Disorders of learning and behavior are linked to brain abnormalities

Rapidly developing science in this field is bringing to light more understanding of the biological basis of learning and behavioral disorders. A paper published not long ago in the journal Pediatrics introduces a classification of attention-deficit/hyperactivity disorder according to underlying organic causes. The authors first observe:

"Attention-deficit/hyperactivity disorder is a neurobiological syndrome with an estimated prevalence among children and adolescents of 5%. It is a highly heritable disorder, but acquired factors in etiology are sometimes uncovered that may be amenable to preventive measures or specific therapy."

The others go on to suggest an organic theory and genetic and biochemical basis for attention-deficit/hyperactivity disorder along with an etiologic (causal) classification, taking into consideration environmental, prenatal, perinatal and postnatal factors including illnesses, injuries and deficiencies.A series of studies published in Biological Psychiatry offer insight into how attentional and behavioral disorders are linked to variations in the very structure of the brain and its component anatomy. The authors of Structural Brain Imaging of Attention-Deficit/Hyperactivity Disorder observe:

"Many investigators have hypothesized that attention-deficit/hyperactivity disorder (ADHD) involves structural and functional brain abnormalities in frontal-striatal circuitry. Although our review suggests that there is substantial support for this hypothesis, a growing literature demonstrates widespread abnormalities affecting other cortical regions and the cerebellum...The most replicated alterations in ADHD in childhood include significantly smaller volumes in the dorsolateral prefrontal cortex, caudate, pallidum, corpus callosum, and cerebellum. These results suggest that the brain is altered in a more widespread manner than has been previously hypothesized."

These authors refer to, among others, an earlier study published under the title Smaller prefrontal and premotor volumes in boys with attention-deficit/hyperactivity disorder.

"Boys with ADHD had (on average) 8.3% smaller total cerebral volumes...Findings suggest that ADHD is associated with decreased frontal lobe gray and white matter volumes. More than one subdivision of the frontal lobes appears to be reduced in volume, suggesting that the clinical picture of ADHD encompasses dysfunctions attributable to anomalous development of both premotor and prefrontal cortices."

Later in the same journal Temporal Lobe Dysfunction in Medication-Naïve Boys With Attention-Deficit/Hyperactivity Disorder During Attention Allocation and Its Relation to Response Variability established that abnormalities could be documented in the temporal lobes as well:

"Patients showed significantly reduced brain activation in left and right superior temporal lobes, basal ganglia, and posterior cingulate...Brain abnormalities in patients with ADHD are not confined to fronto-striatal networks mediating executive functions but are also observed in temporo-striatal and cingulate regions..."

Biological Psychiatry was also the venue for documenting abnormalities in the corpus callosum (the structure connecting the right and left brain hemispheres) in Decreased Callosal Thickness in Attention-Deficit/Hyperactivity Disorder. The authors observe:

"Neuroimaging studies of attention-deficit/hyperactivity disorder (ADHD) have revealed structural abnormalities in the brains of affected individuals. One of the most replicated alterations is a significantly smaller corpus callosum (CC)..."

They used advanced imaging techniques to refine and further validate these observations:

"In close agreement with many prior observations, the CC was shown to be significantly thinner in ADHD subjects...Decreased callosal thickness may be associated with fewer fibers or a decrease in the myelination of fibers connecting the parietal and prefrontal cortices. This might affect interhemispheric communication channels that are necessary to sustain attention or motor control, thus contributing to symptoms of hyperactivity and impulsivity, or inattention, observed in ADHD."

Recently the same journal presented evidence of abnormalities in another brain region in the paper Ventro-Striatal Reductions Underpin Symptoms of Hyperactivity and Impulsivity in Attention-Deficit/Hyperactivity Disorder. This research is significant for its investigation of the reward centers in the brain. The authors observe:

"The neural bases of reward processes have barely been explored in relation to this disorder, in contrast to extensive neuroimaging studies that examine executive functions in patients with ADHD."

The authors examined volumetric differences in the ventral striatum of ADHD children and found substantial correlations:

"The ADHD children presented significant reductions in both right and left ventro-striatal volumes. In addition, we found that the volume of the right ventral striatum negatively correlated with maternal ratings of hyperactivity/impulsivity...Our study provides neuroanatomical evidence of alterations in the ventral striatum of ADHD children...the negative correlations we observed strongly uphold the relation between the ventral striatum and symptoms of hyperactivity/impulsivity."

A paper published in the French medical journal L'Encéphale, sums up the ever-growing scientific literature in this field. Under the title Structural and functional neuroanatomy of attention-deficit hyperactivity disorder (ADHD), the authors observe:

"Three subtypes of the disorder have been proposed in the current clinical view of ADHD: inattentive, hyperactive-impulsive and combined type. Numerous problems are associated with ADHD: poor academic performance, learning disorders, subtle cognitive deficits, conduct disorders, antisocial personality disorder, poor social relationships, and a higher incidence of anxiety and depression symptoms into adulthood. ..From the neuropsychological viewpoint, impairment of the “hot” affective aspects of executive functions, like behavioural inhibition and attention and the more cognitive, “cool” aspects of executive functions like self-regulation, working memory, planning, and cognitive flexibility, are often reported by studies on ADHD. The hot executive functions are associated with ventral and medial regions of the prefrontal cortex (including the anterior cingulated cortex) and named “hotbrain” and the cool executive functions are associated with the dorsolateral prefrontal cortex and are called “coolbrain”.

The potential anatomical areas of interest are extensive:

"Convergent data from neuroimaging, neuropsychology, genetics and neurochemical studies consistently point to the involvement of the frontostriatal network as a likely contributor to the pathophysiology of ADHD...Moreover, a growing literature demonstrates abnormalities affecting other cortical regions and the cerebellum...Anatomical studies suggest widespread reductions in volume throughout the cerebrum and cerebellum, while functional imaging studies suggest that affected individuals activate more diffuse areas than controls during the performance of cognitive tasks...Furthermore, hypoactivation of the dorsal anterior cingulate cortex, the frontal cortex and the basal ganglia (striatum) have also been reported."

As always, biological individuality rules—every child is different. Subsequent posts offer insights into the various underlying causes of these abnormalities in brain anatomy, how to test for them, and what to do about them.