Perception of syllable timing by prebabbling infants.

Adults hear alternating syllables with isochronous syllable onset-onset times as having a long-short, alternating rhythm when the syllables differ in initial consonant. This occurs because adults attend to syllable-internal events, called the "P centers" or "stress beats", rather than to syllable onsets. Thus they report that stress-beat aligned speech is isochronous and stress-beat aligned clicks are synchronized with the speech. The question asked here is whether, like adults, infants attend to the timing of syllable stress beats. In experiment 1, infants showed differences in time to habituate to sequences of alternating monosyllables, [bad] and [strad], having two different onset-onset times (onset- and stress-beat-timed) and two different placements of clicks on the syllables (on syllable onsets and on stress beats). Infants habituated more slowly to sequences with clicks on the stress beats than to sequences with clicks on syllable onsets and most slowly of all to stress-beat-timed speech with clicks on the stress beats. To interpret these findings, a second experiment was run using sequences only of the syllable [strad] so that speech timing measured according to onsets and stress beats was the same. Syllables had isochronous timing or a long-short alternating rhythm, corresponding to two possible ways of hearing the stress-beat-timed speech of experiment 1. In addition, two patterns of click placement were compared, uniform and syncopated, corresponding to two ways of hearing the stress-beat aligned clicks of experiment 1. The patterns of sucking times in the two experiments match exactly if stress-beat aligned speech in experiment 1 is identified with the isochronous speech of experiment 2 and the stress-beat aligned clicks of experiment 1 match with the uniformly timed clicks of experiment 2. It is inferred from this correspondence that infants perceive stress beats and stress-beat timing of syllables as adults do.