Infants' ability to perceive multiple simultaneous auditory objects

Abstract

In order to make sense of the world, infants must be able to segregate incoming acoustic information emitted by simultaneous sources. Auditory scene analysis refers to the ability to break down the complex sound signal arriving at the ear into the discrete objects giving rise to it (Bregman, 1990). Chapter 2 uses a conditioned head-turn paradigm to investigate whether 6-month-old infants are able to discriminate a mistuned harmonic within a complex tone, a stimulus that adults perceive as two separate auditory objects. Results suggest that infants are able to perform such a discrimination, but that their threshold is higher than adults. In adults, the perception of two auditory objects is associated with a neural correlate derived from event-related potential (ERP) recordings referred to as the object-related negativity (ORN). Chapter 3 investigates whether the ORN is elicited from infants in response to these mistuned stimuli, indicating that they hear the mistuned harmonic as a distinct auditory object. By 4 months, infants showed a significant frontally-positive, object-related response which transitioned to an adult-like ORN by 8 months. Chapter 4 uses a visual preference paradigm to determine whether 4-month-old infants have an expectation that the number of auditory objects they hear should match the number of objects they see. These infants looked significantly longer at trials where the number of audio and visual objects did not match, suggesting that they integrate information about auditory and visual sources from a young age. Collectively, these findings indicate that by 4 months, infants are able to discriminate a mistuned harmonic, and that this mistuning is perceived as a distinct auditory object that elicits a measurable response in the infant ERP. However, thresholds for discriminating mistuned harmonics and infant object-related waveforms continue to mature across the first year of birth.