The aim of the study is to investigate effects of brain stimulation-assisted cognitive training in patients with persistent subjective or objective cognitive impairment after polymerase chain reaction (PCR)-positive COVID-19 disease.

Long-term persistent symptoms occur in a substantial number of patients diagnosed with coronavirus 2019 disease (COVID-19). First evidence suggests that long-term symptoms develop not only after severe courses of the disease, but also after less serious illness and include various symptoms such as fatigue and impaired memory, concentration or sleep. During recovery, these symptoms present a heavy burden for patients, who then often experience psychological distress and reduced quality of life. The goal of the present study is to assess the effects of cognitive training alone or in combination with tDCS on cognitive performance, quality of life and mental health in patients with subjective or objective cognitive impairments following post-COVID disease. Patients will either participate in a three-week cognitive training with concurrent online high-definition tDCS application or placebo ("sham") tDCS or participate in an evidence-based muscle relaxation training combined with placebo tDCS. We hypothesize that cognitive training with sham or active tDCS will result in more pronounced improvement on a transfer task compared to the control group. We also hypothesize that training combined with anodal tDCS will lead to significantly higher performance on a transfer task than training combined with sham tDCS. Additionally, we will determine effects on specifically trained and other untrained cognitive functions immediately after the intervention as well as their maintenance one month later.

Inclusion Criteria:

1. History of COVID-19 condition at least 4-6 weeks prior to study inclusion
2. Self-reported concerns regarding cognitive functioning.
3. Age: 18-60 years.

Exclusion Criteria:

1. Acute COVID-19 illness.
2. History of dementia before COVID-19.
3. Other neurodegenerative neurological disorders; epilepsy or history of seizures.
4. Severe and untreated medical conditions that preclude participation in the training, as determined by responsible physician.
5. History of severe alcoholism or use of drugs.
6. Severe psychiatric disorders such as severe depression (if not in remission) or psychosis.
7. Contraindication to tDCS application (Antal et al. 2017).

Anonymized data will be made available to the scientific community upon request.