THE EFFECTS OF SHAPE AND COLOR ON THE TERNUS EXPERIMENT

JACLYN SAUTER, JEREMY O'SHEA, AND EMILY HEISS

ALMA COLLEGE PSYCHOLOGY DEPARTMENT

Motion is an essential part of our daily vision which can often occur as an illusion rather than through physical motion. In apparent motion, a discontinuously displaced visual element is perceived as moving continuously through space. This one element moving is actually two different elements that appear and disappear in succession causing the experience of movement.

The Ternus illusion can be generated when two frames, each with three elements in a straight line, are alternately presented at varying interstimulus intervals (ISI). When there is a long ISI, the appearance of group motion occurs. When there is a short ISI, the end element appears to jump from one end to the other. A change from element to group motion is also observed as viewing eccentricity and element size is increased. With the increase of eccentricity and element size, visual pattern persistence decreased showing that pattern persistence plays a role in determining which is the dominating percept in stroboscopic motion. (Kramer, 1997)

We would like to look into the extent that shape and contrast play a role in visual persistence and the perception of motion.

Methods: For this experiment, we used 15 subjects from the Alma College campus. The experiment was run on the Eye Lines program on a Macintosh computer with the brightness and contrast dials turned all the way up. Three stimuli were used in this experiment. The first stimuli used were circles. There were two frames of circles run in a multistimulus field with the frames at .3 sec and a blank separating them for .1 sec at 25 cycles. The next stimuli used were squares run in a multistimulus field similar to that of the circles. The third stimuli used were a mixture of squares and circles. The first frame had a square, circle, square, while the second frame had a circle, square, circle. (See figure 1) These frames were also run in the multistimulus field similar to the previous two. Each stimulus group was run in four separate trials: one control (black), one dark gray, one light gray, and one in a mixed color group. The trials were placed in a random order and the subjects indicated if they viewed the motion as jumping or shifting.

Results: The subjects viewed the stimuli that contained mixed colors and/or mixed shapes to be engaged in jumping (rather than group motion) (See Figure 2). Stimuli with same color/same shape were more likely to be seen as sliding or shifting. The light gray squares produced no effect of jumping.

Discussion: The results of this experiment confirmed our hypothesis. When the stimuli were a constant shade or constant shape, they were viewed as shifting. By changing the shape and/or color of the stimuli, we were able to change the effect of the apparent motion of the object to a jumping motion. Pattern persistence was increased by the alternating shapes and irregular brightness patterns. This resulted in the observation of group to element motion. Changing the brightness of the objects in the mixed color trials seemed to add to the contrast of the contextual elements, which may be a reason the motion changed from group to element. This also led to the increase in the duration of visible persistence and the increase in the trend of jumping in the mixed color and/or shape trials. A report by Petersik and Pantle tested against the theory by Bowling and Lovegrove that pattern persistence measured with grating stimuli increases as the contrast of the grating decreases. This was proven with the light gray squares having a lower contrast with the background, yet causing only group motion and no element motion (Breitmeyer, 1986). More research may yeild results as to why the apparent motion changes as a result of changing shape and color.

References:

Breitmeyer, Bruno G., and Ritter, Alysia. (1986) Visual Persistence and the Effect of Eccentric Viewing, Element Size, and Frame Duration on Bistable Stroboscopic motion patients. Perception and Psychophysics. 39 (4) 275-280.

Kramer, Peter, and Yantis, Steven. (1997) Perceptual grouping in space and Time:Evidence from the Ternus Display. Perception and Psychophysics. 59(1) 87-99.

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