Pitch Discrimination of Jittered Pulse Trains

Abstract

Listeners were presented with pulse-train stimulus pairs and asked whether they could hear a difference between them. The interval between pulses was random, identically and independently distributed for each stimulus of a pair. Two distributions were observed one nominally Gaussian, the other nominally the distribution of the amplitude of a sinusoidal wave whose phase is uniformly distributed. The principal experiment parameters were the mean interval between pulses and root-mean-square deviation or jitter about this interval. The stimuli of each pair were identical in other respects[long dash]pulse shape (50-[mu]sec. pulse width), loudness (30 or 35 dB sensation level)[long dash]but differed in polarity pattern. Two pattern combinations were observed. For the range of mean pulse intervals investigated, 5-15 msec, the stimuli of each pair were generally discriminable when unjittered. However, the results indicate that, when jittered in amounts greater than 1 or 1.5 msec, the stimuli might be non-discriminable. This critical amount of jitter coincides with a flattening of the power-density spectra of the stimuli for frequencies greater than 250 or 200 cps. The hypothesis is presented that the correlates of discrimination for unjittered or lightly jittered stimuli are distinct neural volley patterns associated with basilar-membrane activity in the 300- to 1000-cps region.