Electrophysiological evidence for increased auditory crossmodal activity in adult ADHD

Abstract

Background: Attention deficit and hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by core symptoms of inattention, and/or impulsivity and hyperactivity. In order to understand the basis for this multifaceted disorder, the investigation of sensory processing aberrancies recently reaches more interest. For example, during the processing of auditory stimuli comparable low sensory thresholds account for symptoms like higher distractibility and auditory hypersensitivity in patients with ADHD. It has further been shown that deficiencies not only exist on an intramodal, but also on a multimodal level. There is evidence that the visual cortex shows more activation during a focused auditory task in adults with ADHD than in healthy controls. This crossmodal activation is interpreted as the reallocation of more attentional resources to the visual domain as well as deficient sensory inhibition. In this study, we used, for the first time, electroencephalography to identify a potential abnormal regulated crossmodal activation in adult ADHD.

Methods: 15 adult subjects with clinically diagnosed ADHD and 14 healthy controls comparable in age and gender were included. ERP components P50, P100, N100, P200 and N200 were measured during the performance of a unimodal auditory and visual discrimination task in a block design. Sensory profiles and ADHD symptoms were assessed with inattention as well as childhood ADHD scores. For evaluating intramodal and crossmodal activations, we chose four EEG channels for statistical analysis and group-wise comparison.

Results: At the occipital channel O2 that reflects possible crossmodal activations, a significantly enhanced P200 amplitude was measured in the patient group. At the intramodal channels, a significantly enhanced N200 amplitude was observed in the control group. Statistical analysis of behavioral data showed poorer performance of subjects with ADHD as well as higher discrimination thresholds. Further, the correlation of the assessed sensory profiles with the EEG parameters revealed a negative correlation between the P200 component and sensation seeking behavior.

Conclusion: Our findings show increased auditory crossmodal activity that might reflect an altered stimulus processing resource allocation in ADHD. This might induce consequences for later, higher order attentional deployment. Further, the enhanced P200 amplitude might reflect more sensory registration and therefore deficient inhibition mechanisms in adults with ADHD.

Keywords: EEG; attention deficit and hyperactivity disorder (ADHD); auditory; crossmodal activity; sensory processing; visual.

Figure 1

Paradigm trial sequences. ITI, intertrial interval; ISI, interstimulus interval; (A) Auditory discrimination task; (B) Visual discrimination task.

Figure 2

Grand average waveform of P50, P100, N100, P200, and N200 at channel O2, ADHD (red) vs. Control Subject (green). First auditory stimulus was presented at time zero. Component amplitudes of P50, N100, P100, N200, and P200 were identified by detecting peaks in pre-defined time windows (gray rectangles for positive, black rectangles for negative components): Between 25 and 74 ms (P50), 75 and 125 ms (P100), 50 and 150 ms (N100), 175 and 250 ms (P200), and 150 and 250 ms (N200) after stimulus onset (black line), respectively.

Figure 3

Grand average results of the auditory discrimination task. (A) Group comparison of correct (hits) vs. false (misses) answers in auditory discrimination task; (B) Group comparison of total auditory discrimination response times and response times (hit only) that included only correct answers; (C) Comparison of auditory discrimination thresholds between the patient (red) and control (green) group.

Figure 4

Grand average results of the visual discrimination task. (A) Group comparison of correct (hits) vs. false (misses) answers in visual discrimination task; (B) Group comparison of total visual discrimination response times and response times (hit only) that included only correct answers; (C) Comparison of visual discrimination thresholds between the patient (red) and control (green) group.

Figure 5

Results auditory discrimination task group comparison. (A) ERPs at channel O2 (crossmodal) averaged over subjects, patient group (red) vs. controls (green), time windows for subject-based peak detection as rectangles, and black line marks the stimulus onset (for details, see Figure 2); (B) Same as (A) at channel Fz.

Figure 6

Results visual discrimination task group comparison. (A) ERPs at channel O2 (intramodal) averaged over subjects, patient group (red) vs. controls (green), time windows for subject-based peak detection as rectangles, and black line marks the stimulus onset (for details, see Figure 2); (B) Same as (A) at channel Fz (crossmodal).