Our brain processes what we hear in waves, with sensitivity to incoming sounds flickering between our ears a bit like the frames of old, silent movies, says new research.
Previous studies have suggested our vision works in a similar rhythmic, oscillating pattern, and now there's evidence that hearing works in the same way – so our whole perception of the world around us is based on a kind of strobe-like effect in the mind.
One explanation for why this happens is that it helps us focus on the most important sounds in our environment, according to the international team of researchers.
They also say it can help us place sounds in the three-dimensional space we live in.
"These findings that auditory perception also goes through peaks and troughs supports the theory that perception is not passive, but in fact our understanding of the world goes through cycles," says one of the researchers, David Alais from the University of Sydney in Australia.
"We have suspected for some time that the senses are not constant but are processed via cyclical, or rhythmic functions; these findings lend new weight to that theory."
Using a simple noise identification experiment, involving 20 participants and 2,100 tests per participant, the research found that our ears take turns in being the most sensitive to sounds.
This happens so fast – about six cycles per second – that we're not really aware of it. But it also happens to match the time it takes humans to make decisions: one-sixth of a second.
Applying a technique called signal detection theory, the researchers showed that both auditory sensitivity and auditory decision-making was oscillating in this way.
While there's still a lot about the brain we don't understand, this fits with observations made about how the brain works in general, with cyclic patterns of activity affecting behaviour and attention.
Such strobing is effective for our brains in working out which parts of a scene are most important – like an onrushing train or someone talking to us. It means we put all of our cognitive energy where it's needed most.
Perhaps that's why our hearing, as well as our sight, works in the same way, the researchers suggest: to concentrate the brain's resources most effectively.
We're now constantly discovering new information about the brain, and these findings appear to feed into the overall picture of how our brain weighs up what's around us and makes decisions on it.
"The brain is such a complex machine one could say – it is a testament to science that we are starting to make sense of it – but a takeaway could be that there is so much we don't know," says Alais.
"A decade ago, no one would have thought that perception is constantly strobing – flickering like an old silent movie."
The findings have been published in Current Biology.