| Summary |
Over the past half a century, considerable effort has been dedicated to the development of universal newborn hearing screening (UNHS) programs across the United States. Newborn hearing screenings have progressed from subjective assessments performed only on at-risk neonates to highly sensitive objective methods of screening auditory sensitivity in nearly all newborns in the country. The Joint Committee on Infant Hearing (JCIH, 2007), National Institutes of Health (NIH, 1993) and the American Speech-Language-Hearing Association (ASHA, 2004) all support the use of either otoacoustic emission (OAE) or automated auditory brainstem response (ABR) for UNHS. Because OAE testing only assesses cochlear function, ABR testing remains the gold standard for evaluating the functioning of the peripheral and central auditory system (i.e., to the level of the brainstem). Historically, ABRs have been evoked using a broadband click stimulus or tone burst stimuli. The click stimulus has been advocated for both neuro-diagnostic and newborn hearing screening applications. Recently, a chirp stimulus was developed to maximize the temporal synchronization of the responses from the underlying neural components. Researchers have shown that this increased synchrony produces larger amplitude ABRs in both adults and infants, even at low intensity levels, potentially reduces test time, and is comparable to the "stacked" ABR. To date, however, few researchers have investigated the effects of manipulating multiple stimulus parameters in both adult and neonate participants. The purpose of this dissertation was to collect normative data based on a multitude of stimulus manipulations, using both air and bone-conducted CE-Chirp and octave band stimuli in newborns and young adults. Additionally, ABR responses to air and bone-conducted chirp stimuli were compared to traditional ABRs to click and tonal stimuli in newborns. Latency and amplitude of ABR waveform components, primarily wave V, were examined. In Experiment 1, it was shown that ABRs to CE-Chirp stimuli are signifcantly (p < .05) larger in amplitude compared to traditional click and tonal stimuli. Statistically significant correlations (p < .0001) were found between testers for wave V latencies and amplitudes to air- and bone-conducted CE-Chirps in Experiment 2. Also, there were no statistically significant differences between testers with wave V latencies and amplitudes to air- and bone-conducted CE-Chirps (p < .05). In Experiment 3, it was revealed that, as expected, adults exhibited signifcantly (p < .05) larger ABR wave Vs than newborns, which underscores the importance for age-related normative values. In Experiment 4, neonates were found to have statistically significant (p < .0001) lower mean ABR thresholds for bone conduction than adults, but no statistically significant (p = .80) threshold differences were seen for air-conduction.. Overall, the results of these series of experiments support the notion that chirp stimuli may be equal or superior to tradtional click and tonal stimuli at assessing auditory function in both infants and adults. The CE-Chirp may also significantly reduce test time for ABR recording, which could enhance the efficiency of UNHS programs. |
