Orthorexia Nervosa (ON); is a condition characterized by individuals being overly focused on healthy eating habits, constantly checking and classifying the foods they will consume. ON has not yet been clinically defined as an eating disorder or in a different category, and has not been included in DSM-5. Cognitive flexibility can be defined as a person's ability to quickly change their thinking and behaviors to adapt to different situations and changing conditions. It has been reported that cognitive flexibility skills are impaired in ON. There is evidence that tDCS, a neuromodulation method, increases cognitive flexibility skills. On the other hand, although there are EEG studies on eating disorders, there is no EEG study directly studying ON, and there is no information on the EEG profiles of individuals with high ON scores. For this reason, EEG studies on eating disorders and obsessive behaviors are generally generalized to orthorexia nervosa and are interpreted. This study aims to compare the EEG oscillations and cognitive flexibility skills of individuals with high ON scores with a control group and to examine the effects of anodal tDCS application on these two variables. For this purpose, a total of 48 individuals, 24 healthy and 24 high ON scores, will be included in the study. Participants will be randomly assigned to tDCS experimental and control groups. Therefore, 48 participants will be divided into 4 different subgroups of 12 people each. Data collection tools will be Informed Consent Form, Demographic Information Form, Beck Depression Inventory, Orthorexia Nervosa Test (TON-17), Cognitive flexibility task. First, participants will be subjected to a cognitive flexibility task by taking simultaneous EEG recordings, then tDCS stimulation will be initiated (active or sham) and after stimulation, a cognitive flexibility task will be performed by taking simultaneous EEG recordings again. EEG analyses will be performed using the Brain-Vision Analyzer program. Repeated Measures Analysis of Variance (within-group and between-groups) will be used in statistical analyses.

Measurement instruments:

Informed Voluntary Consent Form Demographic Information Form Edinburgh Hand Preference Survey Beck Depression Scale Test of Orthorexia Nervosa (TON-17) Cognitive Flexibility Task EEG and tDCS devices Statistics: Statistical analyses will be conducted using SPSS 25.0 (Statistical Package for Social Sciences) for Windows, and EEG data will be analyzed using the Brain-Vision Analyzer software.

Descriptive statistics for group variables such as age and education will be presented as means and standard deviations, while gender will be reported as frequency and percentage.

The normality of the distribution of variables will be assessed using the Kolmogorov-Smirnov test, and appropriate parametric or non-parametric tests will be applied based on the results.

To evaluate pre-test and post-test data for cognitive flexibility and EEG measures in the experimental and control groups, Repeated Measures Analysis of Variance (within- and between-subjects) or its non-parametric equivalent, the Friedman Test, will be used.

For all statistical tests, a significance level of p < 0.05 will be considered as the threshold for statistical significance.

Inclusion Criteria:

• Being between the ages of 18-40,
• Having a score of 17 or less on the Beck Depression Inventory
• Having a score of 61 or more on the Test for Orthorexia Nervosa (TON-17) for the orthorexia group

Exclusion Criteria:

• Having a history of serious neurological and psychiatric illness,
• Currently taking medication that alters the cortical excitability level,
• Having visual defects that cannot be corrected with glasses
• Use of intracranial metal objects, implanted stimulating devices or pacemakers

Participants will not know that which group they will be in