Falling Trees

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If a tree falls in a forest with nobody to hear it, does it still make a sound?

For many, this old chestnut provokes a strong sense of irritation: the classic example of how abstract ruminations are thoroughly irrelevant diversions from real-world issues. To others, however, it represents something else: a window into exploring our world of sense perception. While it’s clear that the act of perception can’t occur without a perceiver, what is much more opaque (and much more interesting) are the manifold intricacies concerning perception itself. In other words, to put it in TOK-speak, the key question isn’t so much, How do we know if falling trees in an empty forest are making sounds? but rather, Under what circumstances can I be certain that the sound I hear is the same as the one you do?

In our day-to-day lives, we make this assumption unceasingly. The ambulance driver unhesitatingly believes that his siren and flashing lights will be perceived and interpreted in the same way by everyone on the road, while the politician regularly adapts her language and message to her audience, secure in the belief that everyone will hear the same words with the same pitch and the same volume. The same rules govern our actions on the personal level. Suddenly startled by an odd-sounding bird, I will unthinkingly turn to my wife and ask, Did you hear that?

The notion that what I perceive is what you perceive is so widespread throughout our common experiences that it seems almost patently absurd to question it. So why should we?

Diana_deutsch_scale_fig3bEnter Diana Deutsch. The renowned UC San Diego psychologist has spent decades investigating how we experience sound and music. As a young researcher who had reluctantly abandoned a professional musical career, she began harnessing the emerging technology of computer- generated music to study the effects of rigorously measured sounds on different people. And she suddenly stumbled onto the shocking world of musical illusions.

“I had a very good computer programmer who created software that would enable me to present two streams of tones at a time. I presented a sequence of tones – a high tone and a low tone – that continuously repeated in both ears simultaneously: when one ear got the high tone the other ear got the low tone.

“When I put the earphones on and listened, I was absolutely amazed: the right ear seemed to be receiving an intermittent high tone while at the same time, in alternation, the left ear seemed to be receiving a low tone. What I was hearing was high tone – silence – high tone – silence in the right ear and low tone – silence – low tone – silence in the left ear.

“Yet what I had typed into the computer was a continuous sequence of high tones alternating with low tones on both channels. My immediate reaction was to check the computer; and when I found no problem there, I tried switching the headphones around. I found, to my amazement, that there was no difference: I continued to hear the high tone in my right ear and the low tone in my left ear.”

Further studies revealed that, while nobody heard what the computer actually programmed, as a general rule, right-handed people would hear high tones in their right ear and low tones in their left, while left-handed people would hear the opposite: high tones in their left ear and low tones in their right. In time, the so-called octave illusion entered the psychology textbooks.

“There were several conclusions. First of all, the basic illusion that most people heard was very bizarre. Secondly, the correlation with handedness had to be important. And thirdly, it opened up this whole question of, When we listen to musical patterns, are different people hearing different things?”

Diana, as you might imagine, continued to probe this question with unrelenting vigour, independently discovering such a vast array of different musical illusions (the scale illusion, glissando illusion, the triton paradox, the cambiata illusion, the phantom words illusion, the speech-to-song illusion) that I couldn’t help wondering why professional psychology wasn’t flooded with thousands of researchers anxious to make their mark on such obviously fertile ground, instead of regarding auditory illusions as a relatively niche topic.

According to Diana, the reasons for this have a lot more to do with our emotional responses than the intrinsic research value of the results.

“I think that people tend to be driven by what they expect, not just by theory as written in a textbook, but what they really expect to hear. And when they hear something that’s so absolutely crazy, such as the octave illusion, sometimes they don’t like it.

“I have to say that I feel that myself too. There’s a certain part of me that is made uncomfortable when I have to come to terms with some of these illusions. I don’t really want to believe that the world is such a disorganized place. I’d like to feel that when I hear something, I know what I’m hearing and that other people will hear the same thing, and we can all understand it. I’d like to believe that one just needs to crunch through various fairly obvious possibilities and all will become clear. But when you come across something that’s totally unexpected, it’s a bit upsetting. And I think that may be partly why not many people investigate auditory illusions.”

Which might also explain why many people get so irritated about pondering the effects of falling trees in empty forests. But it’s clear that we should.

Howard Burton, howard@ideasroadshow.com


To best explore the topics raised in this post for teaching and learning, please see below a number of related expert resources that are part of Ideas Roadshow’s IBDP Portal which offers a dynamic IB-specific database of 500+ authoritative video and print resources featuring more than 80 world-leading researchers, including two Nobel Laureates, explicitly created to meet the needs of both teachers and students in the Diploma Programme – please visit our new website  for further information.

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