Get out your book of the Sistine Chapel and look at all of the paintings one by one. He puts drapery behind many of the figures for compositional purposes. The shape of this drapery is varied from one to the next. He uses the same compositional device quite a few times always varying the shape. It's easy to see why: drapery can take any shape, so you can balance any pose you like by the strategic use of all-purpose drapery. The drapery behind God and the angels in the Creation of Adam is simply one variation of a compositional device he uses several times in the ceiling. The fact it can be seen as resembling the silhouette of a brain is as significant as spotting a cloud or stain on an old wall that looks like a brain, i.e. it means nothing. As Moonbear said in another thread on this topic, she, a neuroscientist, sees brains in curled-up, sleeping cats, and in the shape of many trees.
Look at the panel as an artist. It's got two centers of focus: Adam and God/angels. Adam is framed by the mountainside. God/angels needs a frame for balance, and to visually simplify the overly complex silhouette that would result if all the angel heads were set against the brighter sky. So, the drapery accomplishes two things: it reduces an otherwise busy visual event to a single line, and it enlarges the dark area around God so that it "weighs" roughly the same as the Adam side of the panel. If, at the end of this compositional reasoning on Michelangelo's part, you have something that suggests a brain to a neurologist 500 years later, it's completely unimportant. The drapery is not there for symbolic purposes. It serves an abstract, art fundamental function: it's about line and balance and composition, just like the other drapery you see behind some of the ignudi and some of the characters in the different scenes.
