Auditory Maturation Research Articles

Hearing levels of guinea pigs exposed to elevated noise levels in utero

Pregnant guinea pigs were exposed to loom room noise at 115 dBA SPL for 7.5 h per day for various periods during the last trimester of pregnancy. When the hearing of their offspring was tested by brain stem evoked response techniques at 6‐dB intervals, 5th peak latencies of exposed pups were found to be significantly longer than those of otherwise similar control pups. The latency differences corresponded to some 5 dB at medium levels and 10–12 dB near threshold. The results indicate that it is possible for noise‐induced hearing loss to occur in utero in mammals whose auditory maturation process is complete, or nearly so, before birth.

Neurophysiological correlates of auditory maturation.

At the time of its birth, the auditory system of the cat is not completely developed. Anatomic maturity of the peripheral auditory system, ie, the cochlea, auditory nerve and cochlear nucleus, is attained at about two or three weeks of age. Physiological response properties of neural elements of the peripheral auditory system change radically during the first few weeks of life, with most measures of responsiveness to acoustic stimuli reaching adult status by the end of the third of fourth postnatal week. The physiological maturation of neural responses correlates well with the anatomic maturation of the auditory structures. At least one physiological response property, namely the ability of cochlear nucleus neurons to time their discharges in response to low-frequency tones, is not fully achieved until the sixth postnatal week or later. Although considerably less is known about the development of the central auditory system, it appears that it is in part dependent upon the maturation of more peripheral elements. Auditory evoked responses, for example, follow roughly the same developmental time course as do the majority of response properties of peripheral neurons. This implies that the central system is ready to function at birth, but that it must await the maturation of more peripheral elements before it can function properly. In contrast to that of the cat and other mammals, the auditory system of the human is relatively well advanced at birth. Certain aspects of brain development, such as dendritic aborization and axonal myelination, undergo considerable change postnatally. How these factors influence hearing is not known.

Acoustic brainstem responses in adults and infants

BSRs to filtered clicks (FCs) with center frequencies at audiometric values were recorded from infants and adults. The latencies of Wave V for the respective FCs and intensities were compared for the two groups. The latencies are strongly determined by stimulus parameters: Low frequencies and weak intensities elicit responses with long latencies while high frequencies and strong intensities produce responses with short latencies. The strong stimulus control of response latency is most apparent in adults and is diminished in infants. For low frequency FC's, response latency in infants latencies lag adult values. As the infant matures, latency for the high frequencies decreases. Since Wave V is relatively late in the auditory chain, following cochlear mechanisms and, perhaps, four or more levels of neural processing, interpretation of our data is still uncertain. However, the differentiation of response latency re frequency suggests that one site of auditory maturation is within the cochlear system. [Supported by NINCDS contract NS‐5‐2313.]

Variations in Cochlear Microphonics (CM) and Eighth Nerve Action Potentials (AP) with Age in Rats

CM responses to pure tones, and whole-nerve AP responses to clicks with and without noise masking, were measured acutely from the round window in groups of male Sprague-Dawley rats at ages of 1, 6, 12, 18, and 24 months. According to average CM measures made from 0.5 to 100.0 kHz, the greatest sensitivity occurred in the 1-yr group, with the younger and older animals (up to 18 months) showing comparative losses up to 18 dB at 1.0 kHz. Comparative losses for the 2-yr group ranged from 26 dB at 1.0 kHz to 36 dB at 30.0 kHz. Mean N1 peak amplitudes varied similarly with the highest values found in the 1-yr group and the lowest values in the 1-month rats. When compared with the 1-yr rats, older and younger rats showed a reduction in the slope of the N1 amplitude versus click level curve. Mean N1 latency did not vary with age. Derived masked AP amplitudes [Teas, Eldredge, and Davis, J. Acoust. Soc. Amer. 34, 1438–1459 (1962)] varied with derived masking band center frequency (c.f.) in an age dependent manner. Below c.f. = 24.0 kHz the descending rank order of amplitudes was 12, 6, 18, 24, and 1 month. As with the unmasked AP, the derived masked AP latency did not vary with age. According to these measures, peripheral auditory maturity is not reached in the rat until 1 yr, and significant auditory deterioration does not occur until 2 yr of age or approximately 23 of the rat's lifespan.