it to be manipulated. Short-term memory is part of working memory, but is not the same thing. Working memory refers to structures and processes used for temporarily storing and manipulating information. Working memory has been termed working attention. Working memory and attention together play a major role in the thought process. Short-term memory in general refers to the short-term storage of information, and it does not encompass memory manipulation or organization. Thus, while short-term memory components appear in working memory models, the concept of short-term memory is distinct from other concepts. Within Baddeley's influential 1986 model of working memory two short-term storage mechanisms appear: the phonological loop and the visuospatial sketchpad. Most of the above research involves the phonological loop, because most of the work on short-term memory uses verbal material. Since the 1990s, however, research on visual short-term memory and spatial short-term memory has expanded. Duration The limited duration of short-term memory (~18 seconds without rehearsal) suggests that its contents spontaneously decay over time. The decay assumption is part of many theories of short-term memory. The most notable one is Baddeley's model of working memory. The decay assumption is usually paired with the idea of rapid covert rehearsal: to retain information for longer, information must be periodically repeated or rehearsed, either by articulating it out loud or by mental simulation. Another type of rehearsal that can improve short-term memory is attention-based rehearsal. Information is mentally searched in a particular sequence. Once recalled, the information re-enters short-term memory and is then retained for a further period. Nairn and Lewandosky et. al. dispute that spontaneous decay plays any significant role in short-term forgetting, and the evidence is not conclusive. One alternative asserts that several elements (such as digits, words, or pictures, or logos) are held in short-term memory simultaneously, their representations compete with each other for recall, degrading each other. Thereby, new content gradually replaces older content, unless the older content is actively protected. Capacity Whatever the cause(s) of short-term forgetting, consensus asserts that it limits the amount of retained new information short term. This limit is referred to as the finite capacity of short-term memory. Short-term memory capacity is often called memory span, in reference to a common measurement procedure. In a memory span test, the experimenter presents a list of items (e.g. digits or words) of increasing length. An individual's span is determined as the longest list length that