This study is being carried out as a part of a Psychology Quantitative Project at the University of Glasgow. The School of Psychology at the University of Glasgow supports the practice of protection of human participants in research. The following will provide you with information about the experiment that will help you to decide whether or not you wish to participate. If you agree to participate, please be aware that you can withdraw at any time, or choose to omit to answer any particular question, without providing a reason. The study should not last longer than 20 minutes.

People typically use their experience of the natural three-dimensional world to interpret a two-dimensional image. The Ponzo illusion arises when a person looks at an image with perspective cues, such as a railway track or a road receding in depth. An object in the distance will appear larger than an object close by even when they have the same size in the image. For example, the two lines in the diagram below are actually of the same length. However the one on top appears to be further away and so gives the impression of being longer.

We are now going to show you a series of images in which two balls will appear. You will be asked to answer the question "How much smaller or larger on a scale from -3 to +3 pixels does the ball in the middle look compared to the ball on the left or right.’

Please note that there are no correct or incorrect responses as the question is about the appearance of ball sizes in each image. Therefore, please do not answer the question as if you are measuring each ball with a ruler. If you feel uncomfortable during this study, you have the right to withdraw at any stage. Please indicate by ticking the box below that you understand your rights and agree to participate in the experiment.

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