In this study, quantifiable measures were used to evaluate mirror overflow in children with ADHD and compare them with TD children. We were able to quantify phasic mirror movements, using video analysis, and total mirror movements, using electronic goniometer analysis. We found that children with ADHD exhibit significantly more mirror overflow than age-matched TD control children. The methodologies we used to measure phasic and total mirror overflow produced consistent and reliable results that were in agreement with those from prior studies.
Previously, studies have used qualitative1,3,8
or semiquantitative assessments,2,6,10
to evaluate mirror overflow. Force transducers have been used to quantify mirror movements in prior studies of adults; however, the validity of these measures may be limited because they allow only isometric movements, not naturally occurring open-ended movements involved in classic descriptions of mirror overflow. In this study, we defined a systematic means to quantify variations in the occurrence and amplitude of mirror overflow during naturalistic finger movements. Confidence in the quality of the methodology was provided by the correlation between both types of quantified mirror overflow and the PANESS total overflow, the predominant qualitative assessment of overflow in past studies. In addition, the internal consistency between the phasic and total overflow results provides further assurance in our methodology. This strong correlation also suggests that the less expensive video analysis method may be sufficient. Nevertheless, the goniometers offer a more quantitative measure that could prove useful in correlations examining the physiologic basis of overflow.
The quantification of motor overflow is vital, because evidence from various studies indicates that the motor manifestations of ADHD may provide a window into the neuropathophysiology of the disorder.3
Mirror overflow is thought to result from impaired inhibition of involuntary synkinetic movements. Impairment of voluntary response inhibition has been hypothesized to contribute to the core diagnostic features of excessive hyperactivity, impulsivity, and off-task behavior.9,17
It has been shown that the presence of overflow movements in children with ADHD occurs in parallel to impairments in voluntary response inhibition9,18,–21
; one study found a direct correlation between these variables.3
The association between motor overflow and response inhibition in ADHD highlights the importance of quantitative investigations of overflow.
Transcallosal inhibition is considered necessary for suppressing motor overflow. The 2 prevailing theories explaining the presence of motor overflow, bilateral activation theory, and ipsilateral activation theory, although through different mechanisms, both reflect the notion that a lack of inhibition via the corpus callosum results in overflow.22
It has been reported that there is an improvement with age in the ipsilateral silent period latency, a marker of callosal inhibition, in control boys but not in boys with ADHD. This result was suggested to be due to a delay in the development of interhemispheric connections important for transcallosal inhibition.23
Detailed examination of the association between impairments in response inhibition and the persistence of motor overflow could provide important insights into the brain basis of ADHD and related developmental disorders.
The differences between right-handed children with ADHD and TD children were more prominent during LHFS. This finding suggests that for children aged 8–12 years, ADHD-associated differences in motor control are more noticeable in the later developing nondominant system. Nevertheless, we found phasic overflow during RHFS (but not LHFS) to be predictive of ADHD symptom severity. It may be that delayed development of the dominant motor control circuits more closely reflects a maturational delay or abnormality of the parallel higher-order systems that are necessary to control impulsive, hyperactive, and distractible behavior. Analysis in larger populations across a broader age range would help to test these hypotheses.
Consistent with our hypothesis, we found that excessive mirror overflow in children with ADHD, aged 8–12 years, is specific to boys. Only boys with ADHD, not girls with ADHD, exhibited significantly more phasic and total overflow than did their age-matched TD control children. This result was supported by our findings of gender by diagnosis interactions for total overflow during RHFS and for phasic overflow across LHFS and RHFS. Studies in ADHD populations have largely focused on motor overflow in populations that were either completely or predominantly boys.1,3
A recent study of the difference in overflow measures across gender in children aged 8 through 12 years reported that girls with ADHD showed a steady age-related maturation similar to TD girls, whereas boys with ADHD, unlike TD boys, showed persistence of overflow across this age range.8
Our age correlations support this interpretation that there are differences in the maturation of 8- to 12-year-old TD boys and boys with ADHD. Group differences between girls with ADHD and TD girls and age correlations may not have been found in this study because of a smaller group size than that for the boys or because their trajectory of maturation is earlier than that of boys.24
Thus, it is important to further study girls, especially earlier in their developmental process, to confirm that no difference in mirror overflow exists between girls with ADHD and TD girls.
In addition, we found laterality in the group effects of mirror overflow. In the combined group of boys and girls (all right-handed), there was significantly more phasic and total overflow in the right hands of children with ADHD than in those of TD children during LHFS. However, we found that boys with ADHD also had more total overflow in the left hand during RHFS than did TD children. Greater overflow during nondominant side task performance than during dominant side task performance has been reported in TD children and adults4,25,–29
and has been proposed to be due to the development of more refined patterns of neural control because of the differential use of the dominant hand.28
Because the left hand is used less than the right hand, movements made with this hand would be associated with less efficient networks of cortical control, thus inducing greater motor overflow.28
There was one conflicting study that showed greater overflow during dominant side task performance. In this study, subjects had to successively perform finger flexions, extensions, and forced flexions. Because the muscle contraction of the dominant hand was stronger, the increased overflow could be attributed to strength differences between the hands.30
Because, in our study, there was only a limited role of strength, this finding lends support to the notion that boys with ADHD are likely to have impaired cortical tracts, which are necessary for inhibition of excitatory neural activity, thus leading to overflow in the left hand even during voluntary movement of the dominant right hand.
A major strength of this study is the ability to use the methods in conjunction with additional techniques to investigate the neurologic underpinnings of ADHD. The electrogoniometers and video camera are transcranial magnetic stimulation (TMS) and MRI-compatible, allowing for integrative studies involving motor manifestations of the disorder, inhibitory mechanisms, and anatomic and functional imaging data to investigate structural differences and cortical origins. Of particular interest would be impairments in the white matter tracts, specifically those of the corpus callosum. Additional strengths of the current methodologies are their ease of use and accessibility to investigators. The equipment is easy to use with a variety of hand sizes, and children find it easy to move with the goniometers attached to their hands. The equipment is also cost-effective; particularly for the phasic analysis, which only requires access to a video camera and computer. Overall, the consistency, versatility, and ease of use of the electrogoniometers and video recording make them valuable tools for quantifying mirror overflow.
Several limitations in this study are worth consideration. Phasic overflow was measured as the number of times a bidirectional movement occurred; therefore, there are no data on the amplitude of these movements. In the total overflow measure, we were not able to distinguish small amplitude overflow movements occurring for a long period of time from a large amplitude movement sustained for a short period of time, because total overflow was measured as the total angular displacement from a baseline. A goal of future goniometer analyses is to develop either an automated algorithm or manual protocol to classify short vs long duration and small vs large amplitude overflow on the data tracings. These identifying data could be used in statistical analyses to account for average time and average amplitude. A future study investigating the subtypes of ADHD would help to better pinpoint the neurologic basis of various behaviors associated with the disorder. Furthermore, a larger sample size would have provided more statistical power.
Continued study of motor overflow in ADHD is essential to provide greater insight into the neuropathophysiology of the disorder. Future integrated studies will allow more precise analyses of how these anomalous movements correlate with neurophysiologic measures obtained using TMS and neuroimaging, as well as the opportunity to further investigate gender effects and subtypes of the ADHD diagnosis.