On and Off magnetic auditory evoked responses in early infancy: a possible marker of brain immaturity.

OBJECTIVE: To follow the development of the auditory evoked on- and off-responses in early infancy.
METHODS: The subjects were healthy infants, studied at conceptional age (CA) 39-66 weeks. Serial auditory evoked responses (AERs) were recorded in 17 subjects in response to tones of frequency 1.5 kHz and duration 1.0 s in order to follow the development of the AERs. Two small substudies were performed to examine off-responses evoked by tones of varying duration. The first used 0.2, 0.3, 0.4, and 0.5s tones; the second used 0.5, 1.0, and 2.0 s tones.
RESULTS: For the 1.0 s tones, the on- and off-responses consisted of three components: P250, P400, and N750 m. The on- and off-responses had similar amplitude, in contrast to adult AERs in which the on-responses are typically much larger. Prior to about CA 47 weeks, the on-responses were actually smaller on average than the off-responses, and in infants born prior to CA 38 weeks the AERs were dominated by the off-response. Off-responses were seen for tones as brief as 0.2 s; however, the P400m component was often absent in the off-responses for tone durations < or = 0.5 s. An interaction was observed in which the N750 m-On component was strongly suppressed when it overlapped the P250 m-Off.
CONCLUSIONS: Off-responses are much more prominent in infant than in adult AERs, especially in pre-term infants and at early CAs. On- and off-responses also exhibit more complex behavior in infants than in adult; the individual components can interact and behave independently. SIGNIFICANCE: Measurement of auditory off-responses can improve the assessment of infant neurodevelopment, potentially providing a marker of brain immaturity.