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Our proposed framework has implications for targeted interventions aimed at reducing symptom expression in dyslexia, because it can elucidate the EF profile (strengths and impairments in common: response inhibition, and unique: updating and switching abilities) associated with dyslexia and isolate key EFs implicated in core and non-core symptoms. Previous research suggests that EF is modifiable, with training interventions resulting in changes to EF and unrelated outcomes such as reading ability, reasoning and behavioural problems(Jaeggi, Buschkuehl, Jonides, & Shah, 2011; Klingberg, 2010; Loosli et al., 2012; Mezzacappa & Buckner, 2010). Therefore, key EFs implicated in the aetiology and symptom severity of dyslexia may be modifiable with a targeted training intervention.

Training approaches differ greatly across the EF literature with different studies training different EF processes (response inhibition, working memory/updating, switching), including variability in sample and age ranges, and further variability in how successful training gains are measured in terms of behavioural outcomes. Operational definitions of

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“success” in training transfer also differ within the literature depending on whether the sample is typical or atypical. Training studies with typical samples generally target a cognitive process shown to be important for a behavioural outcome, as is the case for working memory and intelligence or reading outcomes (Baddeley, 2012). These cognitive processes and behavioural outcomes are not impaired in typical samples, so the overall aim is to see if they can be enhanced but not restored. Success of transfer of training is usually based on direct measures of the trained cognitive process (e.g. working memory) and on related behavioural outcomes (e.g. intelligence). In contrast, training studies with atypical samples target a cognitive process shown to be implicated in the aetiology and symptom severity of a clinical condition, as in the case of response inhibition and symptoms of ADHD (Barkley, 1997). These cognitive and behavioural outcomes are impaired in atypical samples, so the overall aim is to see if the underlying cognitive process and behavioural outcome can be restored to typical function. Success of transfer of training is usually based on direct measures of the trained cognitive process which is impaired in the condition (e.g. response inhibition and ADHD) and measures of symptom expression (e.g. inattention and impulsivity).

In typical populations, common EF (response inhibition) appears to improve as a function of training, with evidence of improvement at the cognitive (Berkman et al., 2014; Enge et al., 2014; Johnstone et al., 2010; Spierer, Chavan, & Manuel, 2013) and underlying neural levels (Berkman et al., 2014). However, successful transfer of training is debated, with some reporting that training transfers to improvements in a wide range of regulatory behaviours (such as food consumption, alcohol intake and gambling (Spierer et al., 2013).

However, others have reported little or no transfer to untrained EF measures and fluid intelligence (Enge et al., 2014; Thorell, Lindqvist, Bergman Nutley, Bohlin, & Klingberg, 2009). In typical populations, working memory/updating training has resulted in direct transfer to improved working memory/updating (Dunning, Holmes, & Gathercole, 2013;

Holmes, Gathercole, & Dunning, 2009; Jaeggi, Buschkuehl, Jonides, & Perrig, 2008; Jaeggi et al., 2011; Karbach, Strobach, & Schubert, 2015; Loosli et al., 2012). However, studies differ in the extent to which they find transfer to other behaviours with some studies finding transfer to improvements in reading and fluid intelligence (Jaeggi et al., 2008;

Loosli et al., 2012), while others find no transfer to fluid intelligence, reading or math ability (Holmes et al., 2009; Karbach et al., 2015). Although less work has been conducted on the effects of switching training, there is evidence of direct transfer to switching abilities and transfer to working memory, fluid intelligence and response inhibition (Karbach & Kray, 2009). This suggests that EF processes are modifiable at the direct level in typical samples and may transfer to improved outcomes in closely related behaviours.

EF training has been proposed as a potential intervention for ameliorating cognitive deficits associated with symptoms in complex neurodevelopmental conditions (Keshavan, Vinogradov, Rumsey, Sherrill, & Wagner, 2014). In atypical samples, to our knowledge the transfer effects of isolated common EF training remains unexplored. Working memory training has been explored in atypical populations, and, has resulted in direct

improvements in working memory abilities in those with dyslexia and special education needs (Dahlin, 2011; Luo, Wang, Wu, Zhu, & Zhang, 2013; Shiran & Breznitz, 2011).

Working memory training was found to transfer to reduced symptom expression (reading and phoneme problems) in children (Luo et al., 2013) as well as adults with dyslexia (Shiran & Breznitz, 2011). One study found that these improvements were greater than what was observed with a reading intervention targeted at the behavioural level of impairments (Shiran & Breznitz, 2011), suggesting that targeting underlying cognitive factors which may be implicated in symptom severity may be more beneficial than targeting the symptom. There is evidence of switching training improving switching abilities in ADHD, as well as other EFs and fluid intelligence (Kray, Karbach, Haenig, &

Freitag, 2012), however this study did not track transfer to reduced symptom expression in ADHD.

Mixed training approaches (where more than one EF is trained) have also been explored in atypical samples. Horowitz-Kraus (2015) explored transfer of combined training (working memory, naming, speed, inhibition, flexibility) in children with ADHD and comorbid dyslexia-ADHD and found differential effects in each subgroup. Training transferred to improved reading ability, speed and spatial abilities in those with comorbid dyslexia-ADHD and improved working memory and speed but not reading ability in those with ADHD alone (Horowitz-Kraus, 2015), suggesting that an impairment in outcome may be a

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necessary pre-requisite for transfer at the behavioural level in clinical conditions.

Combined working memory and response inhibition training has been explored in ADHD (Johnstone et al., 2012, 2010), and direct improvements in working memory and response inhibition are found to transfer to reduced symptoms of ADHD (Johnstone et al., 2012, 2010), which were sustained at 6-week follow up (Johnstone et al., 2012). These findings suggest that EF training may have potential for targeting the core and non-core issues in dyslexia. However, it is difficult to conclude which trained factor contributed to overall changes as both working memory and inhibition were trained.

Although there is debate regarding the transfer of EF training to behavioural outcomes in typical samples, the pattern of results emerging from atypical samples suggests that EF training targeted at underlying impairments can improve executive processes

underpinning the disorder, and in some cases, generalise to improvements in the severity of symptoms associated with clinical conditions. Although some reviews suggest that training efficacy is not fully established for clinical conditions, and fundamental issues within the research field need to be addressed to progress in this field (Keshavan et al., 2014; Kirk, Gray, Riby, & Cornish, 2015). According to Kirk et al. (2015, p. 157) “many current cognitive training programs lack a clear underlying theoretical model, which makes it hard to ascertain which domain the programs are truly targeting”. This highlights the importance of exploring EF training in dyslexia, especially if our proposed theoretical framework can elucidate common (response inhibition) or unique (working memory updating, switching) EF impairments which are implicated in core (reading) and non-core (socio-emotional) issues associated with the condition. Before we can train EF in dyslexia, fundamental issues in EF profiling and predictive studies (see section 2.10) need to be addressed in order to target clinically relevant EFs in dyslexia (i.e. EFs which are predictive of core symptoms).