2.2. Stimuli

Following Berger et al. (Berger et al., 2006), we presented the arithmetical equations using a videotape of a puppet theater (see Fig. 1). Preceding each trial, a colorful display was presented to attract the infant’s attention until the experimenter was confident that the infant was looking at the monitor. Each trial began with one or two puppets displayed for 4 seconds. A screen then came upand a hand appeared either inserting or removing one puppet from behind the screen. After this operation was completed, the soundtrack was silenced, and the screen stayed up for 600 ms, during which the baseline in the ERP analysis was calculated; the screen was then lowered revealing the solution to the equation. The positions of the puppets on the stage (right vs. left) were counterbalanced, and the different trial conditions (correct vs. incorrect solutions) were presented in a pseudorandomized order. Looking time was not collected during the ERP task because collecting these behavioral data would have altered the fixed time the stimulus(i.e., the solution to the equation) was presented. It would have required leaving the stimulus on the screen until the infant looked away, thereby lengthening the time needed to administer the task. In pilot testing, subject loss was increased because many infants soon lost interest in the longer version of the task.