Fig. 2.

Infant eye-looking is time-locked to the rhythm of infant-directed singing. During infant-directed singing, we quantified the effects of beat, frequency, and amplitude on how 2-mo-old (A) and 6-mo-old (B) infants looked at the eyes of a singing adult. In both 2 mo olds (A) and 6 mo olds (B), infants significantly increased their looking to singers’ eyes, time-aligned to the beat of infant-directed singing. The observed increases in eye-looking time-locked to the beat were significantly greater in 6 mo olds (B) than 2 mo olds (A), quantified by permutation testing (SI Appendix). At each age, there was no significant change in time-locked eye-looking relative to moments of either high frequency (Upper Right) or high amplitude (Lower Right). Dotted lines in all panels of A and B show 5th and 95th confidence intervals for change in eye-looking expected by chance (one-sided). Note that the scale of the ordinate differs across peristimulus time histograms: plots are scaled to align by probability of observed results (aligned at the 5th and 95th confidence intervals of results expected by chance). Inset phase plots show individual phase of eye-looking for all infants. Phase plots indicate that effects of the beat are observable not only at the group level (as shown in peristimulus histograms), but also in the individual behavior of a majority of infants at each age. (C and D) Lissajous curve for probability of infant eye-looking versus beat phase for (C) 2-mo-old and (D) 6-mo-old infants. Traces at the right of each panel show direction of Lissajous curve travel over time. In Lissajous curves, mean looking probability is plotted in blue while gray areas denote ±1 SEM. In all traces, the arrowhead denotes mean response level at the beat (beat phase = 0), with trace thickness denoting direction of travel (thickening as time moves forward, resetting immediately after the beat). (E and F) Probability of infant saccades versus beat phase for (E) 2-mo-old and (F) 6-mo-old infants. Probability of eye-looking in 6-mo-old infants shows 1:1 synchrony with ∼/5.5 phase shift. Saccades in 6 mo olds are synchronized at two saccade periods per one beat period, with maximum increase prior to (in anticipation of) the beat. For additional Lissajous analyses, see SI Appendix.