Brain, Vol. 123, No. 5, 954-967,
May 2000
© 2000 Oxford University Press
Participation of the left posterior inferior temporal cortex in writing and mental recall of kanji orthography
A functional MRI study
1 Departments of Brain Pathophysiology and 2 Nuclear Medicine, Kyoto University Graduate School of Medicine and 3 Laboratory of Cerebral Integration, National Institute for Physiological Science, Kyoto, Japan
Correspondence to:
Hiroshi Shibasaki, MD, Department of Brain Pathophysiology, Kyoto University Graduate School of Medicine, 54 Shogoin, Sakyo, Kyoto 606-8507 Japan E-mail: shib{at}kuhp.kyoto-u.ac.jp
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Abstract |
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To examine the neuropsychological mechanisms involved in writing kanji (morphograms), we used functional MRI (fMRI) in 10 normal volunteers, all right-handed, native Japanese speakers. The experimental paradigms consisted of kana-to-kanji transcription, mental recall of kanji orthography and oral reading and semantic judgement of kana words. The first two tasks require manual and mental transcription of visually presented kana words into kanji, respectively, whereas the last two tasks involve language processing of the same set of stimulus words without recall of kanji. The transcription and mental recall tasks yielded lateralized activation of the left posterior inferior temporal cortex (PITC). By contrast, neither oral reading nor semantic judgement produced similar activation of the area. These results, in good accordance with lesion data, provide converging evidence that the left PITC plays an important role in writing kanji through retrieval of their visual graphic images, and suggest language-specific cerebral organization of writing. The set of fMRI experiments also provides new neuroimaging data on the cortical localization of basic language functions in people using a non-alphabetical language.
kanji; posterior inferior temporal cortex; writing; mental recall; functional MRI
ANOVA = analysis of variance; fMRI = functional magnetic resonance imaging; L–R = left–right; PITC = posterior inferior temporal cortex; SPM = statistical parametric mapping
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Introduction |
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Numerous observations have been reported on brain-damaged Japanese patients presenting with dissociation of the skills required to read or write two different orthographic systems: kana (syllabograms) and kanji (morphograms). These observations have led to the idea that the processing of kanji and kana may involve different inter- and intrahemispheric mechanisms (Iwata, 1984
; Benson, 1985; Coltheart, 1987; Friedman, 1993). For example, Iwata proposed that the left occipitotemporal areas are especially important for reading and writing kanji, whereas the more dorsal or occipitoparietal neural connection plays a role in reading kana (Iwata, 1984). However, a more comprehensive study by Sugishita and colleagues demonstrated that patterns of kanji–kana dissociation in oral reading vary among patients so much that the neuroanatomical relationship between the phenomenology and the lesion sites may not be straightforward (Sugishita et al., 1992).
By contrast, focal damage to the left posterior inferior temporal cortex (PITC) affects the writing of kanji rather consistently (Iwata, 1984; Kawamura et al., 1987; Kawahata et al., 1988; Mochizuki and Ohtomo, 1988; Soma et al., 1989; Yokota et al., 1990; Sakurai et al., 1994; Hamasaki et al., 1995). The left PITC lesions in most of the cases were caused by cerebrovascular accidents, and were accompanied in the early phase by other signs such as alexia of kanji and anomia. More recent work, however, suggests that only kanji agraphia persists in the chronic stage (Kawahata et al., 1988; Mochizuki and Ohtomo, 1988; Soma et al., 1989; Yokota et al., 1990). Soma and colleagues proposed that this is a single, core linguistic deficit due to left PITC damage in Japanese patients, and stated that the critical lesion for producing the symptom is located in Brodmann area 37 on the inferolateral surface of the left temporal lobe (Soma et al., 1989). In contrast, no single case with left PITC damage has been reported to present the opposite pattern of agraphia, i.e. agraphia preferentially affecting kana. Descriptions of kana agraphia have been sparse, and the clinical features and lesion sites seem rather heterogeneous (Tanaka et al., 1987). Kanji agraphia may also appear after damage to other brain sites, including the left angular gyrus and the left frontal cortex, but damage to the former site usually affects the writing of both kanji and kana (Iwata, 1984), whereas damage to the latter site which causes frontal kanji agraphia is rare (Sakurai et al., 1997). These lesion data suggest that only the left PITC is consistently correlated with the processing of a particular orthography, i.e. the writing of kanji.
The writing impairment is thought to arise essentially from an inability to recall visual graphic forms of kanji, as suggested by analysis of writing errors and by anecdotal reports that the patients complained of `forgetting' letters (Kawamura et al., 1987; Kawahata et al., 1988; Mochizuki et al., 1988; Soma et al., 1989). This unique feature suggests that the left PITC is involved critically in the retrieval of visual graphic memory. In general, the left occipitotemporal cortices are thought to play a role in the generation of visual mental images, as indicated by studies with brain-damaged patients (Farah, 1989) and PET studies in normal people (Roland and Gulyás, 1994). The left PITC in particular is consistently activated while subjects hear or read words to generate visual images of their referents (Démonet et al., 1992; Vandenberghe et al., 1996; Warburton et al., 1996; Mummery et al., 1998), which suggests that this particular area is involved in the retrieval of visual representations from the long-term memory store in association with verbal symbols. It seems likely that the same brain area also subserves the recall of visual images of kanji stored in long-term memory. Kanji agraphia as a result of left PITC damage may be based on dysfunction of a similar neuropsychological process, i.e. retrieval of the mental representation of visual objects, since the normal writing of kanji is, in principle, realized by visualizing the graphic forms to be written (Kaiho and Nomura, 1983; Iwata, 1984).
In the present study, we tested the following hypotheses concerning the neuropsychological mechanisms for writing kanji by the use of functional MRI (fMRI) in normal humans. First, if the left PITC is critically involved in writing kanji, which obviously requires complex sequential motor control, the act of writing kanji will increase neural activity not only in the sensorimotor and classical perisylvian language cortices but also in the left PITC. Secondly, if access to the visual graphic memory store is a mandatory process subserved by the left PITC for the normal writing of kanji, mere mental imagery of visual configurations of kanji will also activate the identical brain area even in the absence of overt motor execution. Thirdly, if the observed activation of the left PITC actually represents the mental retrieval of kanji orthography and is not a simple by-product of other prelinguistic or linguistic processing of stimulus materials, it will be expected that different verbal tasks in the same stimulus condition will not yield a similar level of neural activity in the left PITC if they do not involve the recall of kanji. The last point should also be examined especially with respect to the use of visual stimuli, because language materials potentially trigger the automatic activation of a widespread neural network irrespective of the behavioural tasks engaged (Price et al., 1996), and also because the ventral visual systems including the PITC may respond to both verbal and non-verbal visual materials (Puce et al., 1996).
A few functional imaging studies in the literature have examined the neural correlates of writing letters. Using PET, Petrides and colleagues found bilateral activation of the posterior temporal areas as well as uni- or bilateral activations of the sensorimotor and parietal cortices in a writing-to-dictation task (Petrides et al., 1995). In an fMRI study, Sugishita and colleagues described extensive activation of the left intraparietal region during the mental writing of kana characters, but they did not examine the neural activity of the temporal areas because of technical limitations (Sugishita et al., 1996). Seitz and colleagues employed two kinds of handwriting tasks for their PET study, but they reported activation of the left PITC in neither of them (Seitz et al., 1997). Therefore, the possible role of the left PITC in normal writing does not yet seem to be well elucidated. The present study examines this issue further through the set of experiments outlined above, thereby also providing new neuroimaging data on several aspects of verbal processing in Japanese, disorders of which have attracted universal interest because of their language-specific neuropsychological features.
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Methods |
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Subjects
Ten healthy volunteers (six males and four females, age range 20–25 years) were recruited among students at Kyoto University. None had a history of neurological or psychiatric disease. All were native Japanese speakers and strongly right-handed, as confirmed by the Edinburgh inventory (Oldfield, 1971
). Informed consent was obtained from each subject prior to the experiment. The protocol of this study was in accordance with the guidelines determined by the Committee of Medical Ethics, Graduate School of Medicine, Kyoto University.
Word stimuli
A set of 60 words in kana script was presented visually during all four of the activation paradigms described below (Fig. 1). The kana word stimuli were prepared by transcribing 60 two-character compound kanji words selected from the fundamental vocabulary for Japanese language teaching (National Language Research Institute, 1984). None of them represented homophone words. Half of the words represented concrete nouns and the other half abstract nouns. Concreteness and imagery levels of 48 of the 60 words were controlled using the attributes described by Ogawa and Inamura (Ogawa and Inamura, 1974), although no normative psycholinguistic data covering all of the 60 words were available at the time of the present study. The lexical frequency of the words in most cases exceeded 500 per 1 000 000 (National Language Research Institute, 1970). All the words used in the study are conventionally written in kanji, and kana is used for them only rarely. The 60 kana words prepared as described above each consisted of two to six kana characters.
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Behavioural tasks
Four activation tasks, comprising kana-to-kanji transcription, mental recall of kanji orthography and the semantic judgement and oral reading of kana words, were used in the present study (Fig. 1
). Visual identification of kana characters (for details, see below) was used as a common baseline for the four tasks. In all the tasks, including the baseline task, the same event sequence was employed for each trial: after a period of 100 ms during which the subject fixated a cross (visual angle 1°), either a kana word (activation tasks, 3 ± 1°) or a single kana character (baseline, 1°) appeared for 400 ms, and was followed by a response period for 2500 ms. The kana words or single kana characters were presented randomly during each task epoch (see below). The order of the four behavioural tasks was counterbalanced across the subjects.
Task 1: kana-to kanji transcription
Upon presentation of each kana word stimulus, the subjects were asked to write roughly the first character of the corresponding compound kanji word on a plastic board with the right index finger (Fig. 1). The interstimulus interval of 3 s was chosen because normal adults require ~1.8 s to write out a single kanji word to dictation (Kaiho and Nomura, 1983). The subjects could not see the movements of their fingers. The responses of each subject were monitored with a video camera and scored off-line.
Task 2: mental recall of kanji orthography
Cognitive psychological studies on the structural description of kanji characters have extracted several features to define their visual configurations, among which the left–right (L–R) combination of letter-constituents (radicals) is a basic rule which applies to many kanji characters (Kaiho and Nomura, 1983; Saito, 1997). Some characters are composed of two radicals that are juxtaposed horizontally, whereas others have visual configurations whose constituents are not arranged according to this rule (Fig. 2A). This basic feature was exploited in the mental recall task in the present experiment. In each trial, the subjects were instructed to mentally transcribe each kana word stimulus to kanji script, i.e. to imagine the visual configuration of a two-character kanji word corresponding to the stimulus word (Fig. 2B). The subjects responded by raising the right index finger when the first character of each kanji compound word was of the L–R type and otherwise withheld the response. Half of the 60 trials included the L–R type of character and the other half did not.
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Task 3: oral reading of kana words
Subjects were asked to read kana words aloud without moving the jaw. In addition to a tight constraint on jaw movement to minimize head motion (see below), they were especially asked not to open the mouth wide; this allowed the minimally audible vocalization necessary for the execution of the task.
Task 4: semantic judgement of kana words
The subjects were asked to decide whether the stimulus nouns in kana denoted concrete objects or abstract concepts, and to signal by raising the right index finger only in response to abstract nouns (Fig. 1). The mean probability of appearance of abstract nouns was 50%.
Common baseline
In each trial, a single kana character was randomly presented for 400 ms, followed by a response period of 2500 ms. Single characters were used for stimuli so that the subjects could readily recognize the on and off times of task epochs during scanning sessions (see below). The subject signalled only when the kana character 
(/ma/) appeared. The probability of appearance of the target character was 50%.
fMRI procedure
After a 15 min training session, the subject lay supine in the MRI scanner. Head motion was minimized by the use of foam padding. The activation tasks were generated using SuperLab (Cedrus, Phoenix, Ariz., USA) on a Macintosh computer. The stimuli were back-projected onto a screen using a video projection system via a mirror placed in the head coil. Scanning was conducted with a 1.5 T whole-body MRI system (Horizon; GE Medical, Milwaukee, Wis., USA) using a standard head coil optimized for whole-brain echo-planar imaging. For functional imaging, we used a gradient-echo echo-planar imaging sequence with the following parameters: TR (repetition time) 6 s, TE (echo time) 43 ms, flip angle 90°, field of view 22 x 22 cm, and pixel matrix dimensions 64 x 64. A long TR, as used in recent fMRI studies, enables coverage of the whole brain (Cornette et al., 1998; Lobel et al., 1998). Thirty-two contiguous 3.5 mm thick slices without gaps were obtained in the axial plane for each subject. For each task, there were two scanning sessions, each lasting 222 s and yielding 37 functional images, for each subject. In each session, starting with the baseline task, seven task epochs, i.e. four for the baseline and three for the active tasks, alternated every 30 s (except that the fourth epoch of the baseline lasted 42 s). Therefore, each session consisted of the alternating epochs of a single activation task and the baseline task, and no two activation tasks were performed within the same session.
Data analysis
After image reconstruction, off-line processing of the functional images was performed on an ULTRA-2 workstation (Sun Microsystems, Mountain View, Calif., USA) using SPM96 software (Wellcome Department of Cognitive Neurology, London, UK). Two initial images were discarded from the analysis to eliminate non-equilibrium effects of magnetization. Images were corrected for head motion, resampled every 2 mm using bilinear interpolation, normalized to the standard brain space defined by the Montreal Neurological Institute (Friston et al., 1995), and spatially smoothed with an isotropic Gaussian filter (7 mm full width at half maximum).
Statistical analysis of the fMRI data was performed at both the individual and group levels. This was intended especially to demonstrate the consistency of the results by applying different statistical approaches at the same time. For the individual-based analysis, the fMRI time series of each subject were correlated with the boxcar reference function, to which a high-pass filter (0.5 cycles/min) and temporal smoothing were applied to remove low-frequency noise and to improve the signal-to-noise ratio, respectively. The resulting correlations were transformed into a Z-score map (SPM{Z}) (Friston et al., 1994). The significant Z value was thresholded at Z > 3.09 (corresponding to P < 0.001 at each voxel level, uncorrected for multiple comparison). Activated brain structures were identified using the standard brain atlas of Talairach and Tournoux (Talairach and Tournoux, 1988). A region of interest was also set on the left PITC to further evaluate changes in signal in this area across the four tasks. The 4.6 ml volume region of interest, covering most of Brodmann area 37 in the left posteroinferior temporal cortex, was defined in Talairach coordinates as x = –62 to –44 mm, y = –70 to –52 mm, z = –16 to 0 mm. For each subject, the ratio of averaged signal intensities in the task epochs to those in the baseline was calculated by sampling a voxel with maximum Z score within the region of interest for each task. (First scans within each epoch were excluded to discount the lag in a haemodynamic response.) The resulting percentage changes in signal were subjected to one-way analysis of variance (ANOVA) to examine the main effect of task. One-way ANOVA was used to examine the main effect of task on the percentage changes in signal. They were compared among the four tasks using post hoc ANOVA (Fisher's PLSD). For the multisubject statistical analysis, the random effects kit for SPM96 (http://www.fil.ion.ucl.ac.uk) was used. By fitting the haemodynamic response function, images of the individual-level activation parameter were computed as an adjusted mean image per condition per session for each subject. The two adjusted mean images derived from two scanning sessions were collapsed into a straight mean image per condition per subject. For the intersubject analysis, a paired t-test was applied to the condition-specific mean images by the use of the PET routine of SPM96. The PET statistics thus comprised eight experimental conditions, i.e. four for the active tasks and four for their respective baselines. A threshold of Z > 3.09 was used to determine the presence of significant activation foci. The extent of clusters was corrected at P < 0.05 for multiple comparison. To demonstrate common activation foci across the task conditions, conjunction analysis of the transcription and mental recall tasks to their respective baselines was performed in addition (Price and Friston, 1997). A conjoint activation map that essentially reflects the sum of all the activations was constructed from the two independent SPMs (statistical parametric maps), eliminating voxels where differences between the two contrasts were significant. This approach can be applied to combinations of SPMs obtained by subtractions and enables statistical inference irrespective of interactions among cognitive components. For comparisons among the four tasks, uncorrected Z values are also reported in order to describe the trend of activation. Additionally, the region of interest defined above was applied to report the Z scores in this area in the oral reading and semantic judgement tasks.
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Behavioural data
Response accuracy (mean ± standard deviation) in the transcription, mental recall, semantic judgement and baseline tasks during scanning was 93.8 ± 4.3, 91.7 ± 3.2, 94.3 ± 3.9 and 98.8 ± 1.4%, respectively. Accuracy of all the subjects exceeded 85% in each of the tasks. The subjects also reported that they read aloud all the kana word stimuli successfully in the oral reading task.
fMRI results
Individual analysis
Table 1 summarizes the number of activated voxels and the peak Z scores in the left PITC for each task (uncorrected for multiple comparison). Significant activation of the left PITC was observed in the transcription and mental recall tasks in nine subjects, whereas the oral reading and semantic judgement tasks activated the same area only in three subjects. In the transcription task there were also activations in the left sensorimotor areas and inferior frontal gyrus in all 10 subjects. The mental recall task activated the left middle and inferior frontal gyri in all subjects. In the oral reading task, the bilateral inferior frontal gyri were activated in all subjects and the left superior temporal gyri in six subjects. The semantic judgement task activated the left ventrolateral frontal cortex, including the middle and inferior frontal gyri, in nine subjects, and the left inferior parietal lobule in eight subjects. The left superior temporal gyrus was also activated in six subjects.
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For each task, SPM{Z} maps (Z > 3.09, uncorrected) for the 10 subjects are superimposed on axial planes of the standard brain in Fig. 3
. The left PITC activation across the individual results converged at a single anatomical locus (x = –50, y = –66, z = –12), where activated clusters overlapped across eight subjects in transcription and six subjects in mental recall. In the oral reading task the activated clusters in the left PITC seen in three subjects did not overlap, whereas in the semantic judgement task overlapping voxels across two subjects were located more rostrally on the anterior bank of the left inferior temporal sulcus (x = –60, y = –48, z = –10). Figure 4 compares the mean % signal change at peak voxels within the region of interest among the four tasks. The signal increase in transcription was salient and equalled that in mental recall, but only weak responses were observed in oral reading and semantic judgement. One-way ANOVA revealed a main effect of task [F(3,36) = 4.22, P = 0.01]. A post hoc test disclosed a significant difference in signal increase for the following comparisons: transcription versus oral reading (P < 0.01); transcription versus semantic judgement (P = 0.01); mental recall versus oral reading (P = 0.03); and mental recall versus semantic judgement (P= 0.04). By contrast, the difference did not reach statistical significance in transcription versus mental recall (P = 0.56) or oral reading versus semantic judgement (P = 0.89).
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Group analysis
Brain areas activated in the multisubject analysis are listed in Tables 2, 3, 4 and 5
for transcription, mental recall, oral reading and semantic judgement tasks, respectively (corrected at P < 0.05). Figure 5 illustrates the activation sites projected onto the standard brain space for each comparison. The transcription task yielded extensive activations in the left frontoparietal lobes, including the inferior and middle frontal gyri and the precentral and postcentral gyri. Bilateral activations were found in the supplementary motor areas, cingulate and lingual gyri, basal nuclei, thalami and cerebellar hemispheres. In the temporal lobe there was a significant activation focus in the left inferior temporal and fusiform gyri. In the mental recall task, there were activations in the middle and inferior frontal gyri and the cingulate gyrus in the left frontal lobe. Significant activations were also observed in the inferior temporal and fusiform gyri and the left supramarginal gyrus and inferior parietal lobule. Other activated areas included the bilateral cunei, lingual gyri and cerebellar hemispheres. Conjunction analysis of the two tasks to their respective baselines further revealed activations in the left inferior temporal gyrus and bilateral medial frontal cortices, inferior parietal areas, medial occipital areas, thalami and cerebellar hemispheres (corrected at P < 0.05). The oral reading task yielded significant bilateral activations in the inferior frontal gyrus and superior and middle temporal gyri. There were also bilateral activations in the precentral, postcentral and lingual gyri and cerebellum. In addition, a weaker tendency of activation was detected in the left PITC (Z = 2.86, P = 0.002). In the semantic judgement task, significant activations were observed in the left perisylvian areas, including the middle and inferior frontal gyri, insular cortex and superior temporal gyrus. Activated clusters were also found in the bilateral lingual gyri and cerebellar hemispheres. There was also a weak trend of activation in the left PITC (Z = 2.76, P = 0.003).
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Five additional contrasts were examined to compare the responses of the whole brain to the four tasks. This was done by comparing contrasted tasks with each other. For example, transcription versus its baseline was contrasted with oral reading versus its baseline. For simplicity, this comparison is referred to as `transcription – oral reading'. The following comparisons were included: transcription – oral reading; transcription – semantic judgement; mental recall – oral reading; mental recall – semantic judgement; and (transcription + mental recall) – (oral reading + semantic judgement). Results of the first four contrasts are reported in Table 6
(each thresholded at Z > 3.09, uncorrected). A trend of activation was found consistently in the left PITC through the four comparisons, but there was no other cortical area that acted similarly. Figure 6 illustrates SPM{Z} for the fifth contrast, which also revealed activation of the left PITC relative to the last two tasks, for which retrieval of graphic images is not a prerequisite for subsequent cognitive processing.
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Discussion |
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We carried out a set of experiments to elucidate the neuropsychological mechanisms for the writing and mental recall of kanji, focusing on the role of the left PITC predicted from lesion data. Despite the technical advances of the last decade, only a few investigators have used functional neuroimaging to examine the neural correlates of reading and writing Japanese (Sakurai et al., 1992
, 1993; Sugishita et al., 1996), including some neurolinguistic investigations on brain-damaged patients (Morton and Sasanuma, 1984; Paradis et al., 1985). The activation paradigms in the present study included kana-to-kanji transcription, mental recall of kanji orthography, oral reading and semantic judgement, which involved different kinds of verbal processing of the same kana word stimuli. Because we designed the experiments to test particular hypotheses, the motor response differed inherently among the task conditions. To compensate for this, we adopted the cognitive conjunction approach for group analysis of the fMRI data.
Figure 7 illustrates the verbal operations involved in each task, based on a psycholinguistic model of written word processing in Japanese (Sasanuma, 1987b). Because kana script is rarely used to represent the 60 words selected for the present study, the lexical or whole-word reading strategy (Marshall and Newcombe, 1973) is unavailable for the decoding of the unfamiliar kana words (this exceptional condition is roughly comparable to that in which an English speaker reads words spelled with phonetic symbols). Thus the lexical property of the stimuli should be identified through serial or letter-by-letter phonological conversion of each kana character, which activates phonological lexicons. Transcription and mental recall tasks require the digital and mental retrieval of kanji orthography from the long-term memory store, respectively, whereas oral reading and semantic judgement tasks do not involve the visualization of kanji orthography as an obligatory component. We tested the last two tasks, which reflect the phonological and semantic processing of stimulus words, respectively, in order to compare the activation patterns with those occurring during the first two tasks. The common baseline task was aimed at differentiating the activation patterns among the four active conditions, because all of them share the visual identification of kana characters as an initial prelexical process for subsequent higher-order language processing (Fig. 7). It was used also in an attempt to cancel the influence of early visuo-verbal processing of the stimulus words, as several lines of evidence have indicated that neurons in the ventral visual stream are involved in the processing of various verbal and non-verbal visual stimuli (Luders et al., 1991; Puce et al., 1996; Büchel et al., 1998).
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The role of the left PITC in kanji retrieval
The most important finding from the series of analyses in the present study is that the left PITC is active not only in actual writing but also in the mental recall of kanji. By contrast, neural activity in this area did not significantly change from the baseline during oral reading or semantic judgement of the same word stimuli. Thus, the observed activation of the left PITC should be attributed neither to the motor execution of writing per se nor to non-specific neural response to the visual stimuli. Rather, as a conjoint activation focus of the first two tasks, it was specifically correlated with the retrieval of kanji graphic images, which is commonly implicated in these tasks, whereas no comparable signal increase was found in the same area in the last two tasks, which do not require such cognitive operations. The activation site overlaps precisely the lateral surface of the left posterior temporal lobe, corresponding mostly to Brodmann area 37, to which Soma and colleagues pointed as the lesion site critical for producing the pure agraphia of kanji (Soma et al., 1989
). The activation of the left PITC during the mental recall of kanji is in good accordance with the classical interpretation of the syndrome—that it might arise from impaired recall of the visual graphic memory (Kawahata et al., 1988; Mochizuki and Ohtomo, 1988; Soma et al., 1989; Hamasaki et al., 1995).
The writing disorder uniquely found among Japanese patients with a left PITC lesion may differ from the symptomatology found in subjects using Western languages, in which damage to the same brain site produces pure alexia, anomia (Greenblatt, 1976; Henderson et al., 1985) and rarely lexical agraphia (Croisile et al., 1989). Traditionally, the kanji agraphia after left PITC lesion has been thought to represent a Japanese equivalent of lexical agraphia because of their similar neurolinguistic features and the proximity of the lesion sites (Soma et al., 1989; Yokota et al., 1990; Sakurai et al., 1997). In most reported cases of lexical agraphia, however, the lesions responsible were located more dorsally, at the left occipitoparietal junction (Roeltgen, 1993). Interestingly, the left posterior temporal activation in English-speaking people that Petrides and colleagues found in the `writing words to dictation' task (Petrides et al., 1995) is also localized more dorsally (x = –50 mm, y = –66 mm, z = 5 mm in Talairach coordinates) than the PITC activation found in the present study. Taking these results together, it is possible, despite the apparent resemblance in symptomatology, that writing alphabetical letters and kanji are controlled by different subregions within the left temporoparietal cortex.
The finding that the left PITC is indeed activated by the retrieval of kanji, however, does not preclude the possibility that the area could also be active when subjects form visual images of kana characters. Lesion studies and functional imaging data in normal humans indicate that the left occipitotemporal regions serve for the generation of various kinds of mental images, including non-verbal motor imagery (Farah, 1989; Roland and Gulyás, 1994). In particular, Goldenberg and colleagues reported activation of the left PITC in an alphabet-scrutinizing task in which subjects were engaged in visual imagery of alphabetical letters (Goldenberg et al., 1989). Thus, the left PITC is likely to be involved in the mental recall of other script systems as a key cortical area linking encoded verbal input to visual graphemic images. In the act of writing, however, the area plays an especially important role for kanji, which should be more dependent than phonographic scripts on the effective visualization of graphic forms.
Furthermore, the present results suggest that retrieval of the kanji graphic images occurs mainly in the left hemisphere. Kawamura and colleagues reported that the ability to write kanji was lateralized to the left hemisphere in a patient with left unilateral kanji agraphia due to destruction of the posterior corpus callosum (Kawamura et al., 1989), while another study on callosal disconnection suggested that this ability could be shared by the right hemisphere (Yamadori et al., 1983). In the present study, none of the subjects showed unilateral right-sided activation of the PITC. Although a possible advantage of the right hemisphere has long been speculated for the visual processing of kanji (Benson, 1985; Coltheart, 1987), the overall tendency, as seen from our individual fMRI data, is that the left hemisphere functions predominantly in the active retrieval of graphic images of kanji in right-handed people.
Frontal activations in decoding kana script
The left ventrolateral frontal cortex or Broca's area was commonly activated in all the four contrasts. It probably reflects phonological decoding of the kana character strings that occurs prelexically, as all the tasks involve this component at the initial stage of word processing (Fig. 7). The importance of the area in reading kana has been suggested by lesion studies showing that performance in the reading of kana tends to be more severely affected in patients with Broca's aphasia (Paradis et al., 1985), which may be interpreted as the breakdown of a `phonological processor' essential for translating kana characters into phonology (Sasanuma and Fujimura, 1971). For the transcription and oral reading tasks, however, the activation observed could also be associated with the motor execution of reading and writing, respectively. Mental recall and semantic judgement strongly activated the left dorsolateral prefrontal cortex, and inferior frontal activation in transcription also extended to this area. Previous functional imaging data have consistently reported activation of this area during tasks which involve the semantic processing of words through access to visual or verbal long-term memory (Démonet et al., 1992; Vandenberghe et al., 1996; Warburton et al., 1996; Binder et al., 1997). That only oral reading yielded no similar activation of the area may be accounted for by the assumption that this task involves principally simple letter-to-sound conversion rather than activation of further lexical–semantic processing.
Oral reading and semantic processing of kana words
Oral reading of kana words activated the bilateral perisylvian frontotemporal cortices. Sakurai and colleagues used similar activation paradigms in their PET study (Sakurai et al., 1993). Although the distribution of the activated areas that they reported is mostly consistent with ours, they also described activation of the bilateral (left-side predominant) activation of the PITC, which was not observed in the present study but could be attributed to a difference in the baseline condition. Because a resting state with visual fixation was used in their study, the PITC activation should be interpreted as reflecting the visual recognition of stimulus letters; this was largely cancelled out in the present study. In another PET study using the same baseline, they also reported activation of the PITC during oral reading of kanji words (Sakurai et al., 1992). As the ventral visual areas are specialized for the processing of visual objects in humans (Ungerleider and Haxby, 1994) and are sensitive to various visual stimuli, as shown by past neuroimaging studies (Price et al., 1996; Puce et al., 1996), it should be emphasized that these observations by Sakurai and colleagues (Sakurai et al., 1992, 1993) are reasonable per se, but are different in nature from the left PITC activation seen in the present study.
In the semantic judgement task, activations were observed in the left dorsolateral prefrontal cortex and the left inferior frontal and anterior superior temporal gyri. While similar tasks that require semantic-level processing of words have never been applied to Japanese subjects in functional imaging studies, our finding largely replicates the results of past studies with English-speaking people which described activation of the left lateral frontal and superior temporal cortices (Wise et al., 1991; Démonet et al., 1992; Howard et al., 1992; Pugh et al., 1996; Binder et al., 1997). The left inferior prefrontal or Broca's area is reported to play an important role in the reading comprehension of kana words, because kana script depends more on the phonological decoding process, whereas the reading comprehension of kanji may involve direct access to the semantic system (Sasanuma and Fujimura, 1971; Sasanuma, 1987a; Morton and Sasanuma, 1984).
Some of the present subjects reported that they occasionally imagined kanji characters corresponding to a stimulus kana word to determine the meaning of the word in the semantic judgement task. However, this did not yield a measurable signal increase in the left PITC, probably because the mental visualization of kanji is a possible, but not obligatory, process constantly used, although it might occur in some cases. It is also possible that subjects visualize referents of the stimulus words to determine their semantic property, but neither did this yield significant group-level activation in the temporo-parieto-occipital networks involved in visual imagery (Roland and Gulyás, 1994). This may partly be because half of the stimulus words were abstract nouns with a very low imagery level which could evoke no distinct visual representations. It is more likely, however, that the execution of the task, i.e. selecting abstract nouns from the list of words, was processed by higher-order verbal semantic knowledge and did not necessarily require such visualization of the word referents.
Other activations
The transcription task activated broad areas of the left frontoparietal cortices. Bilateral activation of the supplementary motor area was also observed. These areas are commonly activated by tasks which involve complex serial movement of the fingers (Roland et al., 1980; Shibasaki et al., 1993). Activation of the medial occipital areas was also observed in all the comparisons. Such activation is primarily attributed to the difference in visual stimulus properties between the active tasks and baseline. This may imply that the latter might well not control for the very early stage of visual processing. However, we believe that this is not a serious drawback of the experimental design as the response of the early visual area is outside the interest of the current experiments.
Conclusion
The present study demonstrated that the left PITC was activated in normal people equally for the writing and for the mental recall of kanji orthography. By contrast, phonological and semantic processing of the same words activated the left perisylvian frontotemporal areas, but did not yield similar activation in the PITC. Overall, the results support the hypothesized neural pathway for writing kanji (Iwata, 1984), whereby the left PITC plays a central role in the recall of the `visual engrams of letters'. In the act of writing, even in writing kanji, one may not make a conscious effort to recall the visual images of letters or words to be written, because the sequential visuomotor skill appears to proceed rather automatically in normally educated adults. In this series of experiments we found that the skill is supported by the neural subsystem that is specialized for the retrieval of visual graphic forms. Coupled with lesion data, our results provide converging evidence that the left PITC essentially subserves this retrieval process and allow further insights into the neuropsychological mechanisms for writing.
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This research was supported by Grants-in-Aid for Scientific Research (A) 09308031, for Scientific Research on Priority Areas 08279106 and for International Scientific Research 10044269 from the Japan Ministry of Education, Science, Sports, and Culture, Research for the Future Program JSPS-RFTF97L00201 from the Japan Society for the Promotion of Science, and General Research Grants for Aging and Health `Physiological parameters for evaluation of aging of brain' and `Analysis of aged brain function with neuroimaging techniques' from the Japan Ministry of Health of Health and Welfare.
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Received May 25, 1999. Revised October 25, 1999. Accepted November 16, 1999.
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