I honestly don't know if it can be tested unless it's determined to happen in some area of the brain, that can be monitored externally for increased activity.

I honestly don't know if it can be tested unless it's determined to happen in some area of the brain, that can be monitored externally for increased activity. When I paint myself I usually compose a scene in my mind. When drawing characters, I mostly follow whatever my mental image is, then as I trace the lines I can visualize how they connect. When drawing characters that aren't mine I make a point of not using a reference picture. Basically I can "see" the whole result as soon as I have the eyes drawn, then I just draw single lines until it's "traced", undoing if it doesn't fit the shape I need. Only problem is that at times I don't close lines so using a fill tool is always a pain. When painting proper I just make a quick color base and I add the rest until it fits whatever is in my head, then I add minor detailing to complete the piece in a more mechanical manner like "this gravel can use more texturing" or "add some grating in this robot's armor for cooling". My speed when drawing characters is very high, I at times like to speed-doodle stuff for friends and each piece takes about 5 minutes once I decide on an angle/pose, so the process doesn't seem to interfere with agility. I can also doodle stuff while not conjuring any mental image (such as idle doodling on a notebook or such), however, it just has weird proportions or so. My point being that it's not something necessary at all as you can still paint/draw/write without doing it. Doesn't seem to be a necessity or provide any significant advantage over plain "logic" (as logic as creative stuff can be, but stuff like light usually works in rigid ways) When doing things like coding I plan the structure in advance, but it doesn't resemble the process for visual arts at all. There's no mental image there, just something like "okay now loop here and call this function".