Mom sits here at the table, with Baby in her lap. An assistant sits across the table, moving a toy that keeps the baby's attention. From the speaker comes 'La la la la la' at one-second intervals. Baby is listening, but watching the toy. A teddy bear begins to dance and beat vigorously on a drum.
Baby learns that when the sound changes, the bear performs.
They turn to watch and get this fancy reward. At 6 months, two out of three Japanese and American kids turn to look. To them, the two sounds have become the same. The light goes out and the bear puts down his drumsticks. Back in the mid-'70s, when she was a postdoctoral scholar at the Central Institute for the Deaf in St. Louis, Kuhl showed that monkeys responded to sound changes in the same way, although of course they could not produce recognizable speech.