So it turns out that brains start to look a little like walnuts when you desaturate them, filter them, and shrink them down. This is my original picture, taken off of vintage printables:
And a little collage of some colorways for the pattern:
I recently discovered the magic of the color blending mode for recoloring objects in Photoshop, so that's what I was playing around with in the bottom left square. Tons of fun - thanks again, Helen!