"After one has looked at steadily moving contours for a period of time, stationary contours will appear for a while to move in the opposite direction..[This is known] as the motion aftereffect (MAE)." (Cavanagh and Favreau 1979). "When this effect is elicited by a rotating spiral it is called spiral aftereffect (SAE)." (Milligan and Scott 1971). In 1972, Bonnet and Pouthas found that the duration of apparent motion was a function of the distance of the test field from the viewer; the farther the test field, the longer the duration of apparent motion. In the present study, different test stimuli were viewed by two groups of subjects, each group at a different distance, in an attempt to confirm the relationship found between test field distance and SAE duration by Bonnet and Pouthas (1972).
Subjects consisted of 7 females (1 experimenter) and 1 male experimenter. All were Alma College undergraduates between the ages of 19 and 21 years. All had good or corrected vision. Subjects were split into 2 groups (Groups 1 and 2) which determined test pattern distance. A Spin Science disc (see Figure 1), 5.75 inches in diameter, was used as the adapting stimulus. The disc was spun by a small motor at 175-200 rpm, 1 foot away from the subjects face. Three circular, black and white, test patterns were observed at distances of 2 and 3 feet. Group 1 observed the former, Group 2 the latter. The three test patterns consisted of a grid, a vertical line, and concentric circles drawn with a black marker on white paper. The diameter of the test pattern size and the patterns themselves were proportionate to the visual angle of .45 degrees that was created by viewing the Spin Science disc. A watch, accurate to the nearest 100th of a second, was used to time the adaption period and SAE duration. The entire experiment was conducted in a fully lit room.
During the two minute adaptive period subjects were seated and told to look at the center of the rotating spiral. When the time elapsed subjects turned their head 90 degrees to the left and looked at a test pattern. Subjects said "stop" when the SAE ceased, and the duration was noted. Verbal comments from the subjects were also obtained after the experiment. This procedure was repeated two additional times by each participant with the remaining test patterns. Distances from the patterns were varied by group. Test pattern order was randomized from subject to subject and breaks between trials were given to reduce the increase in duration caused with prolonged exposure.
The mean SAE duration (6.29 s) for the test patterns at a distance of three feet was consistently shorter than the mean duration (10.42 s) for the test patterns at two feet. (see Figure 2) These results display an inverse relationship between SAE duration and test pattern diatance with a low variation across test patterns. More importantly , these results contradict those found by Bonnet and Pouthas (1972).
Several factors may account for our contradictory findings. Our test patterns were proportional to the adaptive stimulus with respect to visual angle. Bonnet and Pouthas (1972) mention nothing about visual angle in their experiment. Bonnet and Pouthas (1972) state, "duration of the MAE increases with the distance of the test feild in conditions where Emmert's law is also valid." In our experiment Emmert's law was not seen as a factor. None of the subjects tested commented on the expending or contracting of the test patterns as a whole. This is most likely due to our test patterns filling the whole visual angle. It was observed in preliminary testing, where visual angle consistency was lacking, that the disc when seen as a test pattern did expand and contract depending on the distance viewed at. Both of the experiments lack a large number of subjects who, if tested, may show a clearer pattern to the relationship between distance and the duration of the SAE. Also because of the small number of subjects, increased exposure and learning may play a role, causing more individual variance. Speed variability of the small motor may also have caused individual variance. Though the subjects were informed beforehand roughly what to look for, they might not have the experience to tell exactly when the effect had disappeared. Subjects more experienced in seeing SAEs could assist in producing more accurate results. In conclusion, this experiments results show that an increase in the distance between subjects and test patterns causes a decrease in the duration of the SAE contrary to results previously seen by Bonnet and Pouthas (1972).
Bonnet, C., and Pouthas, V. (1972). Apparent size and duration of a movement after-effect. Quarterly Journal of Experimental Psychology, 24, 275-281.
Cavanagh, Patrick and Favreau, Olga Eizner (1979). Motion after-effect: a global mechanism for the perception of rotation. Perception, 9. 175-182.
Milligan, W. L., and Scott, R. S. (1971). On the geometry of spiral aftereffect. Perception and Psychophysics, 10, 389-392.
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