A developmental study on the effect of stimulus rate on the auditory evoked brain-stem response.

Four groups of subjects were presented auditory stimuli differing only in repetition rate. One group was adults, another term newborns, and the other 2 groups were preterm newborns tested at respectively, 32 and 36 weeks postconceptional age. All 4 groups were judged to be audiologically and neurologically normal at the time of testing. Six stimulus repetition rates were presented to each subject, 10, 20, 40, 50, 66 2/3, and 80/sec. Auditory evoked brain-stem responses (ABRs) were recorded to each stimulus. Increasing repetition rate increased ABR wave form latencies and decreased ABR wave form amplitudes at all ages tested. For the most reliable measurement, wave V latency, a linear function effectively described the latency-rate function for individual subjects at all ages. The slope of the wave V latency-rate function increased with decreasing age. It was 300 microseconds decade change in rate at 32 weeks, 227 microseconds/decade at 36 weeks, 162 microseconds/decade at term, and 86 microseconds/decade in adults. Wave V latencies were more prolonged by rate than waves I and III and the differences in the rate effects between waves V and I and III increased with decreasing age. Amplitude was a less reliable measurement and amplitude-rate effects were less consistent than latency-rate effects.