Patients 40 and older with CBS who have participated in NINDS protocol 02-N-0001 ("Testing a Model of the Representational Knowledge Stored in the Human Prefrontal Cortex") may be eligible for this study. In protocol 02-N-0001, participants provide a medical history, undergo a neurological examination, PET scanning and MRI, and complete tests, such as sitting in front of a computer monitor and press a key to indicate a decision about what appears on the screen (for example, whether a statement is accurate) and answering questions from a test examiner.