cog8_ptr_demonstration_3

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California State University, Chico *

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334

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Philosophy

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Oct 30, 2023

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Demonstration 3.6: A Size Illusion and a Motion Illusion • 1 Demonstration 3.6: A Size Illusion and a Motion Illusion As the text describes, many aspects of perception involve a process in which you make inferences about the stimulus in front of your eyes. A vivid demon- stration of this point involves a motion illusion. This illusion requires a bit of work on your part—but the effect is worth it. Print the figure below and follow the directions to assemble the three- dimensional dragon. Then, once the dragon is assembled, set it on a desk, and watch the dragon carefully as you slowly move left or right. Odds are good that you will get a powerful motion illusion—so that the dragon moves as you move. To get a good sense of the illusion, you can also view a video of it online by searching for “dragon illusion movie.” The dragon provides a clear case in which you, the perceiver, are inter- preting (and, as it turns out, misinterpreting ) the input. Your visual system (mistakenly) assumes the dragon to be convex (bulging toward you) rather than concave, and this is an understandable error. (After all, how many hollow dragons have you seen in your lifetime? Or, more realistically, how many concave reptiles of any sort have you seen?) But then, your visual system uses this faulty information to interpret the pattern of motion. Here, your reasoning is along the lines of, “The dragon is convex. I can see how my view of the dragon changes as I change my viewing position. How can I make sense of these two facts? The most likely explanation is that the dragon is moving as I move.” As it turns out, this interpretation is entirely consistent with the informa- tion that’s reaching your eyes. If the dragon really were convex, and really were moving as you move, then the stimulation that would reach you would be virtually identical to what you’re receiving from the concave stationary dragon actually in your view. In that way, your perception of the figure is en- tirely reasonable. The problem, though, is the mistake you made at the start: registering the dragon as bulging outward when it’s really hollow. Once you made that mistake, the perception of moving-as-you-move was, in truth, the only way to make sense of the input! Indeed, what’s in play here is a standard “recipe” for producing many per- ceptual illusions: When you’re perceiving one aspect of a scene, you routinely need to take other aspects into account. So, when you perceive brightness, you first need to ask how strongly illuminated the surface is; you then take the illumination into account in judging the brightness. When you perceive size, you first need to ask how far away the object is; you then take distance into account. When you perceive motion, you first need to ask how the ob- ject is arranged in space (is it convex or concave?), and then you take that information into account. In each case, you can think of the sequence as a bit of “if . . . then . . .” reasoning: “If that is the illumination, then this must be the brightness”; “If that is the distance, then this must be the size.” But, of course, if you make an error in the first step in this sequence—that is, if you get the “if” part wrong—then you’re guaranteed to go off track in your cog8_ptr_demonstration_3.6.indd 1 28/10/21 11:36 AM