An Electric "Humm" To Make You Smarter?

This week I came across a brain stimulation device called Humm that promises to improve your cognitive function and memory if you stick it to your forehead.The Humm device. Image adapted from https://thinkhumm.com/preorderThere are several broadly similar devices on the market, which make use of the principle of transcranial alternating current stimulation (tACS) - passing a current through the head (the front of the head, generally) in order to modulate brain activity.In the case o