Working memory has been commonly defined as temporary active storage mechanism of information. Although its definition differs from scientist to scientist, working memory can be considered as a dynamic system for retaining and processing information, which consists of 4 elementary processes ; an input/selection process, a temporary storage process, an output process, and a processing process of information. We have analyzed single-neuron activity in the prefrontal cortex while monkeys performed working memory tasks, and found that many neurons exhibited delay-period activity which lasted tonically during the delay period and showed preferences depending upon stored information. Therefore, delay-period activity is considered to reflect a temporary active storage process of information. In addition, we found cue-period activity and pre-movement activity, each of which may reflect an input/selection process and an output process, respectively. We also observed delay-period activity related to integrating multiple information and a feedback signal from a motor center (post-movement activity) to manipulate stored information. These latter results suggest that an interaction among prefrontal neurons is important for a processing process to integrate or manipulate information. Since working memory is a dynamic system for processing information, we need to further examine neuronal mechanisms of each elementary process as well as dynamic interactions among these processes to understand neuronal mechanisms of working memory.

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    Working memory represents the immediate memory processes involved in the simultaneous storage and processing of information. In the language processing, reading or listening the sentences, the emphasis is on the parallel systems of storing the partial product of comprehension while processing the incoming information.
    In this study, the listening span test (LST) , which can evaluate the individual differences of working memory capacity, was performed to investigate the brain mechanisms of the verbal working memory.
    Magnetoencephalography (MEG) was mesured while the subjects performed the LST. The peak alpha frequency of MEG power shifted when the working memory demands increased. Individual differences were found in the span, in which the shift became to be largest. Moreover, the shift was dominant mainly in the frontal and temporal regions in the left hemisphere, which indicate the locus of verbal working memory.

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    This article reviews the relationship between clinical neuropsychology and the working memory model (Baddeley & Hitch, 1974) , that was developed to account for a wider range of data concerning short-term memory and attention. They subdivided working memory into three component : an attentional controller, the central executive, and two slave systems, the phonological loop which maintains speech-based information, and the visuospatial sketchpad, which stores visual and spatial information.
    The fractionation of working memory was initially motivated by attempt to account for the neuropsychological finding of a double dissociation between long-term and short-term memory deficit (amnesic syndrome vs. short-term memory syndrome) in the late s. In the 20 years since its first publication, working memory model has been considerable inference on clinical and experimental studies in neuropsychology. Now, it has become a crucial interface between many research areas consist of cognitive neuroscience, and might be a clue in the analysis of cognitive deficits connected with mental illness, such as schizophrenia.

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    Working memory consists of central executive and two subsystems, namely phonological (articulatory) loop and visuospatial sketchpad. Phonological loop is a concept developed from the verbal short-term memory of modal model. Phonological loop is further subdivided into phonological short-term store, phonological output buffer, and loop itself connecting the two components. From the results of digit span in aphasic patients, the author assume the left superior temporal gyrus as a candidate for the anatomical substrate of phonological short-term store. The phonological loop is assumed to extend over the left inferior parietal lobule. Phonological output buffer may be located in the left primary motor cortex. Further evidences are needed in both normal subjects (PET activation study or functional MRI) and brain damaged patients, and these two lines of findings must be integrated.

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    In recent years, the concept of working memory has gained popularity in many fields of cognitive neuroscience. Working memory refers to a brain system that provides temporary storage and manipulation of information necessary for complex cognitive tasks. The system consists of two slave systems (the phonological loop and the visuospatial sketchpad) and the central executive. The central executive is a kind of a control center to select and/or integrate information provided by these two slave systems and is a vehicle of decision for appropriate motor behavior or judgment. Recently, I had an opportunity to see a patient with bilateral frontal lobe infarcts with a symptom that has not yet been documented in the literature. He complained a severe difficulty in making a phone call. On examination, he demonstrated extreme difficulty in pointing to a series of numbers, although he had normal vision, normal motor and sensory functions, fair auditory memory for numbers and memory for spatial span. Based on a number of tests, I concluded that the patient's principal disorder was difficulty in converting one mode of information into another mode of information. The present patient's symptom mentioned above can be best explained in terms of a working memory paradigm. Although individual modal systems, or slave systems, retained nearly normal function, simultaneous holding of heteromodal information and its modal transformation in the central executive system seemed to have been at fault. The function of central executive may be related to the prefrontal regions.

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    In order to study aphasics' ability in imaging, a sign drawing test based on Tsvetkova (1972) was carried out.
    The procedure for testing was as follows. Subjects (33 aphasics and 7 normal controls) were given a paper showing three identical and simple figures, for example three circles for task No.1, in which they were asked to draw a tangerine/apple/watermelon. At the same time they were visually and acoustically given the names of the three objects. Using the simple figures and the objects named, the subjects were asked to complete the drawings (here, tangerine/apple/watermelon). Task No.2 consisted of car/train/truck and No.3, bird/animal/insect. Task No.3 consisted of category names in contrast to No.1 and No.2 which involved concrete names. The numbers of “signs” in their completed drawings which correctly characterized the given objects were considered as the test data.
    (1) The number of correctly drawn signs increased from the severe group to the mild group according to communication level. A highly conscious selection ability was seen to be needed for the tasks because the subjects had to make three different objects using the same figures on one paper. We considered it possible that this mental manipulation is related to aphasics' language ability, for example word finding and so on, in daily communication.
    (2) As to the mild group of aphasics' communication level and that of the control group, no correlations in drawn sign numbers were found between tasks No.1, 2 and No.3. We believe this fact suggests that in these 2 groups there exists another imaging procedure which the moderate group does not have.

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    We present a case of crossed aphasia in a 75-year-old male who is Japanese-Korean bilingual and right-handed. Neurological examination found mild left hemiplegia and global aphasia. Cranial MRI revealed an extensive lesion of infarction in the region of the right middle cerebral artery. As his speech function is limited to one spontaneous word, “aigo : ” , the condition was diagnosed as global aphasia. The bilingual acquisition in the patient was in the coordinate pattern. The recovery of his speech symptoms was evaluated as the selective recovery type, as the auditory comprehension only of Japanese improved while that of Korean remained unchanged. Three possible factors were extrapolated for the improvement of speech symptoms in the patient : 1) natural recovery, 2) mastery level of Japanese was higher than that of Korean, and 3) the patient received speech training for Japanese only. Because the patient is a Japanese-Korean bilingual who developed right-handed crossed aphasia, this suggests his bilingual speech function was lateralized in the right hemisphere.

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    We reported two cases of transcortical sensory aphasia caused by a localized lesion in the deep white matter of the left frontal lobe. The characteristics of language disorders in the two cases were summarized as follows : 1) Spontaneous speech was fluent and paraphasia was observed. 2) Severely impaired auditory comprehension was observed, and echolalia was frequently noted. 3) Repetition was preserved. 4) Reading aloud was preserved but with impaired understanding, revealing a dissociation between reading aloud and comprehension. 5) Moderate to severe impairment of naming and writing letters was observed. From these characteristics, the aphasia type in our cases was considered to be transcortical sensory aphasia.
    The lesions of the two cases were found in the deep white matter of the middle frontal gyrus running antero-lateral to the anterior horn of the left lateral ventricle, suggesting the possibility that transcortical sensory aphasia is caused by this lesion. Among individual language disorders that comprise transcortical sensory aphasia, severe impairment of language comprehension poses the most problematic aspect attributable to frontal lobe lesions. SPECT in one case revealed reduced cerebral blood flow in a relatively wide region of the left frontal lobe. Recently it has been reported that impaired comprehension of language is associated with lesions of the middle or lower frontal gyrus. Therefore, it is speculated that impaired comprehension of language results from hypofunction of a relatively wide region of the middle and lower frontal gyri due to a deep white matter lesion of the middle frontal gyrus.

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