Virtual Voice: Using BMI to Control an Artificial Larynx
In a previous article, we looked at the creation of artificial larynx and voice systems, and in particular, how the key aspect is tracking how the air flows through it, and is altered on it's way out. As we said then:
We worked out a short list of the variables necessary to keep track of in order to track the airflow, and it looked something like this.
The list of things to keep track of, is substantial, and of those, the following have to be continually tracked as their minute movements produce voice:
Several of these, including the position of larynx soft tissue, shape of the oral cavity, and movement of the lips are far more complex than first glance might reveal, with a great many separate variables to control. In natural speech, the brain controls these factors with the nervous system.
The Vagus Nerve
The vagus nerve is the only nerve that starts in the brainstem and extends, through the jugular foramen, down below the head, to the abdomen. It is also called the pneumogastric nerve since it innervates both the lungs and the stomach. Like all the main nerves, it is actually a pair of nerves, one on either side of the body. Thus, the larynx is innervated by branches of the vagus nerve on each side.
The superior laryngeal nerve
The superior laryngeal nerve branches out from the vagus nerve, and, unsurprisingly, is dedicated to the sensory needs of the larynx, the glottis and supraglottis, and the musculature in the surrounding area. Damage to this nerve means the vocal cords can no longer be tightened. Severage of this nerve means the voice is completely lost.
The hypoglossal nerve controls the tongue muscle, and tongue movement. It is therefore responsible for part of shaping speech.
The trigeminal nerve is primarily responsible for sensation in the face. Haptic feeling from forehead, cheekbones, jaw, nose tissue, lips, all pass through here.
The nerve is primarily a sensory nerve, but it is also responsible for the motor functions biting, chewing and swallowing. Thus it controls movement of the teeth, and upper throat, and is also important for speech.
If the brain requires all these nerves to control the process of making speech, what chance do we have at controlling the process artificially?
There are a lot of separate nerves involved in speech. Three separate nerve pathways all add their own input to the complex process of speech creation. The vagus works first, to exhale from the lungs, work the larynx and modulate the sound. Then the hypoglossal operates the tongue to alter the airflow, and finally the trigeminal nerve works the teeth and lips to produce the final sound.
Now, all three nerves are cranial nerves, which means they don't run down the spinal column with the rest of the body nerves, but descend directly from the brain. This sounds like a major problem, with a potential three different locations to interface, but this is not the case. All three leave the brain from the same location - the lower brainstem.
They travel in three pairings - each cranial nerve is actually a symmetrical pair. They travel in parallel to the spinal nerves, which are inside the brainstem, and split off at differing heights. It is theoretically quite possible to graft directly onto these nerves, as has been done elsewhere in the brain itself, or lower down in other bodily nerves. This would grant access to their neural pathways, and allow for external control - either an external mechanism feeding signals in, or those nerves controlling a secondary system as if it was a natural part of the body.
With a system such as speech, there is no necessity to override the signals, and prevent them from reaching their distination - it does not matter if the same signals reach the non-functional or missing larynx, as reach the artificial one.
What is required is an understanding of the signal pulses the nerves send. This is an area of research seeing increased funding and attention at this present time, and the signal pulses for several areas of the body have already been decripted.
Wikipedia entry on the Larynx, 18th May 2007
Cranial Nerves, 10th September 2006