Infants' perception of illusions in sound localization: reaching to sounds in the dark

Sixteen infants each at 4, 6, and 8 months of age were tested for reaching to sounding toys in the dark under two auditory illusion conditions: the Haas-effect, which creates the illusion of a single lateralized sound based on an interaural intensity difference (the toy was visible and invisible under some test conditions); and the midline illusion, which creates the illusion of a single sound at midline due to an absence of any interaural time or intensity differences (invisible toy condition only). No-sound control trials indicated the level of spontaneous reaching in the dark. Results indicate that by 4 months infants perceive both the Haas-effect and midline illusions. The ability to reach both for invisible and visible sounding objects in the dark was well developed by 4 months of age, although developmental changes in aspects of reaching behavior were observed and, at all ages, object contact was most frequent when visual localization cues accompanied sound localization cues. The incidence of spontaneous reaching in the dark was low and did not vary with age. Theoretical and methodological implications of this research are discussed.