Sounds sometimes behave so strangely...

How can sounds baffle our brains? Georgia Mills spoke to the queen of auditory illusions, Professor Diana Deutsch and the University of California San Diego...

Diana - One earphone was producing high tone, low tone, high tone, low tone, while the other earphone was producing low tone, high tone, low tone, high tone. I put on the earphones and generated the sounds and listened to them, and I was absolutely amazed at what I heard because it wasn’t at all what I had entered into the computer. What I was hearing was a single high tone in my right ear alternating with a single low tone in my left ear. So then I switched the headphones around and the same thing happened; I kept on hearing the high tone in the right ear alternating with the low tone in the left ear. At that point, I thought well, either I’m crazy or I’ve gone into a different universe or something.

I went into the corridor and brought in a few people and they all heard what I was hearing, except for one person, who happened to be left handed, who heard the opposite: high tone in the left ear alternating with the low tone in the right ear. And then I realised that, in fact, this must be a new illusion that would be of importance to the understanding of how we hear patterns of tones.

Georgia - What did that reveal to us about the brain?

Diana - First of all, as a group, right handers and left handers differ in what they tend to hear, and this is just what you would expect from the neurological literature on patterns of cerebral dominance in the case of right handers and left handers. Most right handers have the left hemisphere dominant; in fact for speech, whereas left handers can go either way. It had been thought that cerebral dominance was simply a function of speech, but these were musical tones and so it showed that, in fact, cerebral dominance was more complicated than had been thought, more general than had been thought.

Georgia : Now this one only works if you have headphones on, but here’s one that works on the radio. It will help if there’s someone else in the room with you. I’ve got fellow producer Izzie Clarke with me to help me out. So here are a set of paired tones, all we need to do is tell if they’re going up or down in pitch.

Number 1 – so that’s clearly going up.

Izzie – no that’s going down.

Georgia - Okay, here’s the next one? That was going down.

Izzie – Yeah

Georgia - Okay, we agree on that one.

Izzie - That was going down.

Georgia - Okay, good… Up.

Izzie - No! That’s still going down.

Georgia - That was definitely going up.

Izzie - No, they’re all going down.

Georgia - What do you know about music?

Izzie - Loads.

Georgia - Well, according to Diana, we’re all wrong, or we’re all right. There is no real answer, it’s ambiguous, kind of like the audio equivalent of that infamous blue/gold dress. Out of interest, which colour did you see the dress?

Izzie - Oh, it was white and gold.

Georgia - It was blue and black. Okay. We clearly have very different brains.

Izzie and I can rest assured, even trained musicians will disagree on this. And Diana’s discovered your brain interprets it according to your life experience.

Diana - It varies depending upon the language or dialect to which they’ve been exposed particularly in childhood. In one experiment, I found that people from the South of England tended to hear this pattern of tones in different ways from people who had grown up in California. Now, the pitch range of speech in the South of England is higher on the whole than the pitch range of speech in California. In another experiment, I looked to see if this correlated with the pitch range of the speaking voice to which you’ve been exposed and, in point of fact, that is the case.

Georgia - Why do you think illusions make such a powerful tool for studying hearing?

Diana - I think that you can learn a great deal about a system when it breaks down. For example, if you have a car that’s broken down you can find out a lot about the way the car works by examining it and examining the way it’s broken down, and that would be true of any piece of equipment. I think the same is true of illusions in perception, both visual and in hearing that you can learn a great deal about them by causing them to fail. So I think that illusions perhaps show more about hearing than anything else. So when you hear something correctly, what can you say about it? Well, okay you're hearing it correctly but, when something goes wrong, then you can start asking what is it that’s gone wrong? And then you can delve into the system and find out more about it that way.

Georgia - So what about the first one we heard ‘sometimes behave so strangely’? Now we’re not exactly sure about why this happens, but Diana says it provides a clue as to how our brain processes song, and that there’s a central director somewhere, deciding whether to send sound information to be processed as speech or processed as music.

Diana - My favourite view of what’s going on, in part, is that the tonal structure, the structure of pitches is rather similar to that of a phrase in the “Westminster Chimes” - boing boing boing boing, so ‘sometimes we hear so strangely.’  And the rhythm is very similar to that of “Rudolph the Red Nosed Reindeer” - ‘sometimes behaves so strangely.’ So this sort of central director decides okay, this is song and sends the information to be analysed that way.