Mental Addition in Bilinguals: An fMRI Study of Task-Related and Performance-Related Activation

Behavioral studies show that bilinguals are slower and less accurate when performing mental calculation in their nondominant (second; L2) language than in their dominant (first; L1) language. However, little is known about the neural correlates associated with the performance differences observed between bilinguals' 2 languages during arithmetic processing. To address the cortical activation differences between languages, the current study examined task-related and performance-related brain activation during mental addition when problems were presented auditorily in participants' L1 and L2. Eleven Chinese–English bilinguals heard 2-digit addition problems that required exact or approximate calculations. Functional magnetic resonance imaging results showed that auditorily presented multidigit addition in bilinguals activates bilateral inferior parietal and inferior frontal regions in both L1 and L2. Language differences were observed in the form of greater activation for L2 exact addition in the left inferior frontal area. A negative correlation between brain activation and behavioral performance during mental addition in L2 was observed in the left inferior parietal area. Current results provide further evidence for the effects of language-specific experience on arithmetic processing in bilinguals at the cortical level

BRAIN CONNECTIVITY AND TREATMENT RESPONSE IN ADULT ADHD:understanding the relationship between individual differences in fronto-parietal and fronto-striatal brain networks and response to chronic treatment with methylphenidate

Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder, characterised by disrupted anatomical and/or functional connectivity, mainly in the fronto-striatal and fronto-parietal networks. Stimulants, such as methylphenidate (MPH), represent a first-line treatment in ADHD, but one third of patients fail to respond, with severe consequences for the individual and the society at large. Hence, a comprehensive understanding of the relationship between individual differences in brain abnormalities and treatment response is needed.This thesis focused on two main brain networks: the fronto-striatal network, a central theme in ADHD research, and the fronto-parietal attentive network, formed by the three branches of the superior longitudinal fasciculus (SLF). The SLF branches have been only recently described in humans, and there is no detailed analysis of their distinct functional roles and involvement in disorders such as ADHD. Therefore, I first investigated the functional anatomy of the SLF branches by combining a meta-analytic approach with tractography, and revealed novel findings about the anatomical and functional segregation and integration of brain functions within fronto-parietal networks. Then, I showed, for the first time, that the three SLF branches are all significantly right-lateralised in ADHD patients but not in controls, and provided preliminary evidence that the pattern of lateralisation of the SLF I may be related to poor attentive performance in ADHD patients.Finally, I conducted functional and structural connectivity analysis to test whether a relationship exists between brain abnormalities and treatment response in adult ADHD. I employed a longitudinal crossover follow-up design. 60 non-medicated adult ADHD patients were recruited and underwent behavioural assessment (Qb test) and magnetic resonance imaging (MRI) scanning twice, once under placebo and once under a clinically effective dose of MPH. Clinical and behavioural response was measured after two months of treatment with MPH. I demonstrated for the first time that there is a relationship between ‘connectivity’ abnormalities within fronto-parietal networks and treatment response in adult ADHD, both at the anatomical and functional level.Ultimately, my investigation contributed towards the identification of potential biomarkers of treatment response, which in the future may help clinicians deliver more individualised treatments.<br/

Neural and behavioral correlates of arithmetic development and learning in children

Arithmetic learning improves mathematical competence, which is necessary for successful daily life. However, little is known about the neural underpinnings of arithmetic learning during childhood, the age when individuals learn most of the mathematical skills and the vast majority of our knowledge comes from adult studies. In this dissertation project, four studies were conducted to investigate the neural and behavioral correlates of arithmetic development and learning in children. In Study 1 arithmetic development was evaluated longitudinally to see whether it is monotonous or there are intermediate phases in which certain domain-general processes become important but disappear later. In Study 2 arithmetic complexity was evaluated to see whether it relies on both magnitude and cognitive processes, such as in adults. In Study 3 it was asked whether the findings in adults are valid for children or are there intermediate stages. Furthermore, it was evaluated whether few training sessions are reflective of more long-term learning processes. In Study 4 the brain activation changes during the course of learning were measured to see whether they reveal similar changes as in after arithmetic learning. The findings revealed that different domain-general cognitive processes are involved in different steps of arithmetic development and learning. Furthermore, arithmetic achievement occurs in two steps in children, first from slow effortful procedural processes to fast compacted procedural processes, and then to retrieval processes. These changes are distinguishable after one and several training sessions, and also during the course of learning. The findings are integrated in a theoretical model of arithmetic achievement in children, which contains two phases: (i) the efficiency increase (from slow effortful procedural processes to fast compacted procedural processes) and (ii) the strategy change (from fast compacted procedural processes to retrieval processes) phases. The model was developed based on two principles of brain function, optimum performance and energy consumption, and supported by several empirical studies. Taken together, this dissertation project provides a comprehensive framework for arithmetic development and learning in children. The findings might be helpful to develop educational and therapeutic interventions and also a new measure of intervention outcomes, particularly in individuals with mathematical learning disabilities