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According to When a fetus hear , When a baby can hear in the womb and several other similar articles, a baby starts to hear sounds at week 18. And according to How hearing works. Hearing involves

nerves communicate to the brain. The hearing nerve then sends the information to the brain with electrical impulses, where they are interpreted as sound.

Does this actually mean a brain is actually processing auditory signals at 18 weeks of human development, or is the brain at 18 weeks only processing sounds by a process that doesn't involve the brain? Perhaps the conclusion might seem obvious for some, but not for me considering the supposed inability of the brain to process several others (or any other?) kind of signals at that stage, such as pain At what stage is the nervous system developed enough to interpret neuronal signals as 'pain'? and being the hearing experience directly connected to pain sometimes.

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  • $\begingroup$ The ear is innervated at 18 weeks, if I recall. There is some evidence of varying quality looking at the response to acoustic stimulation, but good evidence of demonstrating cortical processing puts it much later... i think third trimester. I'd have to do some digging to find this though. A quick search didn't yield anything. $\endgroup$ – De Novo Oct 24 '18 at 5:20
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According to a concise review by Graven & Brown (2008) the development of the auditory system (Fig. 1) in the human is as follows:

  • 15 weeks gestational age*: structural parts of the cochlea and the middle ear are well formed;
  • 20 weeks: middle ear and cochlea are anatomically functional;
  • Around 25 - 29 weeks: auditory system as a whole becomes functional, as the spiral ganglion cells in the cochlea connect inner hair cells to the brain stem and temporal lobe of the cortex. In other words, only at this point, the peripheral end organ becomes connected to the central nervous system. At 25 to 26 weeks' gestation, a loud noise in utero will produce detectable changes in autonomic function.
  • 28 - 30 weeks: neural connections to the temporal lobe of the cortex are functional. The development of tonotopic columns in the auditory cortex commences. They are needed to receive, recognize, and react to language, music, and meaningful environmental sounds.

* Gestational age is a measure of the age of a pregnancy which is taken from the beginning of the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method if available. Gestational age can be obtained by adding 14 days to the point of fertilization.

auditory system
Fig. 1. The adult auditory system. source: Graven & Brown, 2007

Reference
- Graven & Brown, Newborn Infant Nurs Rev (2007); 8(4): 187-93

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