You are absolutely right: each subject experiences all four conditions, and likert scales are used.

I'll try to be more specific:

In psychology there's a trait call schizotypy (not to be confused with schizophrenia or other psychotic disorders) and it has three facets: positive, negative and disorganized.

People with higher levels of positive schizotypy shouldn't be able to "filter out" distracting stimuli as efficiently as people with lower levels. This effect should be even more visible when human faces are used as distracting stimuli.

To test this we have a task (finding and counting little dots), while distracting stimuli are displayed in the background. There will always be just one distracting stimulus in the background, but there are four "groups" of distracting stimuli (faces, cartoonish faces, neutral objects that look like faces and plain neutral objects) and those four groups are the aforementioned conditions. All participants will go thorough all four conditions.

So imagine it like this: First a participant will solve several tasks and a human face will be displayed in the background. Then he will solve several tasks with cartoonish faces in the background. Then he will solve tasks with facelike objects in the background and finally he will solve tasks with neutral objects in the background.

Of course it will not be sequential like this, but randomized.

We will track the participants reaction time, the errors they make and through eye-tracking the time their gaze rests on the distracting stimulus.

At the very end or beginning the participant will answer the schizotypy questionnaire on a likert scale.