ADHD Electrophysiological Subtypes and Implications in Transcranial Direct-current Stimulation

Important advances in the understanding of ADHD pathophysiology, such fMRI studies showing a focal frontotemporal loops dysfunction in brain activity, suggest that frontal brain stimulation might be helpful for the treatment of ADHD. In a recent study of Lyon's university with tDCS they concluded that tDCS is "cheaper and easier-use than transcranial magnetic stimulation (TMS) and the impact on symptomatology seems larger (impact on negative symptoms of schizophrenic patients) and longer (at least 3 months duration) than that TMS currently permits. It is possible that tDCS could in the future be used at home by patients themselves. The efficacy of tDCS depends of parameters like electrode position and current strength.

In this trial, The investigators investigated the effects of 12 days of anodal stimulation of the left dorsolateral prefrontal cortex in ten patients with ADHD (aged 8 +/- 3 years). tDCS was applied through a saline-soaked pair of surface sponge electrodes (35 cm2). The anode electrode was placed over F3/F4 or T5/T6 or P4/P3(based on the 10-20 International EEG System) of each subject. The cathode was placed over the contralateral mastoid area. A constant current between 1.1 and 2.0 mA was applied for 25 min/day (administered for 12 alternated days).

Prior to the first session, ADHD subjects were asked to complete and return a series of questionnaires, including the Conners Brief Symptom Inventory, a health history questionnaire, and the QEEG questionnaire. Subjects were then tested in a first session which lasted approximately three hours. During this period, a comprehensive structured clinical interview was carried out, comprising of an assessment of current and past ADHD symptoms, the history of problems at school, the past psychiatric history (including drug and medication use), as well as past and present comorbidities. Subsequently, EEG data was acquired. EEG data was first recorded while the subject was in eyes-closed and eyes-open resting conditions, lasting four minutes each. Then data was recorded while subjects performed a visual continuous performance task (VCPT). The VCPT took approximately 22 minutes to complete. In addition, subjects randomly performed either an auditory or an emotional continuous performance task.

The control group had a shortened procedure. Subjects were tested in a single session lasting approximately two and a half hours. During this period, a series of questionnaires (Brief Symptom Inventory, Health History questionnaire, Current Symptoms Scales) were filled out and thereafter, EEG data was acquired. Subsequently, a working memory task, which is not relevant here, was administered.

EEG was recorded using a Mitsar 201 19-channel electroencephalographic system. The input signals referenced to the linked ears were filtered between 0.5 and 50 Hz and digitized at a sampling rate of 250 Hz. Impedance was kept below 5 kOhm for all electrodes. Electrodes were placed according to the International 10-20 system using a electrode cap. Quantitative data was calculated using WinEEG software. Linked ears reference montage was changed to average reference montage prior to data processing. Eye-blink artefacts were corrected by zeroing the activation curves of individual ICA component score responding to eye blinks. In addition, epochs of the filtered electroencephalogram with excessive amplitude (>100 μV) and/or excessive fast (>35 μV in 20 to 35 Hz band) and slow (>50 μV in 0 to 1 Hz band) activity were automatically marked and excluded from further analysis. Finally, EEG was manually inspected to verify artefact removal.

Behavioral task

The VCPT is a modification of the visual two-stimulus GO/NOGO paradigm. Three categories of visual stimuli were selected: 20 pictures of animals, 20 pictures of plants, and 20 pictures of humans (presented together with an artificial "novel" sound). The trials consisted of presentations of pairs of stimuli: animal-animal (GO trials), animal-plant (NOGO trials), plant-plant (IGNORE trials), and plant-human (NOVEL trials). The trials were grouped into four blocks. In each block a unique set of five animal stimuli, five plant stimuli and five human stimuli was selected. Each block consisted of a pseudo-random presentation of 100 stimuli pairs with equal probability for each trial category.

The task was to press a button as fast as possible in response to GO trials.

According to the task design, two preparatory sets were distinguished in the trials. In the "Continue set" a picture of an animal is presented as the first stimulus and the subject is supposed to prepare to respond. In the "Discontinue set" a picture of a plant is presented as the first stimulus and the subject does not need to prepare to respond.

During the task, subjects were seated in a comfortable chair, 1.5 m in front of a computer screen. The stimuli were presented on a 17 inch monitor using the Psytask (Mitsar Ltd.) software.

The primary outcome was change in score on the QEEG Rating Scale (AMEN questionnaire). The ERP and questionnaire/behavioural assessments will be made at baseline (before stimulation)and 3 months after stimulation.

This study involved 30 subjects, all aged between 7 and 13. All have been diagnosed with ADHD by a medical professional.

Inclusion criteria

• ADHD diagnosis.
• Age between 7 and 65 years.
• Comorbidities were no reason for subject exclusion.

Exclusion criteria

• Presence of psychosis.
• Subjects taking medication,they had refrained from taking methylphenidate during 24 hours before testing.
• Subjects taking other psychotropics were not included in the study.
• Subjects which had suffered of a head injury with subsequent loss of consciousness, and subjects suffering from neurological or systemic medical diseases were excluded from the study.