Introduction: The motion-after effect, also known as the waterfall effect, is an illusion that includes a pattern of light moving steadily across a field of vision that will produce the apparent movement in the opposite direction when looking at a stationary pattern of the same orientation. Many articles, one being published as early as December 1928, by Dr. Walter R. Miles in The Scientific Monthly, has confirmed this finding (Miles 488). If this is already an established fact than could it also be an additive illusion? To test this there three different phases were used, control (adaptation), central and peripheral field of vision. Three different phases were used, as well as 20 subjects, in order to equate the differences. Our hypothesis was that the motion aftereffect illusion would be additive.
Methods: Our subjects consisted of nineteen, male and female, Alma College students ranging in the ages of 19 to 22 years of age. In order to run this potential illusion there needed to be an organized stimulus. The two stimulus were created using Adobe Photoshop and Quick Time. The first, control stimulus, was a forty-nine second clip of a horizontal downward motion. The second stimulus was a combination of the control stimulus and an added fourteen second clip of forty-five degree diagonal motion.
All subjects completed three different trials. These included the horizontal control stimulus in the central vision, the horizontal-diagonal central vision and the horizontal-diagonal peripheral vision. The first trial consisted of participants watching the horizontal control stimulus in central vision. For the second trial participants watched the horizontal control stimulus followed by the forty-five degree diagonal stimulus in central vision. The last trial had the same stimulus shown as the second, except participants watched using peripheral vision. After each trial the participants were asked to use the mouse to move the line of angle adjustment in the direction of perceived motion, which was done using the computer program Eyelines.
Results: Our data shows that motion aftereffect of the horizontal control stimulus and the horizontal-diagonal in central vision were both similar. They were found to move in the opposite direction because of fatiguing of the motion detectors in the eye. The horizontal control stimulus and horizontal-diagonal in the central vision both had a low error rate, while the horizontal-diagonal in the peripheral vision had an increased error rate. The horizontal control stimulus showed a difference of one degree. The horizontal-diagonal in central vision had a difference of two degrees and the horizontal-digonal in the peripheral vision shows a difference of negative ten degrees.
Figure 1: Motion After effect during three different phases.
During adaptation, the participants saw the opposite motion due to the motion after effect. With central vision, participants saw the illusion as slightly additive. In peripheral vision, the participants saw the illusion as ten degrees from additive.
The deviation was relatively wide in all three of the phases. The horizontal control stimulus showed a standard deviation of thirty to negative thirty degrees. The horizontal-diagonal in central vision showed a deviation of thirty-eight to a negative thirty-two degrees. Lastly, the horizontal-diagonal in peripheral vision showed a deviation of forty-two to negative sixty degrees. On average, our data shows that there was a possible link to the motion aftereffect being additive. This shows that through each of the last two trials, the majority of the subjects perceived the motion as going somewhere between forty-five and ninety degrees. Our horizontal control stimulus, would mean that people would perceive the motion as going upwards, which is true for all but one of our subjects, and the other two trials, in order to be additive, would need to be somewhere between forty-five and ninety degrees, which on average, also proves to be true.
Discussion: All in all our data shows that the motion aftereffect has the potential to be an additive illusion. It also shows that looking at the horizontal-diagonal stimulus through your peripheral vision has a remarkable difference in error magnitude, as well as standard deviation rate, than the other two variables. In addition, the data reveals that the experiment had a negative after effect that is associated with Murch's experiment and the motion after effect percieved in the opposite direction (Murch 47). Psychologist Roderick Power (1994), found that "motion aftereffects follow apparent rather than real movement" (Power 131) in the wagon wheeels experiment.The concluding result of the experiment is that both motions added together caused an additive angle of motion. The participants perceived the motion aftereffect of an upward motion from somewhere between forty-five and ninety degrees.
References:
Miles, Walter R. (1928) Visual Illusions of Motion. The Scientific Monthly, 481-491.
Murch, G.M. (1976) Negative Aftereffects in Visual Perception. Scientific American, 235, 42-48.
Power, R.P. (1994) Motion affecteffects of wagon wheels: Motion aftereffects follow apparent rather than real movement. Perceptual & Motor Skills, 79, 131-140.
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