title: Cortical representations sensitive to the number of perceived auditory objects emerge between 2 and 4 months of age: electrophysiological evidence.
abstract: Sound waves emitted by two or more simultaneous sources reach the ear as one complex waveform. Auditory scene analysis involves parsing a complex waveform into separate perceptual representations of the sound sources [Bregman, A. S. Auditory scene analysis: The perceptual organization of sounds. London: MIT Press, 1990]. Harmonicity provides an important cue for auditory scene analysis. Normally, harmonics at integer multiples of a fundamental frequency are perceived as one sound with a pitch corresponding to the fundamental frequency. However, when one harmonic in such a complex, pitch-evoking sound is sufficiently mistuned, that harmonic emerges from the complex tone and is perceived as a separate auditory object. Previous work has shown that the percept of two objects is indexed in both children and adults by the object-related negativity component of the ERP derived from EEG recordings [Alain, C., Arnott, S. T., & Picton, T. W. Bottom-up and top-down influences on auditory scene analysis: Evidence from event-related brain potentials. Journal of Experimental Psychology: Human Perception and Performance, 27, 1072-1089, 2001]. Here we examine the emergence of object-related responses to an 8% harmonic mistuning in infants between 2 and 12 months of age. Two-month-old infants showed no significant object-related response. However, in 4- to 12-month-old infants, a significant frontally positive component was present, and by 8-12 months, a significant frontocentral object-related negativity was present, similar to that seen in older children and adults. This is in accordance with previous research demonstrating that infants younger than 4 months of age do not integrate harmonic information to perceive pitch when the fundamental is missing [He, C., Hotson, L., & Trainor, L. J. Maturation of cortical mismatch mismatch responses to occasional pitch change in early infancy: Effects of presentation rate and magnitude of change. Neuropsychologia, 47, 218-229, 2009]. The results indicate that the ability to use harmonic information to segregate simultaneous sounds emerges at the cortical level between 2 and 4 months of age.
authors.affiliation: McMaster University, Hamilton, Ontario, Canada.
authors.forename: Nicole A
authors.initials: NA
authors.lastname: Folland
authors.forename: Blake E
authors.initials: BE
authors.lastname: Butler
authors.forename: Jennifer E
authors.initials: JE
authors.lastname: Payne
authors.forename: Laurel J
authors.initials: LJ
authors.lastname: Trainor
mesh_terms: D000161:Acoustic Stimulation; D000367:Age Factors; D000375:Aging / Q000502:physiology*; D001303:Auditory Cortex / Q000502:physiology*; D001931:Brain Mapping; D004569:Electroencephalography; D005072:Evoked Potentials, Auditory / Q000502:physiology*; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008433:Mathematics; D010898:Pitch Perception / Q000502:physiology*