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• W2041196042 abstract "Recently, an increasing amount of research has been conducted using the concept of ’working memory’ to explain short-term memory disorder in patients with Alzheimer’s-type dementia (AD). Research on working memory in AD commonly targets individual systems (central executive, phonological loop and visuospatial sketch pad) in the working memory model. The original studies on each system are summarized. Functions of the central executive include regulation of different mind processes as well as information processing and temporary storage. Disorder in the central executive results in lowered performance in simultaneous dual-tasks that require divided attention. Testing is based on the assumption that simultaneous execution of two tasks will exert a load on the central executive; for example, diverting the attention of the examinee to a second task would result in lowered performance even in remembering a small volume of information for a short amount of time. The Peterson and Peterson task is one of such tasks.1 It was originally used to confirm decline of short-term memory. For example, immediately after presenting three words to the examinee, an interfering task is conducted to disturb rehearsal, and then the examinee is asked to recall the words. Using the Peterson and Peterson task, several studies have reported disorder of the central executive in mild AD. Morris (1986) predicted that the lowered score in Peterson and Peterson task is associated with the complexity of the interfering task, because a task that exerts a bigger load requires a larger processing resource from the central executive.2 He taught a patient three consonants or words, then conducted a simple interfering task (repeating ’the’, tapping on the desk, etc.) and afterwards asked the patient to recall the words or consonants. He compared the delayed recall performance of this task to that using a more complicated interfering task (recitation of numbers backwards, addition, etc.) As expected, in patients with mild AD, the score was markedly lowered with the increase in complexity of the task. However, the delayed recall score was not affected by the nature of the interfering task, and a marked interfering effect was observed even with a simple interfering task. Morris concluded that the capacity of the central executive is greatly reduced in patients with mild AD, and is therefore easily affected even by simple interfering tasks. Morris's method has been criticized for not necessarily reflecting central executive function, because both the learning and interfering tasks are linguistic tasks and require the same processes such as language processing and temporary memory. In view of this criticism, Baddeley designed dual tasks of completely different cognitive processes.3,4 They conducted a tracking task simultaneously with one of the following three tasks; articulatory suppression, simple reaction time to tone and auditory digit span, in 28 patients with moderate AD and 28 age- and education-matched elderly control subjects as well as 20 young control subjects. In the tracking task, the subject was asked to follow with a light pen a white square moving randomly in a color monitor. In articulatory suppression, the examinee was asked to count from one to five, two by two every second. In reaction time to tone, a speaker generated a sound every 4–6 s, and the examinee was asked to respond as soon as possible by pressing on a foot switch. In auditory digit span, the examinee was shown a digit span at a speed of one number per second and then asked to recall it immediately. The tracking task and the length of the digit span had been adjusted to have the same level of difficulty by individually conducting each test in all three groups. The AD group showed markedly lower performance when the dual task was composed of the tracking task and digit span or reaction time to tone. Baddeley regarded this result as indicating an impaired simultaneous execution ability in the central executive when divided attention is required. However, these results might also be interpreted simply as limitation of the general processing ability in a dual task situation which is obviously more difficult than a single task situation. To examine this point, Baddeley (1992) conducted a follow-up study on the same patients with AD 6 months later using a categorization task.4 In the categorization task, the examinee was given a category and the name of an object, and was asked to press a key when the object belonged to the category. The numbers of categories and objects differed, and task conditions of three levels of difficulty were used. No significant correlation was observed between poor performance in this task and progression of dementia symptoms. Based on these findings, Baddeley asserted that the lowered performance in dual task was caused by a qualitative difference between a single task and a dual task, and was not related to the level of difficulty of the task. He once again suggested the presence of disorder of the central executive in AD patients. The function of the phonological loop is to retain phonologically coded materials in the short-term memory. The phonological loop is said to be composed of two subordinate systems, the phonological short-term store (PSS) and the articulatory rehearsal mechanism (ARM) that rehearses the engram in PSS. Few studies on the phonological loop have been conducted on patients with AD. The studies of Morris (1984 and 1986) will be examined.2,5 Phonological short-term store function can be examined by the acoustic similarity effect. When PSS is impaired, the memory span is more reduced in tasks consisting of similar sounds than in tasks with dissimilar sounds. Morris conducted this test in patients with early AD and found that although the AD group showed a significantly lower memory span than normal subjects, their performance in acoustic similarity effect was not lowered, and suggested that the PSS function is normal in AD. Articulatory rehearsal mechanism function can be assessed by the word length effect. It is based on the phenomenon that while repeating a series of words (serial recall), the longer the word, the longer it takes for the ARM to rehearse, resulting in poorer recall score. Morris compared the scores of recalling single-syllable and five-syllable words in patients with early AD, and reported no decline in result due to the word length effect. ARM function can also be assessed by articulatory suppression. In this method, ARM function is suppressed by instructing the subject to say unrelated words, and the decrease in memory span is observed. The magnitude of this suppression is thought to be related to the memory span in the ARM. In Morris's studies using digit span and word span, the AD group and control group showed the same degree of suppression. The speed of verbal information rehearsal by ARM in AD patients has also been studied. An increase in the amount of information is expected to result in a lower speed and shorter memory span. Morris estimated the rehearsal speed by instructing the examinee to read a series of words as fast as possible. He reported no difference in rehearsal speed between AD patients and healthy subjects. Based on these results, Morris concluded that the phonological loop function is not impaired in AD patients, and the decline of memory span in these patients is caused by disorder of the central executive, consequently it becomes difficult to utilize the processing resource to maintain a certain span. The function of the visuospatial sketch pad is to retain temporarily a definite volume of visuospatial information. Until now, several studies using Corsi cube test or delayed reference task have reported low scores in visuospatial short-term memory in AD patients. However, only a few studies have examined this low score as a disorder of the visuospatial sketch pad.6,7 Carlesimo et al. used digit span and Corsi cube test to examine phonological or visual memory span in dementia.6 The Corsi cube test uses nine fixed cubes. After an examiner taps a certain number of cubes in a given order, the examinee has to repeat the tapping in the same order. Although AD patients perform normally in forward digit span, low scores were observed in the reverse digit span and both forward and reverse spatial span (Corsi cube test). According to Carlesimo et al. because forward digit span is processed in the phonological loop, it is not surprising that AD patients, who are not impaired in phonological loop function, score normally in this form of digit span. On the other hand, in reverse digit span that requires repetition of numerals in a backward order, and in Corsi cube test that requires repetition of a spatial and chronological nature, the processing is beyond the function of the visuospatial sketch pad that deals only with temporary retention of spatial relationship, and the involvement of central executive is indispensable. Therefore, AD patients who have a reduced processing resource in the central executive perform poorly in these tasks. Grossi et al. used the Corsi cube test and visuospatial pattern test (a test using nine squares of which 2–6 are black, and the examinee is shown a visual stimulation pattern and then asked to recall the pattern) to examine the visuospatial sketch pad function of AD patients.7 Unlike the Corsi cube test, visuospatial pattern test does not require retention of chronological order, and it reflects relatively selectively the visuospatial sketch pad function. However, the scores for the Corci cube and visuospatial pattern tests in AD patients were both low. Grossi et al. interpreted this result as either a disorder of the visuospatial sketch pad or disorder in the central executive. The later interpretation would suggest that a visuospatial task in itself might require processing resource from the central executive. If this is true, then it is doubtful whether the visuospatial sketch pad task for healthy subjects should be applied as it is to AD patients who might have disorder of the central executive. The above studies on working memory in AD patients indicate that although working memory is an attractive concept, many questions still remain (Kashima et al.8). Further studies are necessary and care has to be exercised in applying this concept to humans." @default.
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• W2041196042 title "Disorder of working memory in Alzheimer's-type dementia" @default.
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