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BACHELOR THESIS Episodic and semantic memory Theoretical and empirical issues and an introduction to the State Trace Analysis Name: Ian MacCorquodale Student No. 5686822 Supervisor: David Neville Date: 20-06-2011 Index Abstract 2 1 Introducing the dissociation 3 1.1 The history of the episodic–semantic dissociation 4 2 Memory 6 2.1 Episodic memory 6 2.2 Semantic memory 8 3 Episodic and Semantic memory 10 3.1 Functional interactions 10 3.2 Models of memory 12 4 Conclusion 14 5 Research proposal 16 5.1 State-Trace Analysis 16 5.2 Proposal for episodic-semantic distinction application 17 5.3 Simulated data 22 Literature 23 1 Abstract Human memory is too complex and too large to be studied or referred to as if it were one single mechanism. Over the years progress has been made in understanding memory by dividing it into multiple components that can be studied separately. According to Tulving (1972) the general knowledge a person possesses about the world not tied to a particular personal experience is called Semantic memory. In contrast, Episodic memory relates to events in their specific context of time and place. A crucial question remains: are episodic and semantic memories functionally dissociated or are they expressions of one underlying system? The application of a statistical technique called State-trace analysis could further clarify the episodicsemantic distinction and lead to re-appraisal of how memory models should be formulated and tested. 2 1. Introducing the dissociation From the moment we are born we are exposed to an enormous amount of information. All the information we gather throughout our lifetime is said to be stored in our memory. We can form memories almost instantaneously, and some of these last a lifetime. A life without memory is unthinkable. When you try to remember the first time you rode a bicycle as a child, you can picture yourself on the bicycle again as in some kind of mental time travel. However when someone asks you how many wheels a regular bicycle is equipped with, you “know” the answer right away without having to actively picture it or even think about it. There seems to be a difference between remembering and knowing when it comes to human memory. In 1972, Endel Tulving proposed that explicit long-term memory consists of two types: Semantic and Episodic. He defined episodic memory as “a system that receives A model of long-term memory and stores information about temporally dated episodes or events, and temporal-spatial relations among them” (Tulving, 1984, p.223). In contrast, “Semantic memory is the memory necessary for the use of language. It’s a mental thesaurus, organized knowledge a person possesses about words and other verbal symbols, their meaning and referents, about relationships among them, about rules, formulas, and algorithms for the manipulations of these symbols, concepts, and relations.” (Tulving, 1972, p.386). The actualization of episodic information is typically deliberate, often requires conscious effort, and is informally termed “remembering,” while that of semantic information is usually automatic, and is often referred to as “knowing”. (Tulving, 1984). In other words, Tulving proposed a distinction between an autobiographical memory of personal events, and a memory that applies to our store of culturally shared general knowledge about the world and facts detached from autobiographical reference. Despite the plausibility of this distinction research has yet to provide conclusive evidence that these two systems are functionally separate. Researchers in a number of areas, including neuropsychology, animal learning, and cognitive psychology, have stated that it is useful to investigate the 3 existence of multiple memory systems because it allows one to distinguish between two forms of memory that intuitively seem different phenomena. Methodologically research has focused on identifying dissociations between the two memory systems: finding manipulations and tasks that have different effects on each of the systems. When one variable has different effects on two tasks a dissociation is shown. These dissociations lend support to the hypothesis of multi memory systems. Over the years progress has been made in understanding memory by dividing it into multiple components that can be studied separately. For example, distinctions have been proposed that are now widely accepted, such as the short-term and long-term memory distinction. However, a crucial question remains: are episodic and semantic memories functionally dissociated or are they expressions of one underlying system? For over 20 years a number of studies have adduced both empirical and theoretical arguments in favour of either the former or the latter hypothesis. For this paper a critical review of past and recent studies will be given in order to propose a clear taxonomy of the effects and of the theoretical models, which dissociate episodic and semantic memory. Before this question can be more fully explored, it is necessary to define more precisely the differences between the two proposed memory systems. 1.1 The history of the episodic-semantic dissociation The dissociation Tulving proposed was not entirely a new one. In Confessions and Enchiridion (cited by Herrmann 1982, from Augustine, 1955) in 410 BC, the philosopher Augustine made a distinction between “things learned or intuited, and principles” and “ images of things perceived and of feeling”. In 1890 The American pioneering psychologist and philosopher William James defined memory as pertaining to facts, or events. He also devoted one chapter of his famous work “the Principles of Psychology” (James,1890 cited by Herrmanns 1982) to factual knowledge and another chapter to memory of one’s past. Not much later, Wilhelm Wundt (1896) distinguished between memory as used in cognition and memory as used in recognition. In the field of psychiatry, Schactel (1947) proposed that memory was practical, based on general knowledge, or autobiographical (Hermann, J, D, 1982). 4 The first proposal that different neurological systems underlie long-term memory phenomena was made by Eduard Claparede (1951). Based on observations of amnesiacs, he discerned that patients could recall "passive associations or idea reflexes" which he called marginal memories, even though they could have great difficulty recalling events from their own pasts. These he called egocentric memories. This observed distinction was later extended by Nielson (1958), who differentiated between temporal amnesia, involving a loss of personal experiences, and categorical amnesia, involving a loss of acquired facts (Crovitz, cited by Herrmann, J, D, 1982). All together a total amnesia for episodic information with an unimpaired semantic memory does suggest a dissociation between episodic and semantic memories. In 1989, Tulving described an elaborate study of a patient called K.C. In a motorcycle accident K.C. experienced severe brain damage to the frontal regions. As a result of this damage he showed a complete loss of his own episodic memory. Interestingly his semantic memory system seemed to work fine. Another well-known case discovered by a neurosurgeon named William Scoville shows an opposite dissociation. After performing surgery on a patient called H.M. the patient completely lost the ability to store new information. In an attempt to cure H.M. of his epileptic seizures he had sectioned parts of both the left and right side of the patients hippocampus. “Surgery had interfered with Damage to these regions caused amnesia in patients K.C and H.M. the process of storing or retrieving new memories but had not touched previously stored memories” (Corkin, 2002). It is clear that for a long time people from different scientific fields have noticed the difference between the two types of memories. In this sense, Tulving’s initial intent was to avoid confusion between episodic and semantic memory research. In his 1972 article Tulving stated that “this distinction is not necessarily a functional one, but merely for the convenience of communication”. This is because until then, both phenomena were referred to as just one memory and researched as such. The whole concept has undergone significant changes over the years. Later on in his work, Tulving and other researchers explicitly assumed them to be functionally separate systems, and directed their research accordingly. Tulving brought to light some of the 5 substantial differences between episodic and semantic memory and in the next part I will further elaborate on the functionally different properties held by each system. 2. Memory 2.1 Episodic memory Our previous individual experiences help us determine who we are. All of our preferences, dislikes and opinions that are a result of experiences we have had, and influence the way we approach new situations. These episodic memories of events we have experienced help guide our actions in the future and therefore it is important to clarify all the characteristics of the episodic memory. Along with Hans J. Markowitsch, Tulving explained the uniqueness of episodic memory in their 1998 article; “Episodic memory is the only form of memory that, at the time of retrieval, is oriented towards the past: Retrieval in episodic memory means ‘‘mental time travel’’ through and to one’s past. All other forms of memory, including semantic, declarative, and procedural memory, are, at retrieval, oriented to the present” (Tulving and Markowitsch, 1998). Also, “The relation between remembering and knowing is one of embeddedness: episodic remembering always implies semantic knowing, whereas knowing does not imply remembering”. Since this system is context sensitive, both encoding and retrieval phases process events in relation to their time and space. This aspect has been formulated by Thompson and Tulving in 1970 as the notion of encoding specificity. Research has shown that when a list of words was presented with sound in the background (instrumental music or white noise), then recall tested 48 hr later was better and forgetting was less if the acoustic background was re-instated rather than changed or removed. If learning occurred with quiet background conditions, recall performance was the same whether testing took place with quiet, music, or white noise in the background (Smith, 1985). Previous studies by Godden and Baddeley (1975) found similar results by showing that divers recalled words better when the recall condition matched the original learning environment, either underwater or on land. Apart from these external contextual differences, internal contexts like emotions or mood also influence our memory of information. Our memory of information is better if we are in the same mood we were in when we where encoding the information (Blaney, 6 1986). Another study where people learned information while they were drunk or sober showed that memory was better when these people were in the same psychological state at recall. Those who studied drunk performed better in the test if they took it whilst being drunk (Goodwin et al., 1969). A common method for measuring the episodic memory is the Recognition Task. During the study phase of this test participants have to asses how familiar a word is. During the test phase participants have to decide whether or not a word has been shown before during the study phase. What is used for analysis is the proportion of words that were correctly recognized. According to Tulving episodic memory differs from semantic memory by the way the memory is encoded. Episodic memories are filled with contextual properties such as emotions and location because they are part of this type of memory. What is stored in the episodic memory is not just the words presented to the subject under water, but the entire event. Episodic memories contain information on how these memories were created. In contrast the semantic memory is less sensitive to contextual influences during retrieval because these contextual factors are not stored in the semantic memory. However there is a problem with this statement because often very little is known about the specific context wherein for example someone has learned what a bicycle is. This makes it very difficult to examine whether the semantic memory is actually less context-dependent. Another important difference between episodic and semantic memory is that the episodic memory is more subject to forgetting, more vulnerable, than semantic memory (McKoon & Ratcliff, 1986). The episodic memory system cannot hold on to all attended information forever. Over time we lose access to many memories of our specific past experiences, but the general knowledge that is extracted from those experiences often stays with us in the form of semantic information. 7 2.2 Semantic memory The general knowledge a person possesses about the world not tied to a particular personal experience is called semantic memory. Literally translated “memory for meaning”. Semantic memory is quite different from episodic memory in the sense that contextual information encoded at the time of learning is lost. An example of semantic memory would be a discussion you have with someone in which the other person mentions owning a bicycle. Rather than visualizing a specific episodic memory of a bicycle, you can access the semantic definition of a bicycle instantly to understand what the other person is talking about. In contrast to episodic memories, all the information is free from context and only an abstract form of information is stored which can be used in novel situations. Without this semantic memory, every time you would see a pencil on a desk you would have to determine its function. An important characteristic of the semantic memory is the organized structure it possesses. Remembering one particular fact brings related facts closer to consciousness. This facilitation of related concepts is called “spreading activation”. Collins and Quillian (1969) viewed the structure of the semantic memory as a network. Within this network each concept is represented as a node. Each of these concept nodes are linked together by pathways of associations between concepts. This explains why related concepts are facilitated when a A network model of memory organization (Collins & Loftus 1972) particular concept is activated. For example, when you read the word “boat” its mental representation becomes activated and therefore activates all the nodes connected to it like “sea” or “water”. Another important feature of semantic memory is the accessibility of the information. Retrieving information from semantic memory is said to be fully automatic and will cost no effort. Retrieving episodic information on the other hand is said to be a process that does require effort and is not automatic. As seen in the previous example about spreading activation, whenever a word is presented it is the meaning of this word that is immediately activated and not a specific event related to the word. 8 The organization of semantic memory appears to be much more structured and free from error in comparison to the episodic memory. The information stored in the episodic memory is more prone to error due to the loose structure it possesses, and the information is easier to be lost because it applies only to one single specific event. In semantic memory, the information is often learned in a better way and less vulnerable to forgetting because it is embedded in rich cognitive structures. However semantic memory retrieval is not entirely free of error either. In 1981 Erickson and Mattson investigated a phenomenon called the semantic illusion. In their study, people were required to answer a set of questions, four of which contained semantic anomalies, such as “How many animals of each kind did Moses take on the ark?” Participants were told that they had the option to say “don’t know” if they did not know the answer or “wrong” if the question had something wrong with it. The researchers discovered that as many as 80% of their participants failed to detect the error that was hidden in the sentence even though they did know that it was Noah, and not Moses, who took the animals on the ark (Erickson & Mattson, 1981). They attribute this phenomenon to inappropriate activation of information in semantic memory. Similar language components such as similar names can cause information to be accessed in an inaccurate way, which can subsequently lead to such mistakes. In summary, episodic memory is more subject to forgetting and more vulnerable in comparison to semantic memory. The information in semantic memory can be inappropriately activated due to overlapping lexical information. Both semantic and episodic memory systems appear to be not entirely free from error. In the next part I will closely look at whether there are more functional properties that are shared by the two systems. Secondly, I will cover the empirical evidence supporting a multiple systems view. 9 3. Episodic and semantic memory 3.1 Functional interactions There appear to be numerous and distinct differences between episodic and semantic memory characteristics. However, before assuming that these observed differences in fact imply functionally separate memory systems, I will discuss some arguments opposing the episodic-semantic distinction. Some investigators, including Tulving, have adopted the functional distinction position, while others have embraced a single-store view of long-term memory. Two researchers by the names of Anderson and Ross explained in their 1980 article that; “there are two kinds of empirical results that are particularly diagnostic in evaluating the semantic-episodic distinction. One is whether similar memory effects are obtained in experiments on episodic as in experiments on semantic memory. Similar effects would be evidence that the two types of knowledge obeyed similar functional laws and would be evidence against a functional distinction” (Anderson and Ross, 1980). In their article they discuss the observed similar effects for the two memory systems and the transfer from episodic memory to semantic memory as two types of empirical evidence against a separation between the memory systems. The purpose of their article was to question whether there is any functional difference between the memory that stores semantic knowledge and the memory that stores episodic knowledge. In order to do so they wanted to find out if the time to retrieve semantic information could be affected by the prior learning of episodic information. In other words, does the learning of episodic material interfere with the retrieval of semantic? They argued that if this were the case it would indicate that the same processes are at work in both semantic and episodic memory. Another argument that would imply the functional separation of the two systems is the transfer of information. According to this view, episodic memory is not stored in the semantic memory, therefore suggesting that the two systems are functionally separate. McKoon and Ratcliff (1979) proposed arguments against a functional distinction by explaining that a functional separation of semantic and episodic stores can be concluded only when evidence shows that semantic or episodic information is independently accessible. To examine whether semantic information is independently accessible from episodic information they used a prototypical lexical decision task. In 10 a lexical decision task subjects are presented with a string of letters on a computer screen and have to decide as fast as they can whether the string of letters form a word or not. In this task the response time is essential. When a word is presented shortly before the target word the response time is affected. Meyer and Schvaneveldt (1976) found that the response to confirm whether the presented word is actually an existing word was faster if the word preceding target word, called the prime, is semantically related to the target word. For example, response time will be faster when the prime word flower is shown shortly before the target word blossom. If the word bread is used as a prime word this should not lead to a faster response. This effect is called associative priming. As explained before, the highly structured organization of semantic memory is the explanation for this effect. The semantic network where each concept is represented as a node and is linked with other nodes causes activation of one node to spread the activation through these links to the related nodes. Because the word pairs in former studies were all well learned semantic associations, McKoon and Ratcliff were interested in the question whether this effect would still occur with newly learned word pairs and thus invoke the episodic memory. In their experiment subjects were taught pairs of words that were not highly associated semantically before the experiment. The association resulting from the newly learned word pairs is called episodic association. Again they used the lexical decision task to examine the amount of priming. Their experiment showed about the same size of priming effects in lexical decision between newly learned word pair associates as in the task with well-learned semantic associations. McKoon and Ratcliff regarded this finding as evidence for the interaction between episodic and semantic information in a standard semantic memory task. This interaction would not be expected under the assumption that the memory systems are independently accessible. Thus, such an effect argues against a functional separation of episodic and semantic memory systems. Durgunoglu and Neely (1987) also found episodic priming effects in a lexical decision task but only when a certain amount of requirements were met. Typically, Carroll and Kirsner (1982) did not find any episodic priming effects in their study. However, in their experiment they presented the two novel words together and the lexical decision had to be made when the two words were again presented together. This indicates that priming as an effect is very sensitive to the variations in 11 experimental procedures and therefore not conclusive evidence against a functional separation. Many researchers have tried to find evidence that provides an answer to the question whether the episodic and semantic memory systems are functionally separate. Often the question is answered with more questions leading to an inconclusive discussion. The formulation of ideas and questions about memory systems in mathematical form, thereby the creation of formal models of memory, assumptions and predictions can be investigated more accurately, therefore providing novel explanations for previous findings alike. At this point the episodic-semantic distinction looks like a descriptive list of features. The simple labelling of effects as episodic or semantic does not provide a theoretical explanation but merely a categorization. The construction of a model for human memory provides a rigorous way to test theories against empirical data. In the next part several models will be described in order to give a clear overview of semantic-episodic memory models. 3.2 Models of memory In 1976, John Anderson proposed a general theory of cognition that focuses on memory processes called the ACT (Adaptive Control of Thought) theory. In his most recent version of this theory, the ACT* model (1983) there is no functional distinction between episodic and semantic information regarding storage or retrieval. It is essentially viewed as one big memory. According to this model, all information, episodic and semantic is stored together in a propositional network similar to the semantic network theory discussed earlier and long-term memory is represented by a network with connected conceptual nodes. The difference is that the network nodes have a continuously altering strength of activation. In other words, if node A has an associative link to node B, activation will spread through the link from A to B with the load of activation of node B being determined by the strength of the link. One important difference between the semantic network theory and the ACT model is the introduction of token nodes. These nodes differ from the so-called type nodes that resemble the nodes in the former semantic model. Token nodes refer to specific events. For example, a “bicycle” type node stands for bicycles in general, but a “bicycle” token node stands for one specific bicycle like “the one across the street”. 12 Through the network structure and the spreading activation mechanism the ACT Theory provides an explanation for the associative priming effects found experimentally. However this single memory system model cannot provide an explanation for the variety of results found in episodic priming experiments. A model that proposes separate storage mechanisms is the SAM (Search of Associative Memory) model proposed by Raaijmakers and Shiffrin (1981). During storage, information is represented in memory images that contain associative and contextual information. According to the model the act of remembering is the endresult of a matching process between the information of the cue and the information of the memory trace. That is why in SAM, when any two items simultaneously occupy the memory the strength of their association is incremented. Thus, items that are presented in pairs more often are more strongly associated. In this model information is also associated with the specific context, and the strength of that association is determined by how long each item is presented in the context. Memories consist of a set of associations between items in memory and between items and contexts and of a parallel mechanism combining episodic and semantic memory information. Like the ACT* model, the MINERVA 2 model by Hintzman (1986) is also a multiple trace model. One of the main goals of this model was to explain memory for individual experiences (episodic memory) and memory for abstract concepts (semantic memory) within one single system. The notable difference between the former two models is that the MINERVA 2 model assumes that a new memory trace, called an echo is formed that composes of all the activated memory traces. All of these multiple trace models propose that information is stored separately. Two models in particular propose that information is stored in a distributed fashion: the Theory of Distributed Associative Memory (TODAM) by Murdoch (1982), and the Composite Holographic Associative Retrieval Model (CHARM) by Eich (1982). In these models, associative information is stored as a memory trace in the form of a convolution of the two item memory trace vectors, and both item and associative information are then represented as traces distributed across a set of shared memory elements. Mathematical models can describe memory in a more precise way than verbal descriptions. The network model by Collins and Loftus neatly describes the structure of memory while the other models discussed focus in particular on the processes and 13 on the retrieval mechanisms. When it comes to the episodic-semantic distinction a satisfactory conclusion has yet to be provided. In order to provide a unified model of long-term memory, it is a necessity to know whether semantic and episodic memory rely on a single underlying system. So far, no conclusive evidence has reached the surface. What is specifically called for is conclusive statistical data able to state whether one or two systems underlie episodic and semantic memory. 4. Conclusion The theoretical work of Tulving (e.g., 1972, 1984) has opened the field of memory to extensive empirical investigation. The heuristic value of the episodic vs. semantic distinction for classifying types of information or experimental manipulations is quite apparent (Tulving, 1985). Still, the crucial question remains, whether this dichotomy accurately reflects the way the human brain actually processes information. Still today there are researchers embracing either the two-store model or the single-store model of long-term memory. When the ability to form new autobiographical memories is lost but the old memories are intact, one could say that the episodic memory system is damaged and the semantic memory system remained intact. One could call this evidence for the operation of dual processes. In other words, something is changed that affects one process but not the other. A limitation with data from lesion studies, however, is that it gives information only concerning whether the specific damaged region is somehow necessary for task performance. Plus, evidence that different tasks are sub-served by different brain regions does not immediately imply functionally different systems. Willingham and Goedert (2001) clarify this example with an analogy: “One could draw an analogy to a computer hard drive: If two Microsoft Word documents occupy different parts of a computer’s hard drive, does that indicate that there is a fundamental difference between them? No, because the representation format and the processes (i.e., software) need to interpret that the two files are the same, despite their different “anatomic loci.” By the same token, Microsoft Word and Microsoft Excel documents would be considered different, even if they occupied neighboring or even interleaved portions of the hard drive, because they use different formats and because different processes are required to interpret them. We could apply the same logic to memory. Anatomic separability is not enough. There must be a difference between 14 the representations at a cognitive level of description to draw the conclusion that they are truly different and represent separate memory systems. It is becoming increasingly clear that complex cognitive and memory processes rely on the integrity of extended networks of brain regions rather than on the workings of individual regions. The reliance on dissociations in the episodic-semantic distinction is therefore problematic. Hintzman (1984) pointed out that when one variable shows different effects on two tasks this is viewed as dissociation. However, when variables are found that have one effect on semantic- and a different effect on episodic memory, there is nothing that predicted which way around the dissociation should have occurred. This way every dissociating result would count as evidence for the proposed distinction. Dissociation says nothing about causation. As for the results found in patients suffering from brain damage, David Horner pointed out that “among the experiments that have demonstrated dissociation of episodic and semantic memory, many of the results can be explained without invoking the existence of separate memory systems” (Horner, 1990). This is because many semantic memory tasks, like word recognition, require the subject to recall well learned information, a type of information preceding brain damage that has been often repeated, well consolidated and over-learned. With the crucial question still unanswered, with researchers still apposing either the multi-store model or the single-store model of long-term memory, statetrace analysis can clear the sky and provide researchers with clear and concluding evidence concerning the underlying memory systems. To give a concrete example of how state-trace analysis can be fruitfully applied to memory research, in the next section I will provide a proposal for an experiment. 15 5 Research proposal 5.1 State-Trace Analysis In 1979, Donald Bamber developed a method of testing simple theories of causation which he called State-Trace Analysis. A theoretical structure is a model proposed to explain empirical observations and the purpose of state-trace analysis is to test the validity of these theoretical structures by analyzing the state traces, which appear as curves in the accompanying plots. Figure 1: Idealized state-trace plots. Each data point represents the average proportion correct (from Newell & Dunn, 2008). Bamber explains a state trace as a plot graph showing the co-variation of two dependent variables. Multiple process models are often advanced on the basis of dissociations in data and the main advantage of such an analysis is that “State trace analysis can be used to evaluate conceptualizations of single- and multi-dimensional theoretical structures, independent of preconceptions or assumptions about the nature of the proposed processes and without using the flawed logic of dissociations”. “The crucial diagnostic feature of this plot concerns whether or not, across a set of experimental conditions, the data fall on a single, monotonically increasing (or decreasing) curve” (Newell & Dunn, 2008). So, if the performance measured by two tasks depends on the same underlying dimension, the relevant data should fall on a monotonically increasing curve in the state-trace plot as seen in Figure 1 (a). If on the other hand, more than one underlying dimension is required to account for the data, a 16 two dimensional state-trace plot would be expected, reflecting the involvement of two systems during task performance (Figure 1, b). So far, state-trace analysis has already been successfully applied in other experiments: by Dunn (2008) on the Dimensionality of the Remember-Know task and by Loftus, Oberg and Dillon (2004) on the Face Inversion Effect. In particular the latter study investigated the finding that inversion disproportionately affects face recognition, thereby suggesting that faces are encoded in a different way compared to other stimuli types. When Loftus et al. tested this effect using state-trace analysis, they initially found evidence for a one-dimensional encoding of unfamiliar faces. However, when they tested for encoding of familiar faces they found evidence for a two-dimensional system. As shown in the study by Loftus et al. and argued by Newell and Dunn, the role of dissociations should be replaced by state-trace analysis because the latter approach is founded on logical principles and overcomes the flaws of dissociation logic. The application of this statistical technique could finally shed light on the episodic-semantic distinction and lead to re-appraisal of how memory models have to be formulated and tested. 5.2 Proposal for episodic-semantic distinction State-trace analysis (Bamber, 1979), which has also been called dimensional analysis (Loftus, Oberg & Dillon, 2004) can be applied to examine whether there is one or more processes underlying episodic and semantic memory. Considering the results from studies on patients with brain damage it is safe to say that more than one cognitive process is involved in memory functioning. To investigate whether a dissociation between episodic and semantic memory can be made I propose a state trace analysis experiment. I will first give an example of how the analysis can be applied and follow with a proposal. Newell and Dunn (2008) use the example of a category learning experiment because, much like the semantic-episodic distinction, it is a debate whether a singleor a multi-process explanation best accounts for the results. 17 In this hypothetical experiment participants are divided into four groups. Two of these groups are trained in a “procedural task” that is assumed to be learned without tapping into the working memory. The other two groups are given a “declarative task” which is assumed to involve the working memory system. Before the first test, one out of each of the two groups is required to perform a concurrent working memory task during the training phase. Figure 2 (a) shows a uni-dimensional model for the relationship between the independent and dependent variables. The two factors, number of training trials and the working memory load show the effect on the underlying dimension. This underlying dimension is responsible for the performance of the two tasks. Figure 2 (b) shows a model with a second underlying dimension only influencing the task that claims to require involvement of the working memory. Figure 2 (a) Figure 2 (b) Now, performances on the two tasks are plotted against each other for both working memory conditions; load versus no load. Each point in this plot represents the average proportion of correct responses of the training trials. As explained before, if performance on a task depends upon two dimensions, the resulting state-trace will not be a monotonic curve (figure 3). Figure 3 18 Dimension factor According to Heathcote & Prince (2009) State-trace analysis is most easily explained with reference to a state-trace plot. “A scatterplot showing the covariation of two factors, a state factor and a dimensional factor. The state factor defines the axes of the plot. Each point on the plot is defined by a pair of dependent variable values, one for each level of the state factor”. The causal variables which are manipulated to differentially influence the latent configurable dimension is the dimension factor. For the proposed state trace I will use the following manipulations that correspond to the three factors needed in state trace. As the dimension factors I suggest the use of two memory tasks. The lexical decision task which is assumed to rely heavily on semantic information and the recognition task that relies on episodic information. A prototypical recognition task consists of a study- relax- and test phase. During the study phase participants have to assess how familiar a presented word is according to their opinion on a numerical scale ranging from 1 to 5. During the relax phase of about 5 minutes a simple computer game should be used to clear the working memory. It is of importance the game does not interfere with the previous task meaning there should be no words involved. A puzzle video game like Tetris would fit that description seeing it involves only sequences of shapes which have to be manipulated to form lines. During the test phase participants have to decide whether a presented word has been seen before during the study phase or not. What is of interest in this task is the level of accuracy. Meaning the proportion of correctly assigned words that indeed where shown during the study phase. In the lexical decision task a string of letters will be shown and participants have to decide as quickly as possible whether the presented word exists or should be classified as a non-word. The time it takes for a participant to classify a word as either existing or non-existing is of interest. 19 State factor The manipulation used as state factor in both tasks should be word frequency. Meaning how frequently a word is used in a specific language. Two lists of words have to be composed. One with high-frequently used words and one with lowfrequently used words. Trace factor The third manipulation called the trace factor that should be applied in both tasks should be repetition: the amount of times a word is presented during the tasks. Multiple levels can be composed in this manipulation. For instance one level wherein a word is shown only once and a level wherein a word is shown as many as five times. Now, in the lexical decision task the assumption is that reaction times decrease when words are more frequently used in language compared to words that are less frequently used because familiar words are easier recognized as being a word or a non-word. This effect should not be of influence on the recognition task because no matter how frequently a word is used you have to be able to asses whether the word has been shown before or not. It can even be more confusing because the words are more familiar at general and therefore lead to a poorer performance. If declarative memory consists of one dimension, the effects on performance of the lexical decision task and performance on the recognition task would be the same. But if multiple dimensions are involved in declarative memory the reaction times on the recognition task will be decreasing for high frequency words and performance will be higher on the lexical decision task. The proposed model should consequently look like the one in Figure 4(a) and 4(b). 20 Figure 4(a) Figure 4(b) If performance on the task depends upon two systems, two different traces should appear in the box plot showing different monotonic curves, since list length only affects the episodic system and therefore performance on the recognition task. If however one monotonic curve appears, the conclusion should be made that the episodic and semantic memory tap into the same system, showing no differentiating effects for the two tasks. Based on previous studies, frequency is assumed to have a differential effect on memory systems. The higher the frequency, the better the performance for semantic memory. The higher the frequency, the lower the performance for episodic memory. 21 5.3 Simulated data By simulating data from 20 subjects divided over the conditions high and low frequency items with 300 items per condition, the following plot (Figure 5) shows a uni-dimensional and a bi-dimensional model. As explained before the semantic memory will be measured with reaction times on the lexical decision task and the accuracy level on the recognition task will measure the episodic memory. However for this plot I simulated accuracy instead of reaction time which for the purpose of merely illustrating the plot is of no importance. Performance on both tasks are simulated. Each of the observations is generated according to a general linear model. A non-parametric measure of statistical dependence (Spearman’s Rho) between the two variables high- and low frequency curves is also shown. Figure 5 The uni-dimensional model shows one monotonic curve implicating that both the lexical decision task and the recognition task performances are based on the same dimension. The bi-dimensional model shows two separate monotonic curves expressing that both tasks depend on separate dimensions. The state trace analysis has been successfully applied in previous experiments and could provide a large contribution to memory research. Along with the data derived from the analysis, these findings could bring researchers a step closer to unravelling the mysteries of the human memory by providing statistical evidence of underlying processes. 22 Literature: Anderson, J. R., & Ross, B. H. (1980). 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