A group of Japanese neuroscientists is trying to peer into the mind — literally. They have devised a way to turn the brain’s opaque gray matter into a glassy, see-through substance. The group, based at the government-financed Riken Brain Science Institute in Wako, Japan, has created an inexpensive chemical cocktail that transforms dead biological tissue from a colored mass into what looks like translucent jelly. Soaking brain tissue in the solution makes it easier for neuroscientists to see what’s inside, a step they hope will uncover the physical basis of personality traits, memories and even consciousness. “I’m very excited about...