What follows is a collection of illusions I put together for my Theory of Knowledge class this year:
Everybody's familiar with optical illusions, but there are other kinds as well. We experimented with tactile illusions in class and I mentioned that there are also aural illusions. Have you experienced any other kinds of illusions? Illusions of taste or smell?
Here are some optical and auditory illusions to enjoy. What do they reveal about the limits of sense perception?
First, an image. When you look at it for the first time, you might not see a pattern at all. It just looks like random black spots on a white background. But eventually, all of a sudden, you can see a picture. Once you see it, you cannot unsee it.
Here's another one with the same effect:
These examples raise the question: How much of what we see depends on what we have learned to see?
The same phenomenon can occur with sounds. Here's an audio track with a stunning demonstration. You will hear a sentence which has been digitally altered to sound like gibberish. You won't be able to figure out what the voice is saying. Then you will hear the original sentence. When you then hear the digitally-altered recording again, it suddenly won't sound like gibberish anymore. You will now be able to recognize the sentence. Try it!
Do we also learn how to perceive smells or tastes? Is it possible that we learn how to perceive by touch as well? Perhaps all sense perception relies on prior knowledge. We perceive because we know how to perceive.
Is this a kind of map knowledge? Remember, map knowledge is all about expectations. If we know how to perceive, that could mean that we have expectations which guide the way we perceive. This raises the question: How much does sense perception rely on our expectations?
Here's a visual test. Are you aware of what you see? (This one's better big, so expand to full screen if you can.)
Sometimes our expectations for one sensory organ can be altered by another, and this affects what we perceive. For example, as you probably know already, what you smell affects what you taste. Did you also know that what you see can affect what you hear? Welcome to the McGurk Effect!
Sometimes a 'b' sounds like a 'b', and sometimes like an 'f', depending on what you see. But here's a tricky question: Do you actually hear the 'f', or do you just think you do?
In other words, should we say that your perception has changed, or should we say that you are wrong about what you are perceiving? What's the difference?
Now here's another audio illusion--an illusion of music. This link will take you to a new page where you will hear a woman talk in a normal speaking voice. She says, "The sounds as they appear to you are not only different from those that are really present, but they sometimes behave so strangely as to seem quite impossible." Then the part where she says "sometimes behave so strangely" repeats over and over again, and eventually it starts to sound like music. You will hear melody in her words and you will hear her singing, even though it is just a repetition of the same, spoken recording. Her normal speech becomes (or seems to become) music through mere repetition!
When the recording is over, start the recording again from the beginning. It will sound like she is talking in a normal voice again, but when she gets to "sometimes behave so strangely," it will sound like she suddenly begins to sing!
The same recorded words can sometimes sound like speech and sometimes like song. In this case, the change is not because of what you see, but because of what you have heard in the past. Our past experiences can change the way we perceive the world. Again, however, we have a difficult question: Do you actually hear music in this case, or do you just think you do? What is the difference?
Finally, another auditory illusion. Listen to this short musical recording, and then play it again, and again. It's like it changes every time, getting higher and higher in pitch.
Do we actually hear the recording at a higher pitch each time we play it, or do we just think we do?