Development of the Human Auditory System

Abstract

Sound is one of the fi rst stimuli from the external environment
to reach and be perceived by the human foetus as early as the fi fth month in utero [approximately 25–26 gestation weeks (GW)]. Thus, auditory input may sculpt developing auditory pathways as well as those important for perceiving speech sounds even prior to birth. Detailed anatomical studies on postmortem human brains and a battery of functional studies such as the auditory brainstem response (ABR), magnetoencephalography (MEG) and fMRI (functional magnetic resonance imaging) have revealed that the cochlea and auditory brainstem are well-developed by the third trimester and adult-like in terms of structure by 6 postnatal months, followed by functional maturation. However, earlier studies have found that neurofi lament proteins which form the cytoskeleton of axons and act as a marker for maturity in the nervous system (in terms of axonal conduction) are present only in Layer I of the human auditory cortex from 22 GW to 3 postnatal years. These studies have further demonstrated that the neural circuits in all other layers of the auditory cortex have a protracted period of maturation (between 1–12 postnatal years) in terms of neurofi lament expression, suggesting a gradual increase in the ability to process sounds. Contrary to these neuroanatomical fi ndings, other studies have shown that third trimester foetuses respond to complex auditory stimuli (including speech sounds) and the auditory cortex is activated by sound as early as 33 GW. In the present review, I have discussed structural and functional data relating to the maturation of the human auditory cortex. In addition to these studies, I have discussed recent results showing that axons in the human auditory cortex may mature before birth, which can be better correlated with the fairly well-developed auditory processing capabilities of the third trimester human foetus.