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Stereopsis Experiment Design

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INTRODUCTION

Stereopsis is the capability of assessing the depth of objects in the visual field, using the relative positions of the objects visualized by each eye.1 The difference in the right eye’s and left eye’s views in terms of the location of an object is defined as binocular disparity.2 The acquirement of the depth concept is a direct consequence of having laterally placed binocular visual system.1

eye1.jpg

Figure 1: Binocular disparity shown in detail2

Stereograms, also called Single Image Random Dot Stereograms (SIRDS) are visual illusions that make it possible to get three dimensional images from two dimensional figures via looking at different parts of the image with each eye and copying the acquired images on top of each other.3

As perception of the vision is a phenomenon in which eye and brain work in harmony where eye is to see and brain is to process and correlate the incoming information. As it is light that makes the vision possible via being absorbed and converted into electrical impulses by photoreceptor cells, the quality and also the quantity of light coming to the eye play determinative roles in visual perception.4

Pulfrich effect is one illustration of the significant role of acquired light for visual perception. In this specific phenomenon, a low contrast object is perceived as slower whereas a higher contrast object is perceived as faster comparing the two objects moving with the same speed. The contrast difference is provided via attenuating one eye’s light acquiration using filter glasses. Pulfrich’s pendulum is constructed to express this effect in quantitative terms via calculating the pendulum’s period. It has been assumed that the subjective slower perception of the filtered eye contributes to the depth perception and the binocular disparity in a way, creating a so thought elliptical movement perception.5,6

Eyepulfrich.gif

Figure 2: The diagram of the Pulfrich pendulum5

Another example of the depth concept in light of data known about visual perception is Ames room. In this particular illusion,if two person stands in the opposite edges of the room, one is perceived as gigantic compared to the other. The reason is that the walls and the windows or any other decorative elements are located in a trapezoidal way with one end of the room is far away located compared to the other with respect to the eyes. In the end, the cues on which the binocular vision depends to interpret relative features of visualized objects were mislead.7

ames.jpg

Figure 3: Ames room7

MATERIALS:

  • Pulfrich’s pendulum
  • Ruler
  • Chairs
  • Filtered sunglasses
  • SIRDS images
  • Cube with three rods inserted

METHODS:

  • Lateral Component of Depth Perception

A small cube with three rods inserted in was observed by experimenters and the position when the relative distance between the rods could be perceived was discussed.

  • Qualitative Evaluation of the Pulfrich Phenomenon

Using the website “http://www.blelb.ch/english/blelbspots/spot16/exspot16_en.htm”, the Pulfrich effect was observed and the perception changes along with the adjusted parameters was determined.

  • Quantitative Evaluation of the Pulfrich Phenomenon

We were provided with a Pulfrich pendulum and filtered sunglasses. By filtering one eye each time, experimenters were asked when they could no longer observe the elliptical movement while a pointer was driven towards them under the pendulum. Then the distance moved by the pointer relative to the pendulum axis was measured. The process was repeated after changing the eye filtered, changing the amplitude of the pendulum and also changing the distance of the eye to the pendulum set up.The results were recorded along with each experimenter’s distance between two pupil of their eyes. Using these data, the period was calculated.

RESULTS

Experiment 1

A cube with 3 differently sized rods are given. It is observed that when the cube is hold horizontally all three rods are seen in the same depth while when it is hold vertically it was clearly seen that one of the rods are not on the same axis with the other two.

Experiment 2

From a website, movement of black dots is observed with using filtered glasses. It is observed that looking with a filter adds a depth of the movement, which is actually happening in two dimensions. The darker the filter is the more deep the circulation occurred; an elliptical path is observed. When the left eye is closed, the dots appear to come close to observer. The dots looks bigger when the degree of filter increased too.

Experiment 3

A pendulum is fixed and set in motion, while eye of the observers is closed with a filter. Pendulum appeared in an elliptical path with a diameter. The results are shown in Table 1. P is the distance between right eye and left eye. When the left eye is closed, clockwise motion is observed while when the right eye is closed counter clockwise motion is observed. The object looks smaller when it seems to get closer to observer.

Table 1. Elliptical pathway observations

Observed

2 m

3 m

3 m

3m

Intensity

GI

GI

GI

GII

Amplitude

1m

0.5 m

1 m

1 m

(cm)

R

L

R

L

R

L

R

L

P

Ekin

30

42

50

44

73

71

-

57

6.5

Fatma

26

10.5

28

20

25

23

25

23

6

Esra

36

13

47

33

18

19

9

16

5.7

It is observed that Fatma and Esra’s results are not very consistent with the expected result but Ekin’s values were consistent except the last part of the experiment. The calculations will be made on Ekin’s results.

The period of the pendulum is found with .

For Amplitude A=0.5 and 1 m T= 2. The angle is so small that amplitude doesn’t make a change.

ω=2 π/T so ω=3.

Δt = Δx / A sin ω

Δx / b = p / (a-b)

The calculations are shown in Table 2. The final part of the experiment is excluded because Ekin didn’t see the pendulum in that one.

Table 2. Calculations

Observed

2 m

3 m

3 m

Intensity

GI

GI

GI

Amplitude

1m

0.5 m

1 m

Δx= p*b / (a-b)

(6.5*30)/12=16.25

(6.5*44)/6=47.6

(6.5*73)/2=237

Δt = Δx / A sin ω

16.25/1*sin3=2.2

47.6/0.5*sin3=13.4

237/1*sin3=33.4

DISCUSSION

In this experiment, the purpose is to examine and understand the working principle of eye according to the light coming to eyes and to understand the perception of the depth according to the orientation of the object.

In first experiment, it is understood that orientation of objects affects the depth of the object in the brain. When rods are vertical, it was seem deeper than rods are horizontally orientated.

In second experiment, Pulfrich Effect is observed. the light coming to our one of the eye was delayed somehow relative to the other one, we saw moving objects in 2-Dimension as if they were moving in three dimensional.

In third experiment, the classical demonstration of the Pulfrich Effect by a swinging pendulum was examined in order to learn the relationship between retina position of corresponding object and depth perception .The pendulum is swung back and forth,when viewed normally with both eyes, it seemed swinging back and forth. When a filter is placed in front of one eye, the pendulum suddenly seemed as swinging in an ellipse parallel to the floor. The background information of this depth perception relies on a reduction in retinal illumination which results a corresponding delay in signal transmission, imparting instantaneous spatial disparity in moving objects. This seems to occur because visual system latencies are generally shorter for bright targets as compared to dim targets. This motion with depth is the visual system's solution to a moving target when a difference in retinal illuminance, and hence a difference in signal latencies, exists between the two eyes.

As it can be seen from the results part, changes in amplitude ,number of filters,distance from the objects leads to the differences in time delay results.For example, when we increase the size of amplitude, the time also increases because the more amplitude means the more in size of the eliptical pathway.Also, the increase in the distance from a real object results in an increase in the deviation of time. In Ekin’s result, the relationship amplitude and pendulum is the most close one to expected result. In Esra’s and Betül’s result, it was expected to increase the pendulum with the amplitude but it did not so. And also, time differences increases when the number of filters increase and with double filters ,higher optical density leads to more eliptical observation of pendulum.

Moreoever, it could also be noted that when the object is far away from observer, it looks larger than the normal and when it is closer to observer it looks smaller than the normal.

REFERENCES:

  1. Kalloniatis, M. (2007). Perception of Depth. Retrieved 05 24, 2013, from Webvision, The Organization of the Retina and Visual System: http://webvision.med.utah.edu/book/part-viii-gabac-receptors/perception-of-depth/
  2. Heeger, D. (2006). Perception Lecture Notes: Depth, Size, and Shape. Retrieved 05 24, 2013, from Department of Psychology, New York University: http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/depth/depth-size.html
  3. Stereograms. (n.d.). Retrieved 05 24, 2013, from http://infohost.nmt.edu/~armiller/stereo.htm
  4. The Eye and The Visual World. (n.d.). Retrieved 05 24, 2013, from Macalester College: http://www.macalester.edu/academics/psychology/whathap/ubnrp/aesthetics/theeye.html
  5. Pulfrich Phenomenon. (n.d.). Retrieved 05 24, 2013, from Medical Dictionary: http://medical-dictionary.thefreedictionary.com/Pulfrich+stereophenomenon
  6. Phenomenology of Motion. (n.d.). Retrieved 05 24, 2013, from Rochester Institute of Technology.: http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_13/ch13p2.html
  7. DorwardF M C, DayR H, 1997, "Loss of 3-D shape constancy in interior spaces: The basis of the Ames-room illusion"Perception26(6) 707–718

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