Two distinct physiological mechanisms underlying loudness sensation were inferred from electric stimulation of the human auditory nerve and brainstem. In contrast to a power function relating loudness and stimulus intensity in acoustic hearing, loudness in electric stimulation of the auditory nerve depends on stimulus frequency. Loudness is an exponential function of electric amplitude for high frequencies and is a power function for low frequencies. A frequency-dependent, two-stage model is suggested to explain the loudness function, in which the first stage of processing is performed by a mechanical mechanism in the cochlea for high-frequency stimuli and by a neural mechanism in the cochlear nucleus for low-frequency stimuli.