In the specific, mistaking your wife for a hat. In general, the loss of coordination between sight (in one half of the visual field)and mental processes. Simply, you don't know that one half of the world exists, even though your vision is 20/20. George Bush, if he suffered this malady, would put broccoli on the affected side of his dinner plate, and claim he never saw it. Bill Clinton does the same, routinely, with China's civil rights record.