In this article, we devise a fidelity criterion for quantifying the degree of distortion introduced by a speech coder. An original speech and its coded version are transformed from the time-domain to a perceptual-domain using an auditory (cochlear) model. This perceptual-domain representation provides information pertaining to the probability-of-firings in the neural channels. The introduced cochlear discrimination information (CDI) measure compares these firing probabilities in an information-theoretic sense. In essence, it evaluates the cross-entropy of the neural firings for the coded speech with respect to those for the original one. The performance of this objective measure is compared with subjective evaluation results. Finally, we provide a rate-distortion analysis by computing the rate-distortion function for speech coding using the Blahut algorithm. Four state-of-the-art speech coders with rates ranging from 4.8 kbit/s (CELP) to 32 kbit/s (ADPCM) are studied from the view-point of their performances (as assessed by the CDI measure) with respect to the rate-distortion limits.