In the early 1980s, a young man came to my lab. His parents were deeply frustrated. Although their son was bright and precocious, he had substantial hearing problems.
He responded inconsistently to sound and exhibited large discrepancies between his excellent visual cognition and poor auditory skills. The young man had normal audiometric thresholds, yet his auditory brainstem response (ABR) was absent.
This year marks the 30th anniversary of the paper in which we presented his case, and several others, identifying the dyssynchrony disorder that is now called auditory neuropathy (AN; Laryngoscope 1984;94:400-406 http://onlinelibrary.wiley.com/doi/10.1288/00005537-198403000-00019/abstract).
The field has come a long way since then, and we now view neuropathy as a spectrum encompassing several pathologies, including disorders of the brainstem, auditory nerve, and ribbon synapses ( Brain 1996;119[pt 3]:741-753 http://brain.oxfordjournals.org/content/119/3/741.long; J Basic Clin Physiol Pharmacol 2000;11:215-230 http://www.degruyter.com/view/j/jbcpp.2000.11.3/jbcpp.2000.11.3.215/jbcpp.2000.11.3.215.xml?format=INT).
In this editorial, I propose we extend that spectrum to include another confounding and frustrating condition that audiologists must tackle—central auditory processing disorder (APD)—viewing both conditions under the biological umbrella of neural synchrony.
Auditory neuropathy and auditory processing disorder are characterized by normal cochlear function, at least as evaluated by conventional tests, coupled with downstream (central) dysfunction.
Both patient groups have problems with auditory attention and excessive difficulty understanding speech when the signal is degraded or the listening environment acoustically challenging ( Nature 1997;387:176-178 http://www.nature.com/nature/journal/v387/n6629/abs/387176a0.html).
Whereas patients with AN are deaf in noise ( J Assoc Res Otolaryngol 2000;1 :33-45 http://link.springer.com/article/10.1007/s101620010004), those with APD understand speech much better, albeit abnormally.
We have investigated the neurobiology of APD, and of language-learning problems more broadly, using the auditory frequency following response (FFR).
Like the auditory brainstem response, the FFR is generated by summed synchronous firing of brainstem nuclei. The FFR is the product of sustained phase locking to periodic sounds ( Ear Hear 2010;31:302-324 http://journals.lww.com/ear-hearing/pages/articleviewer.aspx?year=2010&issue=06000&article=00002&type=abstract).
Unlike the ABR, the frequency following response reflects the confluence of cognitive and sensory processing.
Experiments in my lab have revealed the systematic relationship of reading and language ability with FFR consistency ( J Neurosci 2013;33:3500-3504 http://www.jneurosci.org/content/33/8/3500.abstract).
FFR consistency improves through early childhood ( Cereb Cortex 2013; doi: 10.1093/cercor/bht311 http://cercor.oxfordjournals.org/content/early/2013/12/21/cercor.bht311.abstract); children with auditory processing disorder may experience deviant or developmentally delayed responses.
With older age, consistency declines ( J Neurosci 2012;32:14156-14164 http://www.jneurosci.org/content/32/41/14156.full).
Our findings suggest that subcortical neural synchrony exists on a continuum that tracks with auditory skills and, I believe, represents the bottleneck in a substantial number of auditory processing disorder cases.
Certain cases of dyssynchrony are treatable. We have shown that assistive listening devices ( Proc Natl Acad Sci U S A 2012;109:16731-16736 http://www.pnas.org/content/109/41/16731.long) and auditory training, including music ( Front Aging Neurosci 2012;4:30 http://journal.frontiersin.org/Journal/10.3389/fnagi.2012.00030/full), language experience ( Brain Lang 2014;128:34-40 http://www.sciencedirect.com/science/article/pii/S0093934X1300223X), and computer training ( Proc Natl Acad Sci U S A 2013;110:4357-4362 http://www.pnas.org/content/110/11/4357.long), improve neural synchrony and communication skills.