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published online 27 October 2014J Learn DisabilMichele Poletti

in Reading, Written Expression, and MathematicsWISC-IV Intellectual Profiles in Italian Children With Specific Learning Disorder and Related Impairments

  

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Article

In the recent fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5; American Psychiatric Association, 2013), specific learning disabilities were included in the single, overarching diagnostic category of specific learning disorder (SLD), incorporating deficits that affect academic achievement. The DSM-5 Working Group on Neurodevelopmental Disorders reached this decision considering that specific learning disabilities share many features regarding genetic risk, environmental risk factors, developmental features, comorbidity, and cognitive weak-nesses (Tannock, 2013), in particular of working memory and processing speed (e.g., De Weerdt, Desoete, & Roeyers, 2013; Peng, Conqying, Beilei, & Sha, 2012; Schuchardt, Maehler, & Hasselhorn, 2008; Wilcutt et al., 2010; Wilcutt et al., 2013). Although included in the same diagnosis of SLD (DSM-5 Code 315), subtypes of impairment can still be described, including impairment in reading (Code 315.00), impairment in mathematics (Code 315.1), and impairment in written expression (Code 315.2).

The evaluation of intellectual abilities with standardized intelligence tests is a necessary step during the diagnostic

process of SLD, to exclude the presence of intellectual dis-ability and to identify individual cognitive strengths and weaknesses. The fourth edition of the Wechsler Intelligence Scale for Children (WISC-IV; Wechsler, 2003a) is one of the most used tests for the assessment of intellectual abili-ties in children (Flanagan & Kaufman, 2009; Prifitera, Saklofske, & Weiss, 2008; Weiss, Saklofske, Prifitera, & Holdnack, 2006).

Studies on WISC Profiles

A large number of studies have explored the usefulness of WISC (including WISC-I, WISC-R, WISC-III, WISC-IV) in identifying specific intellectual profiles in children with

555416 LDXXXX10.1177/0022219414555416Journal of Learning DisabilitiesPolettiresearch-article2014

1AUSL of Reggio Emilia, Italy

Corresponding Author:Michele Poletti, Department of Mental Health and Pathological Addiction, Child Neuropsychiatry Service, AUSL of Reggio Emilia, Via Amendola 2, 42100, Reggio Emilia, Italy. Email: michele.poletti2@ausl.re.it, michelepoletti79@gmail.com

WISC-IV Intellectual Profiles in Italian Children With Specific Learning Disorder and Related Impairments in Reading, Written Expression, and Mathematics

Michele Poletti, PsyD1

AbstractThe fifth edition of the Diagnostic and Statistical Manual of Mental Disorders grouped specific learning disabilities in the single diagnostic category of specific learning disorder (SLD), with specifiers for impairments in reading, written expression, and mathematics. This study aimed at investigating the intellectual profile, assessed with the fourth edition of the Wechsler Intelligence Scale for Children (WISC-IV), of 172 children with a diagnosis of SLD, compared to 74 clinical referral controls. WISC-IV intellectual functioning in children with SLD was characterized by a significant discrepancy between general ability and cognitive proficiency (General Ability Index [GAI] > Cognitive Proficiency Index [CPI]), and worse performances on the Similarities, Digit Span, Letter–Number Sequencing, and Coding subtests, supporting models of multiple cognitive deficits at the basis of neurodevelopmental disorders as SLD. GAI was the best and more conservative measure provided by the WISC-IV to identify intellectual functioning in children with SLD, and the intellectual discrepancy between GAI and CPI could be considered a “cognitive sign” for the presence of SLD in a single diagnostic category. Cognitive deficits differed in subtypes of impairment (reading, written expression, and mathematics), supporting their distinction for empirical, educational, and rehabilitative purposes. These findings need further replication in larger samples and in comparison to typically developing children.

KeywordsWISC-IV, specific learning disorder, intellectual abilities, impairment in reading, impairment in written expression, impairment in mathematics

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2 Journal of Learning Disabilities

learning disabilities (hereafter SLD), briefly overviewed regarding most discussed profiles.

A first attempt on the first WISC version was provided by Bannatyne (1968), who categorized the WISC subtest scores grouping them into three categories: spatial (Block Design, Object Assembly, and Picture Completion), con-ceptual (Similarities, Vocabulary, and Comprehension), and sequential (Digit Span, Picture Arrangement, and Coding). Bannatyne (1971) reported that children with impairment in reading had their highest scores in the spatial category, intermediate scores in the conceptual category, and lowest scores in the sequential category (spatial > conceptual > sequential), a pattern partially confirmed by a subsequent review of WISC studies (Rugel, 1974). Therefore, Bannatyne (1974) revised the subtest categorization distin-guishing spatial (Block Design, Object Assembly, and Picture Completion), conceptual (Similarities, Vocabulary, and Comprehension), and sequential (Digit Span, Arithmetic, and Coding) categories. The revised Bannatyne pattern was found to be more present in children with learn-ing disabilities both for WISC-R (Smith, Coleman, Dokecki, & Davis, 1977; Vance & Singer, 1979) and WISC-III (Prifitera & Dersh, 1993), but subsequent studies showed that this pattern was not useful in discriminating between children with learning disabilities and typically developing children (e.g., D’Angiulli & Siegel, 2003; Kavale & Forness, 1984; Smith & Watkins, 2004; Watkins, 2000).

Another approach, attempted by Kaufman (1975), pro-posed the Freedom of Distractibility Factor (Arithmetic, Coding, Digit Span), derived from the WISC-R factor anal-ysis, as a weak cognitive characteristic of children with developmental disabilities, including children with learning disabilities. This hypothesis was not subsequently con-firmed, as this factor unable to distinguish between children with learning disabilities and typically developing children (Kavale & Forness, 1984; McDermott, Fantuzzo, & Glutting, 1990). Then the ACID (deficits in Arithmetic, Coding, Information, and Digit Span) and the SCAD (defi-cits in Symbol Search, Coding, Arithmetic, and Digit Span) profiles were proposed as signs of learning disabilities, especially of impairment in reading (Kaufman, 1981, 1994), but subsequent studies showed that also these profiles are not specific and do not efficiently distinguish between chil-dren with and without impairment in reading (Moura, Simoes, & Pereira, 2014; Thomson, 2003; Watkins, Kush, & Glutting, 1997a, 1997b).

Overall, these studies showed that specific WISC profiles, including Bannatyne’s pattern, Freedom of Distractibility, ACID, and SCAD, may have higher prevalence in SLD chil-dren in comparison with typically children, but their use for diagnostic purpose is not empirically supported, being unable to clearly distinguish between clinical and healthy popula-tions. Furthermore, the cognitive heterogeneity of children with SLD suggested that the investigation of their intellectual profiles is probably more useful for a better comprehension

of underlying cognitive deficits rather than for diagnostic purposes (Flanagan, Alfonso, Mascolo, & Hale, 2010; Flanagan, Alfonso, Mascolo, & Sotelo-Dynega, 2012; Flanagan & Kaufman, 2009; C. R. Hale, Casey, & Ricciardi, 2014; Morris et al., 1998; Pennington, 2009; Prifitera et al., 2008; Weiss et al., 2006).

Searching for Cognitive Deficits

An example of this shift of perspective may be provided by the analysis of WISC-IV performances in children with SLD. The WISC-IV (Wechsler, 2003a), supported by exploratory and confirmatory factor analyses, distinguished measures of crystallized intelligence (the Verbal Comprehension Index), fluid intelligence (the Perceptual Reasoning Index), working memory, and processing speed. In the study accompanying the publication of WISC-IV, Wechsler (2003b) reported that children with SLD showed more difficulties in subtests related to working memory and processing speed,

Another study (Wilcutt et al., 2013) examined the comor-bidity between impairment in reading and impairment in mathematics with a cognitive assessment including some WISC subtests. Authors found both common and distinct cognitive deficits at the basis of these impairments: The common cognitive deficit included weaknesses in verbal comprehension, working memory, and processing speed, whereas distinct cognitive deficits included weaknesses in phoneme awareness and naming speed (associated only with impairment in reading) and weaknesses in set shifting (asso-ciated only with impairment in mathematics). Moreover, children with both of these impairments, in comparison with children with isolated impairments, showed more difficul-ties in global, academic, and cognitive functioning.

Findings of these studies are in agreement with models of multiple cognitive deficits at the basis of neurodevelop-mental disorders in general and of SLD in particular (McGrath et al., 2011; Pennington, 2006) and clarify how the WISC-IV, alone or in addition to other cognitive tasks, may help researchers and clinicians in their identification.

Aim of the Study

This study aimed at investigating the intellectual profile associated with the diagnosis of SLD and with related impairments in reading, mathematics, and written expres-sion, using the WISC-IV, recently standardized in the Italian population (Orsini, Pezzuti, & Picone, 2012a). On the basis of the empirical literature, predictions can be formulated regarding the following:

•• The presence of deficits in working memory and pro-cessing speed in children with SLD, leading to a pat-tern of General Ability Index > Cognitive Proficiency Index (fully described in the method section)

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•• The presence of different WISC-IV profiles regard-ing impairment in reading, impairment in mathemat-ics, and impairment in written expression

Methods

Participants

A total of 246 participants were enrolled for this study: 172 children with a diagnosis of SLD (110 males and 62 females; mean age 10.72 ± 2.78 years; mean grade 5.59 ± 2.66) and 74 children considered as clinical referral controls (CRC; 46 males and 28 females; mean age 10.54 ± 2.41 years; mean grade 5.45 ± 2.47). Participants were enrolled among children referred for neuropsychological assessment due to persistent academic difficulties and assessed at the Clinic for Learning Difficulties, Child and Adolescent Neuropsychiatric Unit of Reggio Emilia, Italy. Children were consecutively recruited for 2 years from August 2012 to August 2014. Written consent to collect data for empirical purpose was signed by parents.

Empirical data for this study were collected during rou-tine clinical assessment for diagnostic purpose. All recruited participants underwent a neuropsychiatric screening and a neuropsychological assessment focused on learning profi-ciency, according to usual diagnostic procedures; moreover, all participants underwent an assessment with the WISC-IV (Orsini et al., 2012a) and a battery of tasks assessing learn-ing proficiency.

Materials

WISC-IV. The WISC-IV Italian version (Orsini et al., 2012a) has been recently standardized with 2,200 participants (1,100 males and 1,100 females), confirming its four-factor structure (Verbal Comprehension, Perceptual Reasoning, Working Memory, Processing Speed; Orsini, Pezzuti, & Picone, 2012b), in agreement with the American standard-ization (Wechsler, 2003b).

The administration of the 10 WISC-IV core subtests permits calculation of 4 indexes (VCI: Verbal Comprehension Index, based on the Similarities, Vocabulary, and Comprehension sub-tests; PRI: Perceptual Reasoning Index, based on the Block Design, Picture Concepts, and Matrix Reasoning subtests; WMI: Working Memory Index, based on the Digit Span and Letter–Number Sequencing subtests; PSI: Processing Speed Index, based on the Coding and Symbol Search subtests) and the Full-Scale Intelligence Quotient (FSIQ). According to WISC-IV manual, these indexes are interpretable if the differ-ence between respective subtests is 4 or less and the FSIQ is interpretable if the difference between the highest index and the lowest index (between VCI, PRI, WMI, PSI) is 22 or less.

Subtests of VCI and PRI permit calculation of the General Ability Index (GAI), which is interpretable if the difference between VCI and PRI is 22 or less, and subtests of WMI and PSI permit calculation of the Cognitive

Proficiency Index (CPI), which is interpretable if the differ-ence between WMI and PSI is 22 or less.According to the WISC-IV manual, the definition of the

intellectual level of each participant is based on interpreta-ble quotients and primarily based on the FSIQ. In case of an uninterpretable FSIQ, the intellectual level is based on GAI; in case of an uninterpretable GAI, the intellectual level is based on the VCI and/or PRI. Levels of intellectual func-tioning, profile discrepancies, and profile strengths and weaknesses are reported in Box 1.

Box 1

The level of intellectual functioning can be qualita-tively described on the basis of index scores as very superior (≥130), superior (120–129), high average (110–119), average (90–109), low average (80–89), borderline (70–79), and extremely low (≤69).

WISC IV Profile: DiscrepanciesThese discrepancies can be calculated:

•• Discrepancy between FSIQ and GAI•• GAI-related discrepancy between VCI and PRI•• CPI-related discrepancy between WMI and PSI•• Discrepancy between GAI and CPI•• Discrepancy between highest index and lowest

index within VCI, PRI, WMI, and PSI•• Discrepancy between highest subtest and lowest

subtest

WISC-IV Profile: Strengths and WeaknessesAccording to WISC-IV manual, the following can be identified for each participant:

•• Normative strengths (an interpretable index within VCI, PRI, WMI, PSI ≥ 115) and normative weaknesses (an interpretable index within VCI, PRI, WMI, PSI < 85).

•• Individual strengths and individual weaknesses: these are present if the difference between an index and the mean of the 4 indexes (VCI, PRI, WMI, PSI) is above or below a cutoff point reported in the WISC-IV manual for each index and varying for age.

•• Key assets and high-priority concerns: Key assets are defined as those individual strengths > 115 that have a normative frequency < 10%, as indi-cated by cutoff point reported in the WISC-IV manual. High-priority concerns are defined as those individual weaknesses < 85 that have a nor-mative frequency < 10%, as indicated by cutoff point reported in the WISC-IV manual.

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4 Journal of Learning Disabilities

Learning proficiency tasks. Specific tasks developed in the Italian clinical context were administered for the assess-ment of learning proficiency. Word and nonword reading and writing and sentence writing were assessed with the Batteria per la valutazione della Dislessia e della Disorto-grafia Evolutiva–2 (Sartori, Job, & Tressoldi, 2007); text reading and writing (dictation) were assessed with the MT Series (for primary schools: Cornoldi & Colpo, 2011; for first grade: Cornoldi & Colpo, 2012; for second grade: Cor-noldi et al., 2010). Mathematical abilities involving number sense (forward and backward counting, number reading, writing and repetition, number semantics) and calculation (multiplication tables, arithmetic facts, oral and written cal-culation) were assessed with the Batteria per la Discalculia Evolutiva (BDE; Bachmann, Mengheri, & Biancardi, 2014; Biancardi & Nicoletti, 2004), which provides a number quotient, a calculation quotient, and a general number and calculation quotient.

Diagnoses. According to the DSM-5 and to the Italian diag-nostic guidelines (Consensus Conference, 2007; DSA, 2011), participants were diagnosed as SLD if at least one interpretable WISC-IV index among Verbal Comprehen-sion or Perceptual Reasoning (see Box 1) was 85 or greater and performances on reading, writing, and/or arithmetic skills were under the clinical cutoff scores indicated by cited guidelines (i.e., usually ≤2 standard deviations below the mean performances of age-matched participants or ≤5–10 percentiles).

Participants were classified as CRC if at least one WISC-IV index among Verbal Comprehension or Perceptual Reasoning (see Box 1) was 85 or greater and any perfor-mance on tasks of proficiency in reading, writing, and arith-metic skills was compatible with a diagnosis of SLD (i.e., >2 standard deviations below mean performances of age-matched participants or >5–10 percentiles).

With the aim of selecting a homogenous group of SLD without significant comorbid and potentially confounding factors (not infrequently co-occurring with SLD), exclusion criteria for this study were the presence of significant medi-cal and psychological problems and the presence of comor-bid disorders, such as attention-deficit/hyperactivity disorder, developmental coordination disorder, present spe-cific language impairment, and disruptive behaviors (e.g., oppositional defiant disorder); these exclusion criteria were adopted for CRC.

According to the DSM-5 and the Italian guidelines, SLD subtypes of impairment were diagnosed. Impairment in reading was diagnosed if at least two parameters were under cutoff scores (≤2 standard deviations below mean for decoding speed, ≤5 percentile for accuracy) in the three administered tasks: word reading, nonword reading, and text reading. Reading comprehension was not considered to diagnose impairment in reading.

Impairment in written expression was diagnosed if at least two parameters were under cutoff scores for accuracy (≤5 percentile) in the administered tasks (word and non-word writing and text writing for children attending primary school; sentence writing and text writing for older chil-dren); isolated difficulties in handwriting were not included in the diagnosis of impairment in written expression.

Impairment in mathematics was diagnosed if at least one quotient provided by the BDE was under cutoff score (≤2 standard deviations below mean). According to guidelines, impairments in reading and in written expression could not be diagnosed before the end of the second primary grade, and impairment in mathematics could not be diagnosed before the end of the third primary grade.

Moreover, on the basis diagnosed impairments, children with SLD were classified as isolated (one isolated impair-ment) or associated (two or more comorbid impairments). According to described criteria, these subtypes were diag-nosed among 172 children with SLD: 115 children with impairment in reading (66.86% of the SLD sample), 77 with impairment in written expression (44.76% of the SLD sam-ple), and 106 with impairment in mathematics (61.62% of the SLD sample). Furthermore, 65 children presented an isolated impairment (37.79% of the SLD sample: 24 reading, 17 writ-ten expression, 24 mathematics) and 107 presented associ-ated impairments (25 reading and written expression, 23 reading and mathematics, 15 written expression and mathe-matics, 44 reading, written expression and mathematics).

Statistical Analysis

Comparisons between groups (SLD vs. CRC; CRC between SLD subtypes, among SLD subtypes) were performed with one-way analysis of variance (ANOVA) and Bonferroni-corrected post hoc analyses were also performed in case of multiple com-parisons. Chi-square analysis was used for comparisons of fre-quencies. Paired t test was used to compare performances in different WISC-IV indexes and subtests in the same subgroup.

Results

Demographic data and WISC-IV performances of children with SLD and of CRC are reported in Table 1. Analyses of WISC-IV profiles of children with SLD and of CRC, including frequencies of interpretable quotients, normative strengths and weaknesses, individual strengths and weak-nesses, key assets, and high-priority concerns are reported in Table 2. Correlations between WISC-IV indexes and between WISC-IV subtests in children with SLD are reported in Tables 3 and 4, respectively. Demographic data and WISC-IV performances of SLD subgroups with iso-lated impairments are reported in Table 5. Comparisons between pairs of isolated impairments and their association are reported in Tables 6, 7, and 8.

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Poletti 5

Comparison Between Children With SLD and CRC

CRC outperformed children with SLD on FSIQ, F(1, 244) = 29.144, p < .001, η2

p = 0.11, GAI, F(1, 244) = 7433., p = .00, η2

p = 0.03, CPI, F(1, 244) = 48.165, p < .001, η2p =

0.16, PRI, F(1, 244) = 11.387, p = .001, η2p = 0.04, WMI,

F(1, 244) = 38.690, p < .001, η2p = 0.14, and PSI, F(1, 244) =

20.154, p < .001, η2p = 0.08 (see Table 1) and for subtests

on Similarities, F(1, 244) = 7.269, p = .008, η2p = 0.03,

Block Design, F(1, 244) = 7.221, p = .008, η2p = 0.03,

Picture Concepts, F(1, 244) = 6.536, p = .011, η2p = 0.03,

Matrix Reasoning, F(1, 244) = 5.084, p = .025, η2p = 0.02,

Digit Span, F(1, 244) = 28.323, p < .001, η2p = 0.10, Letter–

Number Sequencing, F(1, 244) = 27.433, p < .001, η2

p =

Table 1. Demographic Data and WISC-IV Performances in Children With SLD and in CRC.

Measures SLD (M, SD) CRC (M, SD)

Descriptive characteristics Age (years) 10.71 (2.79) 10.51 (2.54) Gender (M/F) 110/62 46/28 Grade 5.58 (2.67) 5.45 (2.47)WISC-IV indexes Full-Scale IQ 93.85 (11.23) 102.26 (11.00)** General Ability 98.91 (12.01) 103.42 (11.38)** Cognitive Proficiency 87.81 (12.48) 99.51 (11.20)** Verbal Comprehension 97.22 (12.97) 99.57 (12.94) Perceptual Reasoning 101.01 (12.79) 107.03 (12.13)** Working Memory 88.05 (12.41) 98.54 (11.01)** Processing Speed 92.40 (13.37) 100.73 (13.52)**WISC-IV subtests Similarities 8.80 (2.30) 9.66 (2.37)** Vocabulary 9.39 (2.51) 9.80 (2.55) Comprehension 10.40 (3.02) 10.31 (2.70) Block Design 9.71 (2.58) 10.68 (2.42)** Picture Concepts 10.47 (2.93) 11.49 (2.62)* Matrix Reasoning 10.36 (2.45) 11.18 (2.67)* Digit Span 8.10 (2.36) 9.82 (2.16)** Letter–Number Sequencing 7.93 (2.47) 9.70 (2.29)** Coding 7.57 (2.67) 9.09 (2.70)** Symbol Search 9.88 (2.52) 11.20 (2.44)**WISC-IV discrepancies FSIQ-GAI 6.30 (4.68) 3.72 (3.17)** GAI-CPI 14.68 (10.90) 9.11 (8.24)** VCI-PRI 11.75 (9.31) 13.00 (10.36) WMI-PSI 13.91 (10.00) 12.68 (11.02) Highest – lowest index 26.05 (10.57) 22.26 (11.96)* Highest – lowest subtest 7.71 (2.27) 6.85 (1.98)**

Note. Ns: children with specific learning disorder = 172, clinical referral controls = 74. CPI = Cognitive Proficiency Index; CRC = clinical referral controls; FSIQ = Full-Scale Intelligence Quotient; GAI = General Ability Index; PRI = Perceptual Reasoning Index; PSI = Processing Speed Index; SLD = specific learning disorder; VCI = Verbal Comprehension Index; WISC-IV = Wechsler Intelligence Scale for Children (4th ed.); WMI = Working Memory Index.*p ≤ .05. **p ≤ .001.

Table 2. Analyses of WISC-IV Profiles in Children With SLD and in CRC.

Descriptive characteristics SLD (n, %) CRC (n, %)

Interpretable FSIQ 78 (45.34) 38 (51.35)Interpretable GAI 148 (86.04) 63 (85.13)Interpretable CPI 139 (80.81) 63 (85.13)GAI > FSIQ 140 (81.39) 42 (56.75)**VCI > PRI 71 (41.27) 20 (27.02)WMI > PSI 60 (34.88) 28 (37.83)Normative strengths

VCI 14 (8.13) 6 (8.10)PRI 15 (8.72) 10 (13.51)WMI 0 (0) 4 (5.40)**PSI 5 (2.90) 4 (5.40)

Normative weaknessesVCI 26 (15.11) 10 (13.51)PRI 6 (3.48) 0 (0)WMI 54 (31.39) 5 (6.75)**PSI 31 (18.02) 1 (1.35)**

Individual strengthsVCI 43 (25.0) 8 (10.81)**PRI 68 (39.53) 25 (33.78)WMI 12 (6.97) 7 (9.45)PSI 27 (15.69) 9 (12.16)

Individual weaknessesVCI 27 (15.69) 18 (24.32)PRI 13 (7.55) 2 (2.70)WMI 77 (44.76) 20 (27.02)**PSI 44 (25.58) 13 (17.56)

Key assetsVCI 14 (8.13) 6 (8.10)PRI 21 (12.20) 13 (17.56)WMI 0 (0) 3 (4.05)**PSI 4 (2.32) 6 (8.10)*

High-priority concernsVCI 13 (7.55) 5 (6.75)PRI 0 (0) 1 (1.35)WMI 35 (20.34) 4 (5.40)**PSI 18 (10.46) 1 (1.35)*

Note. Ns: children with specific learning disorder = 172, clinical referral controls = 74. CPI = Cognitive Proficiency Index; CRC = clinical referral controls; FSIQ = Full-Scale Intelligence Quotient; GAI = General Ability Index; PRI = Perceptual Reasoning Index; PSI = Processing Speed Index; SLD = specific learning disorder; VCI = Verbal Comprehension Index; WISC-IV = Wechsler Intelligence Scale for Children (4th ed.); WMI = Working Memory Index.*p ≤ .05. **p ≤ .001.

0.10, Coding, F(1, 244) = 16.604, p < .001, η2p = 0.06, and

Symbol Search, F(1, 244) = 14.816, p < .001, η2p = partial

0.06. Moreover, CRC presented a lower discrepancy between FSIQ and GAI, F(1, 244) = 18.717, p < .001, η2

p = 0.07, between GAI and CPI, F(1, 244) = 15.465, p < .001,

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6 Journal of Learning Disabilities

η2p = 0.06, between highest and lowest index, F(1, 244) =

6.233, p = .013, η2p = 0.02, and between highest and lowest

subtest, F(1, 244) = 7.798, p = .006, η2p = 0.03.

No significant differences emerged between frequencies of interpretable indexes (FSIQ, GAI, and CPI) in children with SLD and in CRC (see Table 2); in both groups GAI and CPI were interpretable with significant higher frequen-cies in comparison with FSIQ (in SLD: χ2 = 63.21, p < .001 and χ2 = 46.45, p < .001, respectively; in CRC: χ2 = 19.49, p < .001 and χ2 = 19.49, p < .001, respectively).

Comparison Between SLD Isolated Impairments and CRC

Comparing CRC and SLD subtypes of isolated impairments (24 reading, 17 written expression, 24 mathematics), differ-ences emerged for FSIQ, F(3, 135) = 8.723, p < .001, η2

p = 0.17, GAI, F(3, 135) = 4.929, p = .003, η2

p = 0.10, CPI, F(3, 135) = 10.385, p < .001, η2

p = 0.19, PRI, F(3, 135) = 6.478,

p < .001, η2p = 0.13, WMI, F(3, 135) = 9.180, p < .001, η2

p = 0.17, PSI, F(3, 135) = 4.543, p = .005, η2

p = 0.09, and among subtests for Block Design, F(3, 135) = 6.003, p = .001, η2

p =

0.12, Matrix Reasoning, F(3, 135) = 746, p = .045, η2p =

0.05, Digit Span, F(3, 135) = 4.744, p = .004, η2p = 0.09,

Letter–Number Sequencing, F(3, 135) = 8.006, p = .027, η2

p = 0.15, Coding, F(3, 135) = 3.411, p = .019, η2p = 0.07,

and Symbol Search, F(3, 135) = 3.995, p = .009, η2p = 0.08.

Bonferroni-corrected post hoc analysis showed the following:

•• FSIQ: CRC and SLD children with impairment in reading and SLD children with impairment in writ-ten expression outperformed SLD children with impairment in mathematics (p < .001, p = .001, p = .039, respectively)

•• GAI: CRC and SLD children with impairment in reading outperformed SLD children with impairment in mathematics (p = .011, p = .001, p < .001, respectively)

•• CPI: CRC outperformed both SLD children with impairment of reading and SLD children with impairment in mathematics (p = .023 and p < .001, respectively); SLD children with impairment in writ-ten expression outperformed only SLD children with impairment in mathematics (p = .047)

•• PRI: CRC and SLD children with impairment in reading outperformed SLD children with impairment in mathematics (both p = .001)

•• WMI: CRC and SLD children with impairment in reading outperformed SLD children with impairment in mathematics (p < .001 and p = .012, respectively)

•• PSI: CRC outperformed SLD children with impair-ment in reading (p = .014)

•• Block Design: CRC and SLD children with impair-ment in reading outperformed SLD children with impairment in mathematics (p = .003 and p = .001, respectively)

•• Digit Span: CRC outperformed SLD children with impairment in mathematics (p = .002)

•• Letter–Number Sequencing: CRC and SLD children with impairment in reading outperformed SLD chil-dren with impairment in mathematics (p < .001 and p = .004, respectively)

•• Coding: CRC outperformed SLD children with impairment in reading (p = .024)

•• Symbol Search: CRC outperformed SLD children with impairment in mathematics (p = .035)

Investigation of Comorbidity: Isolated Versus Associated Impairments

In the previous analysis CRC and the three SLD subgroups of isolated impairment were compared. The next step, as already reported in the recent study of Wilcutt and

Table 3. Correlations Between WISC-IV Indexes in Children With SLD.

Indexes FSIQ GAI CPI VCI PRI WMI PSI

FSIQ .876** .704** .735** .723** .686** .406**GAI .286** .848** .800** .444** .015CPI .229** .266** .738** .784**VCI .366** .381** –.013PRI .365** .060WMI .166*PSI

Note. CPI = Cognitive Proficiency Index; FSIQ = Full-Scale Intelligence Quotient; GAI = General Ability Index; PRI = Perceptual Reasoning Index; PSI = Processing Speed Index; SLD = specific learning disorder; VCI = Verbal Comprehension Index; WISC-IV = Wechsler Intelligence Scale for Children (4th ed.); WMI = Working Memory Index.*p ≤ .05. **p ≤ .001.

Table 4. Correlations Between WISC-IV Subtests in Children With SLD.

Subtests SI VOC CO BD PC MR DS LN CD SS

SI .668** .547** .366** .461** .391** .377** .362** .118 .181*VOC .472** .376** .356** .339** .334** .327** .153* .221**CO .191* .320** .264** .240** .268** .060 .113BD .455** .494** .305** .409** .161* .242**PC .422** .275** .382** .124 .241**MR .339** .450** .154* .253**DS .544** .078 .179*LN .243** .337**CD .505**SS

Note. BD = Block Design; CD = Coding; CO = Comprehension; DS = Digit Span; LN = Letter–Number Sequencing; MR = Matrix Reasoning; PC = Picture Concepts; SI = Similarities; SLD = specific learning disorder; SS = Symbol Search; VOC = Vocabulary; WISC-IV = Wechsler Intelligence Scale for Children (4th ed.).*p ≤ .05. **p ≤ .001.

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colleagues (2013) in relation to the comorbidity between reading and mathematics impairments, is the comparison between two SLD subtypes of isolated impairment and the SLD subtype with these impairments in association.

•• Reading and mathematics: In the previous analysis SLD children with isolated impairment in reading outperformed SLD children with isolated impairment in mathematics on FSIQ, GAI, PRI, WMI, Block Design, and Letter–Number Sequencing. Comparing SLD children with isolated impairment in reading (n = 24), SLD children with isolated impairment in mathematics (n = 24), and SLD children with associ-ated impairments in reading and in mathematics (n = 23), differences emerged for FSIQ, F(2, 68) = 8.313,

p = .001, η2p = 0.20, GAI, F(2, 68) = 10.079, p < .001,

η2p = 0.23, VCI, F(2, 68) = 5.414, p = .007, η2

p = 0.14, PRI, F(2, 68) = 7.507, p = .001, η2

p = 0.18, WMI, F(2, 68) = 4.959, p = .010, η2

p = 0.13, and among sub-tests only for Letter–Number Sequencing, F(2, 68) = 4.509, p = .014, η2

p = 0.12. Bonferroni-corrected post hoc analysis showed that SLD children with impair-ment in reading outperformed SLD children with impairments in reading and in mathematics in asso-ciation on FSIQ (p = .009), GAI (p = .005), VCI (p = .026), PRI (p = .037), and Letter–Number Sequencing (p = .020), whereas no significant differences emerged between SLD children with isolated impairment in mathematics and SLD children with impairments of reading and mathematics in association

Table 5. Demographic Data and WISC-IV Performances in CRC and in SLD Subtypes of Impairment.

Measures CRC (M, SD) SLD Isolated IR (M, SD) SLD Isolated IM (M, SD) SLD Isolated IWE (M, SD)

Descriptive characteristics Age (years) 10.51 (2.54) 12.21 (3.10) 10.83 (2.77) 9.23 (2.00) Gender (M/F) 46/28 17/7 14/10 11/6 Grade 5.45 (2.47) 7.08 (2.99) 6.04 (2.64) 4.54 (1.85)WISC-IV indexes Full-Scale IQ 102.26 (11.00) 101.17 (8.17) 90.67 (9.40)** 99.08 (7.44) General Ability 103.42 (11.38) 107.00 (8.79) 95.67 (8.72)** 101.92 (13.41) Cognitive Proficiency 99.51 (11.20) 91.71 (10.72)* 86.17 (11.59)** 95.15 (8.41) Verbal Comprehension 99.57 (12.94) 103.58 (9.41) 95.92 (10.24) 102.31 (15.99) Perceptual Reasoning 107.03 (12.13) 109.04 (11.31) 96.33 (8.21)** 101.54 (15.02) Working Memory 98.54 (11.01) 94.50 (12.53) 85.13 (10.22)** 93.08 (8.34) Processing Speed 100.73 (13.52) 91.54 (11.79)* 92.88 (13.20)* 98.85 (10.64)WISC-IV subtests Similarities 9.66 (2.37) 8.79 (2.73) 8.79 (2.91) 8.31 (2.17) Vocabulary 9.80 (2.55) 9.71 (3.12) 8.88 (2.45) 9.00 (3.05) Comprehension 10.31 (2.70) 10.08 (3.13) 10.58 (3.50) 10.08 (2.87) Block Design 10.68 (2.42) 10.42 (3.57) 8.21 (2.79)** 9.85 (2.34) Picture Concepts 11.49 (2.62) 11.21 (3.14) 9.13 (3.37) 9.77 (3.21) Matrix Reasoning 11.18 (2.67) 10.50 (3.40) 9.38 (2.94) 10.15 (1.46) Digit Span 9.82 (2.16) 8.79 (2.84) 8.08 (2.82)** 7.75 (3.02) Letter–Number Sequencing 9.70 (2.29) 9.71 (2.71) 7.79 (2.33)** 8.38 (2.87) Coding 9.09 (2.70) 8.04 (2.15)* 7.63 (2.35) 8.62 (2.39) Symbol Search 11.20 (2.44) 10.17 (1.94) 9.46 (2.34)* 10.31 (3.22)WISC-IV discrepancies FSIQ-GAI 3.72 (3.17) 5.92 (4.82) 5.63 (3.88) 6.08 (4.17) GAI-CPI 9.11 (8.24) 11.17 (8.36) 12.75 (8.40) 8.77 (7.07) VCI-PRI 13.00 (10.36) 12.92 (9.05) 10.96 (9.29) 14.08 (8.71) WMI-PSI 12.68 (11.02) 15.38 (13.45) 11.04 (8.30) 16.31 (11.34) Highest – lowest index 22.26 (11.96) 26.08 (10.71) 22.33 (10.52) 24.62 (8.96) Highest – lowest subtest 6.85 (1.98) 7.67 (1.99) 6.88 (1.65) 7.31 (2.21)

Note. Ns: IR = SLD children with isolated impairment in reading (n = 24), IM = SLD children with isolated impairment in mathematics (n = 24); IWE = SLD children with isolated impairment in written expression (n = 17); clinical referral controls = 74. CPI = Cognitive Proficiency Index; CRC = clinical referral controls; FSIQ = Full-Scale Intelligence Quotient; GAI = General Ability Index; PRI = Perceptual Reasoning Index; PSI = Processing Speed Index; SLD = specific learning disorder; VCI = Verbal Comprehension Index; WISC-IV = Wechsler Intelligence Scale for Children (4th ed.); WMI = Working Memory Index.*p ≤ .05. **p ≤ .001 in comparison with CRC.

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•• Reading and written expression: In the previous analysis no differences emerged between SLD chil-dren with isolated impairment in reading and SLD children with isolated impairment in written expres-sion. Also comparing SLD children with isolated impairment in reading (n = 24), SLD children with isolated impairment in written expression (n = 17) and SLD children with impairments in reading and in written expression in association (n = 25), no signifi-cant differences emerged.

•• Written expression and mathematics: In the previous analysis SLD children with isolated impairment in written expression outperformed SLD children with isolated impairment in mathematics on FSIQ and CPI. Comparing SLD children with isolated impairment in

written expression (n = 17), SLD children with iso-lated impairment in mathematics (n = 24) and SLD children with impairments in written expression and in mathematics in association (n = 15), significant dif-ferences emerged for FSIQ, F(2, 53) = 4.877, p = .011, η2

p = 0.15, GAI, F(2, 53) = 3.349, p = .43, η2p = 0.11,

CPI, F(2, 53) = 4.069, p = .023, η2p = 0.13, VCI, F(2,

53) = 4.093, p = .022, η2p = 0.13. Bonferroni-corrected

post hoc analysis showed that SLD children with iso-lated impairment in written expression outperformed SLD children with isolated impairments in mathemat-ics also on WMI (p = .040) and outperformed SLD children with impairments in written expression and in mathematics in association on FSIQ (p = .040), GAI (p = .028), and VCI (p = .011).

Table 6. Comparison of Isolated Versus Associated Impairments: Reading and Mathematics.

Measures CRC (M, SD) SLD Isolated IR (M, SD) SLD Isolated IM (M, SD) SLD IR + IM (M, SD)

Descriptive characteristics Age (years) 10.51 (2.54) 12.21 (3.10) 10.83 (2.77) 10.96 (3.19) Gender (M/F) 46/28 17/7 14/10 22/11 Grade 5.45 (2.47) 7.08 (2.99) 6.04 (2.64) 5.87 (2.33)WISC-IV indexes Full-Scale IQ 102.26 (11.00) 101.17 (8.17) 90.67 (9.40)* 92.00 (12.69)** General Ability 103.42 (11.38) 107.00 (8.79) 95.67 (8.72)** 106.68 (8.75)** Cognitive Proficiency 99.51 (11.20) 91.71 (10.72) 86.17 (11.59) 86.30 (14.71) Verbal Comprehension 99.57 (12.94) 103.58 (9.41) 95.92 (10.24) 95.39 (10.41)* Perceptual Reasoning 107.03 (12.13) 109.04 (11.31) 96.33 (8.21)** 99.74 (16.34)* Working Memory 98.54 (11.01) 94.50 (12.53) 85.13 (10.22)* 86.57 (13.62) Processing Speed 100.73 (13.52) 91.54 (11.79) 92.88 (13.20) 91.70 (14.53)WISC-IV subtests Similarities 9.66 (2.37) 8.79 (2.73) 8.79 (2.91) 7.91 (2.31) Vocabulary 9.80 (2.55) 9.71 (3.12) 8.88 (2.45) 8.52 (2.76) Comprehension 10.31 (2.70) 10.08 (3.13) 10.58 (3.50) 9.70 (3.54) Block Design 10.68 (2.42) 10.42 (3.57) 8.21 (2.79)** 9.39 (3.29) Picture Concepts 11.49 (2.62) 11.21 (3.14) 9.13 (3.37) 9.70 (3.03) Matrix Reasoning 11.18 (2.67) 10.50 (3.40) 9.38 (2.94) 10.26 (3.07) Digit Span 9.82 (2.16) 8.79 (2.84) 8.08 (2.82) 8.48 (2.57) Letter–Number

Sequencing9.70 (2.29) 9.71 (2.71) 7.79 (2.33)** 7.52 (2.59)*

Coding 9.09 (2.70) 8.04 (2.15) 7.63 (2.35) 7.26 (3.09) Symbol Search 11.20 (2.44) 10.17 (1.94) 9.46 (2.34) 9.35 (2.53)WISC-IV discrepancies FSIQ-GAI 3.72 (3.17) 5.92 (4.82) 5.63 (3.88) 5.91 (4.93) GAI-CPI 9.11 (8.24) 11.17 (8.36) 12.75 (8.40) 11.52 (11.66) VCI-PRI 13.00 (10.36) 12.92 (9.05) 10.96 (9.29) 12.30 (12.95) WMI-PSI 12.68 (11.02) 15.38 (13.45) 11.04 (8.30) 10.65 (7.69) Highest – lowest index 22.26 (11.96) 26.08 (10.71) 22.33 (10.52) 23.09 (13.02) Highest – lowest subtest 6.85 (1.98) 7.67 (1.99) 6.88 (1.65) 7.35 (3.20)

Note. Ns: IR (SLD children with isolated impairment in reading) = 24, IM (SLD children with isolated impairment in mathematics) = 24); IR + IM = 23; clinical referral controls = 74. CPI = Cognitive Proficiency Index; CRC = clinical referral controls; FSIQ = Full-Scale Intelligence Quotient; GAI = General Ability Index; PRI = Perceptual Reasoning Index; PSI = Processing Speed Index; SLD = specific learning disorder; VCI = Verbal Comprehension Index; WISC-IV = Wechsler Intelligence Scale for Children (4th ed.); WMI = Working Memory Index.*p ≤ .05. **p ≤ .001 in comparison with IR.

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Discussion

This study aimed at investigating WISC-IV intellectual pro-files associated with the single overarching diagnostic cat-egory of SLD introduced by the DSM-5. Specific predictions were formulated regarding the presence of (a) deficits in working memory and processing speed in children with SLD and (b) different WISC-IV profiles between subtypes of SLD. These predictions were confirmed by empirical findings, as discussed in the three next sections.

Identification of Intellectual Functioning in Children With SLD

A first issue concerns the identification of the level of intel-lectual functioning. In both SLD and CRC groups only

45.34% and 51.35% of participants, respectively, presented an interpretable FSIQ; these low frequencies were due to mean discrepancies between the highest and lowest indexes (among VCI, PRI, WMI, and PSI) that were higher than the cutoff point (≤22) indicated in the WISC-IV manual (Wechsler, 2003b) to consider the FSIQ as interpretable (26.05 ± 10.57 for children with SLD; 22.26 ± 11.96 for CRC). Different from FSIQ, both GAI and CPI presented higher frequencies of interpretable scores (86.04% and 80.81%, respectively, in children with SLD; 85.13% and 85.13%, respectively, in CRC). The low frequencies of inter-pretable FSIQ in children with SLD and in CRC were mostly due to the discrepancy between indexes of General Ability and indexes of Cognitive Proficiency, and children with SLD presented a higher discrepancy between GAI and CPI in comparison with CRC (14.68 ± 10.90 vs. 9.11 ± 8.24).

Table 7. Comparison of Isolated Versus Associated Impairments: Reading and Written Expression.

Measures CRC (M, SD) SLD Isolated IR (M, SD) SLD Isolated IWE (M, SD) SLD IR + IWE (M, SD)

Descriptive characteristics Age (years) 10.51 (2.54) 12.21 (3.10) 9.23 (2.00)** 9.17 (2.58)** Gender (M/F) 46/28 17/7 11/6 Grade 5.45 (2.47) 7.08 (2.99) 4.54 (1.85)** 3.83 (2.58)**WISC-IV indexes Full-Scale IQ 102.26 (11.00) 101.17 (8.17) 99.08 (7.44) 97.46 (12.17) General Ability 103.42 (11.38) 107.00 (8.79) 101.92 (13.41) 103.17 (13.19) Cognitive Proficiency 99.51 (11.20) 91.71 (10.72) 95.15 (8.41) 89.58 (12.52) Verbal Comprehension 99.57 (12.94) 103.58 (9.41) 102.31 (15.99) 102.50 (13.75) Perceptual Reasoning 107.03 (12.13) 109.04 (11.31) 101.54 (15.02) 102.63 (13.67) Working Memory 98.54 (11.01) 94.50 (12.53) 93.08 (8.34) 91.50 (12.96) Processing Speed 100.73 (13.52) 91.54 (11.79) 98.85 (10.64) 91.96 (12.53)WISC-IV subtests Similarities 9.66 (2.37) 8.79 (2.73) 8.31 (2.17) 9.00 (2.18) Vocabulary 9.80 (2.55) 9.71 (3.12) 9.00 (3.05) 10.00 (2.67) Comprehension 10.31 (2.70) 10.08 (3.13) 10.08 (2.87) 10.71 (2.85) Block Design 10.68 (2.42) 10.42 (3.57) 9.85 (2.34) 10.21 (2.68) Picture Concepts 11.49 (2.62) 11.21 (3.14) 9.77 (3.21) 10.83 (2.79) Matrix Reasoning 11.18 (2.67) 10.50 (3.40) 10.15 (1.46) 10.71 (2.92) Digit Span 9.82 (2.16) 8.79 (2.84) 7.75 (3.02) 8.92 (2.58) Letter–Number Sequencing 9.70 (2.29) 9.71 (2.71) 8.38 (2.87) 8.75 (2.23) Coding 9.09 (2.70) 8.04 (2.15) 8.62 (2.39) 7.38 (3.07) Symbol Search 11.20 (2.44) 10.17 (1.94) 10.31 (3.22) 10.29 (2.45)WISC-IV discrepancies FSIQ-GAI 3.72 (3.17) 5.92 (4.82) 6.08 (4.17) 6.79 (5.02) GAI-CPI 9.11 (8.24) 11.17 (8.36) 8.77 (7.07) 14.54 (11.17) VCI-PRI 13.00 (10.36) 12.92 (9.05) 14.08 (8.71) 10.17 (8.24) WMI-PSI 12.68 (11.02) 15.38 (13.45) 16.31 (11.34) 12.13 (7.46) Highest – lowest index 22.26 (11.96) 26.08 (10.71) 24.62 (8.96) 23.83 (10.29) Highest – lowest subtest 6.85 (1.98) 7.67 (1.99) 7.31 (2.21) 7.57 (2.55)

Note. Ns: IR (SLD children with isolated impairment in reading) = 24, IWE (SLD children with isolated impairment in written expression) = 17; IR + IWE = 25; clinical referral controls = 74. CPI = Cognitive Proficiency Index; CRC = clinical referral controls; FSIQ = Full-Scale Intelligence Quotient; GAI = General Ability Index; PRI = Perceptual Reasoning Index; PSI = Processing Speed Index; SLD = specific learning disorder; VCI = Verbal Comprehension Index; WISC-IV = Wechsler Intelligence Scale for Children (4th ed.); WMI = Working Memory Index.**p ≤ .001 in comparison with IR.

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Therefore the FSIQ in children with SLD was lowered by CPI, as suggested by the 81.39% of children with GAI greater than FSIQ (56.75% in CRC, p < .001), similarly to the greater than 70% reported by Wechsler (2003b) in 133 children with SLD and 45 children with SLD and attention-deficit/hyperactivity disorder. These findings confirmed the first prediction formulated regarding the presence of work-ing memory and processing speed deficits in the WISC-IV intellectual profile of children with SLD.

Moreover, considering its frequencies of interpretable scores and of higher scores in comparison with FSIQ (98.91 ±12.01 vs. 93.85 ± 11.23), this study also suggested that the GAI could probably be considered the best and more con-servative measure provided by the WISC-IV to identify intellectual functioning in children with SLD (Cornoldi, Giofrè, Orsini, & Pezzuti, 2014; Prifitera, Weiss, &

Saklofske, 1998; Raiford, Weiss, Rolfhus, & Coalson, 2005; Wechsler, 2003b; Weiss & Gabel, 2008). A signifi-cant discrepancy between GAI and CPI (with GAI > CPI) was reported also in attention-deficit/hyperactivity disorder (Mayes & Calhoun, 2006; Thaler, Bello, & Etcoff, 2013), intellectual disabilities (Koriakin et al., 2013), high-func-tioning autism (Mayes & Calhoun, 2008; Oliveras-Rentas, Kenworthy, Roberson, Martin, & Wallace, 2012), and pedi-atric epilepsy (Gottlieb, Zelko, Jim, & Nordli, 2012), indi-cating that a significant intellectual discrepancy between GAI and CPI (GAI > CPI) could be considered a “cognitive sign” of SLD taken as a single diagnostic category and probably also of other neurodevelopmental disorders. As a matter of fact, these neurodevelopmental disorders present heterogeneous cognitive profiles but share an impairment of executive functions (Danielsson, Henry, Messer, &

Table 8. Comparison of Isolated Versus Associated Impairments: Written Expression and Mathematics.

Measures CRC (M, SD) SLD Isolated IWE (M, SD) SLD Isolated IM (M, SD) SLD IWE + IM (M, SD)

Descriptive characteristics Age (years) 10.51 (2.54) 9.23 (2.00) 10.83 (2.77)** 10.40 (2.13) Gender (M/F) 46/28 11/6 14/10 Grade 5.45 (2.47) 4.54 (1.85) 6.04 (2.64) 5.13 (1.92)WISC-IV indexes Full-Scale IQ 102.26 (11.00) 99.08 (7.44) 90.67 (9.40)* 91.73 (9.26)* General Ability 103.42 (11.38) 101.92 (13.41) 95.67 (8.72) 92.60 (8.93)* Cognitive Proficiency 99.51 (11.20) 95.15 (8.41) 86.17 (11.59) 92.33 (10.62) Verbal Comprehension 99.57 (12.94) 102.31 (15.99) 95.92 (10.24) 89.47 (11.40)* Perceptual Reasoning 107.03 (12.13) 101.54 (15.02) 96.33 (8.21) 98.07 (8.98) Working Memory 98.54 (11.01) 93.08 (8.34) 85.13 (10.22)* 89.60 (14.01) Processing Speed 100.73 (13.52) 98.85 (10.64) 92.88 (13.20) 98.07 (14.12)WISC-IV subtests Similarities 9.66 (2.37) 8.31 (2.17) 8.79 (2.91) 8.13 (2.69) Vocabulary 9.80 (2.55) 9.00 (3.05) 8.88 (2.45) 8.80 (2.00) Comprehension 10.31 (2.70) 10.08 (2.87) 10.58 (3.50) 9.27 (2.60) Block Design 10.68 (2.42) 9.85 (2.34) 8.21 (2.79) 9.47 (1.95) Picture Concepts 11.49 (2.62) 9.77 (3.21) 9.13 (3.37) 9.87 (3.20) Matrix Reasoning 11.18 (2.67) 10.15 (1.46) 9.38 (2.94) 10.33 (2.69) Digit Span 9.82 (2.16) 7.75 (3.02) 8.08 (2.82) 8.07 (2.31) Letter–Number Sequencing 9.70 (2.29) 8.38 (2.87) 7.79 (2.33) 8.33 (3.06) Coding 9.09 (2.70) 8.62 (2.39) 7.63 (2.35) 6.40 (2.82) Symbol Search 11.20 (2.44) 10.31 (3.22) 9.46 (2.34) 10.27 (3.17)WISC-IV discrepancies FSIQ-GAI 3.72 (3.17) 6.08 (4.17) 5.63 (3.88) 4.87 (6.41) GAI-CPI 9.11 (8.24) 8.77 (7.07) 12.75 (8.40) 13.40 (9.29) VCI-PRI 13.00 (10.36) 14.08 (8.71) 10.96 (9.29) 12.13 (10.58) WMI-PSI 12.68 (11.02) 16.31 (11.34) 11.04 (8.30) 12.60 (8.60) Highest – lowest index 22.26 (11.96) 24.62 (8.96) 22.33 (10.52) 24.27 (9.55) Highest – lowest subtest 6.85 (1.98) 7.31 (2.21) 6.88 (1.65) 7.73 (1.87)

Note. Ns: IM (SLD children with isolated impairment in mathematics) = 24; IWE (SLD children with isolated impairment in written expression) = 17; IWE + IM = 15; clinical referral controls = 74. CPI = Cognitive Proficiency Index; CRC = clinical referral controls; FSIQ = Full-Scale Intelligence Quotient; GAI = General Ability Index; PRI = Perceptual Reasoning Index; PSI = Processing Speed Index; SLD = specific learning disorder; VCI = Verbal Comprehension Index; WISC-IV = Wechsler Intelligence Scale for Children (4th ed.); WMI = Working Memory Index.*p ≤ .05. **p ≤ .001 in comparison with IWE.

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Ronnberg, 2012; Johnson, 2012; Parrish et al., 2007; Pellicano, 2012; Rubia, 2011; Van der Molen, Van Luit, Jongmans, & Van der Molen, 2007), that may directly affect WISC-IV performances in CPI-related subtests rather than GAI-related subtests. At the same time it must be empha-sized that an impairment of executive functions, especially of working memory rather than of processing speed, may also affect abilities tapped by GAI-related subtests (Ackerman, Beier, & Boyle, 2002; Cornoldi, Orsini, Cianci, Giofrè, & Pezzuti, 2013; Fry & Hale, 2000). This finding was confirmed also in this study (see Table 3) by high sig-nificant correlations between GAI indexes and WMI (r = .444 between GAI and WMI, r = .381 between VCI and WMI, r = .365 between PRI and WMI) and low correla-tions between GAI indexes and PSI (r = .015 between GAI and PSI, r = –.013 between VCI and PSI, r = .061 between PRI and PSI).

Comparison Between Children With SLD and Clinical Referral Controls

A second issue regards the comparison of WISC-IV intel-lectual profiles between children with SLD and CRC. SLD children presented lower scores in comparison with CRC in FSIQ, GAI, CPI, WMI, and PSI, and, among subtests, in Similarities, Block Design, Picture Concepts, Matrix Reasoning, Digit Span, Letter–Number Sequencing, Coding, and Symbol Search. Although differences involved many subtests of both GAI and CPI, the analysis of subtest performances suggested that main difficulties of SLD chil-dren were limited to Digit Span, Letter–Number Sequencing, and Coding, presenting the lowest mean scores and the highest discrepancies in comparison with CRC (1.72 for Digit Span, 1.77 for Letter–Number Sequencing, 1.52 for Coding). Although lower in comparison with CRC, perfor-mances on Symbol Search and on PRI subtests (Block Design, Picture Concepts, Matrix Reasoning) were almost on average and therefore cannot be considered as impaired. Finally, a less severe impairment, in comparison to CPI-related subtest, characterized Similarities, with a score was below average (8.80).

This pattern confirmed again the first prediction of the study regarding the presence of working memory and pro-cessing speed deficits in SLD and is in agreement with models of multiple cognitive deficits at the basis of neuro-developmental disorders in general, of SLD in particular (McGrath et al., 2011; Pennington, 2006; Wilcutt et al., 2013; Wolf & Bowers, 1999). The impairment of working memory is in accordance with several previous studies (De Clerq-Quagebeur et al., 2010; De Weerdt et al.. 2012; Peng et al., 2012; Schuchardt et al., 2008; Wilcutt et al., 2013), and probably should be a target of general cognitive reha-bilitation (e.g., Dunning, Holmes, & Gathercole, 2013; Loosli, Buschkuehl, Perrig, & Jaeggi, 2012) in addition to

rehabilitative interventions tailored to specific impairments (reading, written expression, and mathematics). The impair-ment of processing speed was indicated by more severe dif-ficulties in Coding in comparison with Symbol Search: This pattern could be explained considering that “Symbol Search is a relatively simple visual scan test that requires a mini-mum of graphomotor demand in relation to Coding, which has more demanding visual search, memory, and copying components that could be significant factors in understand-ing score differences” (Weiss et al., 2006, p. 155). Therefore the more demanding visual search component of Coding in comparison with Symbol Search could explain more diffi-culties of SLD children in this subtest.

Regarding the milder deficit in Similarities, this sub-test asks how two words representing objects or concepts are alike. The concepts must have been acquired and stored in long-term memory, and the child must be able to access that knowledge from semantic memory on demand. Once these words are recalled, the child can begin to think about similarities: According to Weiss and col-leagues (2006, pp. 144–145),

This reasoning process appears to take place within a transitory working memory space, and the ability to reason may be related to working memory capacity and the efficiency with which ideas are processed in working memory before the trace fade. . . . Children with an age-appropriate knowledge base that is readily accessible but who have deficits in higher order categorization of abstract verbal concepts may score higher on Vocabulary than Similarities and Comprehension. In these cases, it may then also be instructive to compare performance on Similarities with Picture Concepts: both subtests require categorization of abstract verbal concepts, but Picture Concepts does not require that the child explain his or her thinking verbally.

In this study both children with SLD and children with CRC had significantly better performances in Picture Concepts in comparison with Similarities (10.47 ± 2.93 vs. 8.80 ± 2.30, t = −6.423, p < .001; 11.49 ± 2.62 vs. 9.66 ± 2.37, t = −5.687, p < .001; respectively). Overall, more severe difficulties of children with SLD in Similarities, in comparison with CRC, are probably due to the interaction between verbal expres-sion and poorer verbal working memory.

Searching for Common and Specific Cognitive Deficits Between SLD Subtypes

From the comparisons between (a) CRC and subgroups of SLD children with isolated impairment and (b) SLD chil-dren with isolated impairments and SLD children with the same impairments in association, the following emerged:

•• Isolated impairment in reading is associated with a cognitive deficit in processing speed, as evidenced

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12 Journal of Learning Disabilities

by difficulties in Coding, as reported since previous version of the WISC (Kaufman, 1981, 1994; Whitehouse, 1983). As previously underlined, more demanding visual search component of Coding in comparison with Symbol Search could explain spe-cific difficulties of children with impairment in reading in this subtest, confirming that in addition to phonological features (Morris et al., 1998), features of visuospatial attention are involved in reading (e.g., Bosse, Tainturier, & Valdois, 2007; Franceschini, Gori, Ruffino, Pedrolli, & Facoetti, 2012; Menghini et al., 2010), in agreement with the double-deficit model of impairment of reading (Norton et al., 2014; Wolf & Bowers, 1999) and models of multiple cognitive deficits at the basis of neurodevelopmental disorders (Pennington, 2006; Wilcutt et al., 2013).

•• Isolated impairment in written expression wasn’t associated with any cognitive deficits. This finding could instill doubt that in this cases the diagnosis of SLD was correct; however, restrictive diagnostic criteria on learning proficiency (in this case perfor-mances <5 percentile) suggested that these children truly presented a SLD, probably due to underlying cognitive deficits not evaluated by WISC-IC core subtests. In this perspective a recent study examin-ing the structure of working memory impairment in impairment in reading versus impairment in written expression reported distinct underlying cognitive deficits, that is, deficits in domain-general central executive for impairment in reading and more severe deficits of phonological loop for impairment in written expression (Brandenburg et al., 2014). Furthermore, once again this conclusion strongly discourages considering the WISC-IV merely for diagnostic purposes rather than as a tool to define the individual profile of cognitive strengths and weaknesses for subsequent intervention programs, according to major conclusions of the white paper of the Learning Disabilities Association of America (2010).

•• Isolated impairment in mathematics is associated with a more general cognitive deficit involving per-ceptual reasoning, working memory, and process-ing speed. This finding suggests that the impairment in mathematics is associated with difficulties in several broad abilities described by the Cattell–Horn–Carroll theory of human abilities (McGrew, 2005) assessed by the WISC-IV, including Fluid Intelligence, Visual Processing, and Short-Term Memory, as previously reported in other studies on children with difficulties in mathematics (J. B. Hale et al., 2008; Proctor, 2012; Taub, Floyd, Keith, & McGrew, 2008).

•• No significant differences between impairments in reading and in written expression emerged, indicat-ing once again that distinct cognitive deficits (Brandenburg et al., 2014) are not detectable by WISC-IV. Second, no significant differences emerged between isolated impairment in mathemat-ics and impairments in mathematics associated with impairments in reading or in written expression.

Overall these findings confirmed the second prediction of the study regarding the presence of different cognitive defi-cits between subtypes of SLD impairment, especially com-paring impairment in reading and impairment in mathematics, in line with recent studies comparing the same SLD subtypes of impairment (Moll, Gobel, Gooch, Landerl, & Snowling, 2014; Wilcutt et al., 2013).

Limitations

The first limitation is the selection of CRC for the compari-son with children with SLD. CRC cannot be considered as healthy controls considering their difficulties in learning proficiency, leading to clinical referral, as confirmed by their WISC-IV intellectual profile, characterized by a high discrepancy between indexes (22.26 ± 11.96); this finding is probably due to the restrictive cutoff points suggested by Italian guidelines for the diagnosis of SLD (<2 standard deviations below mean or <5–10 percentiles), leading to the inclusion in CRC of children that could have been included in SLD with less restrictive cutoff points. On one hand, this suggests that the comparison between SLD and age-matched controls selected from the general population or adopting different cutoff points for the diagnosis of SLD could have produced different findings, as impairments in other cognitive domains that resulted almost preserved in this study; on the other hand, it strengthens findings on cog-nitive impairments associated with SLD, having being detected in comparison with CRC, that is, a population with difficulties in learning proficiency.

The second limitation is represented by the small num-ber of children with SLD in subgroups split on the basis of impairments, leading to a potential loss of statistical power.

The third limitation is that WISC-IV supplemental sub-tests (Information, Word Reasoning, Picture Completion, Arithmetic, and Cancellation) were not administered; this choice was necessary considering that the WISC-IV was administered during a routine assessment for diagnostic purposes in a clinical service. Indeed, in clinical practice the administration of the WISC is more often limited to core subtests, even if supplemental subtests could provide fur-ther information on intellectual abilities.

Therefore these findings need further replication and a comparison between children with SLD and typically developing children.

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Conclusions

These six major conclusions can be drawn from this study:

•• The GAI is the best and more conservative measure provided by the WISC-IV to identify intellectual functioning in children with SLD.

•• The intellectual discrepancy between GAI and CPI could be considered a “cognitive sign” for the pres-ence of SLD taken a single diagnostic category.

•• Children with SLD present cognitive deficits related to working memory and processing speed, and milder deficits in the Similarities subtest, supporting models of multiple cognitive deficits at the basis of neurodevelopmental disorders, including SLD.

•• Impairment in reading is associated with difficulties in processing speed, especially in Coding.

•• Impairment in mathematics, isolated or in associa-tion, is associated with a more general cognitive deficit involving fluid intelligence, working mem-ory, and processing speed.

•• In addition to the single diagnostic category of SLD, subtypes of impairments have to be specified for empirical, educational, and rehabilitative purposes.

This study reported some cognitive features resulting from the assessment with the WISC-IV in children with SLD. In clinical practice these cognitive features are not sufficient for diagnosing SLD, and at the same time their absence should not suggest the lack of SLD. However, these cogni-tive features cannot be considered clinically meaningless, and their higher incidence among children with SLD can guide diagnosis and subsequent definitions of intervention programs, special educational needs, and strategies of compensation.

Declaration of Conflicting Interests

The author(s) declared no potential conflict of interests with respect to the authorship and/or publication of this article.

Funding

The author(s) received no financial support for the research and/or authorship of this article.

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