It turned out that as a kid, the man had been diagnosed with hydrocephalus, which meant that his brain had a buildup of excess fluid. The fluid had been drained, but it was now back with a vengeance, having taken up most of the space where his cerebral cortex should have been. Only a thin layer of brain tissue remained. But because the displacement happened over the course of several years, experts believe his brain tissue either adapted to take on new functions or had been compressed into that thin layer. Note, however, that when the fluid was drained again, his brain didn't magically spring back into place. But hey, the leg thing was fixed, so there's a win.