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Epilepsy & Behavior

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Basic reading skills and dyslexia: Three decades following right versus lefthemispherectomy for childhood-onset intractable epilepsy

Jacqueline Cummine a,*, Ron Borowsky a, Fern Stockdale Winder b, Margaret Crossley a

a Department of Psychology, University of Saskatchewan, Saskatoon, SK, Canadab Clinical Health Psychology, Saskatoon Health Region, Saskatoon, SK, Canada

a r t i c l e i n f o

Article history:Received 18 March 2009Revised 19 May 2009Accepted 31 May 2009Available online xxxx

Keywords:EpilepsyHemispherectomyReadingWhole wordSubwordLanguage functioningSocial cognitionDyslexia

1525-5050/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.yebeh.2009.05.018

* Corresponding author. Address: Department oSaskatchewan, 9 Campus Drive, Saskatoon, SK, Canad6630.

E-mail address: jacqueline.cummine@usask.ca (J. C

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a b s t r a c t

Dyslexia was explored within the framework of three explanations for language functioning follow-ing hemispherectomy (i.e., equipotentiality, hemispheric specialization, and crowding hypothesis/hierarchy of specialized functions) and the extent to which these models explain reading perfor-mance in S.M. (age 48, right hemispherectomy) and J.H. (age 49, left hemispherectomy). Basic read-ing performance was evaluated by assessing whole-word and subword reading. Both participantsdisplayed severely impaired reading performance on pseudohomophones (e.g., WUN), signifying poorsubword reading. However, J.H. (remaining right hemisphere) also demonstrated impairments inreading exception words (e.g., ONE), suggestive of poor whole-word reading. Thus, although S.M.clearly demonstrated phonological dyslexia and retention of the priority whole-word reading skills,J.H. presented with deficits more characteristic of mixed dyslexia. Taken as a whole, we suggest thatsome modification of the hierarchy of specialized functioning model and crowding hypothesis isneeded, including stipulations about hemispheric specialization, to more accurately accommodatethe present data.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

The study of individuals following hemispherectomy presents aunique opportunity to evaluate the stability of hemispheric spe-cialization of cognitive abilities and the potential of an isolatedhemisphere to subsume typically lateralized higher brain func-tions. Previous research characteristically has examined basic lan-guage functioning following hemispherectomy [1–8], whereasrecent work has focused on complex social behavior [2,9–11]. Lan-guage and social functioning are intricately related [2], and bothfactors are associated with post-hemispherectomy performanceon particular cognitive tasks (e.g., simple reading, identificationof emotions). For example, Fournier et al. [9] explored complex so-cial functioning in J.H. and S.M., 30 years following their hemi-spherectomies. Important to the present article, they reporteddifferences between the participants in general language perfor-mance. J.H., the participant with a left-sided hemispherectomyand an intact right hemisphere, showed deficits in verbal fluencyand agrammatical sentence construction, but intact affective andlinguistic prosody (i.e., appropriately expressive speech). In con-

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ummine).

al. Basic reading skills and dysl(2009), doi:10.1016/j.yebeh.20

trast, S.M., the participant with a right-sided hemispherectomyand an intact left hemisphere, was fully fluent during conversa-tional speech and used grammatically correct sentence construc-tions, but demonstrated impaired affective prosody (i.e., tendingto use flat and nonexpressive speech). Nevertheless, S.M. didexhibit limitations in complex language-based tasks (e.g., confron-tational naming, speeded phonemic fluency, reading comprehen-sion), suggesting that some of the functions typically associatedwith an intact left hemisphere might also have been compromisedor ‘‘crowded out” following right-sided hemispherectomy. Specifi-cally, both S.M. and J.H. demonstrated impairments in basic read-ing skills suggestive of dyslexia. Consequently, the current studyextends previous work with J.H. and S.M. by focusing on these ba-sic reading processes: skills presumed to contribute to overall cog-nitive and academic performance and to quality-of-life outcomesfollowing hemispherectomy [2].

In assessing individuals who have suffered from early acquiredbrain injury or developmental abnormality, and are consequentlyat a higher risk of developing reading impairments, understandingthe underlying reading processes is crucial to the assessment, diag-nosis, and management of these deficits [12,13]. In the comprehen-sion of written language, the most basic component is that ofdecoding the groups of letters that make up words. The current liter-ature regarding basic reading processes suggests that two systemscontribute to our ability to identify words: the whole-word and sub-word reading systems [14–17]. The whole-word reading system is

exia: Three decades following right versus left hemispherectomy for child-09.05.018

Table 1Predictions regarding language function following hemispherectomy according to thehemispheric specialization model, equipotentiality model, and hierarchy of special-ized function/crowding hypothesis.

Side ofsurgery

Whole-wordreading

Subwordreading

a. Hemispheric specialization model Left � �Right U U

b. Equipotentiality model Left U U

Right U U

c. Hierarchy of specialized function/crowding hypothesis

Left U �Right U �

U, Participant should retain function; �, participant should not retain function.

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involved in the processing of words via mapping of whole-wordorthographic representations onto phonological representations.Thus, accuracy in reading letter strings for which the spelling-to-sound correspondence is atypical, such as exception words (e.g.,ONE, given that words with a preceding consonant such as TONEare pronounced with a long vowel), can be considered a measureof whole-word reading. In contrast, the subword reading system isinvolved in the processing of words via grapheme–phoneme corre-spondences. Letter strings are broken down into graphemes andare ‘‘sounded out” based on existing phonological knowledge. In thiscase, accuracy in reading pseudohomophones, letter strings withunfamiliar orthographic representations but familiar phonologicallexical representation (e.g., WUN), is considered a measure of sub-word reading [16]. Although the role of the subword system is totranslate orthography into phonology using grapheme–phonemetranslation rules, a crucial step in reading via the subword systemis to ‘‘check” the derived pronunciation against the phonological lex-ical system prior to speech (e.g., consider the situation when one isdecoding a word he or she has heard but never seen before, as isthe case during reading development) [14,16]. Finally, regular words(e.g., WON) can be read correctly by either or both routes given thatthey would have reliable whole-word and subword representations.Thus, accuracy in reading these stimuli is a reflection of contribu-tions from both systems.

Dysfunction in basic reading processing is typically referred toas a form of dyslexia. The present article focuses on three majortypes of dyslexia: (1) phonological dyslexia, (2) surface dyslexia,and (3) mixed dyslexia. Phonological dyslexia is an impairment inreading aloud unfamiliar words (i.e., an impairment of the sub-word reading system). This is often assessed using a naming taskthat presents participants with novel letter strings (i.e., pseudoho-mophones in the present cases). People with pure phonologicaldyslexia often display little or no difficulty in reading aloud regularwords or exception words, presumably because the whole-wordreading system is intact. Surface dyslexia is an impairment in read-ing aloud exception words (i.e., an impairment of the whole-wordsystem) and the production of regularization errors (e.g., ONE pro-nounced as OWN). However, there is no difficulty in reading aloudregular words or novel letter strings in cases of pure surface dys-lexia, presumably because the subword reading system is intact.Finally, mixed dyslexia involves characteristics of surface dyslexiaand phonological dyslexia subtypes, whereby individuals demon-strate problems with both whole-word and subword reading skills[15].

Within the hemispherectomy literature there are mixed find-ings regarding the extent of language ability and disability in pa-tients who have had right or left hemispherectomy [5,8]. Inkeeping with cognitive neuropsychological models of normal lan-guage functioning, the theory of hemispheric specialization pro-poses that in most right-handed individuals, the left hemispherewill have some specialization in language functions such as read-ing and spelling [5,7,18]. This theory predicts that the right hemi-sphere (in the case of left hemispheric pathology) is limited in itspotential to develop or subsume most language skills (e.g., read-ing). Thus, it would be predicted that an individual who had under-gone a left hemispherectomy would have severe deficits on basicreading performance and typically would present with dyslexia.In contrast, an individual who had undergone a right hemispherec-tomy should have limited deficits in basic reading and performwithin the normal range. Accordingly, the theory of hemisphericspecialization would predict drastic differences in reading perfor-mance between S.M. and J.H. (see Table 1a).

In contrast to the hemispheric specialization model, cognitiveneuropsychological testing involving individuals with acquiredbrain injury often supports an equipotentiality model of lan-guage development. According to this model, at birth the two

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hemispheres are equipotential for most aspects of languageand other higher brain functions [6,19]. The equipotentialitymodel predicts that the noninjured right hemisphere is capableof subsuming most, if not all, language functions that mightotherwise have been managed by the injured left hemisphere,provided that the injury has not occurred too late in develop-ment. In particular, while the left hemisphere is presumed tobe dominant for language processing in individuals with twofunctional hemispheres, the right hemisphere is able to performthese functions in the absence of a working left hemisphere[3,20]. Importantly, the equipotentiality model would be sup-ported by a completely different pattern of results than thehemispheric specialization model. More specifically, such a mod-el would predict normal reading performance by individuals whohad undergone an early hemispherectomy, regardless of surgicalside (see Table 1b).

A crucial refinement to both the equipotentiality and the hemi-spheric specialization models is Ogden’s theory of a hierarchy of pre-served functions [21,22]. Ogden proposed that if left hemisphereinjury occurs in infancy, and there is a recovery period spanning sev-eral years, the right hemisphere can subsume language functions(see also [3,23]), in addition to maintaining the basic visuospatialabilities typically associated with the right hemisphere. An impor-tant caveat is that functions presumed to be low in the hierarchy,or nonessential, will be less well preserved [21]. The hierarchy ofpreserved function model predicts that individuals who have under-gone either a right or left hemispherectomy will display similar pat-terns of strengths and limitations across cognitive tasks. Specifically,if a function is high in the hierarchy, it will be retained by the remain-ing hemisphere or subsumed (i.e., taken over) by the remaininghemisphere. If the function in question is low in the hierarchy, it willbe lost (or less well preserved) to accommodate the transfer of high-er-ranked tasks, or the function will not be taken over by the remain-ing hemisphere (see Table 1c).

A very similar model is the crowding hypothesis, which at-tempts to explain why a solitary right hemisphere loses someof its specialized functions to preserve language [24,25]. Althoughthe crowding hypothesis proposes that each hemisphere hassome specialized function, following left-sided hemispherectomythe remaining right hemisphere will lose some traditional righthemisphere functions (e.g., visuospatial ability) in an attempt totake over the necessary language functions (e.g., speech). Forexample, Vanlancker-Sidtis ([8], see also [26]) examined a partic-ipant who had undergone left hemispherectomy and found thatsome circumscribed right hemisphere linguistic functions (e.g.,comprehension of linguistic prosody) were ‘‘crowded out” bypragmatic language tasks. Comparable to Ogden’s hierarchicalmodel of specialized functions [21,22], the crowding hypothesiswould predict that individuals who have undergone either rightor left hemispherectomies should display similar patterns of re-sults across most tasks.

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1 Participants also read a block of nonwords (e.g., BINT). Given that the results ofthis block were similar to those for the pseudohomophone block (i.e., bothparticipants had poor performance and would be classified as phonological dyslexics),these analyses were not included.

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The primary goal of the current study was to investigate withJ.H. and S.M. the extent to which the right hemisphere can sub-sume basic reading processes (i.e., in J.H.) and, conversely, theextent to which these functions are preserved (i.e., not ‘‘crowdedout” for critical right hemisphere functions such as the evalua-tion of emotions) in the left hemisphere (i.e., in S.M.). In partic-ular, we explored aspects of language functioning that we canassume to be lower in the hierarchy of functions (i.e., subwordreading), or more likely to be ‘‘crowded out,” as well as languagefunctions that are likely to be subsumed by the right hemisphereand unlikely to be ‘‘crowded out” in the left hemisphere (i.e.,whole-word reading). Table 1 summarizes the predictions thatfollow from the three models: hemispheric specialization,equipotentiality, and hierarchy of specialized function/crowdinghypothesis.

2. Method

2.1. Participants

2.1.1. Clinical participantsThe study included two female patients who had undergone

hemispherectomy, J.H. (age 49) and S.M. (age 48). Prior to takingpart in the present study, both J.H. and S.M. participated in a clin-ical interview to determine their general health and specific sei-zure history, and completed a battery of neuropsychologicaltests. The present study was part of a larger investigation that as-sessed general cognitive functioning 30 years following right or lefthemispherectomy and the effects of hemispherectomy on socialcognition. Only information relevant to the present article is re-ported here (we refer the reader to Fournier et al. [9] for a detaileddescription of the neuropsychological test results).

2.1.2. Brief history of participants who had undergonehemispherectomy

J.H. was born with a right-sided hemiplegia. Her seizures began atage 5 and gradually increased in frequency. She underwent a com-plete left-sided hemispherectomy at age 16 for severe intractableseizure disorder. J.H. obtained her Grade 10 equivalency through aregional vocational program. She made several attempts to completeher Grade 12 equivalency, which included special tutoring; how-ever, she has been unsuccessful, primarily as a result of her inabilityto master basic mathematical concepts. J.H. was age 49 when thecurrent study was completed, her verbal intelligence was assessedto be within low normal range, and she had been seizure free for33 years.

S.M. was reportedly healthy until she began to have seizures atage 5. At age 6 she had a partial right-sided hemispherectomy. Thiswas not successful in controlling her seizures and, consequently,she underwent a complete right-sided hemispherectomy at theage of 13. S.M. successfully completed Grade 12. S.M. was age 48when the current study was completed, her verbal intelligencewas assessed to be within the low normal range, and althoughshe continues to experience occasional complex partial seizures,these are generally well controlled by her medication. Bothparticipants have functionally fluent speech, display good languagecomprehension (for the purpose of everyday conversation), andhave hemiplegia and homonymous hemianopsia as predicted bytheir surgical side (left hemispherectomy versus righthemispherectomy).

2.1.3. Normal comparison groupNormative reading data were collected from 12 women who

ranged in age from 38 to 58 years (M = 47) and reported ruralupbringing and educational backgrounds similar those of J.H. andS.M.

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2.2. Measures and procedures

S.M., J.H., and control participants were presented with threeblocks of letter-strings.1 These were pure blocks of regular words(e.g., WON), exception words (e.g., ONE), and pseudohomophones(e.g., WUN). Each block contained 55 letter strings for a total of165 letter strings. The letter strings were presented individually ona computer screen, and the participants were asked to read the letterstrings aloud as quickly and accurately as possible. Emphasis wasplaced on speed for both groups. Accuracy in naming was recordedusing a button press. Participation took approximately 20 min.

2.3. Assessment

Based on the assumption that whole-word reading and sub-word reading represent independent systems, and that both sys-tems contribute to basic reading processes, a method ofexamining phonological and surface dyslexia involves assessingone process as a function of the other [15]. This serves to isolatedyslexic participants who are markedly worse at one task thanthe other, given the linear relationship that exists betweenwhole-word processing (i.e., exception word naming) and subwordprocessing (i.e., pseudohomophone naming). Thus, one can usesimple linear regression of pseudohomophone reading scores onexception word reading scores to provide an estimate of the ex-pected number of pseudohomophones to be read correctly at vary-ing levels of exception word reading (e.g., an assessment ofphonological dyslexia). Similarly, regressing exception word read-ing scores on pseudohomophone reading scores provides an esti-mate of the expected number of exception words to be readcorrectly at varying levels of pseudohomophones (e.g., an assess-ment of surface dyslexia). Thus, the graphical relationship betweenwhole-word reading (as measured by exception word namingaccuracy) and subword reading (as measured by pseudohomo-phone naming accuracy) was used to determine whether dissocia-tions existed between the two systems [15,27]. Based on work byMcDougall et al. [27], 95% sample ellipses were placed around thenormal participants’ data to capture the normal reading perfor-mance. Participants whose performance fell below the 95% ellip-soids were classified as dyslexic.

3. Results

3.1. Naming accuracy

Naming accuracy was computed for S.M. (right hemispherec-tomy), J.H. (left hemispherectomy), and the 12 participants in thenormal comparison group on each of the letter-string blocks. Asshown in Table 2, S.M.’s regular word accuracy (100%) reflects per-formance that is characteristic of the comparison group with sim-ilar age and educational backgrounds (99%). In addition, herexception word accuracy (96%) is also within the typical range ofscores obtained by the comparison group participants (97%). Incontrast to these findings, S.M.’s pseudohomophone accuracy(40%) is markedly lower (by >6 SD) than the accuracy found forthe comparison group (83%), and reflects clear impairment of thesubword reading system.

In contrast to S.M., J.H.’s regular word accuracy (96%) demon-strates an evident weakness when compared with that of the nor-mal participants. In addition, her exception word accuracy (89%)falls significantly below the range typical of scores found in the

exia: Three decades following right versus left hemispherectomy for child-09.05.018

Table 2Naming accuracy for each participant and the comparison group as a function ofstimulus type (regular words, exception words, and pseudohomophones).

Participant Regular wordaccuracy (%)

Exception wordaccuracy (%)

Pseudohomophoneaccuracy (%)

S.M. (right-sidedhemispherectomy)

100 96 40

J.H. (left-sidedhemispherectomy)

96 89 45

Comparison group 99 (0.55)a 97 (2.21) 83 (6.95)

a SD given in parentheses.

Fig. 1. Assessment of phonological dyslexia via plotting of pseudohomophonenaming accuracy as a function of exception word naming accuracy. A 95% sampleellipsoid is placed around the normal, age-matched, comparison group data.

Fig. 2. Assessment of surface dyslexia via plotting of exception word namingaccuracy as a function of pseudohomophone naming accuracy. A 95% sampleellipsoid is placed around the normal, age-matched, comparison group data.

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comparison group (by >3 SD), indicating that J.H. has a mild deficitin whole-word reading. Finally, J.H.’s pseudohomophone accuracy(45%) is severely impaired (by about 5.5 SD) when compared withthat of the normal group and reflects significant impairment of thesubword reading system.

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3.2. Regression-based analysis of dyslexia subtypes

Phonological dyslexia was assessed by plotting pseudohomo-phone performance as a function of exception word performance.As illustrated in Fig. 1, J.H. and S.M. fell below the 95% ellipsoidof the normal comparison group and thus displayed phonologicaldyslexia. Notably, both participants’ pseudohomophone accuracyscores were extremely low when compared with the mean of thenormal group, signifying severe impairments of the subword read-ing system. Surface dyslexia was assessed by plotting exceptionword performance as a function of pseudohomophone perfor-mance. As illustrated in Fig. 2, J.H. fell below the 95% ellipsoid ofthe normal comparison group, indicative of surface dyslexia anda deficit in the whole-word reading system. In contrast, S.M. didnot fall below the 95% ellipsoid and, from this perspective, demon-strates within-normal-range whole-word processing. Based onthese findings, S.M. would receive a classification of phonologicaldyslexia and J.H. would receive a classification of mixed dyslexia.

4. Discussion

The present research provides some support for the hierarchicalmodel of specialized function [5] and crowding hypothesis [24] forlanguage processing following hemispherectomy. Specifically, bothS.M. and J.H. demonstrated deficits in naming pseudohomophones,suggesting that subword reading is lower in the hierarchy of spe-cialized function and was ‘‘crowded out” of the left hemispherein the case of S.M. and not fully subsumed by the right hemispherein the case of J.H. When pseudohomophone reading performancewas assessed as a function of exception word reading performance,both J.H. and S.M. fell below the 95% normal ellipsoid and would beclassified as having phonological dyslexia. However, the presentresults also provide some support for the hemispheric specializa-tion model. That is, compared with J.H. (left hemispherectomy)S.M. (right hemispherectomy) displayed higher exception wordaccuracy (96% versus 89%), providing evidence of left hemispherespecialization of language processing. When exception word read-ing performance was assessed as a function of pseudohomophonereading performance, only J.H. fell below the 95% normal ellipsoidand, in conjunction with her performance on pseudohomophonenaming, would receive an overall classification of mixed dyslexia.

Importantly, S.M.’s performance on pseudohomophone namingwas incongruent with her ability to name exception words. Pre-sumably, S.M.’s left hemisphere had to crowd out or compromisespecialized left hemisphere functions (i.e., subword reading; pre-sumed to be lower in the hierarchy) to accommodate or take overfunctions normally monitored by the right hemisphere. In addition,J.H. displayed a similar impairment on subword reading, perform-ing well below average when compared with age-equivalent peers.Together, S.M.’s and J.H.’s performance on subword reading pro-vides support for the hierarchical model of specialized functionand the ‘‘crowding hypothesis” of language functioning followinghemispherectomy [5,24].

Nevertheless, overall reading performance was higher for S.M.(who had a remaining left hemisphere) than for J.H. (who had aremaining right hemisphere), indicating some support for anenduring left hemisphere advantage for basic language abilities[5,7,18]. In particular, S.M.’s performance on regular word namingand exception word naming was not significantly different fromthat of the age-equivalent comparison group, whereas J.H.’sperformance on both regular and exception word naming was be-low the range typical of her peers. Further, given that J.H.’s readingscores on exception word naming were not markedly differentthan what would be predicted given her performance on pseud-ohomophones, it is suggested that the right hemisphere subsumedsome components of both functions (i.e., whole-word and subword

exia: Three decades following right versus left hemispherectomy for child-09.05.018

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reading) following developmental failure and eventual loss of theleft hemisphere. However, these functions were not fully sub-sumed as is indicated by her overall lower reading performanceon both whole- and subword stimuli when compared with theage-equivalent comparison group. These findings provide evidencefor the specialization of the left hemisphere in basic languageprocesses.

Assessing the relationship between whole-word and subwordreading to identify dyslexia proved to be useful with individualswho had undergone right or left hemispherectomy. In the presentinvestigation, this technique revealed dissociation between whole-word and subword reading for S.M. Although S.M. had normalexception word naming accuracy (96%), her pseudohomophoneperformance (40%) fell well below what would be expected givenher performance on exception word naming. Thus, S.M.’s patternof results (dissociation between whole-word and subword readingperformance) reflects a deviant or specific deficit pattern regardingthe subword processing system. In contrast, J.H.’s pattern of results(poorer performance on both whole-word and subword reading)reflects a broader spectrum of impaired reading skills. AlthoughJ.H. appeared to have relatively good exception word naming per-formance (89%), this score represented impairment when com-pared with that of age-equivalent controls (97%). Similarly herperformance on exception word naming was consistent withmixed dyslexia given her below-normal performance on pseud-ohomophone naming.

Notably, our results allow us to make inferences about the nat-ure of the relationship between whole-word and subword process-ing following hemispherectomy. More specifically, our resultssupport our assumption that whole-word reading is high in thehierarchy of function and thus retained in the left hemisphere(i.e., S.M.) and subsumed by the right hemisphere (i.e., J.H.). Theevidence also suggests that subword reading is low in the hierar-chy of function and consequently is crowded out (i.e., S.M.) or min-imally subsumed (i.e., J.H.). However, both whole-word andsubword reading systems appear to contribute to the correct nam-ing of regular words, as evidenced by the higher regular word nam-ing accuracy, when compared with exception word accuracy andpseudohomophone accuracy. Thus, although the subword systemmay be significantly impaired following hemispherectomy, it isnot the case that such functioning is completely abolished (follow-ing right hemispherectomy), nor is subword processing completelyexempt from being integrated into the remaining hemisphere (fol-lowing left hemispherectomy). The relationship between thewhole-word and subword reading systems following early braininjury is important to our understanding of the underlying pro-cesses at work during basic language functioning, so that we maybetter assess, diagnose, and remediate individuals who suffer fromlanguage impairments as a result of their injuries.

Importantly, when we consider these findings amid what wasreported by Fournier et al. [9], our results continue to be consistentwith the hierarchical model of specialized function and the‘‘crowding hypothesis.” Taken together, these studies indicate thatidentification of neutral, happy, and fearful emotional expressionsand whole-word reading are high in the hierarchy of specializedfunctions and thus retained (or subsumed) in the right and lefthemispheres, respectively. In contrast, identification of emotionalexpressions depicting disgust and anger and subword reading werenot retained in the right and left hemispheres, respectively, norwere they adequately subsumed by the remaining hemisphere.However, our results also indicate that traditional accounts ofhemispheric specialization should be considered when assessinggeneral functioning. That is, J.H. (remaining right hemisphere) dis-played overall superior performance on emotional perception andexpression, cognitive abilities typically modulated by the righthemisphere. In contrast, S.M. (remaining left hemisphere)

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displayed overall superior performance on basic reading tasks,which are typically modulated by the left hemisphere.

4.1. Limitations

These results are qualified by the limitations of the research,including our small case study methodology with two participantswho had undergone hemispherectomy. In addition, although bothparticipants began to experience seizures around age 5, J.H. sufferedprenatal brain injury, whereas S.M. reportedly had normal earlydevelopment. Such differences may influence the retention or lossof cognitive functions. For example, because J.H. experienced an ear-lier brain injury than S.M., in theory her remaining right hemispherehad more potential to take over functions typically modulated by theremoved left hemisphere, although this is not consistent with ourreading results. Given that the extent of equipotentiality is signifi-cantly influenced by age of injury [5], it is important to note thatthe presumed age of injury and the assumptions regarding the po-tential of the remaining hemisphere may be different for these twowomen. It also needs to be acknowledged that individual differencesbetween the two participants might play a role, and cannot be fullydisentangled from the effects of differences in hemispheric develop-mental injury and subsequent surgery.

5. General conclusions

Taken as a whole, it is clear that some modification is needed tothe hierarchy of specialized functions model, which includes stip-ulations about hemispheric specialization. It is evident from ourwork with J.H. and S.M. that an isolated hemisphere is restrictedin its ability to subsume functions typically mediated by the re-moved hemisphere, even when these functions are high in thehierarchy. Our results indicate that there are some functions thatappear to be indispensable (e.g., whole-word reading) and are re-tained or subsumed, consistently, following hemispherectomy.Other functions (e.g., subword reading) appear to be dispensableand, consistently, are lost following hemispherectomy regardlessof surgical side. Nevertheless, although subsuming some of thehigher brain functions associated with the surgically removed side,the strengths and limitations of an isolated right or left hemispherewill continue to demonstrate some lateralization of processingskills typically associated with the remaining hemisphere.

Acknowledgments

We thank J.H. and S.M. for their participation in the presentstudy. Without their willingness, such research would not be pos-sible. We also thank Dr. Noel Lowry, from the College of Medicineat the University of Saskatchewan, for his expertise and guidancein the area of epilepsy and hemispherectomy. The research wassupported by the Natural Sciences and Engineering Research Coun-cil of Canada (NSERC) in the form of a scholarship to J.C. and a re-search grant to R.B.

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exia: Three decades following right versus left hemispherectomy for child-09.05.018