Monday, 22 July 2019

The Octave Illusion

Posted by Thomas Scarborough*

440Hz and 880Hz alternating R and L

The Octave Illusion was discovered by Diana Deutsch in 1973. It produced two alternating tones, one octave apart, in each ear. However, when the tone in one ear went ‘high’, the tone in the other ear went ‘low’, and vice versa. (Click the arrow to play.)

The tones used here are A4 (440 Hz) and A5 (880 Hz) - in this case reproduced with the precision of the modern computer. The effect is the same whether this is played through headphones or loudspeakers -- and interestingly, does not change when the headphones or loudspeakers are swapped around.

It is a simple auditory illusion, yet most powerful. Instead of hearing two alternating tones in each ear - as one should - most people hear alternating tones bouncing from ear to ear. It seems that the brain has therefore removed two tones, one from each ear. That is, the tones are replaced with silences.

There is copious literature on what this might mean, and what might cause the effect - yet the general agreement is that there is no simple explanation.

It might not seem at first that an auditory illusion has anything to do with philosophy. However, what we have in this particular case is auditory ‘objects’ -- a range of auditory ‘things’ which include bangs, ringtones, meows, and so on -- which belong to the much larger set of ‘objects’ in general.

To this day, then, we search for ways to decide what is or is not illusory ...
space, time, numbers

identity, free will

things, events, properties

society, language


matter, force, energy

causes, physical laws...

... and even God.
Thus, illusions such as that above -- of which we all now are aware -- have a lot to do with the way we perceive our world.  It has not always been so. There once was a time - or so it is thought - where we perceived everything as real. Today, to borrow the words of the British philosophy professor Simon Blackburn, we have left this far behind: ‘Everything you can think of has at some time or another been declared to be a fiction by philosophers.’

It falls under the subject of ontology - and it all started, according to the eighteenth century Scottish philosopher, Thomas Reid, when we separated objects from ideas. The Octave Illusion is just one of many evidences that our experience is not the same as the ideas we have about it. Our brains have already interpreted it for us.

* The author once designed a simple electronic unit to produce this effect. This is still obtainable from the publisher at


Keith said...

A stimulating discussion, Thomas, about what is fundamentally the nature of reality: how we tease out what’s real from what’s not (‘illusions’, as you say).

One thing in particular struck me about the items you list as illusory. That is, don’t we perhaps know at least something — in some cases, just a little; in other cases, a whole lot — about every single entry in the list, but one: God?

Hasn’t humanity, over the millennia, devoted great effort mining questions about whether God exists, and if so, how to describe God? Might humanity be asking many of the same questions today?

We've guessed; we’ve developed ‘proofs’; we’ve had hunches; we’ve put words into God’s ‘mouth’; we’ve extrapolated codes of morality; we’ve written ‘histories of religious events’; we’ve anthropomorphized. And much more.

A game effort, and rather interesting journey, but …

Perhaps the closing quote from the essay applies here: ‘Our brains have already interpreted it for us’. Illusions?

Thomas O. Scarborough said...

Thank you, Keith, for a thoughtful reply. I have developed this answer about existents in general: they exist in our minds. In our neural network which is the brain, we tie some strands together and give them a label, and that is then an existent. In former times, one wouldn't have questioned it--or not much: if you have a word for something, it exists. But now, words and existents have been rent asunder. The issue then is, as you say, how we tease out what's real from what's not.

Tessa den Uyl said...

Perhaps we cannot draw a ‘real’ line between what is real or not. In an anxiety attack the subject does feel physical symptoms though they are purely imaginative. For the subject all seems real. Perhaps to come closer to real, we should ‘shut off’ our mental and emotional networks and then see how things appear, but strangely moving in that direction everything that seemed real before vanishes and moves into a completely different reality. Then, the real is subjective to a huge range of inward capacities we do not explore much, generally speaking.

Martin Cohen said...

It is good to have an AUDITORY illusion for a change! Though I confess to finding this one has rather an unappealing effect... surely there are some NICE sounds out there. But it is certainly as Thomas says a very philosophical affair. For Plato in the Republic, optical illusions "are a metaphor for the deceptive nature of what we naively accept as reality". For Descartes in the Meditations, "optical illusions are a spur towards seeking a foundation for knowledge outside the realm of sensory data"!

Thomas O. Scarborough said...

Here is something more sophisticated for you, Martin. An audio illusion by Tchaikovsky:

Thank you, Tessa, for your comment, which adds new depth to the subject, as ever. I am lost for a reply, I can only say yes.

docmartincohen said...

Tchaikovsky beat you to it!
Or was it 'Niggish'...

Thomas O. Scarborough said...

Niggish? I had to look that up. We can add it to our choice words, among them bloviation, adumbration, and paradiastole. I wonder how Tchaikovsky came to use an auditory illusion. Perhaps intuitively?

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