Limitations on rate discrimination

We investigated the limits of temporal pitch processing under conditions where the place and rate of stimulation on the basilar membrane were independent. Stimuli were harmonic complexes passed through a fixed bandpass filter and resembled filtered pulse trains. The task was to detect a difference in $\text{F0.}$ When the harmonics were filtered between 3900–5400 Hz, presented monaurally, and summed in sine phase, subjects could perform the task at all $\text{F0}\mathrm{s}$ studied. However, when the pulse rate was doubled by summing components in alternating phase, thresholds increased with increasing $\text{F0}$ until the task was impossible at $\text{F0=300\hspace{0.05em}}\mathrm{Hz}$ (pulse rate=600 pps). Thresholds improved again at higher $\text{F0}\mathrm{s},$ presumably because some harmonics became resolved. The $\text{F0}$ at which this breakdown occurred decreased when the complexes were filtered into a lower frequency region, and increased when they were filtered into a higher region. In the highest region tested (7800–10 800 Hz), all listeners could detect an increase of less than about 20% re: a pulse rate of 600 pps for alternating-phase complexes. Presenting a copy of the standard (lower-$\text{F0)}$ stimulus to the contralateral ear during all intervals of a forced-choice trial improved performance markedly under conditions where monaural rate discrimination was very poor. This showed that temporal information is present in the auditory nerve that is unavailable to the temporal pitch mechanism, but which is accessible when a binaural cue is available. The results are compared to the inability of most cochlear implantees to detect increases in the rate of electrical pulse trains above about 300 pps. It is concluded that this inability is unlikely to result entirely from a central pitch limitation, because, with analogous acoustic stimulation, normal listeners can perform the task at substantially higher rates.