Developmental Cognitive Neuroscience 6 (2013) 176–194 View metadata, citation and similar papers at core.ac.uk brought to you by CORE Contents lists available at ScienceDirect provided by Elsevier - Publisher Connector Developmental Cognitive Neuroscience jour nal homepage: http://www.elsevier.com/locate/dcn Review Can transcranial electrical stimulation improve learning difficulties in atypical brain development? A future possibility for cognitive training ∗ Beatrix Krause , Roi Cohen Kadosh Department of Experimental Psychology, University of Oxford, Oxford, UK a r t i c l e i n f o a b s t r a c t Article history: Learning difficulties in atypical brain development represent serious obstacles to an individ- Received 2 July 2012 ual’s future achievements and can have broad societal consequences. Cognitive training can Received in revised form 7 April 2013 improve learning impairments only to a certain degree. Recent evidence from normal and Accepted 8 April 2013 clinical adult populations suggests that transcranial electrical stimulation (TES), a portable, painless, inexpensive, and relatively safe neuroenhancement tool, applied in conjunction Keywords: with cognitive training can enhance cognitive intervention outcomes. This includes, for Cognitive training instance, numerical processing, language skills and response inhibition deficits commonly Transcranial electrical stimulation Neuroplasticity associated with profound learning difficulties and attention-deficit hyperactivity disorder Learning difficulties (ADHD). The current review introduces the functional principles, current applications and Dyscalculia promising results, and potential pitfalls of TES. Unfortunately, research in child populations Dyslexia is limited at present. We suggest that TES has considerable promise as a tool for increas- ADHD ing neuroplasticity in atypically developing children and may be an effective adjunct to cognitive training in clinical settings if it proves safe. The efficacy and both short- and long- term effects of TES on the developing brain need to be critically assessed before it can be recommended for clinical settings. © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY license. Contents 1. Introduction . 177 2. An introduction to TES. 178 2.1. TES: types and mechanisms . 186 3. Improving cognitive training using TES in the adult brain. 187 4. Targeting learning difficulties in the developing brain . 188 5. Potential risks and pitfalls of TES . 189 6. Physical side effects. 190 7. Cognitive side effects . 190 8. Guidelines. 191 ∗ Corresponding author at: Department of Experimental Psychology, University of Oxford, 9 South Parks Road, Oxford OX1 3UD, UK. Tel.: +44 01865 271381. E-mail address: [email protected] (B. Krause). 1878-9293 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY license. http://dx.doi.org/10.1016/j.dcn.2013.04.001 B. Krause, R. Cohen Kadosh / Developmental Cognitive Neuroscience 6 (2013) 176–194 177 9. Conclusion . 191 Conflict of interest statement . 191 Acknowledgements . 191 References . 192 1. Introduction profound difficulties in learning and involves delayed corti- cal development (Shaw et al., 2012); (2) current TES studies Learning refers to the “the acquisition of knowledge or in adults have shown the efficacy in improving cognitive skills through study, experience, or being taught” (“Oxford functions that are assumed to be impaired in ADHD (e.g., Dictionaries Online”, 2012). For the majority of individuals, Weiss and Lavidor, 2012), which might also have implica- this definition might be applicable. However, for a signifi- tions for those who are interested in improving learning in cant proportion of the population, children and adults alike, ADHD. While according to diagnostic criteria ADHD does learning does not necessarily follow from studying, expe- not constitute a learning disability, we will here refer to riencing or being taught. The aim of the current review is learning difficulties associated with atypical brain develop- to introduce the potential of transcranial electrical stimu- ment to include ADHD and potential other developmental lation (TES), a non-invasive form of brain stimulation that problems that meet the abovementioned criteria. might be used to improve learning in those who have learn- The current discussion will focus on DD and dyslexia, ing difficulties based on atypical brain development, and along with ADHD, as they are the best-known childhood to evaluate the relevant evidence on TES from research in developmental problems associated with profound diffi- adult populations. culties in learning and atypical brain development, and There is currently no generally accepted definition of evidence in healthy adults suggests that TES has favourable learning disabilities (for a recent discussion see Scanlon, effects on the cognitive functions commonly impaired in 2013). However, based on the Diagnostic and Statistical these learning difficulties. Manual of Mental Disorders (5th ed.; DSM-5), learning DD refers to severe difficulties in manipulating numeri- disabilities are defined as: (1) an academic-based disor- cal information and performing arithmetic operations, and der that originates in the central nervous system, and can some have suggested that the deficit cannot otherwise manifest itself in reading, writing, and/or mathematics; (2) be explained by low intelligence or by reading or atten- a discrepancy in aptitude and achievement that can be tion deficits (Butterworth et al., 2011). Dyslexia, on the identified using psychometric methods (e.g., mathemati- other hand, denotes severe difficulties in reading and text cal achievement scores below the 5th percentile, despite comprehension, despite an (at least) average IQ (Shaywitz, average IQ). Further criteria for the diagnosis of learning 2003). In ADHD, several domains of executive functioning disabilities include stipulations that: (3) learning disabili- can be deficient, including working memory, divided atten- ties cannot be attributed to a disparate array of difficulties tion and response inhibition (impulsivity) (Pasini et al., such as low motivation or self-affect, albeit the individ- 2007). ual might exhibit some of these difficulties in addition to Learning difficulties have important consequences for the learning disability; (4) when assessing learning dis- the individual and the society they live in. The rates of abilities, factors such as age, gender, cultural and language unemployment, reduced income and low socioeconomic group, socioeconomic factors, and level of education should status throughout adulthood are often high in individ- be taken into account, as these factors may influence the uals with learning disabilities (Stein et al., 2011; Parsons symptom evaluation; (5) learning disabilities do not repre- and Bynner, 2005). Further consequences include unem- sent an “all-or-none” phenomenon, and vary in severity; ployment, loss of tax payments, drug abuse, crime, special (6) learning disabilities are regarded mainly as a neu- education, and depression treatment (Gross et al., 2009). rodevelopmental disorder. This might suggest that other In addition, the overall social and health-related conse- cognitive impairments can be observed to a lesser extent in quences can be especially detrimental, as they are likely other domains (Karmiloff-Smith, 1998). Furthermore, we to cover the individual’s entire life span (Stein et al., 2011). would like to note that learning disabilities may appear in For instance, it has been suggested that the lack of success one (e.g., reading) or more (e.g., reading and mathemat- and achievement in individuals with learning difficulties ics) cognitive domains. While the aptitude–achievement predicts high rates of psychiatric diagnoses (Raskind et al., discrepancy is currently the most established means for 1999). These factors in turn affect the state economy, in the the identification of learning disabilities, academic per- sense that extreme annual expenses are required to coun- formance has now been suggested as an identifier in the teract the deleterious societal consequences. These costs DSM-5 (Scanlon, 2013). are estimated to equal nearly £2.4 billion in the UK alone In the current review, we discuss the potential role of for numeracy problems (Gross et al., 2009), £1.8 billion in TES to enhance cognitive training effects in learning disabil- the UK for reading disabilities (Jones et al., 2006), and $42.5 ities such as dyslexia and developmental dyscalculia (DD), billion for ADHD in the US (Matza et al., 2005). This demon- which fit the aforementioned criteria. We further extend strated burden on both the individual and the society our discussion to attention-deficit hyperactivity disorder stresses the pressing need to design successful training and (ADHD). The inclusion of ADHD for the purpose of the intervention methods to counteract these dramatic effects current review was twofold: (1) ADHD is associated with of learning difficulties at the individual and societal level. 178 B. Krause, R. Cohen Kadosh / Developmental Cognitive Neuroscience 6 (2013) 176–194 Table 1 Potential known and possible consequences caused by TES in the developing brain. Unknown factors need to receive scientific attention and careful exploration in order to be able to label the method ‘safe’ in paediatric population. Population Adults Children Short-term effects Long-term effects Potential short-term effects Potential long-term effects Physical tolerability Tingling (70.6%), itching