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<jats:p> <jats:bold>SAN DIEGO</jats:bold> —Limits—a term often bandied in calculus class—are the “endpoints” of infinite processes. The concept hasn't played much of a role in theories of purely finite processes such as computing, which takes place in finite-sized machines. But a new proposal suggests that limits could be key to solving a famous question in computer science: whether certain problems, such as the Traveling Salesman Problem, are “hard,” meaning they cannot be solved efficiently by any computer algorithm. </jats:p>

<jats:p> <jats:bold>T</jats:bold> hree hundred years ago, the French philosopher René Descartes gained immortality with his pronouncement, “I think, therefore I am.” Today, a disparate band of researchers—physicians, psychologists, and neurobiologists—is asking the next question: How do we think? This special issue of Science surveys how they are trying to provide an answer by fusing the perspectives of their formerly separate disciplines. </jats:p> <jats:p> In her Article, <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://www.sciencemag.org/cgi/content/summary/275/5306/1579" xlink:type="simple">N. C. Andreasen</jats:ext-link> discusses approaches to understanding schizophrenia as a dysfunction of specific cognitive systems, such as those for working memory and consciousness. The normal underpinnings of working memory and consciousness are also coming to light, as two stories in the Special News Section make clear. (For interactive demonstrations of behavioral paradigms used to study working memory, reconstructive memory, and visual-spatial attention, see J. D. Cohen's Web site at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://psyscope.psy.cmu.edu/cogdemos/welcome.html" xlink:type="simple">http://psyscope.psy.cmu.edu/cogdemos/welcome.html</jats:ext-link> ) In another Article, <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://www.sciencemag.org/cgi/content/summary/275/5306/1579" xlink:type="simple">W. Schultz</jats:ext-link> and his colleagues examine the neural basis of animals' ability to predict future events, an essential survival skill for finding food and mates and avoiding danger. They report that neurophysiological studies of primates, buttressed by computer modeling of neuronal behavior, have revealed the importance of a particular set of dopamine-releasing neurons. </jats:p> <jats:p> The final pair of Articles describes two efforts to tackle that peculiarly human attribute—language. <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://www.sciencemag.org/cgi/content/summary/275/5306/1579" xlink:type="simple">M. S. Seidenberg</jats:ext-link> reviews the classic paradigm of Noam Chomsky that focuses on innate capabilities for language acquisition and processing, then suggests an alternative based on connectionist theories that attempt to explain learning as alterations in neuronal networks. And <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://www.sciencemag.org/cgi/content/summary/275/5306/1579" xlink:type="simple">A. Prince and P. Smolensky</jats:ext-link> review how optimization, a formal organizing principle derived from neural computation, can characterize grammar as a hierarchy of linguistic constraints. The satisfaction of a single set of constraints appears to resolve well-known inconsistencies between listening and speaking abilities when learning language. </jats:p>

<jats:p>Without it, you couldn't understand this sentence, add up a restaurant tab in your head, or even find your way home. The “it” is working memory, an erasable mental blackboard that allows you to hold briefly in your mind the information essential for comprehension, reasoning, and planning. Now, neurobiologists are beginning to identify the neural machinery underlying this critical ability. They have found that working memory relies on cooperation among scattered areas of the brain, with the prefrontal cortex apparently working as the orchestrator that coordinates the activity of these various regions.</jats:p>