Attention deficit hyperactivity disorders

Attention shifts and anticipatory mechanisms in hyperactive children: an ERP study using the Posner paradigm

BACKGROUND

The aim of this study was to assess attentional, decisional, and motor processing stages during the performance of an attention shifting paradigm, both in normal children and children with attention-deficit/hyperactivity disorder (ADHD).

METHODS

We recorded event-related potentials (ERPs) and performance measures during a variant of the Posner paradigm in 13 control subjects and 24 ADHD children. Subjects responded with a spatially concordant motor response to left or right visual targets, which could be either preceded by a spatial cue ("valid" = same side; "invalid" = opposite side) or presented uncued.

RESULTS

Patients made significantly more errors than control subjects, with predominance of the anticipatory type. As compared to control subjects, ADHD children had faster reaction times, as well as a shortened interval between the N2 and P3 ERPs and the motor response. Patients also showed a decreased attentional priming effect on early sensory responses (P1). Finally, the slow negativity (contingent negative variation/readiness potential) that preceded the target in the "no cue" condition was absent in ADHD patients.

CONCLUSIONS

The combined analysis of electrophysiological and behavioral data suggest a characteristic mode of response of ADHD in attention shifting tasks, characterized by "motor impulsivity" with release of motor responses before stimulus processing is adequately completed, as well as a lack of strategic planning/anticipatory mechanisms in the absence of warning stimulus. These deficits may be partly attributed to dysmaturation of executive frontal functions. In addition, a minor deficit in early attentional priming was also observed in ERPs, with no apparent behavioral counterparts.

Central conduction time of magnetic brain stimulation in attention-deficit hyperactivity disorder

Twenty-seven children and adolescents aged 4 to 18 years, fulfilling the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) criteria for attention-deficit hyperactivity disorder, were included in this study. The diagnosis was determined by a pediatric neurologist and a psychologist examined all 27 subjects with tests that included the Wechsler Intelligence Scale for Children-Revised (WISC-R) and reading, writing, handedness, personality, and anxiety scores. Subjects with histories of epilepsy, head injury, drug abuse, or psychotic disorders were excluded. Culturally based causes or emotional disorders were also excluded. Transcranial magnetic stimulation was performed on all subjects, with recording of the motor evoked potential at the biceps brachii. Central motor conduction time was calculated by cervical root stimulation. Due to shape variability, the amplitude of the motor responses was not measured in the study. The mean value of central conduction time in the subjects was very significantly increased (P < .001) compared to that in a group of normal controls, case-matched for IQ, age, and sex. A central motor conduction time greater than 12 ms indicates abnormality. A second finding in the subjects was the significant difference of central conduction time on the side-to-side stimulation (P = .03). These findings are correlated with delay in the maturation of the corticomotoneuronal system and might provide neurophysiologic data for diagnosis.