Brain and Language 79, 211–222 (2001) doi:10.1006/brln.2001.2481, available online at http://www.idealibrary.com on
The Bilingual Brain: Cerebral Representation of Languages Franco Fabbro IRCCS ‘‘E. Medea’’ and University of Udine, Udine, Italy
The present article deals with theoretical and experimental aspects of language representation in the multilingual brain. Two general approaches were adopted in the study of the bilingual brain. The study of bilingual aphasics allows us to describe dissociations and double dissociations between the different subcomponents of the various languages. Furthermore, symptoms peculiar to bilingual aphasia were reported (pathological mixing and switching and translations disorders) which allowed the correlation of some abilities specific to bilinguals with particular neurofunctional systems. Another approach to the study of the bilingual brain is of the experimental type, such as electrophysiological investigations (electrocorticostimulation during brain surgery and event-related potentials) and functional neuroanatomy studies (positron emission tomography and functional magnetic resonance imaging). Functional neuroanatomy studies investigated the brain representation of languages when processing lexical and syntactic stimuli and short stories. Neurophysiologic and neuroimaging studies evidenced a similar cerebral representation of L1 and L2 lexicons both in early and late bilinguals. The representation of grammatical aspects of languages seems to be different between the two languages if L2 is acquired after the age of 7, with automatic processes and correctness being lower than those of the native language. These results are in line with a greater representation of the two lexicons in the declarative memory systems, whereas morphosyntactic aspects may be organized in different systems according to the acquisition vs learning modality. 2001 Academic Press
Key Words: bilingual aphasia; polyglot reactions; functional neuroanatomy of bilingualism.
CLINICAL NEUROLINGUISTIC STUDIES ON MULTILINGUALISM
Several studies have tried to interpret the different recovery patterns observed in multilingual aphasics (Paradis 1977, 1989, 1993, 1998, 2001). The main issues at stake are the reason why recovery patterns differ so much across patients and why a language recovers better than the other(s). Pitres (1895) and other neurologists in the past century claimed that failure of a language to recover during the intermediate and late phases was not due to its loss, but rather to pathophysiological inhibitory effects caused by the lesion. Pitres had drawn this conclusion on the basis of a general assumption and some empirical studies. The general assumption was supported by several neurologists (including S. Freud, A. Pick, O. Po¨tzl, M. Minkowski, and W. Penfield) and presupposed that all languages of a bilingual or a polyglot subject were localized in common language areas (cf. Paradis, 1989, 2001; Fabbro, 1999). This theory had developed in the wake of the scientific debate which started with R. Scoresby-Jackson in 1867 and continued throughout the second half of the 19th cenAddress correspondence and reprint requests to F. Fabbro, Neurolinguistic Unit, Scientific Institute ‘‘E. Medea,’’ 33078 San Vito al Tagliamento (PN), Italy. E-mail:
[email protected]. 211 0093-934X/01 $35.00 Copyright 2001 by Academic Press All rights of reproduction in any form reserved.
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tury. Scoresby-Jackson speculated that Broca’s area was responsible for the representation of a subject’s mother tongue, whereas the portions anterior to Broca’s area were responsible for foreign language acquisition (Scoresby-Jackson, 1867). This hypothesis was rejected following a postmortem study of the brain of a polyglot, Sauerwein, who spoke 54 languages, at both poetry and prose levels. In the brain of this exceptionally gifted individual Broca’s area and the structures anterior to it were of normal extension and showed a perfectly normal development (Veyrac, 1931). It should be noted, however, that Scoresby-Jackson’s contemporaries had not understood that he was referring to the capacity of the tissue adjacent to Broca’s area to be neurofunctionally involved in processing of other languages vs the native language. This act does not necessarily imply an increase in the anatomic extension of structures adjacent to Broca’s area. A second element—derived from the empirical observation of bilingual aphasic patients—led Pitres to claim that the language that was not available was not lost, but only inhibited. Pitres had repeatedly noticed that these patients could progressively recover a language in a lapse of time shorter than that needed to acquire a foreign language, which meant that the disorder provoked by the lesion did not cause the loss of the language, but only made it partially inaccessible. Pitres defined this disorder as ‘‘inertia’’ of the cortical language centers, which manifested itself by the temporary extinguishing of the motor and sensory images used to understand and utter words and sentences. In his opinion, the cerebral organization of language in polyglots was to be studied adopting a ‘‘neurophysiological approach’’ rather than a ‘‘neuroanatomical approach,’’ which was certainly more direct but less effective. Swiss neurologist Mieczyslaw Minkowski supported Pitres’ theories on polyglot aphasia and thus maintained that the linguistic deficits and the recovery patterns of bilingual aphasics could be explained on the basis of pathophysiological modalities rather than anatomical modalities. In the wake of Pitres, Minkowski held the general assumption that it was not necessary to assume the existence of separate centers responsible for each language known by a subject. With respect to this point, he stated: ‘‘If we assume no spatially separate centers or areas in the cortex for the different languages, but instead assume that within the same area, the same elements are active, though in different combinations and interacting with a differential linguistic constellation, it is easy to explain the phenomena occurring in polyglot aphasia in terms of the interaction of such a large set of factors’’ (Minkowski, 1927, p. 229). A large number of neurologists in the past (Pitres, Freud, Minkowski, and Po¨tzl) and also some contemporary scholars (Paradis) have suggested that when a language is not available, it is not because its neural substrates have been physically destroyed, but because its system has been weakened (cf. Fabbro, 1999). This weakening can be explained in terms of increased inhibition, raised activation threshold, or unbalanced distribution of resources among the various languages (Green, 1986; Paradis, 1998). However, in some cases, stable dissociations in the recovery of the two languages observed in the intermediate and late phases cannot be ascribed to neurofunctional impairments only, but also to the consequences of the destruction of cortical and/or subcortical neural substrates (Fabbro, et al. 1997; Fabbro, 1999). Actually, late-phase dynamic phenomena have hardly ever been described. This seems to suggest that 1 or 2 years after onset, the recovery pattern remains stable as a result of pathological phenomena that are partly due to neurofunctional impairments and partly due to the loss of cerebral tissue originally involved in the organization of linguistic functions. Polyglot Reactions in Bilinguals Some patients present language disorders that seem to be typical of polyglot aphasics only (Lebrun, 1991). Subjects may switch from language to language, alternating
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their verbal expression between the two languages. Other subjects may mix linguistic elements from various languages within a single sentence. Switching and mixing are frequent in normal bilingual speakers too, but they reflect a pathological behavior when produced during conversation with an interlocutor who is unable to understand both languages. In such cases, even if repeatedly asked to use only one language, some patients would go on producing mixing and/or switching phenomena. It is not always possible to draw a clear distinction between mixing and switching, but it seems that pathological switching is a pragmatic disorder of communication which tends to be related to lesions of the frontal lobes (both left and right), whereas pathological mixing is an aphasic disorder, is generally associated with fluent aphasias, and tends to be correlated with left postrolandic lesions (Fabbro, 1999; Fabbro, Skrap, & Aglioti, 2000). Further, pathological mixing seems to be a typical symptom frequently found in bilingual aphasics with dementia (Mendez et al., 1999). Bilingual aphasics may also present translation disorders (Fabbro & Gran, 1997). One of these phenomena is the inability to translate, which may affect both directions of translation, namely from L1 into L2 and, vice versa, from L2 into L1. Another disorder is spontaneous translation, a compulsive ‘‘need’’ to translate everything which is being said by the patients themselves and/or by their interlocutors (DeVreese, Motta, & Toschi, 1988). Another, still, is translation without comprehension, which occurs when patients do not understand commands that are given to them but can nevertheless correctly translate the sentences uttered by an interlocutor to express these commands (Veyrac, 1931; Fabbro & Paradis, 1995b). Finally, paradoxical translation occurs when a patient can translate only into the language that he or she cannot speak spontaneously and not the reverse (Paradis et al., 1982). In a specific study Paradis (1984) analyzed the paradoxical translation phenomenon and the translation without comprehension deficit phenomenon and presupposed the existence of a series of neurofunctionally separate and independent components as follows: (a) a component accounting for translation from language A into language B (A→B) and (b) a component accounting for translation from language B into A (B→A). Therefore, a cerebral lesion in a bilingual subject may for a certain period of time selectively inhibit only a component of the translation process, whereas the other component that is neurofunctionally independent may continue to perform translation without difficulty. The fact that these so-called polyglot reactions only occur in multilingual patients is just an impression. Indeed, mixing, switching, and pathological translation are simply much more evident when different languages are at stake, but they most probably occur also in monolinguals, only with different apparent features. Actually, all verbal functions that are present in a bilingual individual have their homolog in a monolingual speaker. Bilinguals switch and mix languages, while monolinguals switch and mix registers; bilinguals translate from one language into another, while monolinguals may paraphrase from one register to another (i.e., they can express the same concept addressing their own little child or an audience of experts) (Paradis, 1993, 1998). This is another reason why it is no longer reasonable to postulate the existence of neural mechanisms specific to bilinguals, as maintained by several neurologists in the past. Cortical and Subcortical Representation of Languages In the 1960s K. Kainz suggested that often the better recovered language was a language the use of which was not automatic but rather depended on conscious efforts. In Kainz’s opinion, aphasia mainly affected the most automatic language, namely the language which was used unconsciously (Kainz, 1960). Differences in age and manner of learning and language use seem to influence the way languages
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are stored in the brain. When a second language is learned formally and mainly used at school, it apparently tends to be more widely represented in the cerebral cortex than the first language, whereas if it is acquired informally, as usually happens with the first language, it is more likely to involve subcortical structures (basal ganglia and cerebellum) (cf. Paradis, 1994; Fabbro & Paradis, 1995; Fabbro et al., 1997; Fabbro, 2000). EM, an aphasic patient with a lesion to the left basal ganglia whom I and S. Aglioti followed (Aglioti & Fabbro, 1993; Aglioti et al., 1996), had completely lost the capacity to express herself in her mother tongue. In contrast, in her second language, which she had mastered only at the comprehension and reading levels, she showed good production skills. Interestingly, she had never spoken that language. This symptomatological picture remained stable for more than 10 years after her stroke. The difference in brain representation between an informally acquired first language and a formally learned second language seems to be due to different mnestic strategies applied in the process (Paradis, 1994). Most probably, in EM the grammatical components of the first language were more extensively organized in procedural memory systems, while L2 was represented in declarative memory systems to a greater extent.
LATERALIZATION OF LANGUAGES
In the late 1970s Albert and Obler (1978), in an attempt to explain a few cases of nonparallel recovery among their patients, suggested that bilinguals had a more symmetric representation of language in the two cerebral hemispheres. The hypothesis appealed to the scientific imagination of many researchers who over the past 20 years have carried out studies on the cerebral organization of language in bilinguals. These studies were generally performed using typical techniques of experimental neuropsychology, namely dichotic listening, tachistoscopic technique, and finger tapping, but their results were rather controversial (cf. Paradis, 1990). The analysis of a large number of cases of bilingual aphasics showed that the incidence of aphasia following a lesion to the right hemisphere (crossed aphasia) is as high in monolinguals as in bilinguals (Karanth & Rangamani, 1988). However, it is known that the right hemisphere (also called the ‘‘minor’’ hemisphere) is crucially involved in the processing of pragmatic aspects of language use (Chantraine et al., 1998). During the first stages of second-language learning, both in children and adults, the right hemisphere tends to be more involved in verbal communication processes because beginners try to compensate with pragmatic inferences for the lack of implicit linguistic competence in L2. The greater involvement of the right hemisphere during verbal communication in L2, however, does not necessarily imply a greater representation of language processes (phonology, morphology, and syntax) in the right hemisphere (cf. Paradis, 1994, 1998).
EXPERIMENTAL STUDIES ON THE BILINGUAL BRAIN
While clinical neurolinguistic studies initially focused on the development of test methods for a correct, complete, and systematic assessment of language disorders and, subsequently, on the description of the linguistic symptomatology in the different phases that follow a cerebral lesion, experimental studies in this field basically tried to answer the question whether the languages known by bilinguals are (completely or partially) represented in the same or in different cerebral structures.
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Electrical Stimulation Studies in Bilinguals One of the first methods used to assess the cerebral representation of linguistic functions is electrocorticostimulation during brain surgery (Ojemann & Whitaker, 1978; Rapport et al., 1989). In these studies, patients were administered naming tests in both languages while during neurosurgical interventions different cortical areas were stimulated in such a way as to produce their transient functional inhibition and verify whether these areas were involved in language processing. There were sites in which both languages were equally disrupted by electrical stimulation of the brain (EBS), sites in which one language was disrupted more than the other by EBS, and sites in which one language was disrupted and the other was not affected at all. Results were interpreted as showing that specific cortical areas were shared by both languages, whereas other areas, when stimulated, selectively inhibited only one language. These studies were criticized because they could not be repeated on a population of a statistically sufficient size and because the spatial definition of such brain mapping lacked exactness.
Electrophysiological Studies A series of studies carried out by Helen Neville and associates (Neville et al., 1992; 1997; Weber-Fox & Neville, 1997) by means of electrophysiological techniques (event-related potentials, ERPs) revealed possible differences in the cerebral cortical organization of languages according to the age of acquisition and learning strategies. In early bilinguals, closed-class words of both languages tend to be represented in the left frontal lobe, whereas open-class words tend to involve postrolandic cortical structures. On the other hand, in bilinguals who learned their second language after the so-called ‘‘critical age’’ (about 7 years of age), closed-class words of L2 vs L1 do not seem to be represented in left frontal areas but together with open-class words in postrolandic areas.
Functional Neuroanatomy Studies Over the past few years many studies used advanced functional neuroanatomical techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to investigate language representation in the brain of bilingual subjects. Their results raised such interest on the part of both the scientific community and the general public. These investigation techniques certainly present advantages but they also have limitations (Posner & Raichle, 1997; Price, 1998; De´monet, 1999; Paradis, 1999). Limitations depend on some important methodological issues, such as the following: (a) the time needed to study the cerebral representation of a function is expressed in seconds, whereas language processes are expressed in milliseconds; (b) subtractive comparisons between two activation tasks are often difficult to interpret; (c) results of neuroimaging studies often do not correspond to clinical neuropsychological findings; (d) neuroimaging techniques do not allow to determine whether activation of a cerebral structure depends either on an increase in activation processes or in neurophysiological inhibition processes (for example, activation of the right Broca’s area during verbal production tasks might be due to an increase in neurophysiological inhibition processes, cf. Cook, 1984); and last, (e) brain activation was studied with tasks that are too complex (for example, listening to stories) and whose linguistic and pragmatic nature is still scarcely known; these tasks generally simultaneously
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activate many linguistic, pragmatic, and affective structures, thus making it difficult to interpret data. PET and fMRI studies which investigated language organization in the brain of bilinguals are discussed according to the linguistic stimuli that were used: (1) word processing, (2) sentence processing, and (3) short story processing. Word processing. The first neuroimaging study on bilinguals was made by Klein et al. (1995). In English–French bilinguals, PET revealed a greater activation of the left putamen during a word generation task in the language that subjects knew less well. Recently, Klein et al. (1999) carried out another PET study on Chinese–English bilinguals. All the subjects were native Chinese who had acquired their L2 (English) in adolescence. Subjects received verb-generation tasks. Despite the fact that subjects had learned English later in life, the two languages showed activation in the same cerebral structures (left inferior frontal, dorsolateral frontal, temporal and parietal cortices, and right cerebellum). In this study, the authors did not find any difference in the activation of the two languages in the basal ganglia. In their opinion, their results suggested a common macroscopic cortical representation for L1 and L2. Chee et al. (1999) used fMRI to study cerebral activation during word stem completion tasks in Chinese (Mandarin)–English bilinguals. The activated brain areas (left prefrontal region involving the inferior frontal gyrus, the supplementary motor area and, bilaterally, occipital and parietal regions) were the same in the two languages (despite their structural distance and different writing) for both early bilinguals (L2 learned before the age of 6) and late bilinguals (L2 learned after the age of 12). The authors hypothesized that cortical representation of words in bilinguals involves—at the macroscopic level—the same cortical areas regardless of the age of acquisition of L2 and that cerebral asymmetries were the same for both languages and identical to those of monolinguals. Illes et al. (1999) used fMRI to investigate brain activation during semantic judgment tasks (concrete/abstract judgments) in eight late English–Spanish bilinguals. In all eight participants they found a similar cortical activation of the left inferior frontal gyrus for both languages (six participants also showed increased activation in the right inferior frontal gyrus for both languages), which led them to suggest that in the bilingual brain, at the macroanatomical level, there is a common neuronal system responsible for semantic processing in both languages. More recently, Hernandez et al. (2000) used fMRI to investigate brain activation during a naming task (pictures) in six early Spanish–English bilinguals. No differences were found between the two languages in terms of areas of representation or intensity of activation. Thus, these authors reached the conclusion that there was no evidence that in naming processing each language was represented in different macroanatomical areas of the brain. Price et al. (1999) carried out a PET investigation of the cerebral structures activated during word translation tasks and alternating L1/L2 word reading tasks in German–English bilinguals. During the translation tasks the following areas activated: the anterior cingulate structures and, bilaterally, the putamen and the head of the caudate nucleus; the supplementary motor area; and the left insular ventral area. On the other hand, in alternating L1/L2 word reading tasks the left posterior inferior frontal cortex and the bilateral supramarginal gyrus activated. In an fMRI study Hernandez et al. (2000) attempted to better define the cortical structures involved in a switching task (switching between words) in early bilinguals. They found that the only area revealing increased activity for language switching relative to single language processing was the dorsolateral prefrontal cortex. This neuroimaging study supports findings from a previous clinical neuropsychological study which demonstrated that a lesion to the left prefrontal areas causes pathological switching phenom-
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ena (switching between languages) and that such a disorder is not associated to aphasia (Fabbro, 1999; Fabbro et al., 2000). This means that choosing one language or switching from language to language are not really language aspects but pragmatic components of verbal communication. These studies indicate that lexicons of L1 and L2 are macroscopically represented in the same brain areas regardless of the age of acquisition. These results are in line both with theories supported by the first neurologists who investigated the bilingual brain (e.g., Pitres, Minkowski, and Chelenov; cf. Paradis, 1983) and with the most recent hypotheses on the different memory systems involved in language acquisition and learning (Paradis, 1994), according to which lexicons of L1 and L2—learned after the so-called critical age—are stored in declarative memory systems which are represented in the left cortical associative areas subserving language functions (Ulmann et al., 1997). Processing sentences. In an fMRI study, Kim et al. (1997) compared cortical areas activated during silent sentence generation tasks in L1 and L2 in early vs late bilingual subjects. For both languages they found a similar activation of Broca’s and Wernicke’s areas in early bilinguals, while in late bilinguals they found a similar activation of Wernicke’s area but a significant difference in the activation of Broca’s area between L1 and L2. In late bilinguals the authors found in the left Broca’s area two distinct but adjacent centers, separated by approximately 8 mm, subserving language production in L1 vs L2. They thus concluded that the anatomical separation of the two languages in Broca’s area depended on their different acquisition age. The differences in functional organization in Broca’s area between early bilinguals and late bilinguals could be also correlated with L2 competence of late bilinguals. Unfortunately, the authors did not describe the phonologic (presence of foreign accent?) and syntactic skills (presence of syntactic errors?) of the late bilinguals. They only reported that these participants showed a high standard fluency in L2. Chee et al. (1999) tried to replicate these results using fMRI to investigate cortical areas activated during a sentence processing task in fluent early Chinese (Mandarin)—English bilinguals. All the subjects were exposed to both English and Mandarin prior to the age of 6. Participants were asked to judge whether the visually presented sentences were ‘‘true’’ or ‘‘false.’’ The most activated areas were the inferior and middle prefrontal cortex (more extensive on the left side), the left temporal region, the left angular gyrus, the anterior supplementary motor area (more activated on the left side), and the bilateral superior parietal and occipital regions, with no difference between the two languages. The authors concluded that in early fluent bilinguals common macroscopic areas were activated by syntactically complex English and Mandarin sentences. According to the authors, their results support the hypothesis of a ‘‘one-store’’ model for linguistic representation of two languages in early fluent bilinguals. However, the fact that Chee et al. only studied proficient bilinguals exposed to both languages early in life does not allow to exclude that the brain representation of languages in late bilinguals—with lower phonologic and syntactic skills in L2—is compatible with the so-called ‘‘two-store’’ model. Processing short stories. In the first study, Perani et al. (1996) used PET to study brain activation during story listening tasks in Italian (L1) and English (L2) in subjects with moderate command of L2. While in L1 the most activated areas were the classic perisylvian language areas (inferior frontal gyrus, the superior and middle temporal gyri, the temporal pole, and the angular gyrus) and the right cerebellum, in L2 the set of active language areas was significantly reduced. Only left and right superior and middle temporal areas remained active, with bilateral activation of the parahippocampal region. According to the authors, their results clearly supported the hypothesis of a partially different representation of the two languages in the brain.
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Dehaene et al. (1997) carried out an fMRI study to explore brain activation during short story listening tasks in French (L1) and English (L2) in eight subjects with moderate command of L2. In L1 all subjects showed a remarkable consistency in the activation of the same areas of the left temporal lobe (superior temporal sulcus and superior and middle temporal gyri), whereas in L2 the story listening task activated networks that varied greatly from subject to subject (six subjects showed disperse active pixels in the left temporal lobe, the remaining two only in the right temporal lobe). During a story processing task in L1, the average active volume of the left temporal lobe was significantly higher than that of the right temporal lobe (LTL ⫽ 1378 mm3; RTL ⫽ 456 mm3). In story processing in L2, the average active volume was considerably reduced but activation of the left temporal lobe was still significantly higher than that of the right temporal lobe (LTL ⫽ 666 mm3; RTL ⫽ 327 mm3). According to the authors, their results confirmed the hypothesis of a left hemisphere representation for L1, whereas late acquisition of a second language caused great variability in its cortical representation (from complete right lateralization to standard left lateralization for L2). More recently, Perani et al. (1998) used PET to investigate brain activation during short story listening tasks in two groups of bilinguals with high proficiency of L2 but with different age of acquisition [High Proficiency Late Acquisition (HPLA) vs High Proficiency Early Acquisition (HPEA)]. The brain structures that were activated in both languages during the task were similar in both groups. In the HPLA group (L1 ⫽ Italian; L2 ⫽ late acquisition of English) listening to Italian stories activated foci in the left temporal lobe (temporal pole, superior temporal sulcus, middle temporal gyrus, and hippocampal structures). Listening to English stories showed a similar activation pattern in the left temporal lobe and a bilateral activation of the hippocampal structures. No areas with a significantly different activation according to the language participants were listening to were found. In the HPEA group (L1 ⫽ Spanish or Catalan; L2 ⫽ early acquisition of Catalan or Spanish) both languages activated bilateral foci in the temporal poles, hippocampal structures, and the lingual gyrus, and in the left hemisphere the superior temporal sulcus, the inferior parietal lobule, the lingual/cuneus region and areas of the cerebellar vermis were activated. The direct comparison of L1 vs L2 showed significantly different activations only in the right hemisphere (in the middle temporal gyrus for L1 and in the hippocampal structures and superior parietal lobe for L2). These results led the authors to conclude that representation of languages in the bilingual brain seems to be dependent on proficiency in the two languages and independent of the acquisition age. As has already been said, interpreting results of functional anatomical studies with bilinguals performing short story listening tasks is very difficult. In fact, knowledge of the different linguistic and pragmatic levels involved in a story processing task is still scant. Therefore, the linguistic and pragmatic complexity of the task is most probably responsible for the complexity and the variety of the brain activation patterns shown by these studies. The choice of linguistic stimuli that are so complex and, at the same time, scarcely known, makes it difficult to study language representation in the brain of bilinguals.
CONCLUSIONS
One of the first methods used to study representation of languages in the brain was to investigate bilingual aphasia. Some authors hypothesized that differential recovery indicated a different cerebral representation for the two languages (cf. Albert & Obler, 1978). Recent studies on bilingual aphasics showed that differential recovery seems
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to depend not so much on the macroanatomical representation of languages but on pathophysiological factors due to the brain lesion (cf. Paradis, 1998; Fabbro, 1999). Clinical neurolinguistic investigations allowed better definition of the role of some cerebral structures in correct choice of words and sentences during verbal communication in a bilingual (Paradis, 1989). Pathological mixing (mixing of linguistic elements from various languages within a single sentence) is an aphasic symptom associated with classic word-finding difficulties (generally present in fluent aphasias) (Fabbro 1999). Pathological switching (tendency to switch from language to language in verbal production) seems to be due to pragmatic disorders of communication, which are independent of language, as rightly suggested by Michel Paradis (1989), and are generally due to prefrontal lesions (Fabbro et al., 2000). The study of translation disorders in bilingual aphasic patients allowed us to hypothesize the existence of specific neurofunctional systems subserving translation. Translation systems seem to be relatively independent of the comprehension and production systems. Further, bilinguals seem to have specific and independent channels according to the direction of translation (A→B and B→A) (cf. Paradis et al., 1982; Fabbro & Paradis, 1995b). An interesting working hypothesis made by Paradis (1994) is that the acquisition or learning modality seems to determine a different participation of procedural memory systems vs declarative memory systems. If L1 and L2 are acquired in informal contexts and both are at a high level of proficiency, their phonologic and morphosyntactic aspects are stored in procedural memory systems (cf. Kim et al., 1997; Perani et al., 1988; Chee et al., 1999). On the other hand, traditional learning of L2 after the age of 7, along with limited proficiency in production, seems to involve the declarative memory systems to a greater extent. Furthermore, according to Paradis (1998), during the first stages of second-language learning, in both children and adults, the right hemisphere tends to be more involved in verbal communication processes because beginners try to compensate with pragmatic inferences for the lack of implicit linguistic competence in L2. This might explain the greater bilateral activation of some hippocampal structures during processing of short stories in individuals with a moderate knowledge of L2 acquired in school contexts (Perani et al., 1996; Dehaene et al., 1997). Neurophysiologic and neuroimaging studies evidenced a similar cerebral representation of L1 and L2 lexicons in both early and late bilinguals (Klein et al., 1999; Chee et al., 1999; Illes et al., 1999; Hernandez et al., 2000). The representation of grammatical aspects of languages seems to be different between the two languages if L2 is acquired after the age of 7 and automatic processes and correctness are lower than those of the native language (Neville et al., 1992, 1997; Weber-Fox & Neville, 1997; Kim et al., 1997). These results are in line with a greater representation of the two lexicons in the declarative memory systems, whereas morphosyntactic aspects may be organized in different systems according to the acquisition vs learning modality. Last, a crucial aspect concerns the brain volume involved in processing of a complex task in L1 vs a language which is little known. Dehaene et al. (1997) showed that during a story listening task in L1 the proportion of activated brain volume is significantly higher than that activated during the same task in L2. Therefore, the greater the knowledge of a language, the larger the number of circuits activated during its processing. This important result is in line with studies on changes in the brain following automatic learning of motor sequences (Karni et al., 1995). In conclusion, our knowledge of the organization of the bilingual brain is certainly more extensive as compared to the first monograph published by Pitres more than 100 years ago. However, many aspects of the organization of the bilingual brain are
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