Depth Cues: Their influences on the Strength of the Ponzo Illusions
By Harry Smith
To: Vanessa. L. Bates
The aim of this experiment was to investigate the relationship between the number of depth cues and the influential extent this would have on the Ponzo illusion. The hypothesis being that as the number of type of depth cues increased the participants overestimation of the altered stimulus would increase. 28 participants studying Psychology Course 11 at Otago University took part in this experiment as they were presented with six different types of depth cues, ten times for each type. There were two different types of stimuli, horizontal line and truck. The measured variable was the length of the altered stimulus as a percentage relative to the referenced stimulus. During the experiment participants had to change the length of the altered stimuli until it matched the length of the referenced stimulus. The percentage of altered stimulus compared to the references stimuli was then recorded for each participant. Results showed that as the depth cues increased the percentage of altered stimuli in comparison to the referenced stimulus also increased. The hypothesis was thus supported as the number and type of depth cues in the illusions increased, causing the participants estimates of the stimulus length to also increase.
Depth perception is the internal mechanism used by people for perceiving objects relative distance or location within their visual field, and it allows people to see in three dimensions. We perceive depth using different depth cues, sources of information that signal the distance from the observer to the distal stimulus. (Gleitman, Gross and Resiburg 2010). The first type of cue that allows people to perceive depth is binocular cue. When we use our vision both of our eyes look out into the world from a slightly different position this causes each eye to have a slightly different view; this difference in view between the two eyes is known as binocular disparity. This disparity it used in 3-D movies, where two different movies are shown on the screen, each presenting slightly different views of each scene which concurrently are projected onto the screen. Wearing 3-D glasses ensures that viewers see one movie through their left eye and the other movie in their right, resulting in binocular disparity that creates a fascinating perception of depth. The other type of cue used in depth perception is monocular cues. There are multiple monocular cues that are particularly influential in the perception of objects are interposition, motion parallax, relative size, relative height and linear perspective. (Gleitman, Gross and Reisburg, 2010) Motion parallax is a depth cue that our mind associates with movement. We see objects that are closer to us as moving faster than objects that are in the distance. We are able to see the full speed motion of the close items, but distant objects seem to be moving at a slower rate (Gleitman, Gross and Reisburg, 2010) Interposition is when objects obscure others, and the obscured or blocked objects appear further back in the visual field (Gleitman, Gross & Reisburg, 2010, cited by Laboratory Manual Psychology 111/112 2013). Relative size is how people perceive objects further away to be smaller than objects that are closer, when in fact they may be larger (Sternberg, 1998). Relative height is how the higher an object is above the horizon line, the closer it appears, and reciprocally the further below the horizon line an object is, the closer it appears. Linear perspective is an illusion influenced by parallel lines; it is the apparent converging of parallel lines as they are further into the horizon until the lines appear to become one (Sternberg, 2010) . An experiment conducted by Mario Ponzo in 1911 called the Ponzo illusion used linear perspective to show how people perceive an object’s size based on background depth cues (Laboratory Manual:...
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