Methods (Study 3)

Participants

Fifteen control children (mean age 11.0 years, ±0.3, range 9-13 years, 5 female, 13 right-handed, 2 bimanual) and fifteen children who were diagnosed with ADHD (mean age 10.3 years, ±0.4, range 9-13 years, 6 female, 14 right-handed, 1 left-handed) took part in the experiment. There was no significant group difference in age (F(1,28)=2.6, p=.12). Eleven children were diagnosed with ADHD combined type (ADHD-C, DSM-IV code 314.01), four with ADHD predominantly inattentive type (ADHD-I, DSM-IV code 314.00).

Diagnoses were made by the head psychiatrist and his team of psychologists at two paediatric out-patient centres in Konstanz and Friedrichshafen. Diagnoses were based on

STUDIES – Concurrent sound segregation in children with and without ADHD (Study 3) 76 standardised parent and teacher questionnaires, diagnostic interviews and psychometric tests. Six ADHD children were currently under medical treatment with methylphenidate (one child took Equasym® 20 mg; three took Medikinet® 20 mg, 25 mg, 40 mg; two took Concerta® 27 mg, 36 mg). They were asked to omit medication for at least 48 h before testing. 60% of ADHD children had co-morbid disorders (e.g., specific reading disorder and developmental disorder of motor function). All parents gave their written informed consent to participate. Ethical approval of the study was granted by the Ethical Review Board of the University of Konstanz.

MEG recordings

In session one, hearing thresholds were determined and children were familiarised with the experiment. In session two, MEG data were collected in two conditions: a) children listened to sounds and performed an auditory localisation task (Attend condition), and b) children were played the same sounds while performing a visual localisation task (Non-attend condition). Attend and Non-Attend conditions were presented in a counterbalanced order.

Hearing thresholds, attention test, and questionnaires

Hearing thresholds were determined for five frequencies (200 Hz, 595 Hz, 1 kHz, 2 kHz and 4 kHz) with clinical audiometer (AC40). One subtest of the computerised test battery Test of Attention for Children (KITAP), testing sustained attention, was administered (Zimmermann, Gondan, & Fimm, 2002). Handedness was assessed with the Edinburgh handedness inventory (Oldfield, 1971). In order to screen for any maladaptive behavioral or emotional problems, all children were evaluated with the Child Behavior Checklist (CBCL) (Doepfner et al., 1998). ADHD symptomatology was evaluated with the Diagnostic System of Psychiatric Disorders in Children and Adolescents (DISYPS) (Doepfner, Goertz-Dorten,

& Lehmkuhl, 2008), using the questionnaire for ADHD (FFB-ADHD). To screen for abnormalities concerning hearing all children were evaluated with the Questionnaire on Auditory Development (AUDIVA, 2004; Rosenkötter, 1998).

Acoustic Stimuli

Stimuli were created at the Department of Psychology of the University of York, UK (Summerfield, Paul, Kitterick, Briley, & Bailey, 2007). Baseline stimuli were complex tones consisting of 24 sinusoidal partials (f₀ = 200 Hz), which were synthesised digitally

STUDIES – Concurrent sound segregation in children with and without ADHD (Study 3) 77 (44100 samples/s, 16-bit amplitude quantization) with equal amplitudes. Two variants of the Baseline stimulus were formed by perturbing the third partial at 600 Hz. In Mistuned stimuli, the frequency of the third partial was raised by 8% to 648 Hz. In Delayed stimuli, the third partial started 160 ms after the other partials, but ended simultaneously (Fig. 1).

One hundred samples of each stimulus condition (Baseline, Mistuned, Delayed) were created; stimuli lasted 500 ms.

frequency

time

Mistuned Delayed

648 Hz 160 ms

600 Hz

Baseline

500 ms

frequency

time

Mistuned Delayed

648 Hz 160 ms

600 Hz 600 Hz

Baseline

500 ms

Figure 1: Schematic spectrograms of Baseline, Mistuned and Delayed stimuli.

Stimuli were created so that the third partial was always perceived 45° left or right of straight ahead, with equal probability. For specifics of stimulus generation, please see (Lipp et al., 2010).

In order to familiarise the children with the experiment, a training session preceded the MEG-recordings. Stimuli were presented through headphones (Dynamic Stereo Headphones MDR-CD470, Sony), while children performed the localisation task (see below). For the MEG recordings stimuli were presented through tube-phones (Etymotic Research, ER30). For each ear, a 6.1-m length of 4-mm diameter tube terminated in a right-angle adapter (ER30-9) ending with a disposable foam eartip (ER13-14B). A digital filter was designed to compensate for the low-pass frequency response of this sound delivery system, resulting in a frequency response that was flat to within -4–0 dB from 100–4800 Hz when measured with a B&K Ear Simulator (Type 4157) with 0.5-inch microphone (Type 4134), Microphone Preamplifier (ZC 0032) and B&K Hand-held Analyzer (Type 2250).

STUDIES – Concurrent sound segregation in children with and without ADHD (Study 3) 78 Using the same measurement system, the presentation level of Baseline stimuli was set to 70 dB (A).

Task

On each trial of the Attend condition, children reported whether they heard the third partial on the left or right using two keys on a keyboard. Feedback on the accuracy of the response was given by displaying a coloured rectangle on the side where the key had been pressed. It was green if the response was correct or red if the response was incorrect. The inter-stimulus interval (ISI) was the response latency plus 500 ms.

On each trial of the Non-attend condition, a faint grey circle subtending a visual angle of 1° appeared for 50 ms at a randomly-chosen location within 8° right or left of the fixation cross. Children were instructed to report whether the circle appeared left or right of the fixation cross by making a corresponding key press. Feedback was provided in the same format as used in the Attend condition. The ISI was the response latency plus a random value in the range from 500 to 1500 ms. At the same time, the acoustic stimuli were presented asynchronously with an ISI that was chosen randomly from the range 500 to 1500 ms. Children were instructed to concentrate on the visual stimuli and to ignore acoustic stimuli.

MEG recordings

Per attention condition (Attend, Non-attend) one hundred samples of each stimulus type (Baseline, Mistuned, Delayed) were presented randomly. Recording was done with a 148-channel magnetometer (MAGNESTM 2500 WH, 4D Neuroimaging, San Diego, USA).

Acoustic stimuli were delivered with an analog-to-digital converter (Motu 2408) and amplifier (Servo 200, Samson) through tube-phones (Etymotic Research, ER30). The foam tips were introduced carefully into the ear canals of children and were additionally secured with medical tape. Children lay on a height-adjustable bed in a magnetically shielded room (Vakuumschmelze Hanau). Eye movements (EOG) were recorded from four electrodes attached to the left and right temple and above and below the right eye, as well as cardiac activity (ECG) via two electrodes, one on each forearm. A Neurofax amplifier (EEG-11006, Nihon Kohden) served for the recording of EOG and ECG. A video camera installed inside the chamber allowed monitoring the subject’s behaviour and compliance throughout the experiment. Visual stimuli and feedback were projected (D-ILA Projector,

STUDIES – Concurrent sound segregation in children with and without ADHD (Study 3) 79 JVC™, DLA-G11E) onto a screen placed ~40cm away from the participants’ eyes through a mirror system. Data from each channel were band-pass filtered between 0.1 and 200 Hz and sampled at a rate of 678.17 Hz. Recording was continuous.

MEG data analysis

Data were corrected for ocular and cardiac artefacts using BESA^TM 5.3 and were averaged separately for Baseline, Delayed, and Mistuned stimuli in Attend and Non-attend conditions using a pre-stimulus baseline of 100 ms and a post-stimulus time-window of 800 ms.

Epochs were excluded if the signal amplitude in any channel exceeded 3.5 pT or if the signal amplitude differed by more than 2.5 pT between adjacent sampling points.

The sensor groups of interest were two groups of 7 sensors located bilaterally over the temporal lobes, which showed the biggest N2m amplitudes in the averaged cortical response elicited by Baseline stimuli (Fig. 2). This approach was chosen, as it was objective and ensured that sensor groups reflected auditory processing (the auditory N2 component is very prominent in children) (Johnstone, Barry, Anderson, & Coyle, 1996; Sussman, Steinschneider, Gumenyuk, Grushko, & Lawson, 2008).

Figure 2: Difference waveforms (Delayed - Baseline) for each sensor averaged over 15 control children in the Attend condition. Channel Groups used for data analysis are coloured black (7 channels for each hemisphere).

Left hemisphere: A056, A057, A078, A079, A080, A099, and A100. Right hemisphere: A044, A065, A066, A087, A088, A089, and A109.

STUDIES – Concurrent sound segregation in children with and without ADHD (Study 3) 80 In both groups of children, the same 7 sensors showed the biggest N2 amplitudes in each hemisphere. The left channel group covered the in-going magnetic field (positive values) and the right channel group the out-going magnetic field (negative values) of the underlying auditory sources. Therefore, the sign of the signals over the left hemisphere was reversed to facilitate comparisons with right-hemisphere signals.

Mean amplitudes in time-windows of interest were computed for each sensor group, participant, stimulus, and condition. Time-windows were chosen to embrace prominent deflections from zero in either direction in the difference waveforms (Mistuned minus Baseline and Delayed minus Baseline). Paired t-tests and effect sizes (r) were computed for all relevant contrasts using the following formulae:

Cohen's d = M₁ - M₂ / pooled where _pooled = [( ₁²+  ₂²) / 2]

r_Y = d / (d² + 4)

The differences of the mean amplitudes (Mistuned – Baseline, Delayed – Baseline) characterising the components (ESPm, ORNm, LSPm) were analysed statistically with ANOVAs. Hemisphere⁶ (Left, Right) and Attention (Attend, Non-attend) were within-group variables. Group (children with ADHD, control children) was used as between group factor.

To specify individual peak latencies, individual difference waveforms (Mistuned–

Baseline and Delayed–Baseline) for both conditions (Attend, Non-attend) and both hemispheres (Left, Right) were band-pass filtered from 1 to 25 Hz for detecting ESPm and ORNm, and from 1 to 8 Hz for detecting LSPm. Peak latencies were compared between conditions with ANOVAs for each component (ESPm, ORNm, LSPm). Hemisphere (Left, Right) and Attention (Attend, Non-attend) were within-group variables. Group (children with ADHD, control children) was used as between group factor.

6 Except for ESPm in Mistuned condition. Left and right hemispheric values (e.g., differences of the mean

amplitudes or peak latencies) were averaged together for ANOVAs, because for children with ADHD no ESPm on the left side in the Attend session could be identified.

STUDIES – Concurrent sound segregation in children with and without ADHD (Study 3) 81 Behavioural data

The percentages of correct responses to Baseline, Mistuned, and Delayed stimuli were compared in a multivariate analysis of variance (MANOVA). Condition (Baseline, Mistuned, Delayed) was within-group variable. Group (children with ADHD, control children) was used as between group factor. Further, Spearman´s rank correlations were calculated between behavioural results in MEG and attention test scores (KITAP), as well as amplitudes/latencies of MEG components (ESPm, ORNm, LSPm). P-values were corrected for multiple tests using the Bonferroni-Holm method. Hearing thresholds and scores of questionnaires (DISYPS, AUDIVA, CBCL) were compared between groups with t-tests. Attention test scores (KITAP) were compared between groups with Mann-Whitney U-tests, as scores were not normally distributed. Post-hoc analyses were performed with Tukey’s HSD test. All analyses were conducted with Statistica (StatSoft, Inc., Version 6, 2003).