Anyone else have a "Well duh!" response to this? Why else would you grow brain cells? To be stupid? To be non-sentient? What differentiates *brain* cells from say any other random cells? It is exactly that, brain function.
Presumably if you're growing brain cells the idea would be to eventually use them as brain cells - either to transplant into a patient suffering some brain injury or dysfunction, or maybe to study drug effects and utility. If you're going to transplant them into someone they had better function as brain cells, else why bother? "We'll transplant the brain cells into you, but they won't function as brain cells. It means you'll probably vote democrat the rest of your life..." Nooooo!!!! ;-) Ditto for drug effects. "Gee, sorry. In our trial of this new hemorrhoid cream we never realized it destroys upper brain function. But hey, here's your Bernie bumper sticker."