# Shepard scales — an acoustic illusion

## I have always been fascinated by optical illusions. Today I want to show you one that tricks your ears (and brain).

I have written about incredible optical illusions — links can be found at the bottom of this article. They trick your brain into seeing things that are not what you see. A quick example:

Which square, on the above chessboard, is darker, the one marked A or the one marked B? I have cut out the A-square and placed it next to B, so you can see. It is hard to believe — preposterous actually — but the square marked A is actually slightly darker than the one marked B. Our brains, interpreting the shadow over the board, tell us otherwise.

Today I want to switch to an acoustic illusion with which I experimented ages ago. I programmed a generator for it on a Commodore computer. It is just a continuous string of notes that are played. Here is a modern version:

This acoustic illusion was originally created by Oxford and Stanford Professor for cognitive science Roger Newland Shepard (born 1929). He is considered a father of research on spatial relations and provided the world with early optical illusions, like the following:

Can you tell which table in the picture is longer and narrower? You will probably say the one on the left. I have cut out the top of that table and placed it above the right table. You can see that they are the same size. But our brains, looking only at the tables, absolutely refuse to tell us that both are equal in length.

In the “Shepard scales” (above) you clearly hear the tone rising and, in the second case, falling, perpetually. The Centre for the Study of Perceptual Experience of Arts at the University of Glasgow explains it thus:

What is actually happening is that the same sequence of eight complex tones is being played over and over again. The repetition of the same octave being played creates the illusory experience of a continuous ascent or descent. The illusion is created because each tone is composed of many pitch frequencies that are carefully crafted to create ambiguity. Thus, one element of this illusion is that each tone is ambiguous and can be heard as either a higher or a lower sound depending on the context (which consists of the tones that were played before it.)

You’d do well to visit their Illusions Index page for hours of fun and learning.

The illusion of ascending and descending tones is very surprising, but I find the following even more so.

Listen to this sound file. It clearly progresses in speed, playing faster and faster. Try telling yourself — your brain actually — that the speed remains the same. Impossible, it is clearly speeding up. But like the previous examples it is going nowhere. The illusion was created by Jean-Claude Rissett and it is now known as the “Rissett Rhythm”. Once again Illusions Index explains:

This illusion provides us with an example an ambiguous stimulus. The speed of the rhythm played the first time round seems slower than the speed of the rhythm played the second time round, yet it is the same rhythm.

I still find it very difficult to believe that the rhythm is not speeding up, quite dramatically. It is purely an acoustic effect. But look how vision can affect our hearing. It is fun to see the “Two Set Violin” guys (who are always entertaining) experiment with these audio illusions.

## The McGurk effect

Here’s a very nice video by Prof. Lawrence Rosenblum of the University of California demonstrating and explaining the “McGurk effect”, discovered in 1976 by Harry McGurk and John MacDonaldin. They hypothesised that when the brain receives contradicting information from different senses it tries to guess which interpretation is correct. In the above video it is always the same sound, “ba-ba-ba”. But when Prof. Rosenblum moves his mouth in the f-enunciation manner, your brain tells you it must be “fa-fa-f” you are hearing. If you close your eyes you will notice that it is always the same sound you are hearing.