Sometimes we may want to have the stimuli appear only after the participant has looked at a certain place on the screen. Gaze contingent boxes are often used at the beginning of trials to make sure that the participant starts the trial at the correct point on the screen. Gaze contingent boundaries are also often used in ‘boundary change’ experiments. In reading research if you want to have the preview for a word different from when the participant actual fixates the word you will use a gaze contingent boundary. In some visual search research you may with to trigger a change in the display when the participant looks at a certain object in the display.
There are a number of gaze contingent trigger types: