Working memory

Our understanding of the neural basis of memory function has advanced rapidly during the past few decades. Numerous neuropsychological and neurophysiological studies of memory function reveal a system of dissociable processes, not a unitary system. Neuroanatomical studies map these dissociable processes onto distributed neural networks whereas neuropharmacological studies reveal that different neurotransmitter systems play distinct roles in the various memory processes. Thus, memory is not localized to one brain region or restricted to one neurotransmitter system. The theoretical distinctions of memory functions developed by psychologists and neuroscientists provide a meaningful framework for understanding the symptoms of memory disorders. Moreover, new therapies will likely arise from advances in understanding the neuroanatomical and neurochemical underpinnings of memory function. The most common theoretical classification of memory distinguishes short-term and long-term memory. In clinical practice, the terms short-term and long-term memory are used imprecisely to describe memory processes. Many clinicians describe recently acquired information (e.g., what you had for breakfast) as shortterm memory, and memories from the distant past (e.g., what you did during your last vacation) as long-term memory. However, this sensible distinction does not conform to experimentally derived models of memory function (Squire and Schachter, 2002; Tulving, 1985). Short-term memory is properly defined as the ability to store information temporarily (for seconds) before it is consolidated into long-term memory. Short-term memory is examined with a bedside test such as digit span (e.g., “Repeat these digits immediately back to me: 4, 3, 7, 1, 5, 0, 6”). The average span of neurologically healthy subjects is usually six to seven digits (Lezak, 1995). In contrast, long-term memory is properly defined as the ability to learn new information and recall this information after some time has passed. Long-term memory is tested by asking the patient to learn items that must be retrieved after an interval with distraction (e.g. recall of three items – cat, apple, table – after 1 minute of performing some other task). Remote memory, a form of long-term memory, is information that was consolidated in the past. It can be tested by asking the subject to remember past public events (e.g., “When did the first person land on themoon? ”) or personal events (e.g. “Where did you go on your last vacation? ”). Neurologically healthy subjects should easily recall three items after a short distracted delay or accurately recall events from their recent or distant past. This chapter will focus on the more modern notion of short-term memory, called working memory (Baddeley, 1986; 2003). Working memory refers to the temporary maintenance of information that was just experienced or just retrieved from long-term memory but no longer exists in the external environment. These internal representations are short-lived, but can be maintained for longer periods of time through active rehearsal strategies, and can be subjected to various operations that manipulate the information in such a way that makes it useful for goal-directed behavior. Working memory is a system that is critically important in cognition and seems necessary in the course of performing many other cognitive functions such as reasoning, language comprehension, planning, and spatial processing. Likewise, almost any daily activity