Hemisphere lateralization is influenced by bilingual status and composition of words
Highlights
► The age 6 is an important threshold to define bilingual status: early vs. late. ► Hemisphere lateralization of early bilinguals is influenced by composition of words. ► Stroop effect is stronger with words presented to the language dominated hemisphere.
Introduction
The human brain consists of two anatomically different and functionally complementary hemispheres, though the two hemispheres are similar in overall appearance. The two hemispheres complement each other for most functions, including language. Functional lateralization seems to be an ingenious strategy that developed over the time course of human evolution to make the best use of brain capacity (Delis et al., 1986, Ivry and Robertson, 1999, MacNeilage et al., 2009). For instance, the left hemisphere (LH) processes preferentially relatively local and routine/sequential behavior, while the right hemisphere (RH) processes global and holistic behavior (MacNeilage et al., 2009). Such lateralization preferences may increase processing speed by avoiding longer pathways, mainly via the corpus callosum, that would otherwise be needed to connect regions on opposite sides of the brain (Gazzaniga, 2000). Also, when two homologous areas on opposite sides of the brain perform two different functions, the brain's cognitive capacities are in a sense doubled. Rogers, Zucca, and Vallortigara (2004) discovered that normally developed (strongly lateralized) chicks could simultaneously perform a dual task: the chicks had to find grains scattered among pebbles, a LH task, while they monitored for the appearance of a model predator overhead, a RH task, but chicks that had developed abnormally by incubating their eggs in the dark (weakly lateralized) could not attend to two tasks simultaneously.
As for language, current evidence shows that, for right-handed monolinguals, the LH is generally more functionally specialized for language than the RH (Damasio et al., 1996, Gazzaniga, 1970, Hellige, 1993, Leehey and Cahn, 1979, Searleman, 1977, Soares and Grosjean, 1981, Wada et al., 1975). LH dominance of language does not mean that the RH is totally absent for language processing. The two cerebral hemispheres always communicate closely, mainly via the corpus callosum. Furthermore, the cortex exhibits great plasticity, by which the malfunction of some brain regions can be compensated by homologous regions in the opposite hemisphere. Findings from healthy subjects and patients who underwent callosotomy or who experienced unilateral brain damage all provide converging evidence of compensatory hemispheric function in language (Hickok and Poeppel, 2007, Taylor and Regard, 2003).
While LH dominance for language is widely accepted for right-handed monolinguals, the hemisphere lateralization patterns for bilinguals are much more controversial. Hull and Vaid (2007) have analyzed 66 behavioral laterality studies and found that early bilinguals, who acquired both languages no later than age 6, showed bilateral hemisphere involvement for both languages, while monolinguals and late bilinguals, who acquired their second language after age 6, showed LH dominance for the languages. Yip and Matthews (2007) provide a fine-grained longitudinal study of early bilingualism acquired in a naturalistic setting. It is possible that the age 6 is an important threshold during the ontogenetic development of the brain. At age 6, the brain has reached more than 90% of its adult volume/weight and is four times its birth size (Courchesne et al., 2000, Lenneberg, 1967). From age three to six, extensive internal neuron wiring and synapse pruning (as wiring is eliminated based on the “use it or lose it” principle) take place in the frontal lobes, the cortical regions involved in organizing actions, planning activities and focusing attention (Huttenlocher & Dabholkar, 1997). Therefore, it is possible that bilateral, rather than LH-dominant, cortical organization of language is due to the better use of brain capacity for multiple languages by early bilinguals. Moreover, the cortical organization of language can hardly be shifted from one hemisphere to the other after age 6 just because another language is being acquired, leading to late bilinguals becoming more like monolinguals with respect to their first language. This early organization takes full advantage of the period of greatest neural plasticity during early childhood, since language learning is a major task, especially learning two languages simultaneously.
As mentioned above, the two hemispheres preferentially process different kinds of information, with local and routine/sequential behavior dominant in the LH, and global and holistic behavior in the RH (Hsiao et al., 2008, Ivry and Robertson, 1999). The kind of writing system may differentially demand sequential/local vs. holistic/global processing, and thus plays an important role in hemisphere lateralization when reading words (Dehaene, 2009). Unlike alphabetic writing system such as English, where each word is a series of linearly arranged letters, and letters correspond to phonological segments, the logosyllabic writing system of Chinese does not provide any direct information on phonetic segments (Wang, 1973, Wang and Tsai, in press). Each Chinese character comprises a hierarchical arrangement of strokes, and represents both a morpheme and a syllable simultaneously. Meanwhile, many Chinese characters are complete words, which are called single-character words, such as the color words used in this study.
In fact, more than 80% of Chinese characters are semantic–phonetic compounds, e.g., ‘’ (red), which consist of two parts: a semantic component, which hints at the meaning of the character, and a phonetic component, which gives a clue to the pronunciation of the character (Lee et al., 2006, Shu et al., 2003). The phonetic component does not always predict the pronunciation of the character, e.g., the phonetic component ‘’ (ghost) of ‘’ (pagoda tree) is pronounced /gui3/, but the character itself is pronounced /huai2/ (The Mandarin pronunciations of the Chinese characters are represented in Pinyin, and the corresponding lexical tones are represented by Arabic numerals.). Such characters are called irregular characters. There is no way to derive the pronunciation of an irregular character from its phonetic component. Consequently, reading such irregular Chinese characters requires a direct mapping of the whole character to its pronunciation. This situation can be extended to other high frequency (commonly used) regular characters whose phonetic components faithfully represent their pronunciations. Zhou and Marslen-Wilson (1999) found a null decomposition effect for commonly used compound characters, suggesting that reading such characters involves little or no decomposition because the highly efficient processing of the whole characters leaves little time for their phonetic components to be processed sublexically. Converging evidence shows that the processing of commonly used Chinese characters is different from that of low frequency (less commonly used) ones, with faster responses and less demanding procedures for commonly used ones (Kuo et al., 2003). Taken together, the above findings indicate that visual processing of Chinese characters, at least of the commonly used ones, is holistic/global, which is therefore likely to be dominated by the RH.
Unlike the reading process of Chinese characters, reading alphabetic words requires performing grapheme–phoneme conversion prior to obtaining the pronunciation of the whole word (Maurer and McCandliss, 2007, Siok et al., 2004). The requirement of a letter-by-letter grapheme–phoneme conversion leads to a sequential/local scan of each letter. This sequential/local process manner is dominated by the LH. However, unlike graphic processing, language specific processing, such as phonological and semantic processing during word reading of both alphabetic and logosyllabic writings is processed preferentially by the language dominant hemisphere, which is determined by the bilingual status of the participants.
In this study, we first propose a model to predict the lateralization effects in word reading, with focus on Chinese character reading. This model considers both physical (domain general) properties of stimulus words and the bilingual status of participants. Although the model addresses early bilingual, late bilingual, and monolingual, here we test this model using a behavioral Stroop experiment only with late Chinese–English bilingual participants.
Section snippets
Model assumptions
To construct the conceptual model, Composition–Age (CA) model, as depicted in Fig. 1, we use the following two premises:
- 1.
Composition of stimulus words: Composition is a synthesizing process in which smaller constituents, e.g., phonemes, are unified into an integrated unit, e.g., the pronunciation of the whole English word. During reading, commonly used Chinese characters (including Chinese single-character words) are visually processed as a whole in order to derive their pronunciations, hence
Participants
Fourteen late Chinese–English bilingual participants (mean age = 24.5 years, SD = 2.8), who finished pre-university education in Mainland China, and studied at the Chinese University of Hong Kong when the experiments were performed, were paid to participate in this study. The participants were right-handed, and had normal or corrected-to-normal vision, normal color vision, and proficient in their second language. Informed written consent was obtained from each participant in compliance with a
Results
The color identification accuracy for both types of responses was very high, with 97% correct responses for button-press response task, and 98% correct responses for vocal response task. Only reaction time (RT) data were analyzed here. Trials in which the participant made wrong responses, or in which the RT fell outside of two standard deviations from the participant's mean were not included in the analysis of the data. Corrections of statistical results for violations of sphericity were made,
Discussion
The proposed CA model attempts to predict the lateralization of Chinese character reading. The current experimental data show a stronger Stroop effect when Stroop stimuli were presented to the RVF than to the LVF, indicating that word perception of the Stroop stimuli were dominantly processed in the LH for late Chinese–English bilingual participants. Thus, the results of this experiment show that the model accurately predicted lateralization pattern of Chinese character reading for late
Acknowledgment
The work described in this paper was partially supported by grants from the Shun Hing Institute of Advanced Engineering of the Chinese University of Hong Kong (BME-8115020), from National Science Foundation of China (NSFC: 11074267), and from the Research Grant Council of Hong Kong (CUHK DG: 2050461). We thank Paul Kay for his constructive comments on an earlier version of this paper, and Ruoxiao Yang for help with the experiments. We thank the editor and the two reviewers for their
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