Proof version: Bishop, D., & Rutter, M. (2008). Neurodevelopmental disorders: conceptual approaches. In M. Rutter, D. Bishop, D. Pine, S. Scott, J. Stevenson, E. Taylor & A. Thapar (Eds.), Rutter's Child and Adolescent Psychiatry (pp. 32-41). Oxford: Blackwell.
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disorders. Thus, there is considerable overlap between speciļ¬c A āSyndromeā of Neurodevelopmental
reading disability (SRD) and speciļ¬c language impairment Disorder?
(SLI) (Bishop & Snowling, 2004; Eisenmajer, Ross, & Pratt,
2005), between SRD and ADHD (Dykman & Ackerman, 1991; One might start to wonder if, rather than differentiating
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Willcutt & Pennington, 2000) and between SLI and ADHD between neurodevelopmental disorders, it would make more
(Beitchman et al., 1996). Less work has been carried out on sense to group them all together into an overarching category,
developmental coordination disorders (DCD), but there is treating them as variant forms of a common underlying
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evidence for an overlap between motor impairment and both disorder. However, there are sufļ¬cient differences among the
SLI (Hill, 2001) and ADHD (Kadesjƶ & Gillberg, 1998). As neurodevelopmental disorders to preclude such a conceptual-
far as autistic disorder is concerned, the deļ¬ning criteria ization. First, molecular genetic studies have been successful
disallow a diagnosis of SLI in a child meeting criteria for in identifying chromosomal regions associated with risk for
autistic disorder, but it is clear that at the symptomatic level reading disability, SLI, ADHD and ASD, but there has been
there is considerable overlap, with many affected children little or no overlap between the linkages reported for these
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showing the kinds of structural language deļ¬cits that charac- different disorders. For instance, Fisher (2006) noted that
terize SLI (Tager-Flusberg & Joseph, 2003). Furthermore, whereas linkages to dyslexia have been found on chromosomes
many children with SLI or ADHD show in milder form the 1, 2, 3, 6, 15 and 18, those to SLI have been found on chro-
kinds of social/pragmatic impairments that are characteristic mosomes 13, 16 and 19. Fisher pointed out that we need to
of autistic disorder (Bishop & Norbury, 2002; Clark, Feehan, be careful in interpreting such ļ¬ndings: it would be danger-
Tinline et al., 1999; Farmer, 2000; Geurts et al., 2004). SRD ous to assume that there are highly speciļ¬c pathways from
co-occurs with mathematical difļ¬culties at a higher level than genotype to phenotype, especially because few studies have
predicted from the prevalence of either disorder on its own used multivariate methods to look at more than one disorder
D
(Lewis, Hitch, & Walker, 1994). at a time. Undoubtedly there are some genes whose effects
It has become accepted to refer to these patterns of co- are common to more than one neurodevelopmental disorder
occurrence as ācomorbidity,ā but this is misleading because (e.g., Willcutt et al., 2002), but behavior genetic studies also
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it ignores the possibility that much of the supposed com- usually ļ¬nd evidence for speciļ¬c as well as common genetic
orbidity is simply a function of the invalid, and artiļ¬cial, inļ¬uences on co-occurring disorders (e.g., Martin, Piek, & Hay,
diagnostic subdivisions in classiļ¬cation systems (see chap- 2006). Second, there are differences among disorders in drug
ter 2). Thus, for example, it seems likely that much of the response. It is striking, for example, that whereas ADHD shows
co-occurrence of supposedly different anxiety disorders is a marked beneļ¬cial response to stimulant medication (see
simply a consequence of these disorders being slightly differ- chapter 34), no drugs have other than a slight inconsistent effect
EC
ent manifestations of the same underlying condition (see on the basic problems associated with ASD (see chapter 46).
chapter 39). Might the same apply to neurodevelopmental Similarly, although medication may provide some symp-
disorders? Clearly it could. tomatic improvement with the other neurodevelopmental
Thus, SLI and reading disability both comprise disorders disorders, there are not the marked beneļ¬ts that are seen with
of language ā the former with respect to spoken language ADHD. Third, although it is difļ¬cult to compare across
and the latter with respect to written language. It would be imaging studies because of variations in the ways in which they
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rather surprising if there was no co-occurrence between the have been conducted (Peterson, 2003), patterns in the vari-
two. That is not to argue that all cases of reading disability ous neurodevelopmental disorders do not seem at all closely
derive from oral language impairment, because manifestly similar: for instance, fronto-striatal systems are implicated in
they do not (Bishop & Snowling, 2004); but it is to suggest ADHD (see chapter 34), whereas in dyslexia there is reduced
that co-occurrence of some degree is to be expected. activation in left temporo-parietal cortex (see chapter 48).
However, the co-occurrence of neurodevelopmental dis- Fourth, at a behavioral level, there are differences among
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orders does not apply only to language-related disorders; as neurodevelopmental disorders in short and long-term course
noted above, there are also overlaps between language and (Rutter, Kim-Cohen, & Maughan, 2006a). Finally, although
motor impairments, attention deļ¬cit and social deļ¬cits, and most neurodevelopmental disorders are characterized by a
these cannot readily be explained as different manifestations preponderance of males, sex ratios vary across disorders,
of a common cognitive disability. To some extent, overlaps with the male excess being far more striking for ASD than
could reļ¬ect referral bias in clinical samples: for instance, a for other neurodevelopmental disorders (Rutter, Caspi, &
child whose reading or language disability is accompanied by Mofļ¬tt, 2003). Arithmetic difļ¬culties stand out from the
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social impairment or attentional deļ¬cit would be more likely rest, with boys and girls equally likely to be affected (Lewis
to referred to a psychiatrist than one who had an isolated et al., 1994; Rourke, 1989; Shalev, Auerbach, Manor, & Gross-
impairment. However, this cannot be the whole explanation, Tsur, 2000).
because overlaps are seen in epidemiologic samples (e.g., Clearly, it is not feasible to treat the whole gamut of
Beitchman et al., 1996), and second, the rates of co-occurrence neurodevelopmental disorders as a single condition, but
are higher than would be predicted from knowledge of the can we nevertheless identify distinct syndromes within this
frequency of individual disorders. category, in which a pattern of deļ¬cits arises from a common
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NEURODEVELOPMENTAL DISORDERS
neurobiologic cause? Rourkeās (1989) account of ānon-verbal gene gene gene env env env
etiology
learning disabilityā (NLD) is such a model: a distinctive 1 2 3 1 2 3
pattern of strengths and weaknesses in sensorimotor skill,
scholastic achievement, and socioemotional development are
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seen as all originating from destruction or dysfunction of white
matter in the right cerebral hemisphere. The deļ¬cits seen
neurobiology
as characterizing NLD encompass speciļ¬c arithmetical dis-
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order, DCD and Asperger syndrome. The construct of NLD
explains the co-occurrence of these deļ¬cits in terms of a
speciļ¬c neurobiologic basis. However, the validity of the
category is questionable. The different deļ¬cits certainly can cognition cog 1 cog 2 cog 3 cog 4
and do co-occur, and the association of the symptomatology
of Asperger syndrome with the neuropsychologic manifesta-
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tions of NLD has been empirically demonstrated (Klin, beh beh beh beh
Volkmar, Sparrow, et al., 1995). However, the association behavior 1 2 3 4
appears too weak to justify treating it as a syndrome: this
Fig. 3.1. Levels of causation for developmental disorders. The
is demonstrated in studies showing that a high proportion of
dashed line emphasizes that childrenās behavior (beh) can affect
children with a clinical picture of NLD do not show speciļ¬c
the environment (env) they experience. [From Bishop, D. V. M.,
deļ¬cits thought to characterize this disorder (Drummond,
& Snowling, M. J. (2004). Developmental dyslexia and speciļ¬c
Ahmad, & Rourke, 2005; Pelletier, Ahmad, & Rourke, language impairment: Same or different? Psychological Bulletin,
2001). If we embrace the construct of NLD, we end up in 130, 858ā886 with permission.]
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excluding numerous cases because they do not show the
anticipated combination of deļ¬cits, meaning that we either have
to dilute the āsyndromeā to be too general to be useful, or
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we have to devise additional categories to encompass the cases Causal Models of Neurodevelopmental
that do not ļ¬t. Similar problems arise if we try to ļ¬t language, Disorders
literacy and speech disorders into a broader syndrome; we can
ļ¬nd many children who show this constellation of impairments, Cognitive Deļ¬cits
but there are also many who do not (Bishop & Snowling, 2004; One goal of developmental neuropsychology is to uncover
Pennington, 2006). the underlying nature of deļ¬cits seen in neurodevelopmental
EC
Neurodevelopmental disorders thus pose a considerable disorders. As shown in Fig. 3.1, the same behavioral deļ¬cit
challenge for a classiļ¬cation system. On the one hand, we need may arise for different reasons, and one would hope that as
to explain why there is common co-occurrence of different our conceptual understanding advances, we might be able to
deļ¬cits, while at the same time allowing for dissociations categorize disorders not in terms of surface behavior, but
between different types of deļ¬cit, and variable patterns of asso- in terms of underlying cognitive deļ¬cits. For instance, most
ciated features. The causal model shown in Fig. 3.1 provides children with reading disability have difļ¬culties with phono-
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a framework for conceptualizing these questions. In this logic analysis which are evident even when they are tested using
model, ļ¬rst put forward in the context of SLI and dyslexia, methods that do not require any reading or writing (see chap-
a neurodevelopmental disorder is identiļ¬ed on the basis of a ter 48). Other poor readers have visual difļ¬culties, problems
constellation of behaviors; these result from speciļ¬c cognitive learning speciļ¬c spellings of irregular words or poor com-
deļ¬cits, which have particular neurobiologic bases, which are prehension of written texts. A focus on underlying cognitive
in turn affected by genetic or environmental factors. When deļ¬cits thus could help identify new subgroups. It also sug-
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extending the model to cover the whole gamut of neurode- gests that some of the existing distinctions among disorders
velopmental disorder, the ācognitiveā level is taken to include may be unrealistic; for instance, increasingly speech, language
a wide range of underlying mental operations that cannot be and literacy problems are regarded as different manifestations
directly observed, but are inferred from behavior, including of a common phonologic impairment, whose behavioral
perceptual-motor skills, language, memory, social cognition, correlates would vary depending on the age at which the child
reasoning and executive functions. Relationships among the was observed and the severity of the impairment (Bishop
different levels of functioning are not one-to-one, but involve & Snowling, 2004). Nevertheless, if we try to categorize
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complex multifactorial inļ¬uences going from etiology to neu- disorders in terms of underlying deļ¬cit rather than observed
robiology, from neurobiology to cognition, and from cogni- behavior, this does not necessarily simplify our nosology,
tion to behavior. Viewed from this perspective, it is clear that because multiple deļ¬cits are the rule rather than the excep-
overlaps between observed behavioral impairments may arise tion, at the cognitive as well as the behavioral level. Thus,
from shared cognitive deļ¬cits, shared neurobiologic origins although one can identify children who ļ¬t the picture of
and/or shared etiology. We now turn to consider evidence for āphonologic dyslexiaā or āsurface dyslexia,ā most poor
these different causal mechanisms. readers present a mixed picture (Snowling & Nation, 1997).
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Even if one looks at impairments in very different domains, Furthermore, the notion that ābirth injuryā was the main
associated disorders are common. risk factor fell into disrepute in view of the evidence that many
Pennington (2006) conducted a series of studies comparing of the risks supposedly associated with obstetric complica-
children with pure dyslexia, pure ADHD, and comorbid tions actually derived from prenatal problems. A genetically
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dyslexia and ADHD. He was interested in the possibility that abnormal fetus is more likely to have a low birth weight
comorbidity among these disorders might reļ¬ect the inļ¬uence and to be born following premature gestation. That pro-
of one behavior on another (e.g., attentional difļ¬culties might bably accounts for the somewhat inconsistent association
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arise because the child who could not read well became bored between obstetric complications and ASD (Bolton et al.,
and distractible; or conversely, the childās difļ¬culty in attend- 1997). Undoubtedly, extremely low birth weight does lead to
ing could lead to scholastic deļ¬cits). If the ļ¬rst account were an increased rate of motor, language, scholastic and attentional
true, then the comorbid children should resemble the pure difļ¬culties (Marlow, 2004; Marlow et al., 2005). However,
dyslexic cases in terms of underlying impairment. If the the association is not strong when assessed in the opposite
second account applied, then the comorbid children should direction (i.e., by starting with children with neurodevelop-
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resemble the pure ADHD cases. In fact, the pure dyslexia group mental disorders and studying their perinatal history). With
had phonologic deļ¬cits, the ADHD group had inhibition the possible exception of developmental co-ordination dis-
deļ¬cits, but the comorbid group had evidence of both phono- order (for review see Cermak, Gubbay, & Larkin, 2002),
logic and inhibition deļ¬cits. These results are consistent with obstetric complications do not have a particularly important
conventional wisdom that links dyslexia to poor phonology association with any of the neurodevelopmental disorders. It
and ADHD to weak inhibition, but it leaves unexplained the is family history that provides the key differentiator.
co-occurrence of these two impairments.
We can summarize by saying that it was hoped that, by The Concept of Maturational Lag
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studying underlying impairments, we would obtain clearer dis- Given that neurodevelopmental disorders are, on the one
tinctions between disorders and ļ¬nd that apparent comorbidity hand, familial, and on the other hand, not caused by acquired
was a consequence of poor speciļ¬cation of disorders. In fact, brain lesions, we need to seek another causal mechanism at
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studies such as this show that comorbidity is just as apparent the neurobiologic level. There is often an implicit assumption
at the level of cognitive impairment as it was at the level of that genetic or other prenatal inļ¬uences have led to some
observed behavior. This suggests we need to seek an explana- failure of neurodevelopment that leads to abnormality that is
tion for the associations among neurodevelopmental disorders functionally equivalent to a focal brain lesion ā hence the anal-
at a different causal level. ogous syndromes seen in neurodevelopmental disorders and
adult acquired disorders (Temple, 1997). According to this view,
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Neurobiologic Bases the brain of a child with a neurodevelopmental disorder has
Neuropsychologic studies of adults highlight how fairly an underlying abnormality that persists through childhood. An
speciļ¬c impairments in functions such as language, reading, alternative possibility is that these disorders are no more than
arithmetic or motor programming can arise as a consequence an extreme of the normal variation in the timing of develop-
of a focal brain lesion (McCarthy & Warrington, 1990). ment. We know that there are huge differences in the timing
When one sees analogous impairments in children, it is of puberty in both males and females, and marked differences,
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tempting to assume a similar etiology via underlying brain too, in the timing of the eruption of teeth. In similar fashion,
damage, with the precise pattern of observed impairment there is marked individual variation in the timing of speech
depending on the extent, location and severity of the damage. acquisition. When such a delay is followed by later normal
This kind of model was put forward in the 1950s by functioning it may be regarded as a maturational lag of some
Pasamanick and colleagues, who proposed a ācontinuum of kind. The implication is that the problem involves a normal
reproductive casualty,ā suggesting that whereas major neuro- variation in the development of certain brain systems respons-
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logic insult resulting from birth trauma, intracranial hemor- ible for cognitive functioning, rather than in some abnormal
rhage or anoxia can lead to clear signs of neurologic damage difference in brain systems, and that there can be a highly
such as cerebral palsy or epilepsy, milder damage may lead selective delay in the maturation of just one brain system.
to more subtle learning difļ¬culties. However, this conceptu- Associations between neurodevelopmental disorders would
alization of the etiology of neurodevelopmental disorders has then arise in cases where the maturational lag extended to
not received much empirical support. There is good evidence encompass several brain regions, or where there is pluripo-
that such damage often gives rise to behavioral and cognitive tentiality (i.e., a given brain structure is involved in a range
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sequelae (Pasamanick & Knobloch, 1966), but the suggestion of cognitive functions; Noppeney, Friston, & Price, 2004).
that neurodevelopmental disorders often arise from damage A key prediction from the ālagā hypothesis is that not only
during the obstetric process has not proved to be the case. should the pattern of functioning resemble that of a normal
Nichols and Chen (1980) found only weak associations younger child but also, as the children with a neurodevelop-
among neurologic soft signs, hyperactivity and learning dis- mental disorder get older, the main difference from normal
orders, and even weaker associations between these variables functioning should be found for later-maturing functions
and perinatal complications. and not for early-maturing functions (Bishop & Edmondson,
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NEURODEVELOPMENTAL DISORDERS
1987; Bishop & McArthur, 2004, 2005). The limited avail- it is difļ¬cult to become fully competent in the phonology
able evidence is in keeping with that expectation and runs and syntax of a second language acquired after puberty
against the outmoded static lesion notion. However, for most (Mayberry & Lock, 2003; Oyama, 1976). However, critical
neurodevelopmental disorders, although there is a general periods for auditory development have not been demon-
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tendency for gains in function with increasing age, delayed early strated, and there is electrophysiologic evidence that devel-
development is not followed by later normal functioning opment of the auditory system (Albrecht, von Suchodoletz,
(Rutter et al., 2006a). Can a maturational lag account have & Uwer, 2000) as well as of some other cortical systems con-
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any explanatory value in such cases? tinues right up into adulthood (see chapter 12). Longitudinal
Two main possible explanations, both speculative, have studies using magnetic resonance imaging (MRI) have con-
been proposed for why children with a neurodevelopmental ļ¬rmed that brain development continues well after the onset
disorder do not ultimately catch up. First, the persistence could of puberty, with higher-order association areas maturing only
derive from what Stanovich (1986) termed a āMatthew after lower-order somatosensory and visual areas (Gogtay
effect,ā whereby the poor (poor readers) get poorer (make slow et al., 2004; see also chapter 12), and that such changes ā espe-
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progress), while the rich (good readers) get richer (make good cially in the frontal cortex ā are associated with intellectual
progress) as a result of literacy experience boosting further functions (Shaw et al., 2006a). There is considerable vari-
language and literacy development. Furthermore, a poor ation from one cortical region to another, with some showing
reader may lack the necessary experiences later (i.e., reading radical changes at puberty and others unaffected (Nelson
is not usually taught in secondary schools, and such books as et al., 2002). Clearly, further research is required in order to
are available are likely to be too advanced to be intelligible). provide an understanding of both brain development (and
There could also be more indirect effects whereby early impair- its functional consequences) in typically developing indi-
ments create a negative spiral affecting other skills (e.g., the viduals and in those with neurodevelopmental disorders. As
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effects on intimate social relationships of communication the evidence currently stands, the postulate that the relevant
difļ¬culties in early childhood). Because of the importance brain systems lose plasticity around the time of puberty remains
of experiences in the development of psychologic functions, highly speculative, and to test it we would need studies that
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there is little doubt that something of this kind could have compare the impact of training on brain and behavior in
a contributory role. What is much less certain is whether pre- and postpubertal individuals. The trajectory ļ¬ndings of
it could account for the severe problems in intimate social Francis et al. (1996) are compatible with the suggestion of a
relationships in adult life found for many individuals with a change in brain plasticity in adolescence but they provide no
severe receptive SLI in adult life reported by Clegg, Hollis, direct support.
Mawhood et al. (2005) ā because the relationship deļ¬cits We may sum up by concluding that there is little hard
EC
were not a function of the severity of the earlier language evidence in support of a maturational lag account. We need
deļ¬cits (at least in terms of those measured), and because more longitudinal and neurobiologic studies to evaluate this
the nature of the deļ¬cits appeared so different from those idea. In its favor, this kind of explanation has the potential
usually associated with social rejection. It is also noteworthy to highlight parallels between causal mechanisms in normal
that persistence of disorder is often seen in young people with and impaired development, and to account for changing
SLI who have been enrolled in special education throughout proļ¬les seen in neurodevelopmental disorders. It provides the
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the secondary as well as primary school years (Conti-Ramsden, impetus for studies that track neurodevelopment over time:
Botting, Simkin, et al., 2001; Haynes & Naidoo, 1991). for instance, Shaw et al. (2006b) documented changes in
Perhaps the key empirical ļ¬nding is provided by Francis, cortical thickness over time in children with ADHD, with evid-
Shaywitz, Stuebing, Shaywitz et al.ās (1996) individual growth ence of normalization in children who had a good outcome.
curves comparison of 69 children with a reading disability and However, a maturational account seems more plausible as an
334 children with no reading problem. Nine yearly longitud- explanation for transient delays early in development than for
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inal assessments showed that both groups tended to plateau severe and persistent neurodevelopmental disorders.
at about 13 years of age, with no narrowing or expansion of A ļ¬nal point to note is that it is important not to confuse
the gap between the group. They differed in level but not in the hypothesis of a developmental delay with the entirely
trajectory. The ļ¬nding rather runs counter to the Stanovich different hypothesis that the causation of disorders of psy-
(1986) proposition. chologic development, together with ASD and ADHD (as well
A second kind of explanation for persistence of disorder was as numerous forms of other psychopathology), are based on
proposed in the context of language and literacy deļ¬cits by dimensional genetic and environmental risk factors (Rutter,
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Wright and Zecker (2004). They invoked a decline in neuro- Mofļ¬tt, & Caspi, 2006b). With multifactorial disorders,
plasticity as a limiting factor, and suggested that neurobiologic dimensional liability is the rule rather than the exception. The
events at age 10 years associated with the onset of puberty question of whether or not the dimensional risks are the same
halted auditory development at whatever level it has reached, ones that apply within the normal distribution is a separate
so that the adolescent was left with a lasting residue of issue and it is one that has been very little investigated up
deļ¬cit. This viewpoint is consistent with evidence that there to now, although there has been some relevant research in
are sensitive periods for aspects of language acquisition, so that relation to ADHD (see chapters 23 and 34).
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CHAPTER 3
The Neuroconstructivist Approach increased risk for the single disorders A and B, as well as for
The āmaturational lagā account is not the only alternative the A + B combination. Note that these predictions are not
to a āstatic lesionā model of neurodevelopmental disorders. made by other models of comorbidity: for instance, if the A
We have become increasingly aware that the brain changes + B combination represented a separate subtype of disorder
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in the course of development, restructuring itself to form new with distinct causes, then it should ābreed true,ā and there
neural systems, both in response to interactions between should be no increase in rates of disorder A in relatives of those
functional neuronal networks and in response to environmental with disorder B (or vice versa). Furthermore, the predictions
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input (see chapter 12). Karmiloff-Smith (1998) argued that hold up for any disorder that shows familiality, regardless
one needs to take such evidence into account when devising of whether genes or shared environmental factors are more
explanations for neurodevelopmental disorders, and that an important.
apparently speciļ¬c deļ¬cit in a child may be the endpoint of If the disorders are signiļ¬cantly heritable, it is possible to
a process that started with a relatively non-speciļ¬c disruption go further using either quantitative genetic methodologies
to brain development. This āneuroconstructivistā approach (such as twin studies) or molecular genetic studies (focusing
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emphasizes interactions between different neural systems, and on individual identiļ¬ed genes) to determine how far there
to that extent would predict the existence of disorders affect- is a shared genetic inļ¬uence between the two disorders. Few
ing more than one domain of functioning. However, this the- researchers to date have adopted this approach; an exception
oretical perspective is still very young; its main contribution is work by Pennington (2006), who studied comorbidity of
to date has been to question the simple parallels that are some- SRD and ADHD in a twin sample and concluded that there
times drawn between developmental and acquired disorders, was evidence for shared genetic inļ¬uence on the two dis-
and to emphasize the need to put development centre-stage. orders. In a similar vein, Bishop (2002) found evidence for
In order to make more speciļ¬c predictions about neuro- overlapping genetic inļ¬uences on language impairment and
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developmental disorders and their co-occurrence, we need to motor immaturity in a sample selected for SLI. Molecular
develop speciļ¬c computational models of normal development, genetic ļ¬ndings also suggest that susceptibility extends beyond
which then allow us to identify which perturbations could traditional diagnostic boundaries (Rutter et al., 2006b).
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result in a particular proļ¬le of deļ¬cit (Thomas & Karmiloff-
Smith, 2003).
Research Implications of the
Etiologic Inļ¬uences Neurodevelopmental Disorder Concept
For those conditions where genetically informative designs have
been applied (SRD, SLI, ADHD, ASD), there is evidence of Probably the single most crucial research implication is that
EC
substantial genetic inļ¬uences on the liability to disorder (see investigators need to consider the possibility that the causal
chapter 23). With respect to neurodevelopmental disorders inļ¬uences on key features may extend across the range of
(together with other multifactorial disorders), these probably neurodevelopmental disorders, rather than being speciļ¬c to
usually reļ¬ect the actions of normal variants of multiple just one. Thus, it is striking that, with the exception of arith-
genes of small effect operating together with multiple envir- metic difļ¬culties, all the neurodevelopmental disorders show
onmental inļ¬uences (Gilger & Kaplan, 2001; Rutter et al., a marked male preponderance (Rutter et al., 2004). That stands
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2006b) rather than the determinative effect of major mutant in marked contrast to the ļ¬nding that disorders with a marked
genes. This view of etiology is reļ¬ected in the causal model female preponderance all concern syndromes typically begin-
in Fig. 3.1, in that there is no one-to-one relationship between ning in adolescence and that involve emotional disturbance,
genes and neurobiology. Rather, a speciļ¬c neurologic system rather than neurodevelopmental impairment, as the key
is likely to be inļ¬uenced by a range of etiologic inļ¬uences and feature. Baron-Cohen and Hammer (1997) have hypothesized
the given etiology will impact on a range of brain regions. Such that ASDs represent an āextreme male brain.ā There is a lack
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a model allows for both co-occurrence of deļ¬cits and the of good supporting evidence but, in addition, it is necessary
existence of pure disorder. Particular patterns of variation to ask whether that same explanation should be held to apply
may reļ¬ect the inļ¬uence of either speciļ¬c combinations of to ADHD or SRD or SLI? It does not seem particularly likely
genetic or environmental factors, or the operation of chance that the causes of the male preponderance are entirely differ-
inļ¬uences (Wolf, 1997). ent in the case of each of the syndromes, although there may
This raises the question of whether patterns of co-occurrence be syndrome-speciļ¬c factors as part of the explanation.
between disorders indicate a shared liability. Klein and Riso A further question for research is whether studies should
UN
(1993) noted that evidence for overlapping etiology of dis- continue to be focused on āpureā disorders given that they
orders can be found by looking at the familiality of disorders do not appear to be at all typical. The answer will depend on
in pure and comorbid cases. In essence, they note that if the question that is being asked. Pure groups can be useful in
shared etiology is implicated in causing disorders A and B, then identifying correlates of a speciļ¬c kind of deļ¬cit without
we should see an increased risk for disorder B (with or with- additional confounds. For instance, suppose one wants to test
out A) in relatives of a person with disorder A, and vice the hypothesis that reading disability is caused by a low-level
versa. Furthermore, relatives of comorbid cases should show auditory perceptual deļ¬cit. The goal is to compare a group
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NEURODEVELOPMENTAL DISORDERS
of children with SRD and a control group on a task in which receptive language disorder may subsequently merit a diagnosis
they have to listen for small differences between sounds, and of ASD (Conti-Ramsden, Simkin, & Botting, 2006). Most
make a manual response to indicate what they have heard. If parents will be thoroughly confused by such multiple diagnoses,
the sample includes children who have substantial difļ¬culties and conclude that somebody has āgot it wrong.ā It essential
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with language comprehension, motor dexterity or attentional that professionals work together to ensure that the child
control, then it may be hard to disentangle the inļ¬uence of receives a diagnosis that provides access to the most appro-
these impairments on task performance. Careful sample selec- priate services, while at the same time assessing the whole range
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tion to exclude such cases may give cleaner results. Having of areas of function that may be impaired. Intervention will
said that, it is worth noting that those who claim to study need to be individually tailored to take into account the
pure cases may be including large numbers of children with childās speciļ¬c strengths and weaknesses.
additional deļ¬cits which are missed because they are not It is also important that both clinicians and parents recog-
assessed. For instance, many studies of SRD fail to assess nize that diagnostic labels are shorthand descriptors that do
childrenās language or attentional skills. If a sufļ¬ciently two things: they summarize the childās major area of deļ¬cit;
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detailed assessment battery is used, covering the whole range and they indicate that the problem is neither part of another
of neurodevelopmental disorders, the numbers of pure cases syndrome nor attributable to a known organic etiology. All
available for study may become vanishingly small. If so, it may too often, those interpreting the labels assume they imply more
be better to assess associated deļ¬cits, so that one can estab- than this, and treat terms such as ādevelopmental dyslexiaā
lish how far they are associated with the dependent variable and ādevelopmental dyspraxiaā as if they referred to syndromes
of interest, rather than try to control for their effects by exclu- with distinctive features and clear boundaries that are distinct
sion of comorbid cases (cf. Breier et al., 2001). from normality and have a known biologic basis. In practice,
In many research contexts, focus on pure groups is not these diagnoses are made on the basis of quantitative differ-
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just hard to achieve, it can be seriously misguided. In etio- ence form normality. A statement such as āMy child canāt read
logic studies, restricting the phenotype to those with a pure because heās dyslexicā is not an explanation, rather it is a
disorder may be misleading, if risk factors in fact operate across circular redescription of the problem. Furthermore, use of
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a range of neurodevelopmental disorders. As noted above, inclu- a single diagnostic label can oversimplify the complex and
sion of comorbid cases can provide a rich source of evidence multifaceted nature of many neurodevelopmental disorders.
about the reasons for co-occurrence of disorders, both when
studying underlying impairments and when the focus is on
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Chapter title: Neurodevelopmental Disorders: Conceptual Issues
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