The human brain is the most complex and powerful computer known. It has been studied intensively for decades and has been the subject of thousands of books, tens of thousands of journal articles, hundreds of university courses, and even several television specials. Among the computational processes that have been investigated are memory, learning, language, visual and auditory perception and recognition, analogical reasoning, thought, attention, planning, and the control of movement. These studies have provided valuable insights into the computational nature of the brain.

However, even with all the advances in our understanding of human information processing, and even with our ever-increasing knowledge of computations in general, including the organization of knowledge, the nature of algorithmic processes and theoretical limits on computability, the human brain continues to elude our understanding. Among the reasons for this elusiveness are its incredible complexity, the redundancy of its circuitry and the variety of different ways that have evolved to reach the same computational goals. Because all of the computational processes which can be brought to bear on a particular problem seem to operate at the same time, the study of any one of them is difficult at best.

Recent investigations, particularly over the past three decades, have markedly increased our understanding of the human brain. Anatomical studies have given us detailed knowledge of the structure of various networks and the connections between them. Combined with clinical investigations, these studies have given major insights into the computations they perform. Clinical investigations, in particular, have characterized abnormalities resulting from localized brain damage. These abnormalities include deteriorated visual, auditory and tactile recognition, impaired language understanding and production, and abnormal planning and control of movement. Each syndrome gives considerable insight into the computational structure of the brain. Particularly with the advent of noninvasive techniques for determining the . . .