Two patterns are presented on a computer screen. One pattern is designated "correct" and the other "incorrect." Subjects are asked to choose the correct pattern on each try and are provided feedback as to whether the response was right or wrong. Sometimes the rule changes, and the pattern that was correct is now wrong, and vice versa, so that the new correct pattern must be chosen. In addition, misleading feedback is sometimes given intentionally, indicating the subject chose the wrong pattern when in fact the response was correct. Subjects should change their response only when they are sure that the rule has changed, and not because they were incorrectly told that they were wrong. For patients undergoing MRI, blood flow in the different areas of the brain is measured during the test.