By Dr. Ryan Neely, Ph.D.
Stimulating with Sound
Elemind’s headband uses acoustic stimulation to accelerate sleep onset. Specifically, we use sound to create evoked response potentials - you can think of them as “ripples” in the brain. Our device is designed to align each of these ripples with particular phases of alpha waves in the brain, that we detect using the EEG sensors on the headband.
This graph depicts an Evoked Response Potential triggered by a sound pulse (blue) occurring out-of-phase with an ongoing alpha oscillation (grey)
Using sound to stimulate the brain has some unique advantages. You may think that stimulating the brain directly with electricity would be the easiest way to evoke neural activity. However, passing electricity through the hair, scalp, and skull to stimulate the brain can be difficult to do without exceeding safety limits, especially in a targeted way. In contrast, the auditory system has the ability to amplify vibrations caused by even very quiet sounds such that they can generate a cascade of neural activity throughout the cortex. The phototransduction pathway that helps us see has similar amplification characteristics, but it’s more difficult to perceive light with closed eyes. Finally, from a safety perspective, strobing lights can induce seizure in about 1 in every 4000 people whereas the auditory system is not known to have a similar vulnerability.
The Goldilocks Zone
For all of its advantages, acoustic stimulation for the purposes of sleep enhancement needs to be implemented carefully in order to avoid counterproductive consequences. For example, it is well documented that excessive noise can impair sleep. The World Health Organization recommends that ambient sound levels during sleep stay around 30dB, with noise peaks remaining below 40dB. For reference, this is about the level of noise you might experience in a typical library. Other research has suggested that sound level changes are more disruptive than absolute noise levels - for example, changes in noise levels exceeding 17.5dB from background were found to disturb sleep (1). For this reason, the Elemind band is programmed to gradually fade out acoustic stimulation as you fall asleep, ensuring that the rest of your night is as quiet as possible, and that there are no jarring changes in noise level.
However, it is important to note that when it comes to acoustic stimulation, there is such a thing as too quiet - in order for acoustic stimulation to be effective at evoking activity in the brain, it needs to be loud enough to be perceived. Remember that Elemind’s stimulation relies on precise alignment between brain activity evoked by each stimulation sound and the existing alpha waves detected by the headband. Although it may be possible to evoke neural activity with sounds that do not reach conscious awareness, we’ve found that the best rule of thumb is for the sounds to be audible by the user, so we can be sure that they’re having the intended effect. For these reasons, we recommend that users pick a volume level that is just audible - that way they can be sure that the sounds are evoking neural activity as intended, but also remain below a level that could be disruptive to the sleep process. From there, the headband will do the rest - fading out to zero after you’ve fallen asleep.
Mix-N-Match
If you’ve had a chance to use Elemind’s headband, you’ll notice that there are really two components of the sound mixed together - in the foreground are the pink noise pulses that are triggered by each of your alpha brain waves (roughly 10 per second, depending on your brain’s unique frequency), and in the background is a continuous track of rain sounds. The pink noise pulses are the active element of the sound stimulation and are generated dynamically, while the background rain is a passive element intended to improve the experience. When designing the mix of these two sounds, we had to carefully choose the relative balance between them. As mentioned above, the pink noise must be heard to have the intended effect, so masking it completely with rain sounds would be counterproductive. On the other hand, if the rain sounds are too quiet relative to the pink noise, it would defeat the purpose of having them at all. In order to solve this problem, our lead research scientist, Dr. Scott Bressler, measured the brain’s Auditory Steady State Response (ASSR) to different mixes of background rain and pink noise. Without going into too much detail, the ASSR is a way to measure whether the brain is following a repeated presentation of an auditory stimulus. Dr. Bressler observed that a 12dB difference between the pink noise and rain sound still generated a significant response from the brain, while maintaining the audibility of the rain. These data led to the current mix of sounds in the Elemind band.
Results from Dr. Bressler’s ASSR experiment. Reading from left to right, the first five plots depict the strength of the brain’s following response to a pink noise pulse played at 10 Hz when mixed with varying levels of background rain sounds. The mix levels used were no rain, 18dB difference, 12dB difference, 6dB difference, and no difference in sound levels. The sixth and final plot summarizes the data from the first five plots.
Coda
Sound can be a powerful but safe method of evoking activity in the brain. Elemind’s technology uses sounds phase-locked to brain waves to influence brain states - in the case of our headband, sounds are targeted to the alpha frequency waves in order to accelerate sleep onset. Importantly, acoustic stimulation of this type must be heard to be effective. That said, the sounds should be kept at a level low enough to not disturb sleep. The absolute volume level for each person will vary depending on your hearing acuity as well as the presence of any background noise in the environment, so a helpful rule of thumb is to set the volume just loud enough to hear. The mix of sounds in the Elemind headband is tailored to be soothing while still effective. That said, we’re continuing to experiment with new sound mixes and stimulation pulses to improve the experience - we’re looking forward to sharing them soon!
References
Jaiswal, S.J., Garcia, S. and Owens, R.L., 2017. Sound and light levels are similarly disruptive in ICU and non‐ICU wards. Journal of hospital medicine, 12(10), pp.798-804.