Effects of fundamental frequency contrast on discrimination and identification of dichotic CV syllables at various temporal delays

Abstract

Dichotic CV syllables (identical and nonidentical pairs) were presented at nine temporal offsets between 0 and 500 msec. One task consisted in judging quickly whether the syllables in a pair were phonetically the same or different; the other task was to identify both syllables. The fundamental frequency (pitch) of the synthetic stimuli was either the same or different, and either predictable or unpredictable. The pitch variable had surprisingly little effect on the latencies of "same"-"different" judgments, and the expected "preparation" effect of pitch predictability was ba]rely present. Instead, there were strong effects on the frequencies of errors at short temporal delays, which suggests shifts or biases in the phonetic "same"-"different" criterion with context. A comparison with analogous errors in the identification task revealed identical patterns. Further analysis of identification errors showed no overall "feature sharing advantage": The direction of this effect depends on the kind of error committed. Also, a lag effect was found only in nonidentical pairs that received two identical responses. The results are discussed in the framework of a two-stage information-processing model. Effects of pitch are tentatively explained as biases from implicit (pitch) decisions at the auditory level on phonetic decisions in the presence of uncertainty. Four sources of errors are identified: fusion at the auditory level; "integration," confusions, and transpositions at the phonetic level.