Perinatal maturation of the auditory brain stem response: Changes in path length and conduction velocity

Objective: The goal of this study was to correlate developmental data on brain stem auditory path length with data on auditory brain stem response (ABR) conduction time. This was done to estimate changing axonal conduction velocity during the perinatal period. Design: Pathway length was determined by three-dimensional reconstruction of postmortem fetal and infant brain stems in an AutoCAD system. Brain stem conduction time was obtained from previous ABR studies of premature, term, and postterm infants. The process of correlation of path length and conduction time was based on a model of ABR generation (Ponton, Moore, & Eggermont, this issue) that assumes that the III-IV interpeak interval represents activity in an asynaptic pathway and, thus, consists of only axonal conduction time. Results: Brain stem conduction time is adult-like by the time of term birth. However, the brain stem auditory pathway continues to lengthen postnatally, with portions of the pathway not reaching adult dimensions until 3 yr of age. We determined lengths at various perinatal ages for three different segments of the auditory pathway. Each segment began at the cochlear nucleus (site of wave III generation) and ended at a more rostral location that is a possible site of wave IV generation. Conduction velocity was estimated by dividing path length by axonal conduction time (III-IV interpeak interval). All three assumed sites of generation of wave IV gave estimates of a threefold increase in conduction velocity between 29 wk CA and adulthood. However, three highly discrepant measures of absolute conduction velocity were obtained for the different path segments. The most reasonable conduction velocity estimates, from 5 m/sec at 29 wk conceptional age to 20 m/sec in adults, were produced by assuming a site of generation for wave IV near the contralateral medial superior olivary nucleus. Conclusions: Prenatally, increasing conduction velocity more than compensates for increasing path length, causing ABR conduction time to decrease. Postnatally, increasing conduction velocity exactly compensates for increasing path length while ABR conduction time remains stable. Different aspects of myelin development may underlie these two phenomena.