The Effects of the Presence or Absence of Depth Cues in the Ponzo Illusion

The effects of the presence or absence of depth cues in the Ponzo illusion

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ABSTRACT

The Ponzo illusion is one of the example of how human’s visual system can be deceived by using recognisable depth cues. The following research paper describes a study based on the hypothesis that the presence of depth cues is the main reason for the Ponzo illusion. 40 undergraduate students, divided into two groups, took part in the experiment. The first group was asked to adjust the length of two parallel lines lying in between two other converging lines. The second group was told to do the same but without the converging lines. It was found that the standart deviation of the group doing the experiment without depth cues varied less than the standart deviation of the another group which supports the hypothesis. The results of the conducted research are discussed in relation to future experiments concerning perception of depth, space and texture as well as brain-imaging data related to these studies.

INTRODUCTION

We ‘see’ objects in the world because objects and people reflect light, and this light hits our retina (at the back of the eye) and is processed by visual pathways linking the eye and the brain (Goldstein, 2002). The image projected onto the retina is 2 dimensional, yet we live in a 3 dimensional world. To solve this problem, the visual system looks for depth cues in the 2D retinal image, which help work out how far away objects and other people are. Pictorial depth cues include interposition, linear perspective, relative size, and texture gradients (Gleitman, Reisberg & Gross, 2007; Passer, Smith, Holt Bremner et al., 2009). Linear perspective is a powerful depth cue that conveys distance, and helps maintain a stable perception of the word. This technique was used by artists as early as the 15th century (Goldstein, 2007), and we can see many examples of it in our everyday environment (roads, railway tracks, video games). We judge distance using linear perspective, by assuming that the narrower the gap between the converging lines, the further the distance is. But we know that the road does not physically get narrower as the distance increases. This is explained by the principle of size constancy, which was first investigated in the 1930s (Gregory, 1997). Thouless (1931) showed that the perceived size of an object or person is the same despite being viewed at a variety of distances. An object viewed at a longer distance will produce a smaller image on our retina, but because we know it is further away we unconsciously apply the principle of size-distance