Lori is a 20-month-old toddler who is brought to your clinic by her parents, who are concerned that she has not begun to say real words. Although she babbles strings of sounds, such as [baba], [maba], [toto], and [gaga], she does not evidence true words nor does she impose intonation or rhythmic patterns on the babbles. The parents report that just recently (within the last 2-3 weeks), Lori occasionally imitates a babble that she has just produced if the parents have her attention and immediately say her babble back to her. What prelinguistic stage is Lori in?