Habituation, sensitization, and infants' responses to motherese speech.

Four-month-old infants were tested for their visual responses to infant-directed (ID) speech versus adult-directed (AD) speech in a fixed-trial habituation procedure. In Experiment I, infants looked significantly longer in response to a 4 x 4 checkerboard pattern that was compounded with an ID speech segment than an AD speech segment. Looking times increased significantly between the first and second presentations of the ID speech segment only. In Experiment II, infants looked slightly more during ID than AD trials when the two were alternated from trial to trial. Responding to the first AD speech segment was significantly greater than when it was preceded by ID speech than when it was not, while responding to the first ID speech segment was significantly less when it was preceded by AD speech than when it was not. These findings are discussed in relation to the hypothesized differential arousing properties of ID and AD speech.