A Larynx Area in the Motor Cortex: study dispels previous conclusions that laryngeal function generalized across lip, jaw and tongue areas of brain

Source: Brown, S., Ngan, E., & Liotti, M. (2008). A larynx area in the human motor cortex. Cerebral Cortex, 18, 837-845.

Have you ever wondered how your voice actually works? If you have, and looked into it, you will have discovered volumes of information on laryngeal function, aerodynamics, physics, and neurology to name a few. Vocal function is a field that has only recently come under the microscope, quite literally. Though Hipocrates speculated on the workings of the human voice as early as the fifth century BC, it wasn’t until Manuel García thought to shine a mirror down someone’s throat in 1854 that the living voice was seen in action. García presented his findings to the Royal Society of Medicine a year later. Voice medicine has been a slow and late bloomer compared to other specialties, but with increasing interest and new technologies there is unprecedented growth in a number of voice specialties. It’s no wonder than that neuroscientists have “answered the call” (vocally speaking) and begun exploring voice function where it really begins: in the brain.

Article Summary:

Until this study was concluded in 2008, it was widely believed that laryngeal control was spread across several areas of the motor cortex that corresponded to motor control of the articulators – the lips, tongue and jaw. This was based on the motor homunculus (pictured below), which was established by Wilder Penfield and others through neuro-stimulation in the 1930’s and 40’s.

The absence of a specific laryngeal centre in the brain is a pretty substantial thing when you get to thinking of the significance of phonatory communication to the human race. It is, after all, one of the most obvious evolutionary triumphs setting us apart from other species on this planet. Thus, Steven Brown of the McMaster Institute for Music and the Mind conducted a study of 16 individuals using fMRI imaging with a primary goal to define a somatotopic location for the larynx area.

This article described 4 of 6 oral tasks that the participants were asked to do while scanned. The tasks ranged from singing on a “schwa” vowel to performing glottal stops (ie. forced adduction of the vocal folds), lip protrusion and tongue movements. Each activity was done in a repeated pattern with breaks in between, this specificity requiring the subjects to attend a training session before their scan.

There were two principle findings in Browns analysis of the data gathered. First was that the peak activations in the motor cortex for glottal stops and those for phonation were nearly identical in all 16 subjects. This yields a strong argument that there is a common motor region underlying adduction (closing) and abduction (opening) and tensing/relaxing of the vocal folds – the major functions of the intrinsic musculature of the larynx. Brown refers to this general region as the larynx/phonation area (LPA) of the motor cortex. There was also activation in a superior temporal region known as “cortex of the dorsal Sylvian fissure at the parietal-temporal junction” (Spt). The Spt has been previously connected to audiomotor integration for vocal production, but Brown’s data revealed for the first time that this area could be activated in the absence of vocalization (during glottal stops), vocal imagery, or strong auditory stimulation – though Spt activity was significantly stronger during vocalization. It is unclear if the activity was due to auditory stimulation, increased laryngeal activity, or perhaps a combination of the two.

The second finding was that the human LPA is not ventral (in front of) the tongue area as was previously suggested in multiple sources. The LPA is actually located in a dorsal position (or behind) the tongue area and directly across from the lip area in all 16 subjects. Brown concludes that the human larynx area appears to have a novel localization next to the articulators and is much further away from the pharynx area than might be expected.

Reflection:

This was quite an ambitious read for me as I am in my first months of study of music and the brain. I was lead by my interest and investment in vocal function especially as it relates to vocal disorders. In the world of vocal disorders, nodes and polyps (physical abnormalities of the larynx) are what a singer often associates with voice disorders, however there are many vocal disorders that are neurological. Spasmodic dysphonia is one such disorder involving hyper function of the laryngeal muscles. Patients with spasmodic dysphonia deal with what seems to be a mis-firing of the larynx resulting in over adduction (too much closure) of the glottis. Sadly, this disorder has a fairly high incidence in professional voice users.

Though recent research into this disorder has shed some light on the cause (a problem in the feedback loop between the brain and organ with the dystonia), in many cases treatments only marginally restore function, and all treatments centre on the larynx instead of the brain. The most standard treatment is botulinum toxin injections (BOTOX) into the muscles that are spasming. These injections last about 4 months and often immobilize the muscles so much that singing isn’t possible. Other treatments include cutting the nerve to the voice box and attaching another nerve, changing the shape of the voice box, and speech therapy. Non of these treatments are particularly reliable from patient to patient.

Spasmodic dysphonia is just one example of several neurological disorders effecting the larynx. My hope is that Brown’s research will eventually lead to easier identification and diagnosis of neurologically based vocal dysfunction, and perhaps steer specialists toward treatments that include the brain.

Reading this article has helped me understand why the brain is left out of the treatment of these disorders and given me hope that the vocal specializations community is on the threshold of understanding the brain as it relates to vocal function in a whole new way.