Introduction

In a paper recently published by Neurology, Gilbert and associates investigate motor cortex inhibition and modulation in children with ADHD. The research works to expand on an already existing study that reported reduced motor cortex short interval cortical inhibition during rest in children with ADHD (Gilbert et al., 2019). Moreover, the new research works to further develop an understanding of inefficient response inhibition in the physiologic sense along with whether short interval cortical inhibition varies in action selection versus action inhibition (Gilbert et al., 2019). The main purpose was to examine motor physiology during behavior activities that are relevant to impairments present in children with ADHD. The research presents that children with ADHD present normal motor cortex physiology along with deficient short interval cortical inhibition across the behavioral states (Gilbert et al., 2019). Moreover, rest to action selection presents less task-related-up-modulation. As a result, the findings present a need for further investigation about the mechanism of ADHD and clinical impairments that occur in the motor cortex. 

Background

Attention deficient/hyperactivity disorder (ADHD) is one of the most common neurobehavioral disorders present in children (Wilens & Spencer, 2010). It affects about 4-12% of school-aged children worldwide and about 4-5% of college-aged students and adults (Wilens & Spencer, 2010). The symptoms of ADHD are present in early childhood and can affect the academics and social skills of the individuals. Moreover, children with ADHD tend to have learning problems, troublesome relationships, and high medical visits (Mahone & Denckla, 2017). Additionally, ADHD has presented poor attention and inhibited control (Liddle et al., 2011). ADHD impacts many children in the world and many times parents have to play an active role in their children’s lives due to the support they need. Therefore, many researchers have been studying to determine the impairments correlated with ADHD. 

In children with ADHD, response inhibition is impaired while it becomes more efficient in maturing children (Guthrie et al., 2018). Response inhibition is the ability to suppress unwanted actions and that interfere with functional goals (Guthrie et al., 2018). Moreover, children with ADHD present impaired transcranial magnetic stimulation (TMS) evoked short interval cortical inhibition (SICI) of motor evoked potentials (MEPs) in the resting motor cortex (M1) (Gilbert et al., 2019). TMS is a non-invasive procedure that is utilized to measure cortical physiology (Guthrie et al., 2018). Many researchers have examined the physiology while the brain is at rest; however, others have examined excitation and inhibition of the brain (Guthrie et al., 2018). As a result, there is an ability to examine motor cortex physiology as done in this study. Moreover, SICI is an inhibitory phenomenon occurring in the motor cortex which can be studied by TMS (Wagle-Shukla et al., 2009). Furthermore, TMS can induce magnetic pulses to pass over the motor cortex to produce a response known as MEP (Hashemirad et al., 2017). Thus, the purpose of this study is to reflect on whether normal motor cortex physiology presents behavioral dysfunction through the examination of motor response selection and inhibition. This idea was tested through the division into two smaller goals. The first goal was to examine the difference in SICI in response selection and inhibition. The second goal was to examine the difference in the modulation of brain activation in children with ADHD and typically developing children. 

Methods

The following methods were presented in the study Motor cortex inhibition and modulation in children with ADHD. The recruitment process of the children was done through advertisements. 131 right-handed children were recruited for this experiment. The ADHD Rating Scale IV was used to screen the children. The motor cortex physiology was measured through the use of TMS. In order to prepare the children, they started with threshold measures with pulses of 10%, and then the pulses were increased by 10% until a consistent MEP was examined. In order to test the response selection and inhibition, the Slater-Hammel task stop-signal reaction time paradigm was utilized. The study began with the participant pushing down on the controller to cause the racecar to start moving. The purpose of the study is to continually hold the button down and releasing it prior to the 800 ms mark. The participants used their index finger to press down on a force transducer (Marinovic et al., 2010). For some trials, the car randomly stopped prior to the 800 ms mark and therefore, the participant is told to continue holding the button down until the car reaches the 1,000 ms mark (Marinovic et al., 2010). The random stopping of the car prior to the 800 ms mark is the stop cue which is presented by the stop symbol. As a result, the data allowed for statistical analysis of the pulse evoked MEP amplitudes and the SICI during the action selection and inhibition. Moreover, the TMS pulses were lined to the top trial by the stop cues, resulting in the examination of the motor cortex physiology during successful stopping. Therefore, the data allowed for a comparison of the task-related modulation of the motor cortex for differences in children with ADHD and typically developing children. 

Results

The following results were presented in the study Motor cortex inhibition and modulation in children with ADHD. Children with ADHD presented less motor cortex SICI than typically developing children at rest. For the response inhibition behaviors, 47% of children with ADHD and 46% of typically developing children successfully completed the stop trials. 40% of children with ADHD and 40% of typically developing children successfully completed the go trials. Moreover, children with ADHD presented less motor cortex SICI during the successful go and stop trials. For the motor cortex modulation during task engagement, the MEP amplitudes displayed that the normalized to rest MEP amplitudes were bigger during the response inhibition task. The diagnosis associated with ADHD is that there is less Task-Related Up-Modulation (TRUM) of the motor cortex MEP amplitudes and the inhibition task represents a consistent relationship with the information. Children that had worse ADHD scores reported less TRUM. 

This study expanded on an already exiting study that presented “reduced TMS-evoked SICI in the resting motor cortex in children with ADHD” (Gilbert et al., 2019). This was supported by a reduced motor cortex SICI during the response selection which was the go trial and response inhibition which was the stop trial. Therefore, the data presented a reduction in the SICI in children with ADHD. Moreover, the study presented reduced TRUM during response selection and inhibition in children with ADHD. This was supported by higher ADHD symptom scores and slower stop signal reaction times. 

Significance

The study provided by Gilbert and associates presented an insight into the relationship between children with ADHD and motor cortex physiology. Findings from studies have presented that mechanisms that are measured by SICI can activate in attention and reward processing (Gilbert et al., 2011). As a result, more studies about children with ADHD could result in the enhancement of the understanding of SICI in ADHD symptoms (Gilbert et al., 2011). ADHD greatly impacts various aspects of a child’s life while they are young and as they grow older (Harpin, 2005). Moreover, difficulties occur in executive functioning (Harpin, 2005). Therefore, it is important to research the impairments that occur in the brain in hopes of finding a solution to the issues. By examining the motor cortex physiology and determining whether there are behavioral dysfunctions, researchers can understand whether there is a need to do further studies to ease those dysfunctions. As a result, this study is significant because it allows children with ADHD to have hope of findings solutions for their impairments. Moreover, there have not been many studies regarding the topic, therefore it is important to conduct further studies to formulate more enhanced conclusions (Dutra et al., 2016). There is also speculation that the deficits of ADHD, which involve poor attention and inhibitory control, can be helped by increasing engagement of attention (Liddle et al., 2011). Thus, it is important to do further research to assist individuals whether that is children or adults in alleviating their impairments. In conclusion, the study was effective in presenting various findings and opening the door for more research in the future. 

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