Brain hemisphericity and developmental dyslexia

Research in Developmental Disabilities 34 (2013) 1536–1540 Contents lists available at SciVerse ScienceDirect Research in Developmental Disabilities Brain hemisphericity and developmental dyslexia Filippos Vlachos a,*, Eleni Andreou b, Afroditi Delliou a a b University of Thessaly, Department of Special Education, Argonafton & Filellinon, 38221 Volos, Greece University of Thessaly, Department of Primary Education, Argonafton & Filellinon, 38221 Volos, Greece A R T I C L E I N F O A B S T R A C T Article history: Received 5 November 2012 Received in revised form 29 January 2013 Accepted 29 January 2013 Available online The present study examined the link between brain hemisphericity and dyslexia in secondary school students, using the Preference Test (PT), a widely used self-report index of preferred hemisphere thinking styles. The hypothesis was that differences would be revealed between the dyslexic group and their peers in hemispheric preference. A total of 45 secondary school students who were diagnosed with dyslexia and attended regular public schools formed the learning disabled group. A comparison group was formed of pupils who attended the same classes (N = 90), and these were matched for age and sex with dyslexics (1 dyslexic: 2 control). The results revealed that significantly more dyslexic pupils displayed a preference for a right hemisphere thinking style compared to their peers who adopted a left hemisphere thinking style. This finding is in line with the suggestion of the greater right hemisphere involvement in the expression of developmental dyslexia. ß 2013 Elsevier Ltd. All rights reserved. Keywords: Brain hemishericity Dyslexia Preference Test 1. Introduction Although the human brain acts as an integrated whole, each of the two cerebral hemispheres appears to be specialized for qualitatively different types of cognitive processing (Waldie & Mosley, 2000). The idea that people differ in the extent to which they make use of each hemisphere’s cognitive capacity has been termed hemisphericity (Bogen, 1969) or hemisphere preference (Zenhausern, 1978) and has generated a vast number of studies. Several cognitive neuroscientists have maintained that the left hemisphere operates in a linear, sequential manner with logical, analytical and propositional thought (Iaccino, 1993; Springer & Deutsch, 1993). On the other hand, the right hemisphere operates in a nonlinear, simultaneous fashion and deals with non-verbal information as well as dreams, fantasy and creative thinking (Iaccino, 1993; McCarthy, 1996; Mihov, Denzler, & Förster, 2010; Oxford, 1996; Springer & Deutsch, 1993). The left hemisphere appears to be specialized for language, whereas the right hemisphere is specialized for visuospatial and appositional thought (Gazzaniga, 2000). Oxford (1996) maintained that left hemispheric dominants are highly analytic, verbal, linear and logical learners, whereas right-hemispheric dominants are highly global, visual, relational, and intuitive learners. Whole-brain dominants are those who process information through both hemispheres equally and exhibit characteristics of both hemispheres. Those individuals have flexible use of both hemispheres (McCarthy, 1996). Although, research in the past decade had made it increasingly clear that brain functional asymmetries cannot be reduced to simple and absolute dichotomies (Brancucci, Lucci, Mazzatenta, & Tommasi, 2009; Fox et al., 2005), there is some indication that individuals differ in their relative performance on tasks that are known to be associated with left- versus right-hemisphere injury (Tivarus, Starling, Newport, & Langfitt, 2012). Nevertheless, there is also evidence of individual * Corresponding author. Tel.: +30 241074739; fax: +30 241074825. E-mail address: fvlachos@uth.gr (F. Vlachos). 0891-4222/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ridd.2013.01.027 F. Vlachos et al. / Research in Developmental Disabilities 34 (2013) 1536–1540 1537 variation in characteristic arousal asymmetry and in an individual’s propensity to use a mode processing associated with one hemisphere or the other when a choice is given (Hellige, 1993). Developmental dyslexia is the most common of the learning disabilities, defined by difficulties with accurate and/or fluent word reading and spelling (Fletcher, 2009; Vlachos, 2010). The dyslexic population is presumed to have atypical hemispheric specialization. At an anatomic level, dyslexic brains are structurally atypical. Under the microscope, one sees an increased number of ectopic neurons and other minor anomalies, scattered across the cortex but maximal in the left hemisphere (Galaburda & Kemper, 1979). Unusual patterns of hemispheric asymmetry that reflect differences in the relative size of certain brain regions, as well as anomalous gyral and sulcal patterns, are also noted in dyslexics (Brown et al., 2001; Leonard et al., 2001). Most exciting are the functional neuroimaging studies that reveal the distinctive pattern of activation that emerges when a person with dyslexia is reading. Compared with the normal reader, there is a decreased activity in the posterior left hemisphere and a lack of synchrony between the posterior and anterior areas in the left hemisphere, increased activity in the homologous areas of the right hemisphere, and a relative increase of activity in the frontal cortex (Shaywitz et al., 2002). For all the above reasons, Galaburda (1993) suggests that the processing patterns of dyslexic people in the left and right hemispheres show differences compared with non-dyslexics. The implication of this is that dyslexic children and adults could have right hemisphere skills that place them at a disadvantage in left hemisphere tasks, such as reading accuracy. The present study aimed to examine the link between brain hemisphericity and dyslexia in secondary school students, using the Preference Test (PT). The PT is a 20-item instrument created by Zenhausern (1978) that claims to provide an index of hemisphericity. Although studies on hemisphere preference have been criticized on the ground that they employ indirect measures to index hemisphere reliance, research has found the PT to have acceptable psychometric properties (Merckelbach, Muris, Pool, DeJong, & Schouten, 1996) and correlate with biophysical measures of hemisphericity (Merckelbach, Muris, Horselenberg, & de Jong, 1997). Additionally, PT scores have been shown to be associated with reading disability. More specifically, Oexle and Zenhausern (1981) as well as Golden and Zenhausern (1983) found that 85% of reading disabled children rate themselves as using more right than left hemisphere strategies. Based on the aforementioned studies and given that the PT consists of items that intend to tap left-hemisphere and right-hemisphere cognitions, our hypothesis was that differences would be revealed between the dyslexic group and their peers in hemispheric preference. 2. Method 2.1. Participants A total of 135 secondary school students (102 boys and 33 girls, age range 13–18 years, M = 15.07 years, SD = 1.47 years) participated in this study. The dyslexic students (N = 45, 34 males and 11 females; age range 13–18 years, M = 15.14 years, SD = 1.46 years) had a statement of dyslexia after assessment at the Centre of Diagnosis, Assessment and Support of Magnesia, Greece. This centre belongs to the Ministry of Education and is listed amongst the formal assessment centres for specific learning difficulties. The assessment was carried out by a psychologist and a special educator and the criteria used included: (a) assessment of intelligence using the standardized Greek version of Wechsler Intelligence Scale for Children – Revised (WISCIII-R; 3rd Edition), (b) assessment of cognitive skills [i.e. visual discrimination, visual and auditory short-term memory, spatial orientation, using the Benton visual form discrimination test (Benton, Sivan, Hamsher, Varney, & Spreen, 1994), the Rey-Osterrieth complex figure test immediate recall (Osterrieth, 1944; Rey, 1941) the Rey’s auditory learning test (Rey, 1964) and the Guilford–Zimmerman spatial orientation test (Guilford & Zimmerman, 1948) respectively], and (c) assessment of oral reading accuracy, reading rate, reading comprehension, listening comprehension, dictation and free writing using informal reading inventories. Students with dyslexia had a consistent history of persistent specific literacy difficulties, with reading levels at least 18 months behind chronological age, but with a performance Intelligent Quotient above 80 on the standardized Greek version of WISCIII-R. None of the dyslexic participants had comorbid disorders. A comparison group (N = 90; 68 males and 22 females; age range 13–18 years, M = 15.05 years, SD = 1.49 years) was formed by pupils who attended the same classes with dyslexics. They had not been matched for IQ with students with dyslexia; instead they presented typical academic performance according to their teachers’ ratings. Additionally, they did not have a history of major medical illness, psychiatric illness, developmental disorder, or significant visual or auditory impairments according to the medical reports of their schools. The participants of the control group were matched for age and gender with dyslexics (1 dyslexic: 2 control). All children participated in the study were native speakers attending mainstream public schools, while immigrant pupils were not included in the sample. 2.2. Materials and procedure Subjects were run individually and completed the PT (Zenhausern, 1978). The PT is a self-report questionnaire that comprises 20 items, with 10 items presumably referring to a left-hemisphere mode of thinking (e.g. ‘‘I find it easy to think of synonyms for words’’) and 10 items presumably referring to a right-hemisphere mode of thinking (e.g., ‘‘I have a good sense of direction’’). Subjects use 10-point scales to indicate the extent to which the items apply to them (ranging from 1 = ‘‘not at all’’/ ‘‘never’’ to 10 = ‘‘very much’’/‘‘always’’). To obtain an index of hemisphere preference (total PT), the sum of right brain-oriented answer scores was subtracted from that of left brain oriented answers to produce a hemisphericity index ( 100 to +100). Thus, a 1538 F. Vlachos et al. / Research in Developmental Disabilities 34 (2013) 1536–1540 positive difference score is taken as an indication of a stronger preference for left-hemisphere cognitions (i.e., an analytic, verbal approach), whereas a negative difference score is interpreted as a stronger reliance on right-hemisphere cognitions (i.e., a holistic, non-verbal approach). A score of 0 indicated individuals with no clear preference to the left or the right hemisphere mode processing (whole brain dominance). PT has acceptable psychometric properties. More specifically, Merckelbach et al. (1996) found the PT to have sufficient test–retest stability and internal consistency, acceptable variability and a two factor structure consistent with a left and right preference model. Additionally, Merckelbach et al. (1997) found that subjects who show a right hemispheric preference on the PT display greater a power over the left midfrontal area during resting electroectroencephalogram (EEG). Because higher a levels are associated with lower levels of activation, this asymmetry is interpreted as indicative of greater right hemispheric activity (Hellige, 1993). In a subsequent study (Russo, Persegani, Torlini, Papeschi, & Trimarchi, 2001) researchers confirmed the correlation between PT scores and midfrontal EEG asymmetries. 2.3. Statistical analysis All data screening, processing and analysis procedures were performed using SPSS 19. One-way analysis of variance (ANOVA) was used to compare the PT scores between groups. Cross tabulation was carried out to investigate the significance of the distribution of hemispheric preference categories within the two groups of participants. 3. Results PT scores were calculated for each participant in the way described above. Mean total (i.e., L-R) PT scores were 5.02 (SD = 19.09) and 6.94 (SD = 16.23), for the dyslexic and the control groups respectively. This difference was statistically significant (F1, 143 = 2.74, p < 0.01, h2 = 0.096, power = 0.951), indicating that the two groups present significant differences on their hemispheric preference mode. On the basis of the PT scores, subjects were assigned to a left hemisphere preference group, a right-hemisphere preference group and a whole-brain dominance group. The distribution of hemispheric preference in each group is detailed in Table 1. As it can be seen on this Table, the three brain dominance categories were distributed in the following way: in the control group there were 29 or 32.2% right-brain dominants, 59 or 65.6% left-brain dominants, and 2 or 2.2% whole-brain dominants. Between dyslexics there were 29 or 64.4% right-brain dominants and 16 or 35.6% left-brain dominants. The cross-tabulation of hemispheric preference categories with groups had a statistically significant relationship (x2 = 0.32, df = 1, p < 0.05), indicating that the differences on the frequencies between the two groups of participants were significant. As it is shown in Table 1 the incidence of the preference for a right hemisphere thinking style (as indexed by PT) was elevated among dyslexic pupils. On the other hand, there was a clear over-representation of a left hemisphere preference among the participants of the control group. 4. Discussion The present study examined the link between brain hemisphericity and dyslexia in secondary school students, using the PT, a reliable and valid measure of hemisphere preference. The hypothesis was that differences would be revealed in hemispheric preference between the dyslexic group and their peers. The results revealed that significantly more dyslexic pupils displayed a preference for a right hemisphere thinking style (as indexed by PT) compared to their peers who adopted a left hemisphere thinking style, confirming our hypothesis. Our findings support previous studies (Golden & Zenhausern, 1983; Oexle & Zenhausern, 1981), which found that reading efficiency is associated with PT scores, with reading disabled children having a negative PT difference score (i.e. a right hemisphere thinking style). The results of this study are also in line with what one would expect on the basis of lateralization research, which shows that the right hemisphere is specialized for nonverbal, holistic, and emotional processing (Springer & Deutsch, 1993). That is to say, subjects with a right hemisphere preference (in terms of PT scores) could demonstrate reading problems more frequently than subjects with a left hemisphere preference. Additionally, our results confirm the suggestion of neuroimaging studies for a greater right hemisphere involvement in the expression of developmental dyslexia. More specifically, Shaywitz et al. (2002) using four reading-related tasks with a Table 1 Students’ profile concerning hemispheric preference in each group. Hemispheric preference Group Dyslexic Control Right hemisphere Left hemisphere Whole brain F % F % 29 59 2 32.2 65.6 2.2 29 16 0 64.4 35.6 0.0 F. Vlachos et al. / Research in Developmental Disabilities 34 (2013) 1536–1540 1539 large series of dyslexic children found consistently lower activation in left hemisphere sites in functional Magnetic Resonance Imaging (fMRI) measures. In the older dyslexic children, however, there was also activation of right frontal sites when involved in more difficult tasks, and researchers suggested that this reflected compensatory mechanisms. Additionally, during a pseudo-word rhyming task Simos, Breier, Fletcher, Bergman, and Papanicolaou (2000) showed greater right hemisphere activation during word reading in 12–17 year-old dyslexics. A shift to left hemisphere activation was observed in dyslexic children following remediation (Simos et al., 2000). The aforementioned studies suggest that the typical left-hemisphere dominance for reading tasks is not present in dyslexia, although it can emerge with training. It must be stressed however, that the 1/3 of dyslexic students in our study were left-brain dominants. This finding is compatible with the suggestions of Stein (1991) that altered physiological mechanisms in both cerebral hemispheres prevent successful reading. More specifically, he maintained that impaired processing operations would implicate both left and right hemispheres; the left-sided abnormalities giving rise to phonological coding deficiencies; and the right-sided abnormalities giving rise to visuospatial deficiencies, which are no less detrimental to reading progress. Moreover, the findings of a more recent study (Lavidor, Johnston, & Snowling, 2006) support that both cerebral hemispheres contain phonological, orthographic and semantic representation of words, suggesting that cerebral hemispheres are not functionally equal and that may explain the differences between people with dyslexia, the cause of various dyslexic subtypes, and the reason that all dyslexics do not respond successful in educational interventions. Furthermore, the 1/3 of typical readers in our study were right-brain dominants. This finding could be explained on the basis of studies which suggest that the right hemisphere may participate in reading in normal adults. For example, Waldie and Mosley (2000) examined hemispheric specialization for reading in right- and left-handed adults as reflected by their performance on tasks requiring the processing of visually presented single nouns and nonwords and the processing of narrative material. They found out that although the left hemisphere was relatively more efficient, the right hemisphere was dynamically involved in the reading process. Additionally, a very recent fMRI study of 18 healthy subjects (Van EttingerVeenstra, Ragnehed, McAllister, Lundberg, & Engström, 2012) suggests that the right hemisphere may participate in reading in neurologically intact adults who reach high proficiency. It must be stressed, however, that although the two groups of participants presented significant differences on their hemispheric preference mode, the fact that we could not discriminate subtypes of dyslexia in our sample constitutes a limitation of our study. The official Centres that carry through the diagnosis of dyslexia in Greece do not discriminate dyslexics in various subtypes, and therefore relevant information was not available. Future research is needed in order to shed light on the association between brain hemisphericity and developmental dyslexia. More specifically, in order to avoid the inconclusive findings future studies have to count on larger samples of participants, taking into account the various subtypes of dyslexia. Moreover, additional variables such as gender must be at the same time considered, because hemisphericity style has been reported to interact with gender of the subjects on performance of various perceptual and motor tasks (e.g., Oxford, 1996; Roig, 1990). Additionally, noninvasive and relatively inexpensive techniques such as functional transcranial Doppler ultrasonography (fTCD), that have been shown to be a reliable method for determining cerebral lateralization of function (Deppe, Ringelstein, & Knecht, 2004), could be used in future studies to examine the association between brain hemisphericity and dyslexia explicitly. In sum, our data offer some insight concerning the association between dyslexia and brain hemispericity indicating that most dyslexic adolescents present a different type of hemispheric preference (either cause or consequence of dyslexia), which is reflected by reduced left-hemisphere involvement during reading. The results of this study could have interesting educational implications, by providing information to educators concerning the preferred hemisphere style of their students. Some research has found that hemispheric cognitive style can explain some of the variance in academic performance not accounted for by differences in ability (Bracken, Ledford, & Mccallum, 1979; Zhang, 2002). In this way matching instruction to hemispheric style may improve the effectiveness of instruction (Ford & Chen, 2001; Thimor & Fidelman, 1995). Given that more dyslexic pupils displayed a preference for a right hemisphere thinking style, as indexed by PT scores in this study, educators should be encouraged to use materials, procedures and strategies that give an impulse to this mode of thinking (e.g., pictures, diagrams, charts, colour-coding, guided imagery, etc.), in order to improve their students’ academic performance (Gregory, 2005). Recent evidence seems to indicate that dyslexic children use more often multimodal approaches and possibly that is the reason they usually benefit from multi-sensory methods (Rose, 2009). The effectiveness of these methods in dyslexic population have been proved by neurofunctional studies (Shaywitz et al., 2004; Simos et al., 2002), which found increased activation in left hemisphere regions of the dyslexic brain after intensive multisensory intervention programmes.There is of course much to be carried out both in terms of developing an accepted learning abilities profiler, identifying common types of profile, adapting good practice in intervention to the different profiles, and then collecting the necessary evidence to inform educational developments. Additionally, school teachers have to make learning meaningful, taking into account the interests, and needs of children, to provide a rich and varied environment, to integrate reading into other activities, to show that it is an essential, everyday skill with practical value, to use assessment data to determine the current strengths and needs of children, to continually adapt their teaching strategies to match a child’s growth, to pay attention to the needs of children who are at risk of reading failure, and to seek timely intervention and support. Given that other evidence suggests that individuals with dyslexia may have a learning style that is based more on creative, spatial thinking (West, 1997), the analyses based on the development of a screening battery designed to probe the different forms of learning, (e.g. creating for each child a profile of learning abilities 1540 F. Vlachos et al. / Research in Developmental Disabilities 34 (2013) 1536–1540 and disabilities) would be of value for designing intervention approaches based on either alleviating the specific learning disabilities, or around existing strengths rather than existing weaknesses. The view that cognitive style literature needs to be grounded in terms of the underlying brain mechanisms (Nicolson, Fawcett, Brookes, & Needle, 2010) would be beneficial to the improvement of learning, attitudes, behaviour and motivation. Therefore, further research is needed in order to elucidate the role that hemispheric preferences play in the acquisition of knowledge and subsequently increase the efficiency of learning styles in the dyslexic individuals’ learning process. References Benton, A., Sivan, A., Hamsher, K., Varney, N., & Spreen, O. (1994). Contributions to neuropsychological assessment: A clinical manual (2nd ed.). New York: Oxford University Press. Bogen, J. (1969). 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