Determining the precise nature of the developmental course of EF in autism and also of the potential causal links between EF and other social and learning outcomes demands greater understanding of the nature of EF itself and of the mechanisms underpinning its growth. Although there is no question that EF plays a vital role in well-regulated, organized behavior [55
], there has been much disagreement regarding the characterization of the EF construct. Like adult models [56
], competing developmental accounts differ with respect to which they emphasize the unitary [58
] or fractionated [60
] nature of EF. Empirical work with adults, using sophisticated statistical techniques like confirmatory factor analysis (CFA), has reported evidence of three latent EF variables or component processes—set-shifting, updating (working memory), and inhibitory control—which are partially independent but still intercorrelated [62
Studies using similar methods with school-age children in part support this integrated framework [63
], although recent studies with 2- to 6-year-old children have instead reported evidence in line with a unitary model of EF [65
]. These latter results question the apparent continuity in the structure of EF during development but are consistent with a dynamic, neuroconstructivist approach in which cognitive functions begin relatively undifferentiated and become progressively modularised or specialised over time [67
] (see ). This framework suggests that individual differences in development itself might be key to explaining the wide variation in findings both within [68
] and across [12
] studies on EF in autism.
A simplified model showing a unitary executive function (EF) construct early in development and an emergent fractionated construct of EF with development with latent EF variables of working memory (WM), inhibitory control (IC), and set-shifting (SS).
No study has yet explicitly investigated the nature of EF in autism. Early fractionation of EF makes it plausible for distinct EF components, such as cognitive flexibility, to be specifically affected in autism. Yet if, as the evidence suggests, EF is a single, unitary construct during early childhood (at least in typical children), then it becomes more difficult to see how a distinct profile of EF difficulties might emerge in autism. Given the prolonged development of EF and the degree of neural plasticity during childhood [69
], it is likely that emerging prefrontal cortical networks affect, and are affected by, the development of other key cognitive functions. In this case, then, disruption to distinct EF components in autism might be driven by other factors.
One goal therefore should be to understand precisely which mechanism(s) drives
the development of EF in typical children and in children with autism. Some theorists propose that progress in EF occurs via the development of the prefrontal cortex [60
] and the strengthening of prefrontal representations [58
] in an experience-dependent manner [71
]. An influential yet contrasting view suggests that development in children's attentional control
—the ability to focus on a task and ignore irrelevant information—is the source of common variance in EF [61
]. Posner et al. have demonstrated significant advances during the preschool period in the central “attention network,” which includes alerting, orienting, and executive attention processes [73
]. Developmental changes in attention are considered to provide children with greater executive control over action. On this view, then, rate of growth in EF should be predicted by developments in attentional capacities () and such developments might even mediate the fractionation of EF.
Figure 2 Three simplified models showing executive function (EF) development. Model (a) includes a latent construct of attention that influences baseline levels (EF intercept) and rate of growth of EF (EF slope). Model (b) shows a latent construct of language (more ...)
Impairments in core attentional processes have been reported in autism, including problems with disengagement or so-called “sticky” attention [75
]. Fundamental problems in critical aspects of attention could therefore place limits on the rate of EF development, which could in turn hinder the emergence of autistic children's social and learning outcomes. It is of course possible that the causal relationship might exist in the opposite direction, such that early developments in EF might influence the emergence of attentional networks. The relationship between aspects of attention and components of executive control has, however, been hitherto unaddressed in autism.
Yet Posner and Rothbart's [72
] model (see also [61
]) neglects the potentially mediating role of another key function: language
. Children's verbal skills can affect the expression of EF, for example, by limiting their ability to store phonological information in working memory [78
]. Yet language might play a more fundamental role, affecting the very development
of EF. For several theorists [31
], language provides an internal plan for behavior. Vygotsky [81
] emphasized the importance of self-directed speech during early childhood, which becomes increasingly internalized during the preschool years, allowing children (verbally) to “think through” problems and to guide future-oriented behavior. Similarly, Zelazo et al. [59
] stress that language is the medium through which higher-order (if-if-then) rules are formulated and is key to recursive thought. Developmental gains in young children's language skills (specifically their ability to formulate hierarchical rules) are therefore directly implicated in the rate of EF development ().
Difficulties with communication are a core characteristic of autism [82
] and has been previously implicated as a potential limiting factor on the development of EF (e.g., [83
]). Furthermore, children with autism are less likely to use verbal rehearsal strategies on executive tasks [84
] suggesting that they, unlike typical children, may not be using internal language in the service of executive control. Pellicano's [54
] longitudinal work showed that autistic children's initial receptive-vocabulary skills were not predictive of EF performance three years later, suggesting further that verbal skills may not influence the emergence of EF in autism as they do in typical development [26
]. Individual differences in the growth trajectories of autistic children's verbal skills therefore might partially mediate (or fail to mediate at all) the rate of progress of EF in autism. Further still, it is possible that attention and
language could mediate the development of EF. In this case, both functions could make independent contributions to the rate of growth of EF () and both could be limiting factors in autism. Importantly, evidence for any one of these patterns (Figures –) would suggest that the potential influence of EF on children's functional outcomes is indirect
rather than direct.
All of these models suggest that the development of EF itself might be shaped by certain endogenous factors, which in turn could mediate children's developmental outcomes. Yet the developmental trajectory of EF, and its resulting neurocognitive architecture, will be an emergent property of interactions within the children and between the children and their environment. Alternative explanations therefore place the development of EF squarely in the social realm. For example, Luria emphasized that “we must go beyond the limits of the individual organism and examine how volitional processes are formed for the child in his/her concrete contacts with adults” [87
, page 89] (see also [88
]). Hughes [27
] (see also [80
]) extended this view to suggest that the effect of EF upon the development of ToM should be indirectly influenced by the child's social environment. Since negotiating social interactions requires children to regulate their own behaviors (e.g., turn-taking, following rules in games), peer relations are likely to have positive effects on children's developing executive skills, which in turn will foster their developing ToM.
There has been renewed interest in the sociocultural predictors of EF development, which so far include socioeconomic status [89
], parent scaffolding [90
], and parent-child interactions [92
]. These exogenous factors are also likely to influence the development of EF in children with autism. Contrary to popular opinion, children with autism do not grow up in a social vacuum. Rather, they can show attachment security with caregivers [93
], can engage in positive and collaborative interactions with siblings (e.g., [94
]), and actively seek out their nonautistic peers [4
]. It is therefore plausible that social contact could influence autistic children's developing EF, which in turn might exert its effects on key real-life outcomes.