The Skipping Pylon has become something of an internet phenomenon with several articles being written about it (see the reference section below). The image consists of a moving GIF of a Pylon skipping and a shaking effect is added when the pylon hits the ground. Some people report hearing a thumping or boinging sound when watching the video. The sound isn't really there, so it seems that those people are suffering from an auditory hallucination caused by the GIF.

Dr Lisa DeBruine, a psychologist at the University of Glasgow, posted the animation, asking: 'Does anyone in visual perception know why you can hear this gif?' She added a poll asking what people experienced when watching the GIF. As of 4 December 2017, 75 per cent - around 15,000 people - claimed they heard 'a thudding sound'. Fourteen per cent of respondents, just under 3,000 in total, said they heard no sound, while under a thousand of the 20,000 polled said they heard 'something else'. The remainder of respondents, around 1,400, chose not to reveal whether they had heard anything.

Here at the Illusions Index, we report that we find ourselves imagining the sound, though we don't actually hear it.

It is expectation that is causing some people to hear the thuds. We know that expectation can affect what you experience. Demonstrations of this effect are common within one sensory modality. For example, if you look at the figure below, many people report seeing the central figure as a “13” when they read downwards and as a “B” when they read across. This shows that what you see can affect what other things that you see.

In the GIF of the jumping pylon, however, there is a cross-modal expectation (that is, involving more than one sense, such as vision and hearing) effect taking place. What people see is affecting what they seem to hear. We know that there are many cross-modal effects, such as the McGurk effect (in which what lip-movement people see affects which syllable they report hearing) or the Sound-Induced Flash Illusion (in which the number of flashes that people report is affected by how many beeps they hear).

It is interesting to note that it seems to be the shaking or shudder effect that seems to create the effect, rather than the visual imagery of the skipping pylon. This is brought out by a cropped version of the GIF that @HappyToast created (see below) that showed just the grass with the shaking effect and people reported that they heard the noise when looking at it. They also reported not hearing the noise when they looked at the GIF without the shake added.

When we experience an illusion and realise that it is an illusion we also get a glimpse of the fact that our knowledge that an illusion is occurring often doesn’t alter the way that our perceptual systems processes the stimulation. We witness the fact that our perceptual systems are to some degree at least insulated from the knowledge that we possess. (Although it is affected by expectation, it is an interesteing question how to conceive of that expectation. Is it the expectation of the viewers or is it an expectation only of their perceptual system?) The question of how insulated our perceptual systems are from cognition - the question of  whether there is cognitive penetration - is currently a hot topic for research in philosophy and psychology.

You might wonder why not everyone hears the sound. We know that there are many differences between people’s perceptual systems. These often go un-noticed. Some are mundane, like the fact that some people are short-sighted and others long-sighted. Some people are colour-blind, some are not. Others are revealed unexpectedly. For example, consider ‘The Dress’ photo below. Some people experienced it as a blue and black dress and others as a white and gold dress (and some could experience it as an ambiguous figure, first as blue and black and then as white and gold). This showed that different people’s visual systems were making different assumptions about the lighting conditions that the dress was in.