Maturation of contralateral brain-stem responses in preterm infants.

Average auditory brain-stem responses (ABRs) evoked by left and right ear stimulation of varying intensities were recorded from ipsilateral and contralateral scalp placements in human neonates. Subjects included pre-term infants, healthy full-term infants and high risk, full-term infants. Pre-term infants were classified into 4 age groups ranging from 29 to 41 weeks post-conceptional age (PCA). Significant ipsilateral/contralateral differences were observed. Further, this effect was highly dependent upon age. The youngest pre-term infants were devoid of contralateral activity despite robust ipsilateral responses. The emergence and evolution of the contralateral wave form could readily be visualized from 31 to 37 weeks PCA. The contralateral response was uninfluenced by stimulus intensity but appeared to be related to the amplitude of the ipsilateral response. No systematic ear differences were noted. Pre-term infants tested between 38 and 41 weeks PCA (term equivalence) showed some persisting immaturity in terms of the ABR. Healthy and sick full-term infants were indistinguishable with respect to ipsilateral or contralateral potentials. On the other hand, very ill 6-week-old infants showed specific contralateral abnormalities.