Within the first few months after birth, infants can make use of features such as texture, shape and size, they can segregate objects based on their relative motion against a background, and they can use physical and subjective contours to segregate and/or discriminate one visual object from another ( Atkinson & Braddick, 1992 Sireteanu & Rieth, 1992 Kaufmann-Hayoz, Kaufmann, & Stucki, 1986 Curran, Braddick, Atkinson, Wattam-Bell, & Andrew, 1999 Ghim, 1990 Kavšek & Yonas, 2006 Otsuka & Yamaguchi, 2003 Yonas, Gentile, & Condry, 1991). Previous research indicates that from a very young age, infants are able to segregate a complex visual scene into representations of the objects in the scene (for a review see: Atkinson, 1998). Here we examine infants' ability to tell whether there are one or two auditory objects present on the basis of auditory harmonicity cues, by capitalizing on their abilities to understand small numbers and to match the number of auditory and visual objects in the stimulus. Infants must learn to encode and represent the relevant information from the sensory input in each modality in order to make sense of and interact with people and things in their environment. Most environments consist of complex multisensory scenes containing objects with both audible and visible properties. The young infant's ability to organize and process the sensory world is fundamental to virtually all aspects of development. We conclude that infants use harmonicity as a cue for source separation when integrating auditory and visual information in object perception. Four-month-old infants showed surprise at the incongruous pairings, looking longer at the display of two balls when paired with the in-tune complex and at the display of one ball when paired with the mistuned harmonic complex. We paired in-tune and mistuned complex tones with visual displays that contained either one or two bouncing balls. In the present study we use an audiovisual procedure to investigate whether infants perceive a complex tone with an 8% mistuned harmonic as emanating from two objects, rather than merely detecting the mistuned cue. They behaviourally discriminate a complex tone with a mistuned harmonic from the same complex with in-tune harmonics, and show an object-related event-related potential (ERP) electrophysiological (EEG) response to the stimulus with mistuned harmonics. Previous work by our group demonstrated that 4-month-old infants are also sensitive to this cue. Consequently, adults hear a mistuned harmonic in a complex sound as a distinct auditory object ( Alain et al., 2003). One important cue to this source separation is that complex tones with pitch typically contain a fundamental frequency and harmonics at integer multiples of the fundamental.

Sounds emitted by different objects sum in the air and the auditory system must figure out which parts of the complex waveform belong to different sources (auditory objects). Here we use a behavioural method to examine infants' use of harmonicity cues to auditory object perception in a multisensory context. Infants learn to use auditory and visual information to organize the sensory world into identifiable objects with particular locations.

Intermodal perception