Central and peripheral components of working memory storage.

A key issue in cognitive psychology is the nature of limitations in working memory, the small amount of information temporarily held and used in various cognitive tasks. An important question is to what extent working memory storage depends on a mental faculty that is general across domains as opposed to domain-specific (e.g., Kane et al., 2004). A long tradition of dual-task methods lends itself to the examination of this question, and has been used for many years (e.g., Allen, Baddeley, & Hitch, 2006; Baddeley, 1986; Baddeley & Hitch, 1974; Cocchini, Logie, Della Sala, MacPherson, & Baddeley, 2002; Cowan & Morey, 2007; Fougnie & Marois, 2011; Morey & Cowan, 2004, 2005; Morey, Cowan, Morey, & Rouder, 2011; Saults & Cowan, 2007; Stevanovski & Jolicoeur, 2007). With this method, it has been fairly well established that there is some dual-task interference between very different tasks. To some degree, for example, the need to retain visual items interferes with concurrent storage of verbal items, and vice versa. Yet, this methodology has never lived up to its promise because, we contend, a thorough analysis of the problem has not been available. We examine the consequences of what we believe to be an improved methodology to ask how much of the working memory storage capacity is central, or capable of being allocated to different materials to be remembered according to task demands. This concept is separate from peripheral storage, which has different varieties, each of which can only be allocated to a single type of stimulus regardless of task instructions. In our case, storage capacity could be allocated to colored objects (sometimes differing also in shape) and/or words (spoken or, in one experiment, written). Assuming that there is a limited amount of central storage to be allocated freely, the number of colored objects remembered should be reduced when the participant must also remember words at the same time, and vice versa. A dual-task design, with memory for one or two different stimulus sets of different types required on a given trial, is required in order to estimate central and peripheral components of working memory. The reason is that memory for a single stimulus set theoretically can be based on multiple storage mechanisms, only some of which are shareable resources. With a single type of stimulus, multiple storage mechanisms cannot be separated. Here we apply a model in which memory for items in each modality is assumed to come from the sum of the contributions of central and peripheral components. The central component can be estimated as the portion of memory for stimuli of a certain type (e.g., colors) that have to be shared with stimuli of a second type (e.g., words) if that second type also is to be remembered. The peripheral component can be estimated as the portion of memory for stimuli of a certain type that does not have to be shared. Before the analysis of results into central and peripheral components can be accomplished, the number of items of each type retained in working memory must be estimated, as in past work (Cowan, 2001; Cowan, Blume, & Saults, 2013), separately for single- and dual-task conditions (cf. Saults & Cowan, 2007). Then certain subtractions between conditions can be carried out to estimate the central and peripheral components, as we explain below. Before explaining that, though, we discuss in more detail the theoretical implications of central and peripheral storage.