#braindriver

A while ago, I was asked by the makers of TheBrainDriver to write a review of the device. Being curious about this device myself, I got a review unit...and prombptly fogot about it for a while while I flew around the country for interviews. But now that that's mostly done, I'd like to share my impressions on this device!

Opening the box

The first impression that I got of the BrainDriver was that it looks sort of like a prototype iPod. Unlike most consumer tDCS devices, there are no physical controls for current intensity, etc. on the device. Rather, everything is controlled through menus displayed on the device's screen. While the device feels cheap when you pick it up (which, objectively, it is), it also seems sturdy enough that it's not going to crack apart if dropped a few times.

One of the most pleasantly surprising parts of the design is the sponge electrode and cable assembly that comes with the device. Many “entry-level” tDCS devices either don't ship with electrodes or use self-adhesive electrodes that only stick to bare skin and are therefore useless for most tDCS montages (unless you're bald or willing to shave your head). TheBrainDriver instead ships with several round, ~13 square centimeter electrodes , a pair of silicone electrode holders, and a stretchy head strap.

BrainDriver included electrodes. The red and black leads can actually be unplugged and used with other electrodes that accept approximately the same size of pin.

The electrodes connect to the stimulator via an included cable, which plugs into a jack at the top of the stimulator and provides color-coded pins for the electrodes (current flows out of the stimulator and into the head through the red electrode, which is conventionally considered the anode). It's overall a very high-quality electrode system, although the use of the DC jack means that to use electrodes other than the included ones you may need to either hack the cable (by default, it seems to be compatible with my Caputron rubber-carbon electrodes but not Amrex-type ones which need a larger pin)

It’s actually hard to overemphasize how much I liked the electrodes on this device--the sponge+headband arrangement makes for a really quick and easy setup. One thing to keep in mind however, is that because the headband is stretchy it might cause the electrodes to “drift” when placed in certain positions.At 13 square centimeters, the electrodes (while not extremely small) may also be samller than optimal for some protocols.

One final thing worth noting is the documentation that comes with the device. While the manual that comes in the box is excellent in some ways (for instance, it provides a very good step-by-step guide to running a session that will be helpful for new users), I found myself quite leery of the manufacturer's online montage documentation, where I noticed a few errors (for instance, in describing the location of the dorsolateral prefrontal cortex) and vague claims (what does “improved socialization” or “savant learning” actually mean?). As in any case, it’s probably best to use this site as a starting points but review the actual studies before using a montage.

Turning it on

The device is started up by holding the power button for a couple of seconds. Once it wakes up, the backlight turns on and it displays the simulation options. You can set the power between 0.5 and 2 milliamps, as well as the stimulation duration (20 or 30 minutes), and there's a display at the bottom left that shows the battery status. There’s also a start/pause button which lets you start and interrupt stimulation.

While the display is definitely easy to use, the silkscreened LCD is quite limited. What you see when it turns on is what you get—there's no way to set the timer to a duration other than 20 or 30 minutes, for instance, or to see a meter of the actual current output. While the UI works well for what it’s designed to do, it definitely doesn’t offer as much control as the foc.us v2, the other popular digital tDCS device

Stimulation and safety

The Brain Driver is advertised as being “A New Generation of Safer tDCS device”. With that, it's worth noting that it's included some good safety features.Unfortunately it also comes with a number of quirks.

One feature that might be more accurately called a comfort feature is a built-in capability to smoothly “ramp” the current level over a few seconds in order to reduce the feeling of an electric “zap” when the current level changes. While this works fine when starting a session, for some reason the ramping isn’t applied when you press the pause button, meaning you’ll get a definite jolt of electricity. It probably isn’t dangerous (a lot of simpler stimulators do this simply because they have no ramping whatsoever) but after doing this a couple of times I felt a little dizzy.

Another interesting advertised capability is the ability to cut off stimulation if the resistance rises too high. This function can serve two functions on a tDCS device: it prevents the device from ramping up voltage when an electrode loses contact with the skin (thereby preventing the stimulator from generating spikes of current) and it can protect against burns by cutting off current when electrodes are not contacting the skin well (i.e. poorly attached or drying out).

Unfortunately, on the BrainDriver it doesn’t work exactly the way you’d expect--the device continued to insist that the connection was fine when the resistance reached one megaohm! (about a hundred times the resistance you would expect from a typical connection across the head). This is particularly weird given that the device’s maximum output is 24 volts, which means there is a large region where it can’t deliver the target current but also doesn’t give any indication that there’s anything wrong with the connection. The shutoff does work if one if the electrodes is dangling in the air, but that’s about it--it’s not sensitive enough to detect more subtle issues like poor electrode contact or  resistance that is too high to deliver the target current.

Moving on to the device’s current regulator...

The device seems to deliver on average the target current in a “steady state” configuration (resistance held  constant), but there's a small (about 0.1 mA at 2 mA output) low-frequency oscillation present in the signal which is likely not biologically relevant (given that 0.1 mA current levels are sometimes used as a sham control). Edit: Someone on reddit correctly pointed out that this may not be entirely true, since this is an AC ripple current and about AC is actually used in studies at very low current levels approaching this. Therefore it seems reasonable to think that this oscillation might actually change the effects of stimulation compared to “pure” tDCS though what the effects would be exactly is not quite clear. (Keep in mind that oscillatory tDCS is not neccesarily throught to be more dangerous than pure tDCS, it’s just a different thing)

Steady state voltage drop measured over a 100 ohm resistor. A change of 100 millivolts represents a change of 1 milliamp current,

But the resistance in a tDCS circuit doesn’t stay constant--if it did, we wouldn’t need a regulator in the first place. Therefore, another important aspect of a tDCS device is its ability to adapt to rapid changes in resistance.

Here the BrainDriver falls short-- its regulator seemed to be very slow to adapt to changes in resistance caused by varying circuit conditions. A consequence of this is that when resistance varies, rather than a smooth DC signal the BrainDriver tends to produce a series of odd-looking pulses.

The most significant consequence of this is that under certain conditions the device can produce quite large current spikes. For instance, a drop from a steady-state resistance of 1 megohm (which significantly exceeds the capability of the device’s 24-volt maximum output to deliver the target current) to 100 ohms produces a current spike over 40 milliamps!

Current spike generated during sharp transition from 1 megohm resistance to 100 ohm resistance (voltage measured over a 100 ohm resistor). The spike ends when the device powers off (possibly due to tripping some protection mechanism)

Interestingly, spikes of this size seem to trip some sort of fuse in the device’s power supply that cause it to completely shut down after about a quarter of a second.

It’s worth noting that while this test highlights some of the limitations of the BrainDriver’s regulator, the changes in resistance are by design far larger and more rapid than you would expect to encounter during a typical tDCS session. To get a better picture of the device’s “typical” behavior, I spliced a current sensor into the electrode cable and ran a session with electrodes on F3 and F4 which were subjected to vigorous head movement movement and readjustment while the stimulation was running.

The device’s performance during a real session (even one where I was deliberately trying to mess with it) was a lot better than in the previous tests. Even during vigorous shaking and pressing on the electrodes produced only very small current excursions, suggesting that despite its relative sluggishness the regulator was able to keep up with these demands.

Voltage over 100 ohm resistor during vigorous head motion test. 100 millivolts represents one milliamp of current.

Conclusions

I really liked most aspects of the Braindriver. Probably the best aspect of the BrainDriver is that for is entry-level price it’s a very capable system, and it seems to cover the basic demands that people often have of a tDCS system such as an output voltage high enough to easily overcome electrode resistance, sponge electrodes, and ramping capability, though the lack of an onboard current meter is a little disappointing.