A computerised clinical test of forgetting based on the ACT model of memory retrieval

Int. J. Man-Machine Studies (1990) 32, 233-244

A computerised clinical test of forgetting based on the ACT model of memory retrieval DAVID G. ANDREWESt AND DANA MAUDE

The University Of Melbourne, Australia. (Received 5 October 1988 and in revised form 22 May 1989) The A C T model of interference (Anderson, 1976) was applied to a visual-search paradigm using an elderly population (N = 22) 65-85 years, in order to develop computerised clinical test of forgetting. The test is to be used to identify similarities and differences between etiologically-distinct amnesic populations on the basis of susceptibility to interference. A visual-search task manipulated the number of examples presented in association with a particular category. This was achieved by requiring the subject to search for a varied number of distractor examples with a target example. As predicted by the A C T model, increasing the number of distractors resulted in slower identification o f the target item, as measured by increased recognition response latency. Also as predicted, increasing the number of distractors also increased the number of recognition errors. The interference effect produced by the distractors was i'educed by strengthening the association between the target word and the category. This was achieved by presenting the target and category a second time in the presence of different distractors. The test's potential as an automated assessment device is discussed.

1. Introduction Interference effects are seen as important diagnostic signs during clinical testing of patients with memory disorders (Levin, Benton & Grossman, 1982; Butters & Miliotis, 1985), however assessment of these effects is rarely made within the terms of a theoretical framework. There is therefore a requirement for a test which can predict and measure the results of memory interference according to a model of expected "normal" performance. This study is the first stage in the development of a task designed for this aim based on the ACT model of memory retrieval (Anderson, 1981). The more immediate aim here is to assess whether this especially designed task can satisfy the predictions made by this model using a normal, elderly population. A computerised presentation is used here to allow the task to be tailored to each individual's speed of information skills. An elderly population is used to simulate to some degree the problems encountered with patients using automated assessment presentation. The basis of the ACT model is the assumption that memory search is determined by a process of activation process between associations stored within memory. Activation and spreading activation are metaphors used to account for retrieval efficiency from Long-Term-Memory networks (Collins & Quillian, 1969; Anderson t Please send all communications to: Dr D. G. Andrewes. The Medical Psychology and Behavioural Science Unit, Department of Psychiatry, University of Melbourne. Clinical Sciences Block, C/o P.O. Royal Melbourne Hospital, Victoria 3050. 233 0020-7373/90/020233 + 12503.00/0 9 1990 Academic Press Limited

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& Bower 1973; Collins & Loftus, 1975; Anderson, 1976). Anderson (1976, 1980) describes the ACT system of memory in which its nodes represent features such as concrete objects, abstract notions and locations. Nodes are related to one another by links which describe their interrelationship. Spreading activation occurs when information in memory is accessed; nodes which represent the information are activated independently but in parallel from a source node. The rate at which activation spreads is dependent both on the number of interconnecting links and on the relative strength of these links in this limited capacity system (Anderson, 1981). Spreading activation is slowest when the information to be accessed is semantically similar to other information which is also in memory. When activation is dispersed to too many paths this "fan effect" results in a failure to access the required information as the activation level falls below a critical threshold (Anderson, 1981). The "fan-effect" is supported by the large numl~er of studies which have found recognition and verification response latencies to increase proportional to the increase in the number of competing or interfering responses learned (Anderson & Bower, 1973; Mohs, Westcourt & Anderson, 1975; Lewis & Anderson, i976; King & Anderson, 1976). This has been achieved by both increasing the number of themes associated with a concept (Reder et al., 1987) and by the addition of irrelevant, semantically related facts (King & Anderson, 1976). Studies to date have concentrated on the competing effect of material which is also intentionally learned. A question remains as to whether semantically similar material which is incidentally learned and presented in the same context as the intentionally learned material can also produce the fan effect. While the relationship between incidental learning and the fan effect is yet to be explored, one might predict similar but lesser interference from words incidentally learned. In the visual search task presented here subjects are induced to search for the find one targets and foil(s) (distraetor items which are semantically related to the target) from a series of categories. Whereas the targets are identified as to-be-learned, subjects are informed that the foils do not have to be learned. In order to assess the effects of interference from incidental learning of foil(s) the incidence of foils to target within a category search is varied while the time between targets is kept constant. In the conditions depicted in Fig. 1, for this experiment, the ACT model predicts slowest and least accurate retrieval of target item in condition (c) since parallel activation in this limited capacity system is dissipated amongst four alternatives. Condition (a) which is without foils should be associated with the fastest and most accurate retrieval. The model also predicts that strength of association between a particular foil and the category searched will determine the probability of false-recognition of that foil. This is tested by varying the categorical strength of association or potency of the foils (see Fig. 1). This last prediction is made since spreading activation is an increasing function of the strength of association between stimuli (Anderson, 1974; Hayes-Roth, 1977). The effect of strength of the target due to repeated presentation is also tested by presenting a new set of foils for the second trial while keeping the targets constant. An aged sample of subjects is used here as a way of gaining an understanding of the practical limitations of this computerised task as a clinical tool.

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Sapphire (a)

(Target)

I Sapphire (Target)

Pearl (M)

(b)

Amethyst (L)

Sapphire (Target)

Pearl (M)

Diamond (H)

(c)

FIG. 1. A simplified schematic representation of the spread of activation in conditions (a) without semantic foils; (b) with one semantic foil; and (c) with three semantic foils. Foils may have low (L), medium (M) or high (H) saliency within a category.

2. Method 2.1. SUBJECTS

Twenty-two subjects (5 male and 17 female) with a mean age of 73 years (range: 65-85 years) participated in this study. All subjects were resident in old peoples' homes denoted as independent-living accommodation and they were without history of neurological damage. 2.2. DESIGN The three conditions tested the effect of 0, 1 and 3 foils on learning within a visual-search task. These three conditions were tested twice in each learning trial according to one of three balanced order presentations (see Table 1). During one learning trail, two of the six to-be-remembered target words of a test list were preceded by the presentation of all three foils (one with high, one with medium and one with low saliency, see section 2.3) corresponding to the appropriate cateogry; two target words were preceded by one foil (the High Foil); and two target words were presented without foils. T h e y were labelled the 3-Foil, 1-Foil, and 0-Foil conditions, respectively. Each trial was followed by a distraction period which in turn was followed by a test of recall then a timed forced-four-choice

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TABLE 1 Shows the number of foils presented with each of the six target items within a trial. Each trial has a different order o f the 1- versus O- vesus 3-foil conditions. Three presentation orders are therefore required for the three trials per set Alternative

Sequential position of target

presentations 1 2 3

words in each trial 1

2

3

4

5

6

0 1 3

3 0 1

1 3 0

1 3 0

3 0 1

0 1 3

recognition test. Each subject undertook three such trials each trial following a different presentation order as depicted in Table 1. These three trials constituted Set 1. Set 2 repeated the format of Set 1 using the same 18 target words but with different foils (foils came from same category as targets and were defined in terms of saliency as in Set 1). For each set, the order in which the subjects were given the three presentations (see Table 1) and the three target/foil lists was randomised. The combinations were constructed with the constraint that subjects must experience all three test lists and all three alternative presentations during each set of test sessions. An added constraint was that none of the foils could be common to more than one cateogry within the session. 2.3. APPARATUS AND MATERIALS

Automatic presentation of test material on a monochrome visual display unit (VDU) was controlled by an Apple IIe microcomputer. Subjects depressed one of two specified labelled keys on a standard keyboard in response to the test stimuli. The 18 target words (for set 1 and 2) and 24 foils (12 each for set 1 and 2) were obtained from 18 categories which included: precious stones, relatives, metals, reading materials, colours, articles of furniture, parts of the body, fruit, countries, sports, articles of clothing, musical instruments, flowers, cities, snakes, vegetables, carpenter's tools and substances for flavouring foods. These categories were obtained from normative study using a group of 96 aged persons of a mean age 78 years (range 68-88 years). This group came from a similar sociocultural background to the experimental subjects. The categorisation in terms of saliency was obtained according to the frequency with which a word was reported first as a member of category. A further six categories were used for a practice trial.

2.4. PROCEDURE

The subjects were tested individually, seated in front of the V D U and the computer keyboard. To ensure the subjects understood the requirements of the task, they

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were asked tO repeat the content of the instructions automatically displayed on the VDU. Instructions were given prior to proceeding on the single practice trail. In a self-paced visual search task, subjects were required to search for a target word from within a seven word list, vertically displayed on the VDU. A continued search of new lists was achieved by the subject pressing a button marked 'next'. The target was defined only as coming from within a particular category. The subject was required to identify target words within the list which were members of this category (see instruction (a), below). The foils came from the same category as the target words which resulted in their inevitable mistaken identification as targets. When a foil was identified the subject was informed that the response was in error and that the search must be continued (see instruction (b), below). When the subject chose the target word the subject was informed that the word was the correct word and that the subject was required to remember the word (see instruction (c), below). Although, subjects resPonded by pressing a button to a particular list, subjects were in no doubt as to the identified word since the foil or target word was embedded and highlighted within the instructions. After the target had been found the subject was then required to search for a word belonging to a new category. During and at the beginning of a new search the subject were given the searching instructions e.g. "Press the 'next' button to continue searching for a type of flower"; these instructions were presented at the top of the screen. When the subject missed a target or foil (as indicated by the pressing of the 'next' button instead of a 'hit' button in the presence of a target or foil) the word was still displayed for the subject within the instructions that the subject had missed the word (see instructions (d) and (e) below). In this way subjects were exposed to targets and foils irrespective of the subjects ability to search for words. On the occasion that a word was identified from a list which was without any words from the requisite category (false-positive), subjects were informed of their mistake with the instructions (f) below. The following examples are a list of the instructions used in the task: (a) (b) (c) (d) (e)

"Press the 'NEXT' button to continue searching for a type of precious stone." " P E A R L is the wrong word. Nearly right, keeping searching!" "Right! Remember this word: SAPPHIRE" "You MISSED it! The word you have to remember is SAPPHIRE." "You MISSED the word PEARL. But this is not one you have to remember. So keep searching!" (f) "NONE of the above words are meant to be from the category of PRECIOUS STONES. So keep searching!" Targets were presented after a set interval of search time (90 seconds) and was independent of the subjects speed of search. In the foil conditions the computer was programmed to deliver foils at equal intervals prior to the target. The foil and target items appeared in a randomly positioned place within a search list. On completion of the visual search task in each trial, subjects were given a distraction period of 30 seconds to prevent rehearsal (Glanzer & Cunitz, 1966). Subjects counted backwards from a random number (greater than 100) presented on the screen and then were required to recall the six target words of the preceding trial, in any order, by first accessing the appropriate category name.

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The forced-recognition test followed and the instructions were presented on the screen. For each category encountered in the visual search task, the subject was given a recognition choice of four options, labelled A, B, C and D. Subjects were instructed to choose the word which was originally learned. The options consisted of a target word and the respective foils and/or other words from the same category (not previously presented). The subject was asked to respond as quickly as possible, while at the same time being careful to be correct. In consideration of the age of the subjects and to ensure consistency in reaction-times, the experimenter operated the response key during the recognition task. The experimenter responded immediately to the subject's verbal selection of one of the four alternatives. The VDU was positioned in such a way as to obstruct the experimenter's view of the recognition alternatives in order to prevent possible bias. Depression of the response key stopped the timer; depression of the appropriate selection key (A, B, C, or D) triggered the presentation of the next recognition item. Feedback was given by the operator at the end of each test trial with respect to recognition and search errors as well as reaction-times, to encourage speed as well as accuracy. 2.5. MEASURES The fan effect was measured in terms of the number of target words recalled, the number of target words correctly recognised, and the time taken to select the target word within each foil condition for set 1 and set 2. The recognition data was corrected for guessing using R-W/n, where R refers to the number of correct choices, W the number of incorrect choices and n the number of incorrect alternatives available (Brown, 1969). The effect of link strength was assessed by comparing the false-recognition count of High Foils, Medium Foils, and Low Foils. The effect of strength was also assessed by comparing the number of targets correctly remembered in Set 1 and Set 2 data. The number of words searched, search errors and recognition errors were also recorded.

3. Results The average number of words searched, with all lists and Set 1 and Set 2 data combined, was 179.9 words. The average number of search errors (inlcuding false-positives) was 1.35, indicating that the level of difficulty of the visual-search task was appropriate for the subject sample. The reaction-times and error rates for each foil condition were averaged within each trail and across the three trails for each subject. The mean subject reaction-times across foil conditions in the Set 1 data ranged from 1472-5995 ms; and in the Set 2 data, from 1022-5480 ms. The standard error of the means was 176-7 ms in the Set 1 data, and 118.9 ms in the Set 2 data. There was no main effect of alternative presentations of foils in either the Set 1 data (F(2, 42) = 0.95, n.s.) or the Set 2 data (F(2, 42) -- 0.62, n.s.); nor was there an interaction between foil condition and presentation options in either set of data (F(4, 84) = 1.36, n.s.; (F(4, 84) = 0.59, n.s. respectively).

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COMPUTERISED CLINICAL TEST OF FORGETI'ING 3.1. THE FAN EFFECT

Response latency: In Fig. 2 are the means of the reaction-times for each of the foil conditions. For both sets of data, there was a predicted increase in verification times as the number of related instances of a category increased. The differences between the foil conditions are significant in both the Set 1 date (F(2, 42)= 7.58, p <0.001), and the Set 2 data (F(2, 42) = 31.80, p <0.001) in support of the fan effect. In the Set 1 data, the time to select the target word in the 0-Foil condition was significantly faster than in the 3-Foil condition (F(1.42) = 14.98, p < 0.001) but not the 1-Foil condition (F(1.42) = 2.48, n.s.). The time to select the target word in the 1-Foil condition was significantly faster than in the 3-Foil condition (F(1, 42) = 5.46, p < 0.05). In the Set 2 data, the difference in verification times between the 0-Foil condition and both the 1-Foil and 3-Foil conditions were significant (F(1.42) = 8.42, p <0.01; F(1-42) =44.06, p <0.001, respectively). The difference between the 1-Foil and 3-Foil conditions was also significant F(1.42) = 13.96, p < 0.001). Accuracy data: The average number correctly recognised in each foil condition is presented in Figure 3. In both the Set 1 and Set 2 data the proportion of errors increased with the number of foils. The difference between the foil conditions was significant in both the Set 1 data (F(2.42)= 20-72, p < 0.001) and the Set 2 data (F(2.42) = 10.40, p < 0.001). In the Set 1 data, there was a greater number of errors produced in both the 1-Foil and 3-Foil conditions than the 0-Foil condition (F(1.42)= 19.76, p <0.001; F(1.42) = 39.40; p < 0.001; respectively), and a greater number of errors produced in the 3-Foil condition than the 1-Foil condition (F(1, 42)= 4.14, p < 0.05). Similarly, in the Set 2 data the difference in errors between the 0-Foil and both the 1-Foil and 3-Foil conditions was significant (F(1,42)=8.32, p < 0 . 0 0 1 ;

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250C 230C

+ (671.4)

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1900 1700

I

0

I

1

Number of foils

I

3

FIG. 2. Means and (S.D.) of recognition response latencies for the 0-, 1- and 3-Foil conditions for Set 1 and 2. Two points are predicted by the ACT model for the 3-Foil condition for Set 1 (-II-) and Set 2 (-I-) (see Section 4).

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D. M A U D E

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F(1, 4 2 ) = 20.31, p < 0.001; respectively), as was the difference between the 1- and 3-Foil conditions (F(1, 4 2 ) = 4.10, p < 0.05). In the Set 2 data there was a significant positive correlation between the number of errors and the mean reaction-time in the 0-Foil, 1-Foil, and 3-Foil conditions ( r = 0 . 4 8 , p < 0 . 0 5 ; r = 0 . 5 4 , p < 0 . 0 5 , r = 0 - 4 7 , p < 0 . 0 5 ; respectively). This relationship was not evident in the Set 1 data (p > 0.05 in all foil conditions). There was a predicted trend of a decrease in the number recalled with increased fan (see Table 2). However, the difference between the foil conditions was not significant (F(2, 42) = 1-08, n.s.). 3.2. STRENGTH MANIPULATIONS Reaction times: The data in Fig. 1 also indicates that t h e - r e c o g n i t i o n reaction-times in Set 2 were consistently faster than those in Set 1. The two sets of TABLE 2

Means and (S.D.) of the number of words recalled for the 0-, 1- and 3-Foil conditiom for Set I and 2 data

Mean (S.D.)

1

Foil condition 2

3

2.50 (1.10)

2-25 (1.20)

2.05 (1.12)

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data differed significantly in the 0-Foil condition (t(21) = 3-09, p < 0.01), the 1-Foil condition (t(21)= 3.41, p <0.001), the 3-Foil condition (t(21)= 2-306, p <0.05), and overall (t(21) = 3.21, p < 0.01). Recognition accuracy: Figure 3 illustrates a similar consistent difference between the two sets of data for recognition accuracy. There were less recognition errors produced in the Set 2 data in both the 1-Foil (t(21)=2.18, p <0.05) and the 3-Foil (t(21) = 2-51, p < 0.05) conditions, and there was an overall difference (t(21) = 3-37, p <0.001). Due to the generally low proportion of errors in the 0-Foil Condition, there were no differences between the sets of data (t(21)= 0.72, n.s.). This result, plus the reaction-time data, supports the view that strength of the trace increases speed and accuracy of retrieval. Recall accuracy: The total number of words recalled in the Set 2 data (M = 7.64, S.D. = 2.65) was greater than in the Set 1 data (M = 6-0, S.D. = 3.15), a n d this difference was significant (t(21)=3.64, p < 0 ; 0 0 1 ) (see Fig. 3). There were consistently more target words recalled in the second trial, however, only t h e increase in the 3-Foil condition was significant (t(21)=4-45, p < 0 . 0 0 1 ) . This partially supports the strength hypothesis. Foils: The number of each type of foil incorrectly responded to, showed the expected trend. High Foils (N = 34) were selected more often than Medium Foils ( N = 25) and Low Foils (N = 18). But this difference did not reach significance (F(2, 42) = 0-59, n.s.) and therefore the strength hypothesis was not confirmed for this data. 3.3. RELIABILITY

There were no significant differences between the lists in the 0-Foil Condition (F(2, 42)= 1.26, n.s.), the 1-Foil Condition (F(2, 42)=0.28 n.s.), or the 3-Foil Condition (F(2, 42) = 0.64, n.s.)

4. Discussion 4.1. PREDICTIONS MADE BY THE ACT MODEL

The results are generally consistent with previous studies of the fan effect (e.g. Anderson, 1974, 1976; Lewis & Anderson, 1976; Mohs et aL, 1975; Thorndyke & Bower, 1974) and are in keeping with the assumption that increasing the number of items related to the concept accordingly interferes with the memory for an item. Anderson (1976) proposed that this is due to a reduction in the amount of activation assigned to any one pathway leading from that concept when parallel spreading activation occurs within in a limited-capacity system. The predictions from these assumptions are supported by the latency data as a measure of interference. Recognition response latency increased as the number of categorical foils increased. Also, a similar relationship was found between foils and recognition errors. A reasonable concordance with the ACT model has been achieved in Fig. 2 for the latency data. Anderson's (1981) model of reaction-time is approximately predicted using the formula: E R r = I + (1/rA)

Where E R T (the expected retrieval time) is directly proportional to the level of

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activation. I is the intercept representing other (constant) effects on retrieval, A is the amount of activation and r is the relative strength of the target word compared to the competing responses. In this case the point predicted by this equation at the three foil level was calculated by using the points at the other two interference levels. The 'no foil condition' gives an intercept. Given this information the value of rA can be deduced for the single foil condition given the latency time for this position. With a value of rA the three foil point can be predicted since the relative level of activation should be reduced by one half since the activation is now divided between four pathways (target and three foils) compared to two (target and one foil). The relationship between error rates and accurate recognition times was found to be linear in the second set of data. The linear relationship may have been reduced in the Set 1 data due to the inevitability greater variability in response within the first trial. This linear relationship is contrary to any speed-accuracy trade-off explanation (Pachella, 1974), where one would expect faster response times to be associated with lowest accuracy. A further prediction of the ACT model concerns the effect of strength of a memory as represented by increased exposure of an item. This increases the proportion of activation assigned to the pathway from the category to the item thereby increasing the likelihood that the item would be retrieved correctly and rapidly (Anderson, 1974; 1976; and 1981). This prediction was satisfied here since increasing the strength of association between the target word and the category (by repeating target presentation in a second trial) decreased identification times and error rates at all levels of the foil condition. High associative strength between foils and the category should have been associated with a high rate of false-positive recognition pattern. The expected pattern was there but it was not statistically significant. This result is inconsistent with other studies where typicality ratings and pre-experimental associations are highly correlated with accessibility (Loftus, 1973; Perlmutter, Harsip & Myers, 1976; Rips, Shoben & Smith, 1973; Smith, Shoben & Rips, 1974). A strong contributing factor to the nonsignificant result could have been the low recognition error rates exhibited in this experiment. With clinical populations higher error rates may be realised and therefore foil strength may still prove to be a potentially useful indicator of susceptibility to interference. The better prediction of the Fan effect in the results of the recognition performance and recognition response latency compared to the recall results is consistent with the ACT framework and the generate-recognition models of recall. According to the generate-recognition model recall involves the process of retrieving items which are then subjected to the recognition process. The first-stage of retrieval is obviously not under experimental control to the same extent as in a forced-choicerecognition since the number of foils subjectively retrieved is not under experimental control. The variability of the retrieval process will therefore undermine the predictions of the fan effect during recall. 4.2. T H E A D V A N T A G E S O F T H E C O M P U T E R I S E D A S S E S S M E N T A N D C L I N I C A L APPLICATION

The main advantage of this system over a manually presented one is that it allows a complex timed visual search task which is tailored to the search speed of the testee.

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This allows a measure of visual-search speed and accuracy which is independent of the presentation rate of the to-be-remembered. The important clinical distinction can therefore be made between speed of information processing and memory impairment. A great difficulty in designing tasks for brain-damaged patients that involve reaction-time comes from the confounding effects of perceptuo-motor performance. This problem is largely overcome in this task since the button pressing reaction-time response is supplied by the experimenter. An alternative approach would have required a timer linked with a voice key. This inevitably leads to a loss of data because of the problems often encountered with variation in speech volume and extraneous noises introduced by patients. The difficulty of comparing latencies of samples which have been assessed by different testers could be overcome by comparing the performance of the tester with a standard prior to testing. A co/istant could then be added to or subtracted automatically from the individual reactiontimes. We are now testing clinical populations using this task and we predict distinctions between patient groups on the grounds of vulnerability to interference. The use of recognition response latency as a dependent variable has been successful in a number of studies using amnesic patients. For example, Moscovitch (1982) and Glass and Butters (1985) demonstrated priming with lexical decision tasks reflected in response latencies. It is hoped that the test will finally be of use both in the classification of patients and in the planning of rehabilitation programmes. Knowledge of an individual patient's vulnerability to interference would be useful in prompting the use of strategies to reduce potential distraction and interference during treatment programmes and other learning situations.

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amnesia of patients with alcoholic Korsakoff disease. Archives of Neurology, 36, 211-216. ANDERSON, J. R. (1974). Retrieval of propositional information from long-term memory. Cognitive Psychology, 36, 451-474. ANDERSON, J. R. (1976). Language, Memory and Thought, Hillsdale, New Jersey: Erlbaum. ANDERSON, J. R. (1980). Cognitive Psychology and its Implications, San Francisco: Freeman. ANDERSON, J. R. (1981). Interference: the relationship bet~veen response latency and response accuracy. Journal of Experimental Psychology: Human Learning and Memory, 7, 326-343. ANDERSON, J. R. & BOWER, G. H. (1973). Human Associative Memory, Washington D.C.: V. H. Winston. BROWN, S. R. (1969). Measurement of retention. In L. POSTMAN & G. KEPPEL, Eds., Verbal Learning and Memory, 453-459. USA: Penguin Books. BurruRS, N. & CERMAK, L. S. (1980). Alcoholic Korsakoffs Syndrome. London: Academic Press. BtJl-l~RS, N. & MIL1OTIS, P. (1985). Amnesic disorders. In K. M. HEILMAN & E. VALENSWEIN,Eds., Clinical Neuropsychology, 2nd Edition. Oxford: Oxford University Press. BLrI-IIr.RS, N., TARLOW, S.~, CERMAK, L. S., & SAX, D. A. (1976) A comparison of the information processing deficits of patients with Huntington's Chorea and Korsakoff's Syndrome. Cortex, 12, 134-144.

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