focus on a comparison of activated areas when using the first language (l1) and second language (l2). studies of language production which employ functional neuroimaging methods, investigate the cerebral representation of language activity in bilinguals. these methods (i.e. pet and fmri) separate subjects mainly on basis of age of l2 acquisition and not on proficiency level in l2. with the use of pet in the study of late learners, regional cerebral blood flow (rcbf) distribution has been found to be comparable between l1 and l2. repetition of words engages overlapping neural structures across both languages; whereas, differences in neural activation are only observed in the left putamen when individuals repeat words in their second language. the putamen, therefore, plays a critical role because the articulation process places greater demand on brain resources, when one is producing a second language learned late in life. word generation tasks including rhyme generation (phonological bases), synonym generation (semantic search bases), and translation (lexical access to other language) are used to observe lexical-semantics. word generation has been shown to cause significant activation in the left dorsolateral frontal cortex (brodmann areas 9, 45, 46, 47). considerable overlie has been found in the frontal areas, regardless of task requirements (rhymes or synonyms) and language used (l1 or l2). selective activation is observed in the left putamen when words are generated in the second language (i.e. increased rcbf in left putamen resulting from l2-l1 subtractions). even when the second language is acquired later in life (up to age five), l2 production in highly proficient bilinguals reveals activation of similar brain regions as that in l1. word generation (phonemic verbal fluency) has also led to larger foci of brain activation for the least fluent language(s) within multilinguals (observed using fmri). regardless of language, however, activation is principally found in the left prefrontal cortex (inferior frontal, middle frontal, and precentral gyri). additionally, activation can be observed in the supplementary motor area and parietal lobe. this activation is larger for l3 than l2 and l1, and less for l1 than for l2. familiarity with a language reduces the brain activation required for its use. age of second language acquisition language acquisition appears to play a large role in the cortical organization involved in second language processing. using functional magnetic resonance imaging (fmri), representations of l1 and l2 have been found in spatially isolated parts of the left inferior frontal cortex of late learners (broca's area). for early learners, similar parts of broca's area are activated for both language—whereas later learners have shown to use different parts of broca's area. in contrast, there is overlap in active regions of l1 and l2 within wernicke's area, regardless of age of l2 acquisition. effects of language proficiency on l2 cortical representation conversely, it has also been reported that there is at times, no difference within the left prefrontal cortex when comparing word generation in early bilinguals and late bilinguals. it has been reported that these findings may conflict with those stated above because of different levels of proficiency in each language. that is, an individual who resides in a bilingual society is more likely to be highly proficient in both languages, as opposed to a bilingual individual who lives in a dominantly monolingual community. thus, language proficiency is another factor affecting the neuronal organization of language processing in bilinguals. with the use of positron emission tomography (pet), research has shown that brain regions active during translation are outside classical language areas. translating from l1 to l2 and vice versa activates the anterior cingulate and bilateral subcortical structures (i.e. putamen and head of caudate nucleus). this pattern is explained in terms of the need for greater coordination of mental operations. more specifically, automated circuits are favoured over cerebral pathways for naming words. language switching is another task in which brain activation is high in broca's area and the supramarginal gyrus. this was originally observed by poetzl, (1925, 1930) and leischner, (1943)—all of whom reported that patients with supramarginal lesions were defective in switching languages. areas of the brain associated with phonological working memory have been shown to have greater activation in bilinguals proficient in both languages using fmri. equally proficient bilinguals use working memory more than bilinguals who have unequal proficiency. this suggests that optimal use of phonological working memory, specifically the left insula and left inferior frontal gyrus, is associated with higher second language acquisition. linguistic fluency most studies involving neuroimaging investigations of language production in bilinguals employ tasks that require single word processing—predominantly in the form of word generation (fluency) tasks. fluency tasks show substantial activation of the left dorsolateral frontal cortex. phonemic verbal fluency (initial letter fluency) activates the left inferior frontal gyrus, and the posterior frontal operculum (ba 44). semantic fluency, however, engages discrete activation of anterior frontal regions (brodmann areas 45 and 46). functional neuroimaging research has shown that very early bilinguals display no difference in brain activation for l1 and l2—which is assumed to be due to high proficiency in both languages. additionally, in highly proficient late bilinguals, there is a common neural network that plays an important role in language production tasks; whereas, in late bilinguals, spatially separated regions are activated in broca's area for l1 and l2. finally, it has been found that larger cerebral activation is measured when a language is spoken less fluently than when languages are spoken fluently. overall, in bilinguals/polyglots, achieved proficiency, and possibly language exposure, are more crucial than age of acquisition in the cerebral representation of languages. however, since age of acquisition has a strong effect on the likelihood of achieving high fluency, these variables are strongly intertwined. language comprehension in bilinguals research generally supports the belief that language comprehension in the bilingual brain is malleable. listening to stories in l1 and l2 results in largely dissimilar patterns of neural activity in low proficiency bilinguals—regardless of age of acquisition. some researchers propose that the amount to which one masters l2 is accountable for the measured differences between groups of early and late learners. specifically, in terms of auditory language comprehension for proficient bilinguals who have acquired l2 after ten years of age (late learners), the activated neural areas are similar for both languages. however, as already noted, there are fewer individuals becoming highly proficient at later ages of acquisition. language comprehension research on bilinguals used fmri techniques. groups of two orthographically and phonologically outlying languages (english and mandarin) were the basis of analysis. sentence comprehension was measured through visually presented stimuli, showing significant activation in several key areas: the left inferior and middle frontal gyri, the left superior and middle temporal gyri, the left temporal pole, the anterior supplementary motor area, and bilateral representation of the superior parietal regions and occipital regions. also, brain activation of these two orthographically and phonologically outlying languages showed striking overlap (i.e. the direct contrast did not indicate significant differences). single word comprehension using l1 generated greater activation in the temporal pole than comprehension of words in l2. language comprehension studies of bilinguals using neuroimaging give more conclusive results than production studies. general findings functional neuroimaging methods such as pet and fmri are used to study the complex neural mechanisms of the human language systems. functional neuroimaging is used to determine the most important principles of cerebral language organization in bilingual persons. based on the evidence we can conclude that the bilingual brain is not the addition of two monolingual language systems, but operates as a complex neural network that can differ across individuals. the bilingual language system is affected by specific factors of which proficiency appears to be the most important. evidence, mentioned previously, has shown that differential cerebral activation in anterior brain structures (e.g. ba and the basal ganglia) is related to poor performance on word generation and production. with regards to language comprehension, differences in levels of language proficiency engage the temporal lobes (particularly the temporal pole). where in the least proficient language, more cerebral activation is related to speech production, less activation is related to comprehending the least proficient language. age of acquisition is not as important in comprehension activities as it is in production activities. however, that is not to say that age of acquisition is not a major factor in the proficiency of l2. in fact studies have determined late learners to be less proficient in l2 than early learners. functional imaging methods have revealed that holding proficiency constant leads to age of acquisition not having a large influence on representation of l2 in the brain, but there are fewer individuals achieving high proficiency at later ages of acquisition. structural plasticity second language proficiency and age at acquisition affect grey matter density in the brain. the human ability to learn multiple languages is a skill thought to be mediated by functional (rather than structural) plastic changes in the brain. learning a second language is said to increase grey matter density in the left inferior parietal cortex, and the amount of structural reorganization in this region is modulated by the proficiency attained and the age at acquisition. it has been suggested that this relation between grey matter density and performance denotes a general principle of brain organization. there is an increase in grey matter density in the left inferior parietal cortex of bilinguals compared to that in monolinguals. grey matter density is more prominent in early bilinguals than it is in late bilinguals. evidence has also shown that density in this region increases with second language proficiency and is negatively correlated with age of acquisition. it has also been shown that bilingualism affects the white matter of the brain, expressed as increased myelination of a series of white matter tracts, including the corpus callosum, in sequential adult bilinguals that are active users of their second language. it is thought that these effects are due to the cognitively demanding skill of handling more than one languages, which requires more efficient connectivity between areas in the grey matter of the brain. similar effects have been found in lifelong elderly bilinguals and simultaneous bilingual children. it is debated whether the above-mentioned effects are the result of a genetic predisposition to increased density, rather than experience-related structural reorganization. a second language is likely acquired through social experience, in early bilinguals, rather than through genetic predisposition. thus, the research suggests that the structure of the human brain is reworked by the experience of acquiring a second language. this theory is also consistent with growing evidence that the human brain changes structurally due to environmental demands. for instance, it has been established that structure is altered as a consequence of learning in domains independent of language. as to structural plasticity induced by bilingualism, it has recently been shown that bilinguals, as compared to monolinguals, have increased grey matter density in the anterior cingulate cortex (acc). the acc is a brain structure that helps subjects to monitor their actions and it is part of the attentional and executive control system. bilinguals have increased grey matter in this brain area because they continuously monitor their languages in order to avoid unwanted language interferences from the language not in use. the continuous use of the acc in turn induces plastic neural effects. this may be the same reason why bilinguals are faster than monolinguals on many attentional control tasks. bilingual aphasia bilingual aphasia is a specific form of aphasia which affects one or more languages of a bilingual (or multilingual) individual. as of 2001, 45,000 new cases of bilingual aphasia are predicted annually in the united states. the main factors influencing the outcomes of bilingual aphasia are the number of languages spoken and the order in which they are learned—both influenced by the pattern of daily use and expertise in each language before the onset of aphasia. the type and severity of the aphasia, the location and size of the lesions, as well as the patient's levels of education and literacy also influence the functional outcomes of bilingual aphasia. lateralization previously, research has revolved around the hypothesis that language in bilingual individuals is more symmetrically represented in the brain, where the symmetrical representation in the cerebral hemispheres can be attributed to differential localization of the languages. thus, if one of the languages is heavily represented in the right hemisphere, it can then be partially represented in a different