Cross-domain associations
1.2. The Embedded Process Model of Cowan
Cowan conceptualized working memory as “the cognitive processes that retain information in an unusually accessible state, suitable for carrying out any task with a mental component” (Cowan, 1999, p. 62). His working memory model is a further elaboration of his information processing theory (Cowan, 1988, 1995, 1999), which we will start by explaining.
The introduction of a new information processing theory was driven by the inconvenience of the predominant information processing model of Broadbent (1958). Although both of these models are termed information processing models, we consider them to be easily
transformable into working memory models by focusing on the relation between short term memory, attention, and long term memory. In his theory, Broadbent (1958) supposed information to flow in the human mind in a rather predisposed way (see Figure 1.10).
Information would first pass through a sensory store (called short term store in the model), on which a selective filter acts. Information that makes it through the filter is then represented within a “limited capacity channel”, a system largely corresponding to what is termed
nowadays short term memory. Some of this information flows then further to higher levels of processing with the long-term memory store as the endpoint, while the remaining information fades away.
Figure 1.10: Broadbent’s (1958) information processing model (Cowan, 1995). © 1995 Oxford University Press
32
A main criticism on this model concerned the order of the stores. For example Bower and Hilgard (1981) highlighted the fact that pattern recognition and coding processes are necessary for information to flow into the short term memory store but that these processes are influenced by long term memory. Yet, the long-term memory store is to be retrieved only later within the information flow according to Broadbent’s model. Broadbent had already noticed that this straightforward linear information flow was not tenable, reason for him to incorporate certain loops within the model. However, according to Bower and Hilgard, this kind of loops could still not account for the observed interplays observed between de different components within the model. A solution to the problem of the order of the stores was to imagine a more integrated relation between the different components. Norman (1968), for example, proposed the short-term store as the activated part of long-term memory. This kind of structure allowed for a rich interplay between these components. Cowan (1988, 1995, 1999), like many others (e.g., Anderson, 1993; Cantor & Engle, 1993; Engle et al., 1999), adopted this hierarchical structure, hence the name of his working memory model: the embedded process model (Cowan, 1999). A representation of the structure and the main assumptions of this model are shown in Figure 1.11. Though short term memory was thus designed as integrated within long term memory, short term memory was not considered to coincide with long term memory. Cowan (1988) elucidated the differences between short term and long term memory by the processes that act upon the representations. For the short term memory store, this concerned rehearsal in order to keep them in an activated state, while for long term memory, different processes like for example semantic elaboration could be used in order to create the necessary retrieval cues.
Figure 1.11: Cowan’s information processing model (1988). © 1988 Amercian Psychological Association
33
Cowan (1988) imported another important deviation from Broadbent’s model into his information processing theory, namely the status of sensory memory. According to Cowan, information remains in sensory memory only for a couple of hundred of milliseconds. All information persisting within the human mind after this short delay appears to rather obey to the rules of short term memory than those of sensory memory. This vision contrasted with earlier visions on sensory storage that had often been divided into a first extremely short delay, followed by a second phase that could last for several seconds (Aaronson, 1974;
Kallman & Massaro, 1983). Though this second phase was characterized by containing information that was already partly analyzed, it was still considered to be in sensory memory.
However, based on an analysis of duration, capacity, access to awareness and coding characteristics of sensory and short term memory, Cowan (1988) reasoned that this second phase of sensory memory fitted thus better with the concept of short term memory than with sensory memory.
Cowan (1988) also introduced a different notion of the concept of awareness within his model. The traditional vision on awareness was that its contents and the contents of short term memory are identical (James, 1890; Stern, 1985; cited in Cowan 1998). Nevertheless Cowan argues that this is not necessarily the case. He refers for example to subliminal
perception, the fact that information can enter the human mind without one being aware of it.
Cowan favors hence a concept of awareness that is more constrained than short term memory.
Information maintained or activated in short term memory does not imply one is actively aware of this information. This vision can also be illustrated by the fact that environmental unchanging stimuli one is no longer aware of can still be activated in short term memory. A change of these stimuli or a questioning about them can rapidly bring this information back into a state of awareness. A central issue within Cowan’s information processing theory relies on the attentional selection of the information one is consciously aware of and that represents the “focus of attention”. Information can enter this focus of attention either through executive processes, which orient attention effortfully to certain stimuli. Additionally, the change of a stimulus to which attention was no longer actively oriented can as well redirect attention towards it and report this stimuli within the focus of attention. The role of attention is then described as enhancing the processing of certain information while excluding other non-relevant information. In contrast to the multi-component model which favored a separation of the processing domains, Cowan describes the effect of attention as “cutting across processing domains and tasks” (Cowan, 1999, p. 65), representing thus a domain-general resource. These
34
attentional capacities are the resource of the central executive. Besides highlighting the role of attention within the maintenance of information, Cowan does not go beyond a description of the central executive as the collection of mental processes susceptible to modification by instructions or incentives (Cowan, 1999). Apparently, the functioning and the role of the central executive are hard to define, as was already noted Baddeley (1986).
Based on the criticisms on Broadbent’s model, and elaborating on several of its concepts, Cowan proposed thus a new information processing theory (1988, 1995). The elaboration of this theory formed the basis for Cowan’s embedded process model of working memory.
1.2.1. The representation of single features
As summarized in Figure 1.11, Cowan assumes that information can be represented on different levels of hierarchy. Cowan (1999) considers the focus of attention, short term
memory as well as long term memory to contribute to working memory. Information that is in the focus of attention is in an elevated state of activation and can thus easily be retrieved. This information is considered the information one is explicitly aware of. The maintenance of information within the focus of attention relies on attention. Next to this maintenance
function, attention is employed to execute other cognitive tasks as well. This implies thus that when attention is diverted from the focus of attention for processing activities, these memory items decrease in activation. Information that is in short term memory but not in the focus of attention can also still be retrieved relatively easy, although it demands for a little more effort than for the information within the focus of attention. Generally, information in long term memory is harder to retrieve. Yet, information in long term memory can still be considered to be in working memory if working memory disposes of cues pointing to the item and if the item can be retrieved if necessary by the aid of retrieval cues. While the focus of attention is limited in terms of capacity, short term memory is rather limited in terms of duration.
Activation diminishes over time and strategies to counteract this decay have to be executed in order to prevent the loss of the information. In addition, interference can act on these activated items as well. Long term memory in contrast is neither time nor capacity limited. However, retrieval of the information depends nevertheless on the existence of retrieval cues.
35
A similar hierarchical working memory structure can be found within Oberaurer’s working memory model (2002). Oberaurer describes the three components contributing to working memory functioning as the focus of attention, the region of direct access and long term memory. A main issue of disagreement between the theories concerns however the capacity of the focus of attention. Both theories agree on its limited capacity, but Cowan assumes the focus of attention to maintain about four items (1999, 2001, 2005) while Oberaurer favors a capacity limit of only one item (2006; Oberauer & Bialkova, 2009).
Unlike within the multi-component model, the embedded process model (Cowan, 1988, 1995, 1999) does not emphasize domain-specificity. Cowan deliberately decides as such not to make a distinction between the representations of verbal, visuo-spatial or any other domain-specific information. Instead, Cowan (1999) emphasizes the generalities that exist across representations from different domains, like the time course of their activation or the interference caused by similar coding of subsequent stimuli. Within working memory, information from different domains can thus be represented in many different modalities (phonemically, semantically, visually, etc.) as a function of its modality at encoding and its specific physical properties. Cowan acknowledges the domain-specific rehearsal strategies that may be used to keep information within the working memory system (e.g., rehearsal or visualization), though these are just considered as different processes to reactivate memory traces. Other more central strategies to maintain information within the focus of attention consist of deliberately focusing attention on these items (Cowan, 1999) or searching through a set of items, which makes them circulate through the focus of attention as well (Cowan, 1992, 1999).
1.2.2. The representation of feature associations
In contrast to the multi-component model, the embedded process model does not assume a different structure to maintain associated features. Like single features, associated features are represented as activated parts of long-term memory. These associations are nevertheless represented in the format of chunks. A chunk represents a familiar unit of information. Although the term chunk remains rather vague, it has proven to be a fruitful concept (e.g., capacity limits in terms of chunks, Miller, 1956; chunks of declarative knowledge within ACT-R, Anderson, 1993). According to Cowan, chunks can be formed when different elements are represented within the focus of attention at the same time,
36
allowing multiple associations between these elements to be created (e.g., Cowan, 1995, 1999; Halford, Cowan, & Andrews, 2007). Cowan (2001) estimated the capacity limit of the focus of attention at about three or four items. As such, one could imagine chunks not to exceed four elements. However, an iteration of the chunking process can be executed.
Imagine a first chunk of four elements being created. Newly learned associations then
continue their existence within long-term memory, after their temporal association within the focus of attention. So later on, this chunk will be represented within the focus of attention as one element and can thus be grouped to the three remaining elements to create a chunk containing now seven elements (or even more if one assumes the other elements in the focus of attention to be chunks composed of several elements as well). This process can be
performed iteratively, creating hence chunks that largely exceed the limit of four items. It is in this way that one should think of expertise, the creation of always bigger chunks allowing thus a multitude of information to be activated at the same time. This conception of expertise is very close to the long term working memory concept developed byEricsson and Kintsch (1995). They considered long term memory to play an active role within skilled activities such as reading or the domain of expertise of experts. According to these authors, long term memory is indispensable to explain the magnitude of information one relies on within this kind of activities. Within long term memory are maintained the end products of processing sequences executed previously. Retrieval cues activated in short term memory allow then for a rapid access to these end products. The rapid access to a large amount of information explains the processing speed and efficiency for well-experienced activities. Both models acknowledge hence the important role long term memory plays within expertise.
As we have stated before, the embedded process model is less detailed concerning the different representations working memory may contain than the multi-component model. In contrast to the multi-component model, single features and feature associations are
maintained within one domain-general memory store, i.e., the focus of attention. Both kinds of information respond to the same characteristics and the general properties concerning the maintenance of feature associations have occupied a smaller part of specific research than was the case within the multi-component model. Instead, the focus of Cowan’s work is on
capacity limits, which have hence been studied more intensely for both single features as well as feature associations.
37