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Hall of Illusions


   What is this picture?  As you search for meaning in this picture your brain strives to make sense of the seemingly meaningless spots.   If, after a good effort, you still see no meaning in the picture read the answer below.  Once you  "see" the solution it will never again be meaningless to you.

So What's Going On?

    This experiment shows that past experience can affect your perception of such properties as form or depth. Consider what happens when you view this illustration. At first most people cannot tell what this picture depicts, but with continued inspection or a hint, the fragments suddenly are perceptually reorganized and recognized, in this case, as a Dalmatian dog. A recognizable image emerges that had no perceptual reality before. Hence, there is some sort of perceptual change among  the neurons in your brain. This also leads to a change in the way in which you perceive the shape and depth of the scene. Perhaps most importantly, the figure now looks like the object it was supposed to represent - it now has the shape and depth relations of a Dalmatian dog.

    Sometimes being told that a Dalmatian hides in this scene can provide the visual system with enough of a hint to recognize the dog. This is a case in which a high-level brain area underlying language comprehension tells a lower-level area, in this case the cortical areas dedicated to visual scene analysis, what might be going on.

    If this dog was animated then it would be immediately apparent. Common motion of a group of otherwise unrecognizable blobs is a very powerful cue for your visual system. It enables your visual system to realize that it's dealing with a single object. This effect is termed grouping. It is important that an animal can do this well, otherwise it might not easily spot a predator, prey or other  food, such as apples. Animals must be able to separate the figure from the ground. What we call camouflage is an attempt to deceive these processes.  A stalking cat moves cautiously and freezes from time to time to avoid giving motion clues to its prey. It has even been suggested that our good color vision evolved to enable our primate ancestors to spot colored fruit against a confusing background of green leaves. What gives us so much visual pleasure may originate as a device to spot our food and to break camouflage.

     In most illustrations, we tend to perceive a figure that stands out from the background. In printed material, the figure is usually darker than its background.  Figures also tend to be smaller and more regular than backgrounds. Sometimes these principles do not hold, and we have difficulty distinguishing figures from their backgrounds. However, this difficulty disappears when our brain somehow organizes these difficult visual images into a meaningful and recognizable pattern. When this is done, only the figure and background can be seen, and whatever was seen before is gone forever!  Once you understand what is being presented, your perception changes, and you will be fixed on the "correct" interpretation forever.

     Since these types of figure/ground figures do not lead to immediate identification, the mental recognition of the "correct" figure must be perceptual in character.  After all, the image does not change on your retina. Thus we can assume that some mental process that precedes or accompanies the moment of recognition entails a perceptual reorganization.

Second Experiment.

Now that you have recognized the Dalmatian dog, try perceiving the image in its original meaningless way. You will find it almost impossible to do.

      Once you have recognized the Dalmatian dog, it becomes almost impossible to see the picture in its original meaningless interpretation.  The picture becomes permanently  meaningful.  This is in contrast to a truly  bistable or ambiguous figure that has two equally likely interpretations. The bistable or ambiguous figure will "flip" between two states perennially, because your brain cannot decide which one is more meaningfully biased over the other one. 

      In the case of the Dalmatian dog, however, once your brain perceives the "correct" image and ascribes meaning to the picture, your brain will not be able to perceive a meaningless image again, because the meaningless interpretation is no longer equally biased with that of your past experience with Dalmatian dogs.

      During all the time you were staring at the picture, the image on your retina did not change. Rather, your brain worked to construct a correct interpretation of the image, trying out different interpretations, until your brain "recognized" something.  This emphasizes that perception is an active process of constructing a scene description. 


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