Serial magnetoencephalographic study of fetal and newborn auditory discriminative evoked responses

The mismatch negativity (MMN) response elicited to auditory stimuli is an indicator for cognitive function of sound discrimination in humans. MMN was successfully recorded in previous studies in newborns and fetuses (33-40 weeks of gestation) with magnetoencephalography (MEG). The aim of our study was to perform systematic serial MMN recordings on fetuses starting at 28 weeks of gestation with a follow up recording within 2 weeks after birth. The recording of weak magnetic fields from the fetal brain were performed with the 151 channel MEG system called SARA (SQUID Array for Reproductive Assessment). Two tone bursts were presented in a sequence of a standard complex tone of 500 Hz intermixed with a deviant complex tone of 750 Hz in 12% of the stimuli, inter-stimulus interval 800+/-100 ms. Eighteen pregnant women between 28th and 39th gestational weeks participated in the study. Measurements were performed every two weeks and once after delivery. The averaged evoked responses to standard and deviant tones were obtained and subtraction between them was calculated. A successful detection of response to the frequency change was found in 66% of the fetal data and 89% of the neonatal data. Responses to the standard tone were detected in 56% of all records. In the 28-39 week gestational age group, the discriminative brain responses to tone frequency change could be detected as early as 28 weeks. Although not statistically significant, a decrease in latency was observed with increase in gestational age. The ability of the fetus to detect changes in sounds is a prerequisite to normal development for cognitive function; related to language learning and clinical aspects of auditory disorders.