Etiologic subtypes of attention-deficit/hyperactivity disorder: brain imaging, molecular genetic and environmental factors and the dopamine hypothesis

Multiple theories of Attention-Deficit/Hyper-activity Disorder (ADHD) have been proposed, but one that has stood the test of time is the dopamine deficit theory. We review the narrow literature from recent brain imaging and molecular genetic studies that has improved our understanding of the role of dopamine in manifestation of symptoms of ADHD, performance deficits on neuropsychological tasks, and response to stimulant medication that constitutes the most common treatment of this disorder. First, we consider evidence of the presence of dopamine deficits based on the recent literature that (1) confirms abnormalities in dopamine-modulated frontal-striatal circuits, reflected by size (smaller-than-average components) and function (hypoactivation); (2) clarifies the agonist effects of stimulant medication on dopaminergic mechanisms at the synaptic and circuit level of analysis; and (3) challenges the most-widely accepted ADHD-related neural abnormality in the dopamine system (higher-than-normal dopamine transporter [DAT] density). Second, we discuss possible genetic etiologies of dopamine deficits based on recent molecular genetic literature, including (1) multiple replications that confirm the association of ADHD with candidate genes related to the dopamine receptor D4 (DRD4) and the DAT; (2) replication of differences in performance of neuropsychological tasks as a function of the DRD4 genotype; and (3) multiple genome-wide linkage scans that demonstrate the limitations of this method when applied to complex disorders but implicate additional genes that may contribute to the genetic basis of ADHD. Third, we review possible environmental etiologies of dopamine deficits based on recent studies of (1) toxic substances that may affect the dopamine system in early development and contribute substantially to the etiology of ADHD; (2) fetal adaptations in dopamine systems in response to stress that may alter early development with lasting effects, as proposed by the developmental origins of health and disease hypothesis; and (3) gene-environment interactions that may moderate selective damage or adaptation of dopamine neurons. Based on these reviews, we identify critical issues about etiologic subtypes of ADHD that may involve dopamine, discuss methods that could be used to address these issues, and review old and new theories that may direct research in this area in the future.

Eye and head turning indicates cerebral lateralization

When solving verbal problems, right-handed people usually turn head and eyes to the right, whereas with numerical and spatial problems, these people look up and left. Left-handed people differ in all these respects. The results suggest that the direction in which people look while thinking reflects the lateralization of the underlying cerebral activity.

Sensory and attention abnormalities in autistic spectrum disorders

Individuals with autistic spectrum disorders (ASDs) often experience, describe and exhibit unusual patterns of sensation and attention. These anomalies have been hypothesized to result from overarousal and consequent overfocused attention. Parents of individuals with ASD rated items in three domains, 'sensory overreactivity', 'sensory underreactivity' and 'sensory seeking behaviors', of an expanded version of the Sensory Profile, a 103-item rating scale developed for the present study. Parents also rated symptom severity, overselective attention and exceptional memory, and completed the Vineland Adaptive Behavior Scales. Of 222 rated subjects, 144 had complete data. Cluster analysis showed the predicted overfocused pattern of sensation and attention, comprising overreactivity, perseverative behavior and interests, overfocused attention and exceptional memory in 43 percent of this sample. This pattern was striking in 10 percent. The neurological basis of overreactivity and overfocusing is discussed in relation to the overarousal hypothesis. Attention is drawn to its considerable prevalence in the ASD population.

Hemispheric control of spatial attention

According to the activation-orienting hypothesis the distribution of attention in space is biased in the direction contralateral to the more activated hemisphere. The present investigation tested this proposal and evaluated the nature of hemispheric differences in orienting control. Activation imbalance was produced by a unilateral visual stimulus. The distribution of attention was measured using a modified line bisection task in which subjects judged the location of an intersect on a tachistoscopically presented horizontal line. The first three experiments suggest that (a) attention is biased in the direction contralateral to the stimulated hemisphere, and (b) the biases do not depend on the task relevance or hemispatial position of the stimulus producing the activation imbalance. The final three experiments suggest that when orienting conflict is introduced the rightward bias becomes more robust than the leftward bias. The findings are consistent with the activation-orienting hypothesis. Each hemisphere generates a contralateral attentional bias and the rightward bias of the left hemisphere is stronger. The relevance of these findings to understanding unilateral neglect resulting from parietal damage is discussed.

Generalized and Lateralized Effects of Concurrent Verbalization on a Unimanual Skill

After practice, subjects balanced a dowel rod on the right and on the left index finger while speaking and while remaining silent. As compared to control, the verbal condition yielded shorter balancing times for the right hand, but longer ones for the left. A speculative model postulates enhancement of the practised skill by virtue of the distraction effect of the concurrent activity. This is counteracted on the right by interference with right-sided motor control by the left cerebral hemisphere due to verbal activity programmed by the same hemisphere.

Food dyes impair performance of hyperactive children on a laboratory learning test

Forty children were given a diet free of artificial food dyes and other additives for 5 days. Twenty of the children had been classified as hyperactive by scores on the Conners Rating Scale and were reported to have favorable responses to stimulant medication. A diagnosis of hyperactivity had been rejected in the other 20 children. Oral challenges with large doses (100 or 150 milligrams) of a blend of FD & C approved food dyes or placebo were administered on days 4 and 5 of the experiment. The performance of the hyperactive children on paired-associate learning tests on the day they received the dye blend was impaired relative to their performance after they received the placebo, but the performance of the nonhyperactive group was not affected by the challenge with the food dye blend.

Asymmetrical transfer of training between hands: implications for interhemispheric communication in normal brain

Two experiments are presented which investigated claims of asymmetrical transfer of training between the hands/hemispheres. In Experiment 1, 96 right- and left-handed male undergraduates practiced an inverted-reversed printing task with either the right or the left hand. Transfer to the opposite hand was then compared to same-hand transfer, in a between-subject design. In Experiment 2, 176 right-handed boys and girls were tested at ages 7, 9, and 11 years. For right-handed subjects in both experiments, the left hand benefited more from opposite-hand training than did the right. The reverse was true for left-handers in Experiment 1, although one group (who wrote with the "inverted" position) showed little transfer in either direction. Two current models of interhemispheric interaction do not satisfactorily explain these findings. A third model, based on cross-activation, may provide a more effective alternative.

Stimulant-related state-dependent learning in hyperactive children

Hyperactive and nonhyperactive children performed a learning task in two states, while being treated with stimulant medication (methylphenidate) and while taking a placebo, and were tested for retention of each class of learned material in both states. Symmetrical state-dependent learning was demonstrated in the hyperactive group but not in the nonhyperactive group. The state-dependent effect was contingent on the presence of drug-induced facilitation during initial learning. This is apparently the first report on record of state-dependent learning with a drug agent that facilitates rather than impairs performance of human subjects.

Human handedness: a partial cross-fostering study

The hand preference of college students correlated significantly with the writing hand of their biological parents but not that of their stepparents. The results are consistent with a genetic theory of the origin of human handedness.

Functional significance of individual variations in callosal area

We considered the hypothesis that the richness of callosal interhemispheric connections has a role in determining the degree of behavioural laterality and time-sharing ability in dual-task performance. Behavioural laterality as measured by dichotic word listening, line bisection and turning bias tests correlated inversely with the midsagittal cross-sectional area of the corpus callosum, as seen on MRI. The amount of dual task interference was strongly inversely correlated with the callosal area in both within-hemisphere and between-hemispheres conditions. These relationships between normal variations in callosal area, and outcomes on tests both of laterally and time-sharing capacity in normal adults suggest that the corpus callosum assumes a cross-excitatory role when subjects perform these tasks.

Auditory hallucinations and subvocal speech in schizophrenic patients

Fourteen of 18 hallucinating schizophrenic patients reported that the voices they heard went away when they undertook a maneuver that precluded subvocalization. The same applied to 18 of 21 normal subjects who hallucinated under the influence of hypnotic suggestion. Control maneuvers had no such effect. The authors suggest that auditory hallucinations may be projections of schizophrenic patients' verbal thoughts, subvocalized due to deficient cerebral cortical inhibition.