Magnetoencephalographic study on the cerebral neural activities related to the processing of visually presented characters

Language comprehension dependent on emotional context: a magnetoencephalography study

In communication, language can be interpreted differently depending upon the emotional context. To clarify the effect of emotional context on language processing, we performed experiments using a cross-modal priming paradigm with an auditorily presented prime and a visually presented target. The primes were the names of people that were spoken with a happy, sad, or neutral intonation; the targets were interrogative one-word sentences with emotionally neutral content. Using magnetoencephalography, we measured neural activities during silent reading of the targets presented in a happy, sad, or neutral context. We identified two conditional differences: the happy and sad conditions produced less activity than the neutral condition in the right posterior inferior and middle frontal cortices in the latency window from 300 to 400 ms; the happy and neutral conditions produced greater activity than the sad condition in the left posterior inferior frontal cortex in the latency window from 400 to 500 ms. These results suggest that the use of emotional context stored in the right frontal cortex starts at ∼300 ms, that integration of linguistic information with emotional context starts at ∼400 ms in the left frontal cortex, and that language comprehension dependent on emotional context is achieved by ∼500 ms.

Comparison of brain mechanisms underlying the processing of Chinese characters and pseudo-characters: an event-related potential study

Most Chinese characters are composed of a semantic radical on the left and a phonetic radical on the right. The semantic radical provides the semantic information; the phonetic radical provides information concerning the pronunciation of the whole character. The pseudo-characters in the study consisted of different sub-lexical parts of real Chinese characters and consequently they also had the semantic radical and the phonetic radical. But they were not readable and had no actual meaning. In order to investigate the spatiotemporal cortical activation patterns underlying the orthographic, phonological and semantic processing of Chinese characters, we used event-related brain potentials (ERPs) to explore the processing of Chinese characters and pseudo-characters when 14 healthy Chinese college students viewed the characters passively. Results showed that both Chinese characters and pseudo-characters elicited an evident negative potential peaking around 120 ms (N120), which appeared to reflect initial orthographic distinction and evaluation. Then, Chinese pseudo-characters elicited a more positive ERP deflection (P220) than did Chinese characters 200-250 ms after onset of the stimuli. It was similar to the recognition potential (RP) and might reflect the integration processes of phonological and semantic processing on the basis of early orthographic information. Dipole source analysis of the difference wave (pseudo-characters minus characters) indicated that a generator localized in the left temporal-occipital junction contributed to this effect, which was possibly related to phonological and perceptual-semantic information integration. Between 350-450 ms, a greater negativity (N360) in pseudo-characters as compared to characters was found over midline fronto-central scalp regions. Dipole analysis localized the generator of N360 in the right parahippocampal cortex. Therefore, the N360 might be an N400 component and reflect the higher-level semantic activation on the basis of orthographic, phonological and perceptual-semantic processing. La plupart des caractères chinois sont composés d'un radical sémantique à gauche et d'un radical phonétique à droite. Le radical sémantique peut fournir de l'information sémantique et le radical phonétique peut fournir de l'information concernant la prononciation du caractère entier. Les pseudo-caractères consistaient en différentes parties sous-lexicales des caractères chinois réels et, conséquemment, ils avaient aussi un radical sémantique et un radical phonétique. Mais ils ne pouvaient pas être lisibles et ils n'ont actuellement aucun sens. Afin d'étudier les patrons d'activation spatiotemporels corticaux sous-jacents aux traitements orthographique, phonologique et sémantique des caractères chinois, nous avons utilisé des Potentiels cérébraux reliés aux événements (PRE) pour explorer le traitement des caractères et des pseudo-caractères chinois auprès de 14 collégiens chinois visionnant les caractères passivement. Les résultats ont montré qu'à la fois les caractères et les pseudo-caractères chinois provoquaient un potentiel négatif évident atteignant un sommet autour de 120 ms (N120), ce qui apparaissait refléter la distinction et l'évaluation orthographiques initiales. Puis, les pseudo-caractères ont provoqué une déviation PRE plus positive (P220) que les caractères chinois entre 200 et 250 ms après l'apparition des stimuli. Ceci était similaire au PR (Potentiel de reconnaissance) et pouvait refléter les processus d'intégration des traitements phonologique et sémantique sur la base de l'information orthographique initiale. L'analyse dipôle de source de la différence de courbe (pseudo-caractères moins caractères) a indiqué qu'un générateur localisé dans la jonction temporo-occipitale gauche contribuait à cet effet, ce qui était possiblement relié à l'intégration de l'information phonologique et perceptuelle-sémantique. Entre 350 et 450 ms, une plus grande négativité (N360) fut observée pour les pseudo-caractères comparativement aux caractères des régions du scalp au-dessus des régions médianes fronto-centrales. L'analyse dipôle a localisé le générateur de N360 dans le cortex para-hippocampique droit. Ainsi, le N360 pourrait être une composante N400 et refléter une activation sémantique de haut niveau sur la base des traitements orthographique, phonologique et perceptuel-sémantique. La mayoría de los caracteres chinos están compuestos por un radical semántico a la izquierda y un radical fonético a la derecha. El radical semántico puede proporcionar información semántica y el radical fonético puede proporcionar información relacionada con la pronunciación del carácter entero. Los pseudo-caracteres consistieron en diferentes partes sub-léxicas de los caracteres reales chinos y como consecuencia, también tenían radicales semánticos y fonéticos. Pero éstos no se podían leer y tampoco tenían un significado real. Para investigar los patrones de la activación cortical espaciotemporal subyacentes de los procesamientos ortográfico, fonológico y semántico de los caracteres chinos, utilizamos potencial relacionado con eventos (ERPs) para explorar el procesamiento de caracteres chinos y pseudo-caracteres mientras que 14 sanos estudiantes chinos veían los caracteres de forma pasiva. Los resultados demostraron que ambos, caracteres y pseudo-caracteres chinos evocaban potenciales negativos con picos alrededor de los 120 ms (N120), lo cual parece reflejar distinción inicial ortográfica y evaluación. Además, los pseudo-caracteres chinos provocaban una desviación ERP más positiva (P220) en comparación con los caracteres chinos entre 200-250 ms después de la aparición de los estímulos. Esto fue parecido a RP (potenciales de reconocimiento) y puede reflejar un proceso de integración del procesamiento fonológico y semántico basado en la información ortográfica temprana. Un análisis dipolo de la fuente de la onda de diferencia (pseudo-caracteres menos caracteres) indicó que un generador localizado en el cruce temporal-occipital izquierdo contribuyó a este efecto, lo cual probablemente tuvo relación con la integración de la información fonológica y perceptivo-semántica. Se encontró mayor negatividad (N360) entre 350-450 ms, en pseudo-caracteres en comparación con los caracteres situada sobre las regiones de la línea media fronto-central craneal. Un análisis dipolo localizo un generador de N360 en el cortex parahipocampal derecho. Por tanto, el N360 podría ser un componente de N400 reflejando una activación semántica de más alto nivel basado en el procesamiento ortográfico, fonológico y perceptivo-semántico.

Left-lateralized N170 effects of visual expertise in reading: evidence from Japanese syllabic and logographic scripts

The N170 component of the event-related potential (ERP) reflects experience-dependent neural changes in several forms of visual expertise, including expertise for visual words. Readers skilled in writing systems that link characters to phonemes (i.e., alphabetic writing) typically produce a left-lateralized N170 to visual word forms. This study examined the N170 in three Japanese scripts that link characters to larger phonological units. Participants were monolingual English speakers (EL1) and native Japanese speakers (JL1) who were also proficient in English. ERPs were collected using a 129-channel array, as participants performed a series of experiments viewing words or novel control stimuli in a repetition detection task. The N170 was strongly left-lateralized for all three Japanese scripts (including logographic Kanji characters) in JL1 participants, but bilateral in EL1 participants viewing these same stimuli. This demonstrates that left-lateralization of the N170 is dependent on specific reading expertise and is not limited to alphabetic scripts. Additional contrasts within the moraic Katakana script revealed equivalent N170 responses in JL1 speakers for familiar Katakana words and for Kanji words transcribed into novel Katakana words, suggesting that the N170 expertise effect is driven by script familiarity rather than familiarity with particular visual word forms. Finally, for English words and novel symbol string stimuli, both EL1 and JL1 subjects produced equivalent responses for the novel symbols, and more left-lateralized N170 responses for the English words, indicating that such effects are not limited to the first language. Taken together, these cross-linguistic results suggest that similar neural processes underlie visual expertise for print in very different writing systems.

Pediatric Magnetoencephalography and Magnetic Source Imaging

Magnetoencephalography (MEG) and magnetic source imaging (MSI) together represent a uniquely powerful functional imaging modality because of their capabilities of directly observing the electrophysiologic activity of neurons with exquisite temporal detail and accurately localizing corresponding neuromagnetic field sources onto high-resolution MR images. These features have and should continue to advance our understanding of the complex spatiotemporal basis of normal and abnormal brain function and development in children. By more clearly delineating and characterizing epileptogenic foci and their relation to eloquent cortex, MSI enables earlier and more effective neurosurgery to be performed, thus resulting in improved seizure outcomes. Although MEG and MSI cannot replace scalp electroencephalography, neuropsychologic testing, and the need for meticulous intraoperative cortical mapping in patients undergoing excision of epileptogenic lesions, their increasing availability should ultimately persuade many clinicians of their key, if not essential, role in the evaluation and treatment of children with epilepsy.

When meaningless symbols become letters: Neural activity change in learning new phonograms

Left fusiform gyrus and left angular gyrus are considered to be respectively involved with visual form processing and associating visual and auditory (phonological) information in reading. However, there are a number of studies that fail to show the contribution of these regions in carrying out these aspects of reading. Considerable differences in the type of stimuli and tasks used in the various studies may account for the discrepancy in results. This functional magnetic resonance imaging (fMRI) study attempts to control aspects of experimental stimuli and tasks to specifically investigate brain regions involved with visual form processing and character-to-phonological (i.e., simple grapheme-to-phonological) conversion processing for single letters. Subjects performed a two-back identification task using known Japanese, and previously unknown Korean, and Thai phonograms before and after training on one of the unknown language orthographies. Japanese subjects learned either five Korean or five Thai phonograms. Brain regions related to visual form processing were assessed by comparing activity related to native (Japanese) phonograms with that of non-native (Korean and Thai) phonograms. There was no significant differential brain activity for visual form processing. Brain regions related to character-to-phonological conversion processing were assessed by comparing pre- and post-tests of trained non-native phonograms with that of native phonograms and non-trained non-native phonograms. Significant differential activation post-relative to pre-training exclusively for the trained non-native phonograms was found in left angular gyrus. In addition, psychophysiologic interaction (PPI) analysis revealed greater integration of left angular gyrus with primary visual cortex as well as with superior temporal gyrus for the trained phonograms post-relative to pre-training. The results suggest that left angular gyrus is involved with character-to-phonological conversion in letter perception.

The effects of homonymy and polysemy on lexical access: an MEG study

We examined the neural correlates of semantic ambiguity by measuring changes in MEG recordings during a visual lexical decision task in which the properties of ambiguous words were manipulated. Words that are ambiguous between unrelated meanings (like bark, which can refer to a tree or to a dog) were accessed more slowly than words that have no unrelated meanings (such as cage). In addition, words that have many related senses (e.g., belt, which can be an article of clothing or, closely related in sense, a fan belt used in machinery) were accessed faster than words that have few related senses (e.g., ant). The findings are inconsistent with accounts that posit that both kinds of ambiguity involve separate lexical entries, but instead offer both behavioral and neurophysiological support for separate entry accounts only for homonymy, and a single-entry model of polysemy. The findings also provide neural correlates for a behavioral study of lexical ambiguity that demonstrated that the frequently reported ambiguity advantage in lexical decision tasks is not due to the presence of many unrelated meanings but to the presence of many related senses.

Magnetoencephalography (MEG) and magnetic source imaging (MSI)

BACKGROUND

Real-time, direct assessment of brain electrophysiology is critical for noninvasive functional mapping and for the identification of paroxysmal epileptiform abnormalities in the evaluation of patients for epilepsy surgery. Historically, electroencephalography (EEG) and evoked potentials (EPs) have performed these functions. However, both often required direct intracranial recording for precise localization. Magnetoencephalography (MEG) takes advantage of the fact that neuromagnetic signals penetrate the skull and scalp without distortion. The magnetic source image (MSI) is created when the MEG data is superimposed on a magnetic resonance image (MRI).

REVIEW SUMMARY

MEG performs noninvasive functional imaging by recording the magnetic flux on the head surface associated with electrical currents in activated sets of neurons. MEG has rapidly evolved in the last 2 decades because of the introduction of whole head systems and advances in computer technology. MEG is now the imaging modality of choice where a precise and high degree of localization is required. MEG can be used to localize the primary sensory cortices (visual, auditory, or somatosensory), areas involved with receptive language function, the irritative zone in epilepsy patients, and identify children with anomalous language development. This article reviews the basis of MEG, the instrumentation used, the clinical applications and current limits of the technology.

CONCLUSION

MEG studies can now be performed on a routine basis as a clinical tool. MEG is now indicated for: 1) localization of the irritative zone in lesional and nonlesional epilepsy surgery patients, 2) functional mapping of eloquent cortex, and 3) assessment of normal and abnormal language development. In the future MEG may help the understanding of normal development and reorganization after brain injury. The neurologist can use MEG data to complement structural and metabolic imaging techniques.

The precise time course of lexical activation: MEG measurements of the effects of frequency, probability, and density in lexical decision

Visually presented letter strings consistently yield three MEG response components: the M170, associated with letter-string processing (Tarkiainen, Helenius, Hansen, Cornelissen, & Salmelin, 1999); the M250, affected by phonotactic probability, (Pylkkänen, Stringfellow, & Marantz, 2002); and the M350, responsive to lexical frequency (Embick, Hackl, Schaeffer, Kelepir, & Marantz, 2001). Pylkkänen et al. found evidence that the M350 reflects lexical activation prior to competition among phonologically similar words. We investigate the effects of lexical and sublexical frequency and neighborhood density on the M250 and M350 through orthogonal manipulation of phonotactic probability, density, and frequency. The results confirm that probability but not density affects the latency of the M250 and M350; however, an interaction between probability and density on M350 latencies suggests an earlier influence of neighborhoods than previously reported.

Gradual recovery from dyslexia and related serial magnetoencephalographic changes in the lexicosemantic centers after resection of a mesial temporal astrocytoma. Case report

Letter-perception centers are not held in as high regard as motor- and language-related cortices during planning of neurosurgical procedures, and there have been no reports suggesting cortical reorganization of reading ability. The authors describe a patient with a left mesial temporal glioma in whom two letter-perception centers (the anterior portion of the left superior temporal gyrus and the left fusiform gyrus) were successfully localized before surgery by performing magnetoencephalography (MEG) during reading tasks. Control MEG examinations of 15 healthy volunteers were also performed to assist in a careful interpretation of patient results. Although a radical resection of the mesial temporal glioma, which involved the left fusiform gyrus, caused severe dyslexia, the patient's impaired reading skills improved gradually during a 1-year postoperative period. In the meantime, the spared left superior temporal gyrus displayed an overshot recovery of MEG responses. During the postoperative period there was no obvious recovery in MEG signals and no compensatory activity in the contralateral fusiform gyrus. This case demonstrates that lexicosemantic centers involved in the reading process can be noninvasively localized using MEG and that the results obtained are highly reliable for surgical planning. The results of the repeated MEG reflected sequentially the patient's recovery from dyslexia. This is the first report in which MEG studies have been shown to predict preoperatively the risk of dyslexia and demonstrate its serial physiological recovery.

Frequency effects of Chinese character processing in the brain: an event-related fMRI study

Knowing how the brain processes Chinese characters of different frequencies of occurrence may shed light on the extent to which orthographic variations of different languages can influence reading processes in the brain. In the present study, event-related fMRI was used to investigate frequency effects on Chinese character processing. Reading low-frequency characters invoked higher activation in several brain regions including the left premotor/inferior frontal gyrus, supplementary motor area, left anterior insula, left posterior inferior temporal gyrus, left superior parietal cortex, and lingual cortex, while reading high-frequency characters resulted in higher activation in the left supramarginal/angular gyrus and left precuneus. The activation pattern of reading infrequently encountered characters reflects a more demanding processing procedure of retrieving, formulating, and coordinating the phonological output. Access to the lexical route may benefit the reading of high-frequency characters. By uncovering the differential brain responses in reading Chinese characters of different occurrence frequencies, not only has a substantial overlap between functional neuroanatomy of reading Chinese and alphabetical languages been demonstrated, but also features permitting the separation of language-specific content from universal mechanisms.

Neuromagnetic evidence for the timing of lexical activation: an MEG component sensitive to phonotactic probability but not to neighborhood density

Evidence from electrophysiological measures such as ERPs (event-related potentials) and MEG (magnetoencephalography) suggest that the first evoked brain response component sensitive to stimulus properties affecting reaction times in word recognition tasks occurs at 300-400 ms. The present study used the stimulus manipulation of Vitevich and Luce (1999) to investigate whether the M350, an MEG response component peaking at 300-400 ms, reflects lexical or postlexical processing. Stimuli were simultaneously varied in phonotactic probability, which facilitates lexical activation, and in phonological neighborhood density, which inhibits the lexical decision process. The present results indicate that the M350 shows facilitation by phonotactic probability rather than inhibition by neighborhood density. Thus the M350 cannot be a postlexical component.

New approach to localize speech relevant brain areas and hemispheric dominance using spatially filtered magnetoencephalography

We used a current localization by spatial filtering-technique to determine primary language areas with magnetoencephalography (MEG) using a silent reading and a silent naming task. In all cases we could localize the sensory speech area (Wernicke) in the posterior part of the left superior temporal gyrus (Brodmann area 22) and the motor speech area (Broca) in the left inferior frontal gyrus (Brodmann area 44). Left hemispheric speech dominance was determined in all cases by a laterality index comparing the current source strength of the activated left side speech areas to their right side homologous. In 12 cases we found early Wernicke and later Broca activation corresponding to the Wernicke-Geschwind model. In three cases, however, we also found early Broca activation indicating that speech-related brain areas need not necessarily be activated sequentially but can also be activated simultaneously. Magnetoencephalography can be a potent tool for functional mapping of speech-related brain areas in individuals, investigating the time-course of brain activation, and identifying the speech dominant hemisphere. This may have implications for presurgical planning in epilepsy and brain tumor patients.

A magnetoencephalographic component whose latency reflects lexical frequency

Magnetoencephalography (MEG) measured subjects' brain responses during a lexical decision task. The words employed come from six frequency categories, which were defined in terms of a linear decrease in log-frequency. Although frequency effects in reaction-time are well-documented in studies of lexical access, a neural component whose latency predicts reaction time has not been discovered. This study identifies an MEG component (the M350) whose latency mirrors the frequency-effect.

Somatosensory, auditory, and visual evoked magnetic fields in patients with brain diseases

The features of somatosensory (SEFs), auditory (AEFs), and visual evoked fields (VEFs) in healthy subjects and patients with brain diseases provide the basis for clinical investigations using magnetoencephalography (MEG). The SEFs provide clinically useful information to identify the central sulcus and somatotopic organization of the primary somatosensory cortex. Localization accuracy of the SEFs can be tested by cortical stimulation during surgery. Functional reorganization suggested by SEF studies must be verified by other modalities. The AEFs can localize the auditory cortex in the bilateral temporal lobes. Separation of bilateral activities is much clearer in AEFs than in auditory evoked potentials. Modulation of the interhemispheric differences of latency, amplitude, and source localization of AEFs can be used to evaluate auditory function in patients with intracranial lesions. Pattern reversal VEFs provide stable localization of the primary visual function. Separation of bihemispherical activities is the advantage of VEFs over visual evoked potentials. Investigation of VEFs provides objective evaluation of visual field deficits such as homonymous or bitemporal hemianopsia in patients with intracranial lesions. Evoked magnetic fields can provide useful diagnostic information. Such clinical findings, in turn, provides the opportunity to test the source estimation accuracy of MEG.

The neural circuitry involved in the reading of German words and pseudowords: A PET study

Silent reading and reading aloud of German words and pseudowords were used in a PET study using (15O)butanol to examine the neural correlates of reading and of the phonological conversion of legal letter strings, with or without meaning. The results of 11 healthy, right-handed volunteers in the age range of 25 to 30 years showed activation of the lingual gyri during silent reading in comparison with viewing a fixation cross. Comparisons between the reading of words and pseudowords suggest the involvement of the middle temporal gyri in retrieving both the phonological and semantic code for words. The reading of pseudowords activates the left inferior frontal gyrus, including the ventral part of Broca's area, to a larger extent than the reading of words. This suggests that this area might be involved in the sublexical conversion of orthographic input strings into phonological output codes. (Pre)motor areas were found to be activated during both silent reading and reading aloud. On the basis of the obtained activation patterns, it is hypothesized that the articulation of high-frequency syllables requires the retrieval of their concomitant articulatory gestures from the SMA and that the articulation of low-frequency syllables recruits the left medial premotor cortex.

Patterns of brain activity during visual imagery of letters

Cortical signals associated with visual imagery of letters were recorded from 10 healthy adults with a whole-scalp 122-channel neuromagnetometer. The auditory stimulus sequence consisted of 20 different phonemes corresponding to single letters of the Roman alphabet and of tone pips (17%), delivered once every 1.5 sec in a random order. The subjects were instructed to visually imagine the letter corresponding to the auditory stimulus and to examine its visuospatial properties: The associated brain activity was compared with activity evoked by the same stimuli when the subjects just detected the intervening tones. All subjects produced broad imagery-related responses over multiple cortical regions. After initial activation of the auditory cortices, the earliest imagery-related responses originated in the left prerolandic area 320 msec after the voice onset. They were followed within 70 msec by signals originating in the posterior parietal lobe close to midline (precuneus) and, 100 msec later, in the posterior superior temporal areas, predominantly in the left hemisphere. The activations were sustained and partially overlapping in time. Imagery-related activity in the left lateral occipital cortex was observed in two subjects, and weak late activity in the calcarine cortex in one subject. Real audiovisually presented letters activated multiple brain regions, and task-induced visuospatial processing of these stimuli further increased activity in some of these regions and activated additional areas: Some of these areas were activated during imagery as well. The results suggest that certain brain areas involved in high-level visual perception are activated during visual imagery and that the extent of imagery-related activity is dictated by the requirements of the stimuli and the task.

Functional magnetic resonance imaging of neural activity related to orthographic, phonological, and lexico-semantic judgments of visually presented characters and words

Functional magnetic resonance imaging was used to investigate neural activity during the judgment of visual stimuli in two groups of experiments using seven and five normal subjects. The subjects were given tasks designed differentially to involve orthographic (more generally, visual form), phonological, and lexico-semantic processes. These tasks included the judgments of whether a line was horizontal, whether a pseudocharacter or pseudocharacter string included a horizontal line, whether a Japanese katakana (phonogram) character or character string included a certain vowel, or whether a character string was meaningful (noun or verb) or meaningless. Neural activity related to the visual form process was commonly observed during judgments of both single real-characters and single pseudocharacters in lateral extrastriate visual cortex, the posterior ventral or medial occipito-temporal area, and the posterior inferior temporal area of both hemispheres. In contrast, left-lateralized activation was observed in the latter two areas during judgments of real- and pseudo-character strings. These results show that there is no katakana "word form center" whose activity is specific to real words. Activation related to the phonological process was observed, in Broca's area, the insula, the supramarginal gyrus, and the posterior superior temporal area, with greater activation in the left hemisphere. These activation foci for visual form and phonological processes of katakana also were reported for the English alphabet in previous studies. The present activation showed no additional areas for contrasts of noun judgment with other conditions and was similar between noun and verb judgment tasks, suggesting two possibilities: no strong semantic activation was produced, or the semantic process shared activation foci with the phonological process.

Responses to silent Kanji reading of the native Japanese and German in task subtraction magnetoencephalography

The neuromagnetic activities evoked by semantic processing were localized by magnetoencephalography (MEG). We observed distinct time courses of the activities in native speaking Japanese subjects (Japanese speaker) and German subjects (German speaker) during silent reading of Japanese letters; Kanji and meaningless figures made by deforming the Arabian letters. There were significant differences in amplitude of the activities between Kanji and meaningless figure stimuli. The responses with meaningless figure stimuli were subtracted from those with Kanji stimuli to demonstrate the semantic responses. Earlier responses peaked at about 273.3+/-50. 8 and 245.0+/-23.8 ms (mean+/-S.D.) and were mainly located in the right fusiform gyrus (FuG) in the Japanese and German speakers, respectively. All the Japanese speakers constantly showed additional later responses in the left superior temporal gyrus (STG) and the supramarginal gyrus (SmG) at approximately 616.1+/-105.5 ms, whereas no further activity was observed in the German speakers who did not know the meaning of each Kanji. Because the later responses in the STG and SmG in the Japanese speakers were only observed in their dominant hemisphere, we believe the source of these responses to be part of the neural basis of Kanji semantic processing. The task subtraction MEG analysis could be a powerful method to discriminate distinct responses and visualize the neural networks involved in semantic processing.

Reading of Japanese Kanji (morphograms) and Kana (syllabograms): a magnetoencephalographic study

Magnetoencephalograms were recorded from six healthy Japanese subjects in order to investigate the areas in the cortices which are involved in the recognition of Japanese characters (Kanji and Kana). Forty-four Kanji (morphograms), 44 Kana (syllabograms) and 20 alphabet letters were used as stimuli. They were presented randomly and the subjects were required to read each stimulus and count the number of letters. The magnetic responses were recorded with dual 37-channel SQUID (Superconducting Quantum Interference Device) gradiometers from the temporal, parietal and occipital areas of the brain. The magnetic responses to Kanji and Kana were similar and consequently the locations of equivalent current dipoles (ECDs) to Kanji and those to Kana did not differ at any recording site. In all the subjects, ECDs were found in the posterior inferior temporal (PIT) areas approximately corresponding to Brodmann area 37 in the latency range of 150-300 msec. These activities were found in both hemispheres without consistent laterality. The location of the ECD moved forward from posterior to anterior in the PIT area as the latency increased in all but one subject. Only one subject showed activities in the left angular gyrus. Since activities in PIT areas were also found in alphabet letters, the bilateral PIT areas are considered to play an essential role in reading.

Neural processing of words in the human extrastriate visual cortex

Measurements of neuromagnetic fields were made for the responses to visually presented words comprised of Japanese characters (phonograms), and for comparison of responses to single characters and character symbols, while subjects performed, respectively, semantic category matching, rhyming, and character matching tasks. The magnetic field responses recorded from the occipital and occipitotemporal regions consisted of 2-3 major peak components, occurring between 150 and 300 ms after the onset of the visual forms. The localization of equivalent current dipole sources of these components within the brain structure of individual subjects indicated that the main regions of the neural activity occurring at 150-250 ms were located in the extrastriate visual cortices. They included the lateral area mostly at the occipital gyrus, medial area consisting of parieto-occipital and calcarine sulci and lingual gyrus, and ventral area which is continuous from the lingual gyrus (LG) to fusiform gyrus (FG). In the ventral LG/FG area the left side was activated primarily by words, while the right side was responsive in more or less equally to words, characters, and symbols. It is suggested that the left occipitotemporal LG/FG mediates the neural function that subserves the specific visual word processing and/or general analysis of complex graphical features of visual forms.