Abnormal distracter processing in adults with attention-deficit-hyperactivity disorder

Abstract

Background: Subjects with Attention-Deficit Hyperactivity Disorder (ADHD) are overdistractible by stimuli out of the intended focus of attention. This control deficit could be due to primarily reduced attentional capacities or, e. g., to overshooting orienting to unexpected events. Here, we aimed at identifying disease-related abnormalities of novelty processing and, therefore, studied event-related potentials (ERP) to respective stimuli in adult ADHD patients compared to healthy subjects.

Methods: Fifteen unmedicated subjects with ADHD and fifteen matched controls engaged in a visual oddball task (OT) under simultaneous EEG recordings. A target stimulus, upon which a motor response was required, and non-target stimuli, which did not demand a specific reaction, were presented in random order. Target and most non-target stimuli were presented repeatedly, but some non-target stimuli occurred only once ('novels'). These unique stimuli were either 'relative novels' with which a meaning could be associated, or 'complete novels', if no association was available.

Results: In frontal recordings, a positive component with a peak latency of some 400 ms became maximal after novels. In healthy subjects, this novelty-P3 (or 'orienting response') was of higher magnitude after complete than after relative novels, in contrast to the patients with an undifferentially high frontal responsivity. Instead, ADHD patients tended to smaller centro-parietal P3 responses after target signals and, on a behavioural level, responded slower than controls.

Conclusion: The results demonstrate abnormal novelty processing in adult subjects with ADHD. In controls, the ERP pattern indicates that allocation of meaning modulates the processing of new stimuli. However, in ADHD such a modulation was not prevalent. Instead, also familiar, only context-wise new stimuli were treated as complete novels. We propose that disturbed semantic processing of new stimuli resembles a mechanism for excessive orienting to commonly negligible stimuli in ADHD.

Figure 1. Modified oddball task.

Altogether 460 target, non-target and novel (non-target) symbols were presented to each subject in the current modified oddball task. Meaning-wise, novels were either familiar (relative novel) or non-familiar (complete novel). The symbols appeared at intervals of 2 seconds in randomised order.

Figure 2. Categorisation of novel stimuli.

Categorisation of the stimuli strongly overlapped between groups. This can be deduced from the present description, in which the sixty novel stimuli were ordered according to semantic familiarity scores in the control cohort: First and second columns provide scores per group, the highest familiarity being 15 (meaning that all group members could associate a meaning with the given stimulus), the lowest 0 (meaning that none of the group members could associate a meaning with the given stimulus). In the third column, between-group differences for stimuli with the indicated rating constellation are presented, expressed as the rating score of controls minus that of patients. In the last column, the number of stimuli with the between-group rating-constellation, specified in the respective row, is indicated.

Figure 3. Oddball P3.

Grand-average of ERP from parietal electrodes (P8, P4, Pz, P3, P7) upon targets (bold line), rare (thin line) and frequent non-targets (dotted line) in controls (A1) and patients (A2). (B) shows target-P3 differences between controls (dotted line) and patients (bold line).

Figure 4. Familiarity effect on frontal P3.

Grand-average of ERP from frontal electrodes (F8, F4, Fz, F3, F7) upon familiar (bold line) and unfamiliar novels (dotted line) in controls (A1) and patients (A2). B1 shows ERP-differences for familiar, B2 for unfamiliar novels between controls (dotted line) and ADHD-patients (bold line).

Figure 5. Familiarity effect on parietal P3.

Grand-average of ERP from parietal electrodes (P8, P4, Pz, P3, P7) upon familiar (bold line) and unfamiliar novels (dotted line) in controls (A1) and patients (A2).

Figure 6. Statistical comparison of novelty P3.

Columns indicate mean novelty-P3 amplitude to familiar and unfamiliar novels, bars show the respective standard errors of mean (ns = not significant, * indicates P<.05).