Our human interface with reality

If you don’t know what a “Denture Sensor” is, you need to read this Extract from Chapter Two – Sense and Sense Ability, first.


The Denture Sensor – How to make your own

Equipment required:

If you want to make your own denture sensor, you need an oscilloscope, but not a modern computerised one with a flat panel screen. Instead you need a good old fashioned oscilloscope that uses a cathode ray tube, because you ideally need the bright spot to be physically scanning across the screen at 20 milliseconds per division (200 milliseconds full scale).

Today’s oscilloscopes digitise the signals, and store them prior to displaying them on a computer screen. The benefits are enormous, flicker is eliminated and there are almost infinite possibilities to perform signal processing on the data prior to displaying it on the screen, for example as a frequency spectrum, or with additional data. However, as there is no bright spot scanning across our field of view, the “denture sensor” effect will not be seen.

So assuming you have your oscilloscope set to scan continuously at 20 ms/div, you now need to view this from a distance, the further away, the larger the amplitude of the apparent signal waveform.

You will also need a dummy sensor to reinforce the illusion. Almost anything will suffice provided it has a piece of electrical cable to connect it to the oscilloscope. Of course it does nothing, but it helps to convince the observer that something is being sensed externally.

When subjects have bought in to the illusion, they find it surprisingly difficult to see the reality. When I unplugged the cable to show subjects that there was no signal going into the oscilloscope, I expected that they would immediately realize that there was no sensor, but instead they would often look round the back of the ‘scope for another hidden cable.


Firstly, this works best in a science environment, because it whets people’s expectations. Next, before wheeling in a new subject, you need to thoroughly familiarise yourself with the following, or there is a good chance you will only create confusion: If you click your own teeth, you will see a waveform, as your own eyeballs are bouncing, but no-one else will see it. Similarly when another subject clicks their teeth, you see nothing, but you have to pretend that you do in fact see the same as they do. Equally when you click your own teeth you have to pretend that you see nothing (because that is what the other person sees).

It gets much more complicated when there are others in the room, and success depends on all the others being familiar with the above, and prepared to go along with the ruse. This is likely to be true if they have all previously been taken in by the illusion. I found that previous subjects are often very enthusiastic to see new “victims”, sometimes too enthusiastic as they forget the rules above, and confuse everyone. It is just easier to make this work when only you and the new subject are in the room.

Further variations:

You can have fun with exploring what causes the waveform to vary in magnitude: The apparent amplitude increases linearly with distance from the screen, which is counter intuitive (assuming that the dummy “sensor” is located adjacent to the screen). Those with a scientific background would would have expected it to decrease inversely as the square of the distance.

The perceived signal falls to zero when the head is tilted to the left or right at 90 degrees, as the direction of the eyeball bounce is then along the direction of the scanning bright spot.

The perceived signal inverts when the subject inverts their head (most entertaining when you persuade the subject to turn round and look at the screen from between their legs).

Lastly you see a similar waveform if you just thump the top of your head with your hand.

Now you just need to find some innocent bystanders, and have some fun!