While I have previously wondered about the fabric shape, I don't think the similarity is great enough. Unless there is significant variation in brains, the silhouettes differ in way too many places and only really share a couple of major features.

However, the shape is similar enough for me to accept anatomical inspiration. I could see him wanting to draw something around god, sketching a cloud, and then remembering the shape of a brain and going with that. If indeed the artist was fascinated with anatomy, it would make sense that anatomical shapes could be found all over his more freeform work.

This one almost seems more plausible.