Can we hear silence?

‘The sound of silence.’ Sounds like an oxymoron, doesn’t it? Well, a new study published this week in the journal PNAS suggests Simon and Garfunkel might have been onto something. James Tytko reports.

James - Philosophers have been grappling with a problem for centuries. If silence is just an absence of sound, a nothingness, why is it that it can be so emotive? For example, a rest in a powerful piece of music...

< Opening to Beethoven's 5th>

James - Or ducking into a quiet space from a busy street. Just as a well-placed full stop, paragraph break or chapter ending in a novel can have a profound effect, so too a dramatic pause in a powerful speech or piece of music often communicates something through nothing.

Rui Zhe - And it does seem like as the audience, we feel these silences, right? Like these silences don't just seem to be the absence of experience, but a positive thing. That's a positive experience that's being felt.

James - Rui Zhe Goh is a researcher in philosophy and psychological and brain sciences at Johns Hopkins University in America. And studying silence with science presents some challenges...

Rui Zhe - Silence has no pitch. It has no loudness. And so it is not clear what the scientific method can zoom in on to study this phenomenon.

James - To address this, Rui Zhe and other scientists in the field have to rely on illusions of time. A famous example is the so-called 'one is more' illusion, and it's one I can demonstrate now with you as the test subject. You're about to hear two beeps followed by one beep. Your task is to identify which is longer: The first two beeps combined or the longer beep. Here you go:

Robot Voice -
One (two beeps with a gap of silence) Two (One beep, no gap)

James - So which was longer? The first two beeps together are the longer beep on its own. Here it is again...

Robot Voice - One, two.

James - Now subjects overwhelmingly judge the one long beep as longer than the two short beeps, but in fact, they're exactly the same length.

Rui Zhe - The sound waves that we hear are just continuous waves of sounds without obvious discrete breaks. But what we perceive are discrete sounds. So we don't just perceive a continuous jumble of sound waves, but we perceive discretised sounds such as words or musical notes. And the process that makes this possible is this process called event segmentation, where our auditory system breaks discontinuous input into discrete representations. And this process of event segmentation is thought to be underlying this one is more illusion.

詹姆斯,你可以看到这是going. Using the theory of event segmentation, Rui Zhe's intuition was to invert the one is more illusion by substituting the silence with sound and vice versa.

Rui Zhe - And if these moments of silence trigger the same auditory process that happens with sound, then we have evidence that the auditory system can produce experiences with silence.

James - So let's have another go. We're going to repeat the experiment, but with silences instead of beeps cutting through ambient noise in a bustling restaurant. It's the same format as last time, two short silences followed by a longer silence. Which seems longer overall?

James - Now I'm still interpreting the two shorter silences as being faster than the longer silence. And it turns out I'm not alone.

Rui Zhe - The results of our experiment was that subjects judged the one silence to be longer than the sequence of two silences, and that the proportion of subjects that did so were exactly the same as the proportion of subjects that judged the one sound as longer than the two sounds. Our results about silence challenge common conceptions of perception, of hearing more specifically. We generally think that perception is about seeing stuff out there in the world. Where we hear things, there must be a sound for us to hear. When we see things, there must be objects out there for us to see. But what our results seem to suggest is that our auditory system can produce a perceptual experience even when there's nothing out there in the world to be perceived.

James - What next? Have you got any follow up questions or research planned to enhance our understanding of this topic even further?

Rui Zhe - I think it's still an open question whether we can positively hear pure silence not in contrast with sound, such as silences heard during meditation or silences heard when it's late at night and you're gazing at the stars and there's no sound. We also have ordinary experiences of visual absences such as when something moves across your visual view and then suddenly disappears. And we have some current work trying to ask whether the one that is more illusion also happens with visual disappearances.

James - Well, we anticipate the findings of that study with a lot of interest.

Rui Zhe - Thank you so much.