I'm sorry if I'm slow but I don't understand how OpenWater's device can "write" neural activity. I understand how her device works when it comes to "reading" neural activity. It's still unclear to me.
Does it require genetically modified cells? so that they can be light-sensitive (optogenetics)?
I don't believe it requires genetically modified cells. As far as I know if you heat up the brain cell membrane (with infrared and/or ultrasonic) it will fire, no GM required. I have no idea as to what resolution they expect to hit.