P-center judgments are generally insensitive to the instructions given.

The perceptual moment of occurrence of a syllable, its P-center, has frequently been examined by instructing subjects to adjust a series of speech sounds until they sounded isochronous. The present two experiments examined the effect of changing the instructions. In addition to the overall isochrony instructions, we asked subjects to align pairs of syllables so that the syllable onsets, vowel onsets, or syllable offsets sounded isochronous. In the first experiment, 3 of 4 subjects showed no difference among the first three instruction sets, and the changes introduced by the fourth went in the wrong direction. All subjects found it impossible to make alignments with respect to offsets. In the second experiment, vowel durations of two versions of some stimuli differed by 100 ms, to enhance the difference in syllable rhyme durations. Two subjects received the same instruction sets as in Experiment 1, and again found alignment with respect to offsets impossible. These subjects showed differences among the other instruction sets, although the direction and magnitude of the differences indicated that they had not succeeded in changing their timing criteria. The results indicate that P-center alignments are the syllable timing judgments that subjects most naturally make, and they may, indeed, be the only isochrony judgments that subjects can make reliably.