1.3.1 The need to study development and related issues
Research in the field of neurodevelopmental disorders has historically adopted models used for adult brain-damaged patients (for a detailed discussion see Chapter 2). For example, evidence from double dissociations has repeatedly been used to show that certain cognitive abilities can be either spared or impaired and thus, can exist independently from each other â which led to the development of theories of âmodularity of mindâ by Fodor (1983). A frequently mentioned example of such a double dissociation in neurodevelopmental disorders is the fact that language abilities outperform non-verbal intelligence abilities in WS, in contrast to individuals with Specific Language Impairment (SLI) who show language difficulties in the absence of any general intelligence deficits (Pinker, 1999; though for further discussion on this issue see Chapter 8). Yet, notions of double dissociations are static and claims that the brain exists of specific modules are based on studies investigating the brain in its mature state. Recently, more subtle matching methodologies have suggested that it is highly unlikely that any area of functioning can be âintactâ and have instead suggested that neurodevelopmental disorders can be described in terms of relative proficiencies and deficits.
Additionally, studies that have investigated cognitive abilities in infants have shown that the course/pathway of development in neurodevelopmental disorders is often atypical. For example, while typically developing children generally point before they start speaking infants with WS only point after the emergence of their first word (Laing et al., 2002). In addition, it is less clear how the infant brain is structured and how the endstate is the result of developmental processes (Karmiloff-Smith, Scerif & Ansari, 2003). A recent view of cognitive development is that the specialisation of brain structures is the result of brain maturation over development through interaction with the environment, genes, brain and behaviour (Thomas, 2003). Thus, advocates of a neuroconstructivist approach have provided strong evidence that a more developmental approach is required when studying neurodevelopmental disorders (see discussion in Chapter 2; Karmiloff-Smith, 1998, 2009; Farran & Karmiloff-Smith, 2012; Thomas, Baughman, Karaminis & Addyman, 2012).
One reason why a developmental approach is needed is the fact that the brain changes over time. Brain plasticity is often defined as changes to the brain system as a result of external (environmental) or internal (brain damage) factors (Huttenlocher, 2002). Plasticity of the brain is larger in children than in adults, which is evidenced by the fact that children recover better and faster after brain damage compared to adults (but see Thomas, 2003, for a discussion). This means that subtle differences over time can impact the development of cognitive abilities. This is especially true for specialisation of the brain in neurodevelopmental disorders:
brain volume, brain anatomy, brain chemistry, hemispheric asymmetry and temporal patterns of brain activity are all atypical [âŠ]. How could the resulting system be described as a normal brain with parts intact and parts impaired, as the popular view holds? Rather, the brains of infants with [neurodevelopment disorders] develop differently from the outset, which has subtle, widespread repercussions.
(Karmiloff-Smith, 1998, p. 393)
As a consequence, a developmental approach is required to study neuro-developmental disorders.
More developmental approaches have emerged within the field of neurodevelopmental disorders in recent years. First, research studies have started to adopt more developmental research methods, including inclusion of wider age ranges and tracing development back to infancy, and are gradually using fewer matched group designs. For example, in the past studies of neurodevelopmental disorders often investigated narrow age ranges and matched groups of neurodevelopmental disorders to control groups (e.g. typically developing individuals or other neurodevelopmental disorders) based on either chronological age or mental age abilities. Such approaches are common in adult neuropsychology but again they represent a static timepoint in development in that they do not capture any of the changes over time. In addition, matching participants with neurodevelopmental disorders to a typically developing control group based on chronological age (CA) often underestimates the disorder group, as disorder groups rarely perform at their CA level. However, matching groups on mental age requires groups to be matched on a specific standardised task, for example matching groups on their receptive vocabulary using the British Picture Vocabulary Scale (Dunn, Dunn, Whetton & Burley, 1997), or the Ravens Coloured Progressive Matrices task (RCPM: Raven, Court & Raven, 1990). Not only does this mean that the matching is theoretically driven, which can interfere with the findings of the study, depending on what abilities the groups are matched, performance in the disorder group will again be under- or over-estimated, due to the often uneven cognitive profiles of neurodevelopmental disorders (see Thomas et al., 2009 for a discussion). Finally, matching studies often apply the rule that two groups are matched when their performance is not significantly different (or p value is larger than 0.05). Yet, the question is how matched the two groups really are as differences in performance often have to be quite large in order for the statistical result to be significant and statistically non-significant group differences might still include large differences and significance in the real world.
The best developmental solution to a matching approach is to study cognitive changes in neurodevelopmental disorders over development using longitudinal studies. Yet, longitudinal studies are time consuming and often expensive. An alternative method that is becoming frequently used is that of a developmental trajectory or cross-sectional approach in which different participants across a large age range are examined and trajectories of the neurodevelopmental group are compared to those of the control group (see Thomas et al., 2009). Yet, the developmental trajectory approach is not without criticism and recent studies have shown that outcomes from cross-sectional studies differ from those using longitudinal designs (see discussion in Chapter 10; Cornish, Cole, Longhi, Karmiloff-Smith & Scerif, 2013). Cross-sectional studies include snapshots of cognitive abilities across different age groups and thus the individual differences between these individuals might mask any real changes over time across an entire group. As a result, although cross-sectional studies can give an indication of the developmental profile in neurodevelopmental disorders, these studies should be followed up by longitudinal research. In addition, other difficulties for crosssectional studies include the fact that standardised tasks need to include a wide age range in order to avoid floor and ceiling effects between the two groups (see the discussion in Thomas, Purser & Van Herwegen, 2012) and importantly, this design assumes that individuals with the same disorder will follow the same developmental trajectory (this may not always be the case: see Chapter 7 for a discussion of heterogeneity within disorders and Little et al., 2013 for a study using cluster analysis to explore variability within one disorder group).
A second developmental trend in recent studies is the examination of domain-general abilities (such as eye movement behaviour, attention, processing speed, cognitive control, memory abilities, etc.) and how these building blocks affect the development of cognitive abilities later in life. This is important in that even when development in neurodevelopmental disorders appears to be within the typical range, because of plasticity and compensation strategies in the brain, this behaviour might be reliant upon alternative cognitive strategies or abilities. For example, individuals with WS are often reported to have relatively good face processing abilities on some aspects of recognition, despite their lower general intelligence. Yet studies have shown that they rely upon atypical strategies to complete basic face re...