Psychology

Attention-deficit and hyperactivity disorder (ADHD) is considered to be a genetic and heritable lifetime disorder that has incidence of around of 5 (Comings et al., 2005 Luders et al., 2008).  It is distinguished by presence of inattention, hyperactivity and impulsivity, and is also related to the problems of academic and interpersonal relationship as well as comorbid mood and substance dependency disorders, and is lasting throughout adulthood (Comings et al., 2005 Garrett et al., 2008 Luders et al., 2009). 

Increasing amount of evidence from the neuroimaging research suggests that ADHD has neurobiological origin, thus major brain structural changes are present involving cortical, subcortical regions, cerebellum and basal ganglia as well as impairment in fiber tracts networks (Fassbender  Schweitzer, 2006 Garrett et al., 2008 Hamilton et al., 2008 Luders et al., 2009).

The purpose of this research is to review primary research sources presenting evidence regarding involvement and impairment of certain brain areas in individuals with ADHD.

Biological Factors of ADHD
Functional neuroimaging enables to link brain activities observed in the real life to behavior, symptoms and cognitive function in clinical situations and better understand the morphological changes as potential compensatory strategies in the cases of lesion (Fassbender  Schweitzer, 2006). 

Findings from these studies suggest that higher cognitive activities are associated with more important activation in regions responsible for motor, visual and spatial processing in patients with ADHD, because patients lose their ability employ complementary regions of the brain and develop strategies helping to perform cognitive tasks while healthy individuals engage regions responsible for cognitive organization and employ phonological strategies (Fassbender  Schweitzer, 2006).

Anatomical and functional changes of prefrontal cortex (PFC), impaired construction in PFC are found in patients with ADHD findings confirm that PFC, especially the right hemisphere, plays a main role in the cognitive deficit found in ADHD (Fassbender  Schweitzer, 2006). 

Activity of anterior and posterior cingulate cortex is also reduced especially performing synchronized movement task in patients with ADHD (Fassbender  Schweitzer, 2006).

Data from three primary research studies performed by Garrett et al. (2008), Hamilton et al. (2008) and Luders et al. (2009) will be discussed further in this paper.  The first study explores neuroanatomical changes in late adolescent patients with ADHD and the other two investigate respectively impairment in white matter and corpus callosum with regards to ADHD.

Garrett et al. (2008) conducted a study exploring the specific sample of individuals aged 15-19 years with family ADHD history.  Researchers were looking for functional implications resulting from changes - decrease or increase- in volume of the neural matters of the brain regions exploring the caudate nucleus and the frontal gyrus in patients with ADHD.  Adolescents participating in the study had a family history of ADHD and they expressed persistent symptoms of the disease.  Garrett et al. hypothesized that atypical volume of caudate nucleus and frontal gyrus will be present in young patients with ADHD.

Evidence was gathered from 24 youth patients with ADHD and the control group of 10 healthy individuals to whom investigation with structural high-resolution Magnetic Resonance Imaging (MRI) was performed as well as from archive materials of examination of 12 additional healthy individuals (Garrett et al., 2008).  Researchers also performed functional activation of MRI in order to find out correlation between brain volume and function in adolescents with ADHD. 

Study findings showed the similar entire brain tissue between the two groups as well as increase in the volume of the right caudate gyrus and right inferior frontal gyrus in the ADHD patients (Garrett et al., 2008). Additionally, enlarged left caudate volume in individuals with ADHD was associated with decreased function.  Garrett et al. (2008) argue that these neuro-developmental changes are particular present in individuals with ADHD in late adolescence and having family history of ADHD.

It is thought that impairment in the neural transmission circle, which is important in attention, motor and executive functions, is part of ADHD patho-physiology (Hamilton et al., 2008). 

Hamilton et al. (2008) cited structural studies performed by Castellanos et al. (2002), Filipek et al. (1997), Krain and Castellanos (2006), as well as studies by Luders et al.(2008) that showed decreased volume of white matter, reduced areas of midsagittal part of corpus callosum, and declined cortical thickness in patients with ADHD compared with the individuals from control groups.  Hamilton et al. argue that impairment in fiber tract are better seen by diffusion tensor imaging (DTI), because it is more informative and because data can be further computed to fractional anisotropy (FA) presenting the directions of water diffusion within the body tissue.

In the study Hamilton et al. (2008) applied DTI in order to evaluate fractional anisotropy (FA), a measure of wholeness of the fiber tract in individuals with ADHD.  Participants were 17 male patients with ADHD in the investigation group and 16 male individuals in the control group.
Findings from this study proved the presence of disturbance in networks responsible for attention and motor function in individuals with ADHD (Hamilton et al., 2008).  Furthermore, Hamilton et al. (2008) concluded that decreased FA in the cortico-spinal tract and the superior longitudinal fasciculus may be related to the ADHD symptoms, such as reduced attention, hyperactivity and impulsivity.

Luders et al. (2009) stressed the importance of corpus callosum (CC) as the largest brain intersection responsible for the transmission of the inter-hemispheric information and outlined the results from neuroimaging studies reporting significant reduction in CC leading to asymmetry and contrasting hemispheric effects in ADHD.  Further, Luders et al. cited the meta-analysis performed in 2007 by Valera et al. where splenium of CC as a particular part of CC was greatly diminished in patients with ADHD.

In the clinical study comparing morphology of CC between the group of ADHD patients (19 subjects) and control group of normally developing individuals (19 subjects) Luders et al. (2009) aimed to define the exact location of changed CC thickness applying new methods for image analysis, which is a surface-based geometrical modelling.

Study findings proved that CC is significantly thinner in patients with ADHD and especially anterior and posterior CC parts are affected (Luders et al., 2009).  It is argued that the thinner CC is related to the reduced amount or decreased myelination of the fibers that tie the parietal and prefrontal parts of the cortex, and thus negatively affect ways of inter-hemispheric communication.  Theses changes hind the sustainability of motor control and attention, therefore contribute to the development of ADHD symptoms, such as inattention, hyperactivity, and impulsivity (Luders et al., 2009).

Conclusions
Findings from majority of nuroimaging and behavioral studies prove existing neuroanatomical brain changes and fiber tract disturbances in patients with ADHD.

Knowledge about these biological changes can help better understand the variety of existing symptoms of this disorder, plan interventions and treatment.