Cortical indices of sound localization mature monotonically in early infancy

In human neonates, orienting behavior in response to an off-midline sound source disappears around the first postnatal month, only to re-emerge at 4months. To date, it is unclear whether sound localization processes continue to operate between postnatal months 1 and 3. Here, we used an event-related potential, reflecting change detection in the auditory cortices, to measure the cortical responses elicited by large (+/- 90 degrees relative to midline), infrequent changes in sound source location in 2-, 5-, 8- and 13-month-old infants. Both fast-negative mismatch negativity (MMN) Naatanen etal. (2007) and slow-positive mismatch response (MMR) Trainor etal. (2003) were elicited from all age groups. However, both components were smaller and the fast-negative component occurred later in the 2-month-old group than in older age groups. Additionally, the slow-positive component tended to diminish in amplitude with increasing age, whereas the fast-negative component grew larger and tended to occur earlier with increasing age. These results suggest that the cortical representation of sound location matures similarly to representations of pitch and duration. A subsequent investigation of 2-month-old infants confirmed that the observed MMR and MMN were elicited by changes in sound source location, and were not merely attributable to changes in loudness cues. The presence of both MMR and MMN in the 2-month-old group indicates that the cortex is able to detect changes in sound location despite the behavioral insensitivity observed around 1-3months of age.