I love optical illusions, particularly ones which make us see things that aren't there, miss things that are there, or misinterpret things. They show us that our brains do not faithfully relay everything in the outside world directly to our consciousness, but there is plenty of pre-processing, pattern matching and filtering that goes on in real time.

Today i discovered an amazing audio illusion that just blew my mind. Perhaps because i am quite used to optical illusions, they don't have the same powerful effect on me, but when you see this you'll realise that what you 'hear' and what you 'see' effect each other enormously.

Play it a few times, sometimes watching and listening, sometimes just listening (close your eyes). Isn't it incredible how you 'hear' something different depending on whether you are watching or not?! It's a very persistent effect too. Even though i know i should be able to hear 'ba ba' when i'm watching, i just can't. My brain takes the visual input and processes it with the audio to tell me what it thinks i have heard.

Here's another good audio illusion, though it may not work for everybody. Play this a couple of times. You might be surprised at what you hear on the second play!

I think it's all about the harmonics and balance, but i'm sure there is a fair amount of brain trickery involved with our tendency to match patterns wherever possible.

There is also an audio illusion which i can't try because it requires speakers placed some distance apart, but apparently it causes people to hear phantom words that aren't there at all. Different people hear different words. This to me is very interesting from the point of view of spiritual experiences. People say they hear angels, or the voice of God. I once had an experience where the volume of about 60 people singing suddenly sounded more like 200. Just because you hear something doesn't mean it is there.

I've also recently been interested in the spectrum of what people can hear. There is a 'mosquito' alarm outside a nearby shop that emits a squealing noise at a very high frequency. I had no idea it was there until somebody told me they could hear the noise. It's very unpleasant apparently, and its purpose is to stop kids from wanting to hang around outside the shop. The frequency is more likely to be heard by children than adults because their audible range is larger.

It's quite fascinating to be reminded that everything we think we see, hear, smell, taste and feel is really just the brain's filtered interpretation of the things around us.

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