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Limitations on rate discrimination
Journal of Acoustical Society of America, 112(3 Pt 1), 1009-1025
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We investigated the limits of temporal pitch processing under conditions where the place and rate of stimulation on the basiliar 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 F0. When the harmonics were filtered between 3900-5400mhz, presented monaurally, and summed in sine phase, subjects could perform the task at all F0s studied. However, when the pulse rate was doubled by summing components in alternating phase, thresholds increased with increasing F0 until the task was impossible at F0=300Mhz (pulse rate = 600pps). Thresholds improved again at higher F0s, presumably because some harmonics became resolved. The F0 at which this breakdown occurred decreased when the complexities were filtered into a lower frequency region, and increased when they were filtered into a higher region. In the highest region rested (7800-10800 Hz), all listeners could detect an increase of less than about 20% re: a pulse rate of 600pps for alternating-phase complexes. Presenting a copy of the standarrad (lower-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 whern a bin-aural cue is available. The results are compared to the inability of most cochlear implantees to detect impulses in the rate of electrical pulse trains above about 300pps. It is concluded that this inability to result entirely from a central pitch limitation, because, with analogous stimulation, normal listeners can perform the task at substantially higher rates.