Marijn Lijffijt

A meta-analytic review of stopping performance in attention-deficit/hyperactivity disorder: deficient inhibitory motor control?

This review discusses whether deficient inhibitory motor control is the core deficit of attention-deficit/hyperactivity disorder (ADHD). Inhibitory motor control is commonly assessed using the stop-signal paradigm. Since the last meta-analysis that was performed, 33 new studies have appeared. The current meta-analysis revealed a significant difference between ADHD patients and matched controls in stop latency (stop-signal reaction time) in both children and adults. Basic reaction time was significantly longer in children with ADHD, but not in adults, and there was a significant interaction between the elongation of the latency to stop and to respond in adults, but not in children. Deficient inhibitory motor control may be less crucial in children than in adults with ADHD.

On the processing of spatial frequencies as revealed by evoked-potential source modeling

OBJECTIVES AND METHODS Visually evoked potentials (VEPs) are known to be sensitive to spatial frequency, especially in the time range between 50 and 100 ms post-stimulus. In two experiments we localized the cortical activity elicited by stimuli of varying spatial frequency in scalp-recorded brain potentials, using multi-electrode recordings and dipole-source analysis. RESULTS Low spatial frequencies (<1 c/d) activated relatively lateral occipital areas, the orientation of the neural ensembles involved being predominantly perpendicular to the scalp surface. In contrast, high spatial frequencies (>4 c/d) induced activation of more medial occipital areas with the predominant orientation of the sources being much more parallel to the scalp surface. Furthermore, at about 100 ms latency the lateral-occipital response to low spatial frequencies was stronger in the right hemisphere; no such asymmetry was found for the responses to the high spatial frequencies. These findings were consistent across varying recording conditions, individual subjects, subject populations, stimulus characteristics (grating orientation, grating vs. checkerboard), and task conditions (active vs. passive). CONCLUSION The results indicate that there are differences in sensitivity to specific spatial frequencies between primary and secondary visual areas, as well as between the right and the left hemispheres.

Split-Second Sequential Selective Activation in Human Secondary Visual Cortex

This work addressed early selection based on nonspatial visual features, using event-related potentials (ERPs) with high temporal resolution and dipole-source modeling. Subjects were presented rapid sequences of gratings varying in spatial frequency and orientation, and were instructed to attend to gratings with one spatial frequency and ignore those with another. Attention effects started at 120-msec latency as anterior positivity and proceeded as posterior negativity (200 msec) and anterior negativity (265 msec). Dipole-source modeling suggested that these effects reflect the sequential selective activation of, on average, posterior dorsalmedial, posterior ventrallateral, and anterior medial cortical areas. In contrast, stimulus-specific activity was observed well before 100-msec latency and characterized by dipoles with locations significantly posterior to those of the attention-modulated activity. These results indicate that even with highly discriminable spatial frequencies, selection is not as early as before the 100-msec latency, unlike what is often found for location selection. It is also separated in time and anatomically from the earliest stimulus-specific cortical activity. Reducing discriminability of the selection feature resulted in longer selection latencies, becoming manifest only at 175 msec as an apparent combination of posterior and anterior negativities, and in an elevated criterion for overt responding.

Differences between low and high trait impulsivity are not associated with differences in inhibitory motor control

Objective: The present study investigates whether there is an association between trait impulsivity in the normal population and inhibitory motor control as assessed by the stop task. Method: Low- and high-impulsive participants (as assessed by the I7 questionnaire; both groups n = 31) performed the stop task. Differences in performance were analyzed by an independent samples t-test. Furthermore, a short meta-analysis was performed on this study and three previous studies with a similar aim. Results: The low- and high-impulsive groups did not differ on the speed to stop the response (SSRT). However, the meta-analysis revealed that high-impulsives are marginally slower in stopping than low-impulsives (effect size = -0.26, p = 0.06). Conclusions: There is only minor evidence that impulsivity in the common population is associated with poor inhibitory motor control.

Childhood-onset growth hormone deficiency, cognitive function and brain N-acetylaspartate

Cognitive deficits have been reported in adults with childhood-onset growth hormone (GH) deficiency. We evaluated cognitive deficits simultaneously with parameters for neuronal integrity using (1)H magnetic resonance spectroscopy (MRS) in a cross-sectional design. We studied 11 adults (mean age 24.5 years) with childhood-onset GH deficiency, which persisted after reaching final height. All subjects were evaluated after interruption of GH supplementation for at least 3 months. We performed neuropsychological assessment (NPA) using tests evaluating memory, mental processing speed, reading ability and executive functioning. MRS was used to assess brain N-acetylaspartate (NAA)/choline ratios. Data were compared with an age-, sex- and education-matched control group (n=9, mean age 27.3 years). NPA demonstrated attenuated performance of the patients in the delayed verbal memory recall score (P<0.05) and the trail making A test (P<0.05), a measure of planning of behavior, processing speed and attention. Other neuropsychological tests were not affected. NAA/choline ratios were significantly reduced (P<0.01) in GH deficient subjects. Specific cognitive defects indicating affected memory and attention were found in patients with childhood-onset GH deficiency. These defects occur simultaneously with reduced neuronal integrity.

Somatotropic-axis deficiency affects brain substrates of selective attention in childhood-onset growth hormone deficient patients.

Reduced levels of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) are associated with deteriorated cognitive performance in senescence. Little work has been done on the effect of GH and IGF-1 on a crucial aspect of cognition, selective attention. This study investigated the effect of GH/IGF-1 on performance and brain potentials (EEG) during a selective-attention task in patients with low levels of GH and IGF-1 (childhood-onset growth hormone deficiency) compared to healthy controls. Detection of occasional visual target patterns was impaired in patients. This was paralleled by a reduction in an attention-related brain potential, which has been associated previously with anterior cingulate cortex functioning.