Making Sense Of Sensory Illusions

Tales From The Dork Web #31

What are sensory illusions and how do they work? How does online media use illusion to tell stories without you realising it? In this issue I’ll explore different types of illusion covering audio, visual and even taste. Along the way I'll gently bend your head.

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The history of optical illusions goes back to cave paintings, and probably further. Western writings on optical illusions start in 5th century BC Greece. Epicharmus thought our senses were wrong. Protagoras believed the senses were fine but the environment was wrong. Aristotle’s belief that ‘senses can be trusted, but are easily fooled’ was closer to the truth. Different parts of the human brain process individual sensory input. Our interpretation (e.g. parsing speech, liking a smell) happens elsewhere. Illusions occur when our brains incorrectly process information along the way.

Historically illusions occured more often in art and architecture than maths and science. Only in the 19th century did we really start investigating how they work.

One of these 19th century tools is the illustrated Müller-Lyer illusion, shown above. The viewer is asked to mark the mid-point of 3 lines. Most mark towards the tail end of the arrow, even though the lines are perfectly aligned and the same length. Psychologist Richard Gregory suggested brains misinterpret perceived depth cues based on environmental experience. Under his hyptohesis outward pointing arrows would appear to come towards us. Inward pointing arrows would point away.

Trompe l’oeil (deceive the eye) techniques date back to antiquity, but the term took off in the early 19th century. Trompe l’oeil plays with tone and perspective to deceive the viewer in some way. When you see intricate reliefs that are really painted on, that’s Trompe l’oeil. Pere Borrell del Caso’s Escaping Criticism above is Trompe l’oeil.

Manipulating Perspective

Of course, Trompe L’oeil wasn’t the only perspective-bending art form. William Hogarth’s Satire on false perspectives was part of an educational art pamphlet. It highlights common faults many artists made at the time. Note the woman in the window (top right) lighting the man on the distant hill's pipe. How many perspective problems can you find?

Op art explores interactions between colour, figures and perspective through visual art. Leaders in this space include Bridget Riley, Richard Anuszkiewicz and Victor Vasarely. The installtion above, from the church of Pálos in Pécs, Hungary, juxtaposes colour and plane in one piece. The incorrect shadows betray an artificial sense of depth. I chose this as it may remind readers of someone more known in maths than art for their love of impossible objects.

I am of course writing of Lionel and Roger Penrose. They popularised impossible objects, some created by Swedish graphic artist Oscar Reutersvärd. The Penrose Staircase uses perspective to manipulate your sense of scale. As your eye follows the staircase it makes sense. Seeing the whole staircase at once looks… freaky.

M.C. Escher’s famous Waterfall piece uses two interlocking Penrose triangles. Penrose triangles were also discovered by Swedish graphic artist Oscar Reutersvärd. The resulting ambiguous perspective uses the same tricks as Hogarth and Vasarely.

Where there’s information, there’s scope for misinformation. For example, strange things happen when we play with scale and perspective in audio.

If you’ve ever played Super Mario 64 on the Nintendo 64 you might remember the endless stairs room. Like the stairs, the music playing in the room goes up forever. This uses the auditory equivalent of the Penrose stairs, the Shepard tone. Liam Robinson’s piano tutorial gives away how this illusion works. Press play on the video above and watch it for a few seconds. See how the notes move like a barber’s pole?

The Shepard tone uses an escalating set of notes that fade out the higher up the scale they go. At the same time, lower notes come in and fade in as they go up. Reversing the scale creates an ever-descending melody. Playing with pads, chords and tempo lead to interesting effects. Keep an ear out for the Shepard tone in movies and TV shows. Once you learn how to hear it, you’ll hear it everywhere.

Claude Risset’s rhythm illusion uses tempo instead of tone to similar effect. Press play on the video above. Is the beat getting faster, or is it slowing down? Play with the illusion by skipping back and forth.

Shepard Tones and Risset Rhythm are movie and TV composer staples. They’re valuable tools for managing on-screen tension.

Hanz Zimmer uses both in his movie soundtracks. Most notably in Chris Nolan's Dunkirk, which may as well have been called Shepard Tone: The Movie. Layered Shepard Tones combine with the Risset illusion in "Home" to ratchet up anxiety.

Inversion, negative space and ambiguous figures

As well as deformed and ambiguous perspectives, other illusions operate in negative space. In art, this is usually easy to spot, such as in Rubin’s vase above. The sudden interpretive shift or inversion is called a gestalt switch.

Outside of art, psychologists use these illusions to explore perception. Ludwig Wittgenstein famously published Joseph Jastrow’s Rabbit-Duck illusion. Wittgenstein employed the illusion to distinguish perception from interpretation.

If you first see a duck, you might struggle to see the rabbit. You should be able to see a duck or a rabbit. Once you understand how to see the other, you should be able to choose between the two. This is what Wittgenstein referred to as two different ways of seeing - “seeing that” and “seeing as”.

As well as seeing that and seeing as, we can also hear that and hear as. Play the video above with your eyes closed for a few seconds, then open and focus on each line for a few seconds. Now stop the video and read through each line and try to remember the sound. If each line sounds clear in your head you've experienced an illusory memory.

Our brains constantly try to make sense of the world around us. Simple things like a flash or loud bang use lower functions. Higher order functions like speech need more information. While we mostly hear speech, visual cues are also used when available. When they conflict, something weird happens. This is the McGurk effect, above.

Our minds match patterns and attempt to resolve conflicts without engaging higher functions. This means that part of our brain will lie to us to convince us of what is real. This forms part of what Nobel laureate Daniel Kahneman referred to as System 1 thinking. I’ll cover System 1 and System 2 thinking in a future issue.

Gestalt Switches and ambiguity also feature in musical composition. David Bruce explores several musical illusions in the video above. Along the way he shines light on tricks by the master of musical auditory illusion, György Ligeti.

Illusions in Smell and Taste

So far I’ve used visual and auditory illusions as these are the easiest to convey by mail and web page. But we absolutely can and do experience illusions with other senses. Greg Foot recreates some of Charles Spence’s multisensory experiments. Grab headphones and a coffee and play along for yourself.

Illusions in Words

Ambiguous perspective can also occur in written language, but it’s a little bit harder to define. In writing, ambiguous perspective relates more to understanding than depth of field. Consider the following:

John helped jack off a horse.

Now if you’re thinking what I’m thinking, you’re one sick puppy. If you’re not, then I am. Lets try this again:

Variants of this image are found all over the Internet. While writing this issue one even showed up on Hacker News. Gregory’s hypothesis suggests our brain’s top-down processing misinterprets the word.

Our brains substitute expectations for reality more than we (actively) think.

Journalists use ambiguity and perspective illusions in reporting. Consider the headline above. This actual Bloomberg piece refers to two case clusters identified by Singapore's Ministry of Health.

Your expectation of an 11-month high might be informed by local case number awareness. An 11-month high suggests very large case numbers.

25 Cases came from the whole entertainment district (the “Karaoke Cluster”). 42 Cases came from a fishery port. “Karaoke Cluster” implies people not taking Covid seriously. “Essential food processing” doesn’t. The headline abuses scale to make the smaller number look more significant.

92 Cases were reported that day, out of a population of nearly 5.9 million people. As I write, the UK averages nearly 40,000 cases a day for almost 11 times the population. 1012 daily cases would look like the end of COVID from here.

It’s not just Bloomberg. BBC News published “The Foreigners in China’s disinformation drive” on the 11th of July, 2021. The article accuses several China-based vloggers of pumping out disinformation. On first publish, they recoloured a vlog thumbnail (left) to show China as bleaker than it really is.

In an article about disinformation.

I love the text under the doctored photo, “CGTN presents Mr Lightfoot as a vlogger critical of ‘Western media outlets distorted reports’".

They later restored the original image but didn’t acknowledge the change in the article. Chinese outlets rightly picked up on this. Well played, BBC News. Well played.

While the BBC News and China’s running propaganda battle makes for a fascinating rabbit hole it isn’t the point. The point is that illusions in online media aren’t isolated occurences. Illusions can be as powerful as the words themselves. The line between ‘real’ and ‘fake’ news can be more nebulous than we’re being told.

Things You May Have Missed

The illusion of the year is really worth a look. And while we’re at it, why not print some impossible objects? I couldn’t cover Diana Deutsch’s amazing work on auditory illusions but this Braincraft video breaks down four of them.

Nathan Ferguson writes about updating our surveillance tech nightmares. Maybe he should start with Edward Snowden’s highly noddable take on Apple’s iPhone snooping. People Make Games highlights how Roblox exploits kids for money(h/t Dennis Groves). This amazing piece covers Google’s Orwellian attempts to remove the concept of antitrust by redefining speech.

Ethiopian Wolves are intermingling with Baboons in echoes of early dog domestication.

I’ll leave you with Radiohead’s Videotape. This song plays interesting tricks with rhythm, timing and syncopation. If you’ve never heard it before, give it a go. If you have, see if you can spot them.

I thought I’d end this issue with a quote from Stanley Kubrick that I feel sums up the illusory state of tech today:

If you can talk brilliantly about a problem, it can create the consoling illusion that it has been mastered.

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