In the Missing Number App, participants will be asked to find out the missing number from a set of four numbers in a counting sequence. In particular, a fixation across appears in the center of the screen for 500ms, follows by 500 ms later by a stimulus. For each trial, four single-digit numbers will be presented in a sequential pattern with one of them missing (e.g., in ascending or descending order). The position of the blank in the number sequence will be the second position for half the sequences and the third position for the other half of the sequences (e.g., 1 _ 3 4; 3 4 _ 6). Participants will be asked to verbally report the missing number as quickly as possible, and the experimenter will enter their response using a separate keyboard that is connected to an iPad. More specifically, for each trial, once the participant says the answer out loud, the experimenter will need to 1) press the spacebar (RT recorded), 2) then type the number (participant's verbal response recorded) in the keyboard; and 3) press enter to proceed to the next trial. Experimenter could extract test result through iTunes file sharing function.