What is 'waterlog'?
"Waterlogged" describes something that has become soaked or filled with water. Ex: The homes were waterlogged after severe flooding. In this case, however, the memory circuits of the brain are described as being "waterlogged" in a metaphorical sense, since he compares the brain to a sponge here. When a sponge is too soaked with water, it cannot soak up any more water just like a brain cannot hold any more information when it is too full already.