Know your Neurobiology: The Auditory System Development

Development

The auditory system develops in a very specific way inside the womb. This process is interrupted and occurs differently when taking place outside of the womb due to a premature birth. The infant’s exposure to frequencies of sounds, noise, and noise levels is very different within the NICU.

There are differing estimates of the onset of hearing.

• All major structures of the ear are in place between 23 and 25 weeks but the anatomy of the ear structures will continue to develop until puberty
• The auditory processing system starts forming at the 18th week of pregnancy and continues to develop until the baby is between 5 and 6 months of age. What we hear – the auditory sensations – will influence and contribute to this process. The auditory system requires stimulation through speech, music and other sounds to grow, even in the womb. Newborns prefer to listen to sounds e.g., music and speech they heard in the womb over unfamiliar sounds.
• By 22 – 24 weeks the fetus will start to hear low-frequency noises from outside of the womb.
• 26-30 weeks hair cells in the cochlear are fine tuned for specific frequencies.
• 28 weeks can tell the difference between father’s voice and mother’s voice.
• Passive listening to music can stimulate the motor cortex.
• 34 to 36 weeks can distinguish between different moods or emotional qualities to speech.
• In 1965 Robert Chuckrow wrote a paper “Music: A synthesis of prenatal stimuli,”outlining that element of music, rhythm and melody are based on foetal sensory experiences. The fetus matches and remembers sound patterns linked to emotions of the mother.
• The most vital time for the development of hearing is between 25 weeks of pregnancy and 5 to 6 months of age.

The primary auditory cortex begins to mature at about 26 weeks. The non-primary auditory cortex starts maturing later towards end of term. In premature infants both these cortical areas are less well developed. It is thought that the later developing non-primary auditory cortex is even more impacted when born early and could be contributing to the frequently noted speech and language problems noted at the age of 2yrs in this population.

While some basic abilities are well developed at birth, there is a clear prolonged maturation of auditory development well into the teenage years. Maturation involves auditory pathways. However, non-auditory changes (attention, memory, cognition) play an important role in auditory development.

Social communication begins with vision and somatosensory (tactile & proprioceptive) processing. This is then combined with phonological (speech sounds) processing -> semantic processing (hear a word and encode its meaning) -> Syntactic processing (encode sentences and phrases).

Across the planet parents use ‘parentese’ or baby talk to support language learning, engaging child emotions and highlighting structure in language to help babies decode syllables and sentences.

Studies show that infants can perceive and anticipate a rhythmical pulse and that they, and toddlers, will spontaneously move to a rhythm irrespective of their cultural background. This suggests that it could be a reflexive activity. There is some suggestion that our ability to perceive rhythm starts during our time in the womb and the repetitive pattern generated by the bipedal walking of human mothers.

The impacts of music, silences and sounds on early developing neurology is being given greater consideration. ‘Mechanisms of Timing, Timbre, Repertoire, andEntrainment in Neuroplasticity: Mutual Interplay in Neonatal Development.’ J Loewy &A Jaschke.

The role of audio-motor interactions and particularly in their effect on communication, bodily perception and motor performance is an area of growing interest. The noticeable patterns of timing, vocal timbre and melodic gesture between infant and parent has been introduced to us by the ground-breaking work of Colwyn Trevarthen. Emeritus Professor of Child Psychology andPsychobiology at the University of Edinburgh.

Almost all our actions give some form of auditory feedback. The coupling of auditory information with other sensory qualities enables us to give meaning to sounds including speech. Difficulties understanding sounds including speech is now recognised as Auditory Processing Disorder (APD)

Studies are showing that adolescents who were born prematurely have more difficulties with central auditory processing skills in areas of biaural integration (detecting where sounds are coming from) and speech-in-noise (understanding speech in the presence of background noise). Something we often observe in our practice.