This dataset contains raw behavioral responses of 29 participants (M = 29.38 years, SD = 5.52, 17 females). The experiment was aimed to test the strength of Object-Substitution Masking (OSM). The independent variables were the size of Ebbinghaus inducers (small inducer, large inducer), viewing condition (simultaneous viewing, delayed viewing), target circle size (small, large) and congruency (congruent, incongruent). In each trial, two of the eight circles were surrounded by Ebbinghaus inducers: one was designated as the target and the other as the nontarget stimulus. Trials where the target and nontarget stimuli were the same size were categorized as congruent, while those with different-sized target and nontarget stimuli were classified as incongruent. During the experiment, participants were presented with a size-discrimination task where they supposed to identify the size of the target. Each row in the dataset represents one trial. Below is a description of the columns: 1. Mask Type: Indicates the viewing condition (simultaneous viewing, delayed viewing) 2. Inducer Size: Indicates the inducer size (Small inducer, Large inducer). 3. Target Size: The size of the target circle (Small target, Large target). 4. Nontarget Size: The size of the nontarget circle (Small nontarget, Large nontarget). 5. Congruency: Indicates the congruency of sizes of target and nontarget stimuli (congruent, incongruent). 6. Accuracy: 1 for correct responses, 0 for incorrect responses. 7. Pressed Key: 'j' for small responses, 'k' for large responses. 8. Reaction Time: The time (in milliseconds) between the presentation of the response window and the participant’s key press. 9. Distractor Sizes: The sizes of the six distractor circles on the screen, measured in degrees of visual angle. 10. Distractor Average Size: The average size of the six distractor circles. 11. Target Location: The coordinates of the target position in each trial. 12. Nontarget Location: The coordinates of the nontarget position in each trial.