Despite the physical maturity of the cochlea about two-thirds of the way through gestation, sound conduction through the external and middle ear to the inner ear is inefficient at birth, hindering the transmission of information to the auditory neural pathway. Perception of low frequencies is poor in young infants relative to high frequencies. In fact, low-frequency discrimination does not mature until about 10 years, but discrimination of high frequencies is superior in infants relative to that of adults. The most common measure used when testing intensity processing for pure tones is the absolute threshold , the smallest intensity of sound detectable in a quiet environment.
The absolute threshold improves throughout infancy and reaches adult levels by puberty, and the higher the frequency, the earlier adult levels are achieved.
It is amazing that even the youngest infants can localize sounds. These sounds appear to be the precursors of vowel production. Infants learn to produce phonemes by trial and error, by listening and looking at the speaker. Between 4 and 6 months, infants expand their vocal repertoire to include growls, yells, whispers, squeals, and isolated vowel-like sounds. Event-related potentials ERPs , 4 extracted from the electroencephalogram EEG , provide millisecond-accurate information on brain processes underlying auditory perception and memory functions i. The guidelines are suitable for the parents of children with hearing
For example, the absolute threshold level at 4, and 10, hertz Hz reaches adult levels by age five, whereas the level for 1, Hz requires 10 years or more to reach maturity. Between one and three months, the absolute threshold improves by 15 decibels dB ; between three and six months, a dB improvement occurs for the threshold at 4, Hz.
In contrast to pure tones, many sounds in the environment are complex, made up of multiple frequencies and various intensities. For example, perception of timbre, such as hearing differences in the way different musical instruments sound, involves comparison of different intensities across frequencies. As early as seven months, infants can discriminate between sounds of different timbres with the same pitch, but adult levels of competence at discriminating a series of complex timbres are not reached until well into childhood.
The ability to locate the source of sounds is required to accurately perceive sound in the environment.
Spectral shape and intensity and binaural comparisons provide information on positions in elevation the vertical plane and azimuth the horizontal plane , respectively. Infants tend to use spectral shape more than binaural comparisons when locating the source of a sound, possibly because they are more sensitive to differences in sound frequency than to differences in sound intensity. Once different types of auditory information have been received, they need to be organized into perceptually meaningful elements.
For example, for a conversation to be followed, speech produced by members of the family must be grouped together and noises from children playing outside must be filtered out. The process of grouping is partly functional in infants, but it is more easily disrupted in children than in adults.
Auditory development in the fetus and infant entails the structural parts of the ears that develop in the first 20 weeks of gestation, and the neurosensory part of the. It has interesting potential features that need auditory experience to show them. This article is a short review of auditory development in infants.
Part of this process is ignoring irrelevant sounds while attending to the relevant sound source. Infants, unlike adults, often seem to act as if they are not sure about disregarding irrelevant sounds. For example, studies with seven- to nine-month-old infants suggest that they cannot detect a pure tone when presented simultaneously with a wide-frequency noise band. Infants appear to have difficulty segregating speech from other competing sounds.
Thus, when interacting with infants, adult caregivers often compensate for this difficulty by making major acoustic adjustments in their speech, such as the use of infant-directed speech, which contains exaggerated pitch contours , a higher register, repetitions, and simpler sentences. A central question in this area concerns whether infants respond to phonetic differences in a manner similar to that of adults. Studies examining cross-language and native-language speech perception suggest that infants are born with universal sensitivity to the phonemes that are present in all languages.
These phonemes are all produced by placing the tongue against the alveolar ridge, just behind the teeth, and releasing it in time with voice onset. They vary with respect to the precise part of the tongue and alveolar ridge involved and to voice-onset timing. Infants often exhibit preferences for speech sounds over nonspeech sounds; the former can help in attending to signals in the environment necessary for language acquisition. But infants do not always prefer speech.
In addition, speech preference does not appear to be a result of prenatal auditory exposure to human speech, and infants are attentive to other forms of communication, including sign language.
Newborns also are sensitive to prosody, the patterns of rhythm and intonation in speech, and may use prosody to discriminate one language from another. Prosody appears to be the primary way for young infants to perceive speech. This is especially useful in bilingual environments , because it helps infants avoid potential confusion.
Adults experience the world through the integration of sensory impressions. Infants, to some extent, are capable of coordinating information perceived through different senses. Most intermodal relations in the world, however, are quite specific rather than arbitrary. An example is speech, which can be simultaneously heard and seen in a talking face.
This coincides with the EEG activity pattern, which changes from discontinuous to triggering to continuous and more synchronized at the end of the second week after birth[ 20 ]. These changes in the EEG power reflect the changes in brain function during the neonatal period and are closely related to cognitive development in neonates.
The ERP wave area represents the sum of the biological potential discharged by cerebral neurons in units of time and is considered to represent the energy of the brain[ 21 ]. Lamm et al. In our preliminary study involving normal children aged 4 to 12 years, Dong et al. Our findings demonstrate that the N2 area gradually increases during the neonatal period. We consider this to be related to the characteristics of cortical development because the cerebral cortex is the largest and most complex structural component of the brain. At birth, neonates possess normal gyri, but these gyri are superficial[ 24 ].
During the process of constant deepening of the gyri, the cortical area continuously increases. In addition, the cerebral cortex contains many neurons and synapses, which constitute the neural circuitry. At birth, the number of neurons has reached the level of adults, but few synapses are present. There is a long path to accomplishment of construction of the neural circuits responsible for information transmission and processing.
Although some neural circuits have been established, they are not firm. With an increasing number of neurons, the somas enlarge and synapses form, stabilize, and mature.
This contributes to differentiation of the brain, formation of a complex neural network, and final maturation of the brain. During the neonatal period, the N2 area gradually increases, indicating strengthening of cortical function and neural networks and the development of neonatal cognitive processing. The results of this study demonstrate that N2 latency gradually shortens with age during the neonatal period. We considered that this possibly occurred because of myelinization, the process by which axons are enveloped in a myelin sheath, which serves to accelerate neuronal transmission along nerve fibers and ensure directional transmission.
At birth, myelination has not yet been implemented; thus, the bioelectrical conduction velocity is slow, it takes a long time for infants to judge stimuli and process signal information, and N2 latency is prolonged. With increasing age after birth, the degree of myelination increases and segmental structures transmit nerve impulse rapidly and precisely in a manner of leaps, contributing to constantly accelerating information processing.
Hence, the N2 latency tends to gradually shorten.
Latency represents the speed at which the brain categorizes and recognizes external stimuli and is a hallmark of the information processing time[ 25 ]. Most studies of N2 latency have been performed in preschool- or school-aged children, and the conclusions are controversial. Cunningham et al. Moreover, we found that the N2 latency shortened with age; this became most obvious at 11 to 20 days. All of these changes reflect the synchronization of maturity on the development of cognitive function and brain function. The first 11 to 20 days after birth may be the most critical period during which the infant accept stimulus signals and responds; it may also be a special period of cognitive and brain function development.
Further research is needed to test this hypothesis. Because neonatal aERP research is still in its infancy, the N2 component was evaluated in this study. However, the other components of neonates need to be further explored. Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field. Abstract Objective The present study was performed to investigate neonatal auditory perception function by quantitative electroencephalography QEEG and auditory event-related potentials aERPs and identify the characteristics of auditory perception development in newborns. Methods Fifty-three normal full-term neonates were divided into three groups according their age in days.
Results The four main findings of this study are as follows. Conclusion Rapid cognitive development occurs during the neonatal period. Introduction Cognitive function refers to the process of acquiring, coding, manipulating, extracting, and using sensory input information when recognizing objective items. Methods 2. Download: PPT. Results 3.
Fig 1. Fig 2. Fig 3. Fig 5. Comparison of N2 wave area in the three groups by target and non-target stimuli. Fig 6. Comparison of N2 latency at Fz and Cz leads in the three groups. Discussion QEEG can directly and objectively display brain function. Supporting information. S1 Data. References 1.
Kolb B. Brain plasticity and behavior. Psychology Press, Review of evoked and event-related delta responses in the human brain. Int J Psychophysiol. Gamma, alpha, delta, and theta oscillations govern cognitive processes. View Article Google Scholar 4.