pure tone auditory stimulus is examined in terms of statistical estimators of neural coding parameters. The neural spike train encoding process is described in terms of a multimodal histogram of interspike intervals. Such a multimodal process has the important property that both stimulus amplitude and stimulus frequency can be encoded in the same spike train. Performance of the neural coding parameters is evaluated using the CramÃ©r-Rao bound as a measure of discriminability of statistical estimators. The dependence of the Weber fraction for stimulus frequency and stimulus amplitude is derived as a function of frequency, amplitude, and observation interval. These results are compared to results of psychophysical experiments. It is concluded that while statistical estimates based on one primary auditory nerve fiber would be insufficient to account for psychophysical performance, estimates based on about 10-12 fibers sampled in parallel would be sufficient.