One illusion in particular takes a very unique look at movement. (Cormack, Blake and Hiris, 1991); took a look at objects moving against different backgrounds. This study is going off of this previous research and building upon it. Another article looks at why this happens, Adelson and Movshon (1982) says the illusion is because of the "eye aperture" problem. The motion vectors don't give accurate information about direction or movement and goes on to say that directional movement requires many receptors, some global and some local. These experiments only actually work when the eye is not directed towards the illusion. The eye's peripheral vision is the part that is inaccurate and hazy about direction of movement. There hasn't been much research done on varying speeds in the peripheral vision. This study is looking at whether speed has any influence on the strength of the illusion. This is taking a close look as to whether speeding up the object or slowing down the object as it passes over the differential background. Is the eye only able to pick up on movement at certain speeds or is any speed going to fool the eye into misidentifying the trajectory of an object going across a differential background?
Participants
The participants of this study were 11 undergraduate students at a small Midwestern college; 7 males and 4 females. They were between the ages of 18-25. they were all volunteers and given the chance to be in a raffle to in a $10 gift certificate to a local restaurant.
Apparatus and Stimuli
In this study the visual effects ere produced on a plasma screen monitor controlled by a Macintosh computer. The object used was a rectangular box moved at a 45 degree angle to the upper right of the screen. The background used was a striped ox with the lines facing toward the upper left corner of the screen as seen in Figure 1. The rate of speed of which the box is moving was one of 3 speeds randomly chosen in any order. The rates are as follows: 5, 10 and 15 times the normal mouse speed of the computer. It is moving at an average speed of 1cm/sec, 1.2cm/sec, and 1.6cm/sec. The length it travels is not critical. The movement will be controlled by a button located on the keyboard and will be operated by the lab assistant.
Figure 1: The Cormack illusion as used in this experiment. This image shows the Box ready to travel across the grated background toward the upper right of the screen.
The participants will be instructed to sit at the computer and look a few inches above the monitor so that the grated box can be seen in their peripheral vision. The lab assistant will then operate the keyboard and cause the box or object to move across he screen in the upward direction. This will be done in 3 sets for a total of nine times so as to ensure enough trials at varying speeds. The participants will then be instructed to look at another screen and take the line that is on the screen at a 45 degree angle using the mouse and move the line so it appears to go in the same direction that the box previously went as seen in Figure 2. They will then be debriefed and asked if they have any other questions at which time if they do not they will be dismissed and asked to not divulge the results or methods of the experiment.
As you can see from Figure 3 and Table 1 below there was a high trend of error during the 5x and 10x speed trials and a much higher accuracy during the 15x trials. There was one outlier that didn't show the general trend during the 5x and 10x trials. The average for the 5x trials was 14.4 degrees of variance, the average for the 10x trials was 10.9 degrees of variance while the average for the 15x trials was 3.6 degrees of variance.
Figure 3: Graph showing general trend. This shows the steady decline in error of angle detection during the speed variance Cormack Illusion. As you can see form the image there is a very strong trend when the speed increases the strength of the illusion decreases.
The data clearly shows a general trend. The faster the illusion is administered the weaker the illusion becomes. As Figure 1 shows, the strongest illusion is when it is traveling at 5x and the weakest is when it is traveling at 15x. This shows that at 15x the box traveling at the 45 degree angle is traveling too fast for the eyes to be tricked. This was a general trend throughout all subjects and trials except for one. One of the subjects had data showing no correlation to the previous subjects. This can be explained however, it is suspected that they were nervous and perhaps either anxiety played a part or they did not follow the directions accurately and thus didn't witness the illusion out of their peripheral vision but rather out of their primary line of vision. This would explain the lack of error in identifying the angle of movement and the shocking accuracy of her results.
However, ignoring the outlier, all subjects performed as predicted. My hypothesis that speed would indeed affect the overall strength of the illusion was proven correct. The faster that the box traveled across the differential background the less the eye was fooled. The average degree of error for the 15x speed was approximately 5. This shows that the subjects were fairly accurate in their assessment of which the box traveled; at times they were within 2 degrees of the actual angle traveled. Whereas, with the 5x and 10x speeds subjects were far less accurate in their predictions, with some subjects being as far as 20 degrees off from the actual degree traveled.
Some of the possible confounds in this study was there were only 11 subjects involved in this study, 7 males and 4 females. This might have caused the findings to not be that of the greater community but of a select for of individuals. If I were to do this again I would attempt to get more participants for this study.
Cormack, R., Blake, R., & Hiris, E. (1991). Misdirected Visual Motion in the Peripheral Visual Field. Vision Res. Vol. 32, No. 1, pp. 73-80, 1992.
Adelson, E.H., & Movshon, J.A. (1982). Phenomenal Coherence of Moving Visual Patterns. Nature, 300. 523-525.
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