Ponzo Illusion
There are two different types of cues that humans use to judge depth and distance. Binocular cues involve both eyes and include binocular disparity. You can demonstrate binocular disparity to yourself by covering one of your eyes and looking ahead for a few seconds. Now open both eyes and you will notice increased depth perception due to slightly different images available to each eye. The majority of cues for depth come from one not both eyes. These monocular cues include perspective, closeness to the horizon, texture gradients, shading, and interposition of objects, among others. This lab activity demonstrates the Ponzo illusion, which illustrates the role of monocular cues on the perception of distance and judgment of the size of objects.
Judging Horizontal Line Length
This activity is simple. You will be presented with two horizontal lines, A and B, on each of 12 experimental trials. Simple plus and minus controls will allow you to complete the task of adjusting the length of line B so that it equals that of line A. The two lines will be separated by a vertical distance and, on some trials, they will appear with a background scene. Use the plus and minus controls to adjust line B until you feel it is equal to line A and then press the spacebar to begin the next trial.
Inference of Size Based on Scene Cues
The first set of results you will be presented with after completion of the trials shows the baseline condition, when the two lines appear without a scene background. In this baseline condition, most participants perceive both bars to be the same length with the perceived length of line A/line B=100%. When a scene is placed behind the two lines, for this experiment a set of straight railroad tracks, then lines placed higher up on the scene are perceived as longer. A typical participant responds by making line B longer to match line A. The Ponzo illusion shows how the inference of size is based on cues in a scene. The cues can be so powerful that they cause the observer to be unable to replicate the actual image projected onto the retina. People are dependent on depth cues provided by the scene.
Monocular Cues
There are multiple monocular cues that may affect your perception of apparent distance in this lab activity. These include perspective, the convergence of parallel lines that imply distance. Another is the texture gradient, where smaller texture implies greater distance and vice versa. The location of the horizon is also a powerful cue with objects close to the horizon being perceived as more distant. Shading is also powerful with darker features implying more distance and vice versa.
Moon Illusion
The location of the horizon is a powerful monocular cue, which can be demonstrated on a clear night with an unobstructed view of the sky. This illusion is related to the Ponzo illusion and is called the moon illusion. In the moon illusion, the location of the moon close to the horizon makes it appear larger to us compared to when it is high in the sky, where there is no horizon nearby to act as a monocular depth cue.