Effects of stimulus level on nonspectral frequency discrimination by human subjects.

Frequency difference limens were determined as a function of reference-stimulus level for pulsatile electrical stimuli in 5 postlingually deaf human subjects with Nucleus-22 cochlear implants and for sinusoidally amplitude-modulated acoustic white noise stimuli in 4 normal-hearing humans. Subjects were tested at levels throughout the dynamic range and extending to the lowest detectable levels. Response stability was measured over the course of 10 sessions. For electrical stimulation in the deaf ears, difference limens decreased as a function of level throughout much or all of the dynamic range of hearing. This result contrasts with the case for nonspectral acoustic stimulation of normal-hearing subjects, where nonspectral frequency difference limens were strongly affected by level only near the detection threshold. These data suggest differences in the acoustic and electrical response spaces that must be considered in the design of auditory prosthesis processors.