In the study of the human hearing mechanism there has been an historic tendency, rightly or wrongly, to divide the auditory system into peripheral (or “just the cochlea”) and central processing. Central processing refers to all of the manipulation of a speech or music signal by the nerves and other central “way-stations”. The central processing comes complete with neural feedback loops from the cochlea to the brain (afferent) and from parts of the brain back down to the cochlea or inner ear (efferent). So far so good….
The above description can be found in any introductory neuro-anatomy or physiology textbook. But to make the apparent distinction between cochlear peripheral hearing and the central auditory processing even greater, it is generally felt that cochlear issues are permanent and irreversible, whereas central issues are “plastic”- they can change over the lifetime, for better and for worse.
One may argue that the cochlea is “hardware” and can only wear out over time and that the central structures responsible for the processing of sound are “software” that can be re-written over time. And at first glance at the available data, this hardware/software distinction seems reasonable. After all, many senior citizens do have a gradually progressing peripheral (sensory) hearing loss as a result of the breakdown of the outer and inner hair cells in the cochlea and their supporting and associated structures. Once someone has a sensory (sometimes inaccurately called sensori-neural) hearing loss, there is no training, surgical procedure, or medicine (pharmaceutical or otherwise) that can resolve the cochlear dysfunction.
As far as the realm of neuroplasticity is concerned, there are reams of data and articles demonstrating that the more central neurological structure can re-wire (i.e., reprogram) themselves. This is observed with auditory training, musical training, and a number of other training methods that seek to optimize neural function for a wide range of forms of perception.
Now (assuming that this is read later in the day), grab a glass of wine and click on Nina Kraus’s website http://www.soc.northwestern.edu/brainvolts/ at Northwestern University. Brainvolts is one of the best overviews of where we have been and where we are going with respect to learning about the various effects of training (and of aging) on the brain. Oh, and you will probably need to set aside a couple of hours, so perhaps two glasses of wine would be needed – or tea for those who like to remember what we just read.
In July 2014 an article was published in the Journal of the Acoustical Society of America (JASA) entitled “Psychophysical auditory filter estimates reveal sharper cochlear tuning in musicians”, by Gavin M. Bidelman, Jonathan M. Schug, and Skyler G. Jennings. In this article, the authors were able to demonstrate that there were cochlear (peripheral) changes for musicians that were (presumably) more beneficial for playing and listening to music. This indicates a cochlear change rather than just a purely central processing change.
It actually is not so surprising because all parts of our body are ultimately connected with a series of feedback loops, whether it’s standing upright or hearing. In this article, the authors were able to show that the cochlear psychophysical tuning curves were sharper for musicians than non-musicians.
Psychophysical tuning curves are one way that researchers have been able to quantify how well-tuned something is. On a radio, there is a series of tuners that allow the listener to adjust to any of the carrying waves of the available radio stations. For example, in Toronto we have our public radio (CBC) on the dial at 99.1 (MHz). That is, CBC radio transmits on a carrier wave of 99.1 MHz (and then being an FM station, has a frequency modulation of the signal on that wave). If I had a lousy radio and it was picking up not only 99.1 MHz but also 99.2 MHz (another station), then both radio stations would be rather unintelligible. The radio’s tuning is sharp enough to pass 99.1 MHz but not 99.2 MHz.
If our auditory system was sufficiently healthy, then our tuning where 1000 Hz would be perceived as 1000 Hz and not 900 Hz, would be quite adequate- our psychophysical tuning curves would be sufficient sharp. In contrast, if there was poorer tuning, then our psychophysical tuning curves would be broader (and less tuned).
Well, it turns out that even though non-musicians with normal hearing have quite good tuning in their cochlear psychophysical tuning curves, musicians have even better cochlear tuning. Musical training is not just a “brain thing”; its an “ear thing” as well.
But if the cochlea is the hardware (and is hard-wired) and we cannot regrow cochlear hair cells (which is why hearing loss prevention is so important), how is it that musicians can have super human tuning in their cochleae?
The authors propose that the feedback loop carrying information from the brain back to the cochlea (i.e., “efferent” neurological feedback) may be one source where the “input” to the cochlea- this time from the brain side and not the environment- may be one of the many factors. They also state that: “Findings suggest musicians’ pervasive listening benefits may be facilitated, in part, by superior spectral processing/decomposition as early as the auditory periphery.” (p. EL33). That is, they feel that something is happening as early (“peripheral”) as our cochlea and that perhaps our cochlea is not just a static bundle of wires and other non-changing hardware.
Regardless of the source of this improvement of the psychophysical tuning curves in musicians (central or peripheral), it is one more piece of evidence to demonstrate that musical training has benefits that go far beyond just the appreciation and technical ability to play music.