I think, when you start experimenting on yourself, you officially upgrade to actual, real-life mad scientist, not just playing one at Halloween.

Just kidding, I will explain.

I had finished watching this video on Galvanic Vestibular Stimulation (GVS):





GVS is "...a simple, safe, and specific way to elicit vestibular reflexes." (Probing the human vestibular system with galvanic stimulation, Richard C. Fitzpatrick, Brian L. Day, Journal of Applied Physiology, Vol. 96. June 2004). The effect has been known for almost two centuries now. Vestibular reflexes are the sense of balance that is provided to us by our inner ears. By altering the vestibular sense, you change a person's perception of how they are oriented in space.

The potential for Virtual Reality applications is frankly quite exciting. Imagine a racing game where going around the corners feels like you're actually in a car, shifting around in your seat. Imagine a virtual sailboat experience, where you feel the gentle swell of the water under your feet. Imagine standing, in your living room, and feeling like you are walking in a straight line, but the balance influence system forces you to walk in a circle; you'd have an infinitely long track!

But as is the case with most of the things one reads about in the pop-sci press, you can't get it. There are no actual stereo, head-mounted, motion-tracking displays for sale; the one I have is a developer's kit (at least not yet. HTC will beat Oculus to market). You can't really buy a haptic glove anywhere. All of this really amazing stuff just seems to go into a black hole of venture-capital funded blue-sky projects that never make it to market. So as usual when I get frustrated, I take things into my own hands (literally).

The video lead me head-first into everyone's favorite game, Going Down the Wikipedia Rabbit Hole. I read up on Galvanic Vestibular Stimulation, which lead me to this paper in the bibliography on actually using it, found out where the Mastoid Process is located, and finally read up on the dangers of electrical shock and the electrical properties of skin.

And I thought, "well hell, I can do that."

I made some ersatz medical electrode probes out of alligator clip wires, aluminum foil, and paper towels folded up into a pad. Why did I make them? Because I don't keep medical electrodes lying around and I didn't want to wait for an order to show up in the mail. As usual, I hack things together to test principles before putting real money into them. Here, I'll show you:





I strapped them down tight to my head, just behind my ears, then I clipped the other ends of the wires to a 9v battery. Aaaaand...



It threw my balance way off! I felt like I was tipping over!

The 9v was a little uncomfortable. It wasn't exactly painful, not even as bad as pinching your finger in a drawer, mostly just a buzzing. So I switched to 4 AA batteries, making 6v overall, and it felt much better. By Ohm's Law, Current = Voltage / Resistance. Assuming wet skin, with really good contact (measuring the resistance of skin is difficult at best), is at least 1000 ohms of resistance, a 9V battery can--at most--provide 9mA of current. Similarly, a 6V supply is only providing 6mA of current. The highest current we could get is if we break the skin, as the internal resistance of the body is only about 300 ohms. With a 9V battery, that would bring us up to 30mA DC, which is quite painful--so don't do that--but won't kill you. Let's just not use knives or sewing needles for electrodes.

It's a pretty small spot, the mastoid process. Find the point where your jaw meets your skull, directly under your ear. There is a V-shaped divot where the two meet. Push your index finger in and back. That part of your skull is the mastoid process. Don't press too hard. If you press forward instead of back you hit a pressure point that can be surprisingly painful for relatively little pressure (don't worry, it's harmless, just uncomfortable).

For me, it's directly behind my ear lobe, but I don't know if different people's ear lobes can be in different places. Pressing my ear lobe flat against my neck, I touch the mastoid process through it. The electrodes need to be at the "knob" end of each, where it peaks. Look at the Wikipedia page for help on locating it. I tried a number of spots around with the 6V battery pack and bare alligator clips, and it was only a small fingertip of an area that was sensitive.

Both cathode and anode need to be connected to the mastoid process directly. It can't just be one on and one somewhere else on the skin. I was hoping that would be the case, I would have wired an anode to both mastoid process points and put the cathode at the nape of the neck, but that doesn't look like it will work.

I imagine building a pair of electrodes into over-the-ear headphones.Just at the back of the ear cup, it touches to the mastoid process, a small bump in the skull that sits right in front of the inner ear. Apparently, when scientists first started doing this experiment in the 19th century, they did it with electrodes in the ear. This was before they knew what exactly was going on or that the mastoid process was better. So maybe a pair of earbud headphones could be outfitted with the electrodes instead. When electricity flows through the skin between the two electrodes attached on either side of the skull, some of it flows over the posterior semicircular canal in the inner ear.

The sensation is undeniable. The direction of the tip is dependent on the direction of the current, as evidenced by just swapping the electrodes. Obviously, since these were batteries, this was a DC current. Sensation grew stronger over a brief period of time until it saturated. It sort of feels like someone pushing or pulling you over, except there is no tug on your arm. The ramp-up in the feeling made me think of charging a small capacitor.

I ran a test where I stimulated one mastoid process with the anode while connecting the cathode to different parts of my body, both near and far from the anode. There was no noticeable balance effect, though there was the same warm sensation from the electrical flow, indicating good contact was made. However, I don't know if that is because the electricity was not flowing in the right direction or if it was because the two inner-ears did not agree. It seems like the electricity should have been flowing in mostly the right direction for the left ear when the cathode was connected near-but-not-on the right mastoid process. I suspect it's the disagreement between the two that is the problem. I'd like to confirm by trying to stimulate each ear separately, with two isolated circuits that have their own anode and cathode for each side of the skull.

I'd like to use it to try to make the aforementioned demo of sailing in a boat. I have no ideas yet of how different the sensation could be if using a pulse-width modulated signal: it could provide control over the sensation, or it could just make the electrical shocking feel rougher. The sensation was only left-right, so I'm assuming that is because the current was flowing over only one axis of the semicircular canals. The electrodes are a little annoying, so perhaps the electrical charge can be induced, rather than conducted directly. Regardless, these are exciting times to be an armchair scientist.

Post Script: Mr. Galen Marchetti has written up a replication of my efforts. Awesome.