Beneath the brushstrokes, van Gogh's sky is alive with real-world physics

Vincent van Gogh's painting "The Starry Night" depicts a swirling blue sky with yellow moon and stars. The sky is an explosion of colors and shapes, each star encapsulated in ripples of yellow, gleaming with light like reflections on water.

Van Gogh's brushstrokes create an illusion of sky movement so convincing it led atmospheric scientists to wonder how closely it aligns with the physics of real skies. While the atmospheric motion in the painting cannot be measured, the brushstrokes can.

In an article published this week in Physics of Fluids, by AIP Publishing, researchers specializing in marine sciences and fluid dynamics in China and France analyzed van Gogh's painting to uncover what they call the hidden turbulence in the painter's depiction of the sky.

"The scale of the paint strokes played a crucial role," author Yongxiang Huang said. "With a high-resolution digital picture, we were able to measure precisely the typical size of the brushstrokes and compare these to the scales expected from turbulence theories."

To reveal hidden turbulence, the authors used brushstrokes in the painting like leaves swirling in a funnel of wind to examine the shape, energy, and scaling of atmospheric characteristics of the otherwise invisible atmosphere. They used the relative brightness, or luminance, of the varying paint colors as a stand-in for the kinetic energy of physical movement.

"It reveals a deep and intuitive understanding of natural phenomena," Huang said. "Van Gogh's precise representation of turbulence might be from studying the movement of clouds and the atmosphere or an innate sense of how to capture the dynamism of the sky."

Their study examined the spatial scale of the painting's 14 main whirling shapes to find out if they align with the cascading energy theory that describes the kinetic energy transfer from large- to small-scale turbulent flows in the atmosphere.

They discovered the overall picture aligns with Kolmogorov's law, which predicts atmospheric movement and scale according to measured inertial energy. Drilling down to the microcosm within the paint strokes themselves, where relative brightness is diffused throughout the canvas, the researchers discovered an alignment with Batchelor's scaling, which describes energy laws in small-scale, passive scalar turbulence following atmospheric movement.

Finding both scalings in one atmospheric system is rare, and it was a big driver for their research.

"Turbulence is believed to be one of the intrinsic properties of high Reynolds flows dominated by inertia, but recently, turbulence-like phenomena have been reported for different types of flow systems at a wide range of spatial scales, with low Reynolds numbers where viscosity is more dominant," Huang said.

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Illusion helps demystify the way vision works

For the first time, research shows that a certain kind of visual illusion, neon color spreading, works on mice. The study is also the first to combine the use of two investigative techniques called electrophysiology and optogenetics to study this illusion. Results from experiments on mice settle a long-standing debate in neuroscience about which levels of neurons within the brain are responsible for the perception of brightness.

We're all familiar with optical illusions; some are novelties, while some are all around us. Even as you look at the screen in front you, you are being fooled into thinking that you're seeing the color white. What you're really seeing is lots of red, green and blue elements packed so tightly together it gives the impression of being white. Another example is a fast rotating wheel or propeller, which can briefly look like it's reversing direction while it's accelerating to full speed. In any case, it might be surprising to know that optical illusions are not just fun to look at but can also be a useful tool to learn more about eyes, nerves, minds and brains.

Associate Professor Masataka Watanabe from the Department of Systems Innovation at the University of Tokyo is on a mission to understand more about the nature of consciousness. It's a vast subject area so naturally there are many ways to explore it, and amongst other things, he uses optical illusions. His most recent research looked at whether a certain kind of illusion that works on humans would also work on mice. And it turns out, it does. But why is this significant?

"Knowing this kind of illusion, called a neon-color-spreading illusion, works on mice as well as humans, is useful for neuroscientists like myself, as it means that mice can serve as useful test subjects for cases where humans cannot," said Watanabe. "To really understand what goes on inside the brain during perceptual experiences, we need to use certain methods that we cannot use on people. These include electrophysiology, the recording of neural activity with electrodes, and optogenetics, where light pulses enable or disable firing of specific neurons in a living brain."

Watanabe's experiment was the first of its kind to make use of both electrophysiology and optogenetics at the same time in animal test subjects exposed to the neon-color-spreading illusion, which allowed his team to see precisely what structures within the brain are responsible for processing the illusion.

"After a visual stimulus lands on the eye, it's carried to the brain by nerves and is then received by a series of layers of neurons called V1, V2 and so on, where V1 is the first and most basic layer, and V2 and above are considered higher layers," said Watanabe. "There is a long-standing debate in neuroscience about the role higher levels play in the perception of brightness and it was not an easy thing to study. Our experiment on mice has shown us that neurons in V1 responded not just to the illusion, but also to a nonillusory version of the same kind of pattern shown. But only when the illusory version was shown to the mice did neurons in V2 also play a crucial role: that of modulating the activity of neurons in V1, thus proving that V2 neurons do in fact play a role in the perception of brightness."

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When we feel things that are not there

Virtual reality (VR) is not only a technology for games and entertainment, but also has potential in science and medicine. Researchers at Ruhr University Bochum, Germany, have now gained new insights into human perception with the help of VR. They used virtual reality scenarios in which subjects touched their own bodies with a virtual object. To the researchers' surprise, this led to a tingling sensation at the spot where the avatarized body was touched. This effect occurred even though there was no real physical contact between the virtual object and the body. The scientists led by Dr. Artur Pilacinski and Professor Christian Klaes from the Department of Neurotechnology describe this phenomenon as a phantom touch illusion. They published their results in the journal Scientific Reports of the Nature Publishing Group in September 2023.

"People in virtual reality sometimes have the feeling that they are touching things, although they are actually only encountering virtual objects," says first author Artur Pilacinski from the Knappschaftskrankenhaus Bochum Langendreer, University Clinic of Ruhr University Bochum, explaining the origin of the research question. "We show that the phantom touch illusion is described by most subjects as a tingling or prickling, electrifying sensation or as if the wind was passing through their hand."

Body sensation arises from complex combination of different sensory perceptions

The neuroscientists wanted to understand what is behind this phenomenon and find out which processes in the brain and body play a role in it. They observed that the phantom touch illusion also occurred when the subjects touched parts of their bodies that were not visible in virtual reality. Second author Marita Metzler adds: "This suggests that human perception and body sensation are not only based on vision, but on a complex combination of many sensory perceptions and the internal representation of our body."

This study involved 36 volunteers wearing VR glasses. First, they got used to the VR environment by moving around and touching virtual objects. Then they were given the task of touching their hand in the virtual environment with a virtual stick.

Comparison between virtual and suggested touch sensations

Participants were asked if they felt anything. If not, they were allowed to continue touching and the question was asked again later. If they felt sensations, they were asked to describe them and rate their intensity on different hand locations. This process was repeated for both hands. There was a consistent reporting of the sensation as "tingling" by a majority of participants.

In a control experiment, it was investigated whether similar sensations could also be perceived without visual contact with virtual objects purely due to experimental situation demands. Here, a small laser pointer was used instead of virtual objects to touch the hand. This control experiment did not result in phantom touch suggesting that phantom touch illusion was unique to virtual touch.

The discovery of the phantom touch illusion opens up new possibilities for further research into human perception and could also be applied in the fields of virtual reality and medicine. Christian Klaes, member of the Research Department of Neuroscience at Ruhr University, says: "It could even help to deepen the understanding of neurological diseases and disorders that affect the perception of one's own body."

Further collaboration with the University of Sussex

The Bochum team plans to continue their research on the phantom touch illusion and the underlying processes. For this reason, a collaboration with the University of Sussex has been started. "It is important to first distinguish between the actual sensations of phantom touch and other cognitive processes that may be involved in reporting such embodied sensations, such as suggestion, or experimental situation demands," says Artur Pilacinski. "We also want to further explore and understand the neural basis of the phantom touch illusion in collaboration with other partners."

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Illusions are in the eye, not the mind

Numerous visual illusions are caused by limits in the way our eyes and visual neurones work -- rather than more complex psychological processes, new research shows.

Researchers examined illusions in which an object's surroundings affect the way we see its colour or pattern.

Scientists and philosophers have long debated whether these illusions are caused by neural processing in the eye and low-level visual centres in the brain, or involve higher-level mental processes such as context and prior knowledge.

In the new study Dr Jolyon Troscianko, from the University of Exeter, co-developed a model that suggests simple limits to neural responses -- not deeper psychological processes -- explain these illusions.

"Our eyes send messages to the brain by making neurones fire faster or slower," said Dr Troscianko, from the Centre for Ecology and Conservation on Exeter's Penryn Campus in Cornwall.

"However, there's a limit to how quickly they can fire, and previous research hasn't considered how the limit might affect the ways we see colour."

The model combines this "limited bandwidth" with information on how humans perceive patterns at different scales, together with an assumption that our vision performs best when we are looking at natural scenes.

The model was developed by researchers from the Universities of Exeter and Sussex to predict how animals see colour, but it was also found to correctly predict many visual illusions seen by humans.

"This throws into the air a lot of long-held assumptions about how visual illusions work," Dr Troscianko said.

He said the findings also shed light on the popularity of high-definition televisions.

"Modern high dynamic range televisions create bright white regions that are over 10,000 times brighter than their darkest black, approaching the contrast levels of natural scenes," Dr Troscianko added.

"How our eyes and brains can handle this contrast is a puzzle because tests show that the highest contrasts we humans can see at a single spatial scale is around 200:1.

"Even more confusingly, the neurones connecting our eyes to our brains can only handle contrasts of about 10:1.

"Our model shows how neurones with such limited contrast bandwidth can combine their signals to allow us to see these enormous contrasts, but the information is 'compressed' -- resulting in visual illusions.

"The model shows how our neurones are precisely evolved to use of every bit of capacity.

"For example, some neurones are sensitive to very tiny differences in grey levels at medium-sized scales, but are easily overwhelmed by high contrasts.

"Meanwhile, neurones coding for contrasts at larger or smaller scales are much less sensitive, but can work over a much wider range of contrasts, giving deep black-and-white differences.

"Ultimately this shows how a system with a severely limited neural bandwidth and sensitivity can perceive contrasts larger than 10,000:1."

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Time-traveling illusion tricks the brain

The Rabbit Illusion (see video: https://youtu.be/yCpsQ8LZOco).

We experience the world through our senses, a constant torrent of sights, sounds, smells, and more. Our brains take these signals and process them, giving rise to our individual perceptions of the world. But sometimes our senses play tricks on us, notably in the case of perceptual illusions.

Now, Caltech researchers have developed two new illusions that reveal how the senses can influence each other -- in particular, how sound can give rise to visual illusions. These illusions occur so quickly that they illustrate a phenomenon called postdiction (as opposed to prediction) in which a stimulus that occurs later can retroactively affect our perceptions of an earlier event.

The Caltech work is among the first to show this kind of time-traveling illusion across multiple senses.

The work was done in the laboratory of Shinsuke Shimojo, Gertrude Baltimore Professor of Experimental Psychology and affiliated faculty member of the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech. A paper describing the research appears in the October 3 issue of the journal PLOS ONE.

"Illusions are a really interesting window into the brain," says first author Noelle Stiles (PhD '15), a visitor in biology and biological engineering and a postdoctoral scholar-research associate at USC. "By investigating illusions, we can study the brain's decision-making process. For example, how does the brain determine reality with information from multiple senses that is at times noisy and conflicting? The brain uses assumptions about the environment to solve this problem. When these assumptions happen to be wrong, illusions can occur as the brain tries to make the best sense of a confusing situation. We can use these illusions to unveil the underlying inferences that the brain makes."

The two illusions in this study were developed to illustrate how stimuli that occur later can affect the perception of stimuli that have already occurred. Postdictive processing has been demonstrated within individual senses, but this work focuses on how the phenomenon can bridge multiple senses. The key to both of the new illusions is that the audio and visual stimuli occur rapidly, in under 200 milliseconds (one-fifth of a second). The brain, trying to make sense of this barrage of information, synthesizes the stimuli from both senses to determine the experience, using postdiction to do so.

The first illusion is called the Illusory Rabbit. To produce the illusion, first a short beep and a quick flash are played nearly simultaneously on a computer, with the flash appearing at the left side of the screen. Next, 58 milliseconds after the first beep, a lone beep is played. Finally, 58 milliseconds after the second beep, a second nearly simultaneous beep-flash pair occurs, but with the flash appearing on the right side of the screen. The beep location is always central and does not move. Though only two flashes are played, most people viewing the illusion perceive three flashes, with an illusory flash coinciding with the second beep and appearing to be located in the center of the screen.

The fact that the illusory flash is perceived in between the left and right flashes is the key evidence that the brain is using postdictive processing.

"When the final beep-flash pair is later presented, the brain assumes that it must have missed the flash associated with the unpaired beep and quite literally makes up the fact that there must have been a second flash that it missed," explains Stiles. "This already implies a postdictive mechanism at work. But even more importantly, the only way that you could perceive the shifted illusory flash would be if the information that comes later in time -- the final beep-flash combination -- is being used to reconstruct the most likely location of the illusory flash as well."

The second illusion is called the Invisible Rabbit. In this related illusion, three flashes are shown on the screen, the first on the left, the second in the middle, and finally the third on the right, with only the first and third flashes coinciding with beeps. In this case, most people do not see the second flash -- the one without a corresponding sound -- at all. The absence of the second beep leads the brain to decide after the fact that there actually was no flash, even though it was in fact present.

By showing that a sound can excite a visual illusion, the researchers have uncovered new clues as to how the brain combines the senses over space and time to generate an integrated sense of perception.

"The significance of this study is twofold," says Shimojo. "First, it generalizes postdiction as a key process in perceptual processing for both a single sense and multiple senses. Postdiction may sound mysterious, but it is not -- one must consider how long it takes the brain to process earlier visual stimuli, during which time subsequent stimuli from a different sense can affect or modulate the first. The second significance is that these illusions are among the very rare cases where sound affects vision, not vice versa, indicating dynamic aspects of neural processing that occur across space and time. These new illusions will enable researchers to identify optimal parameters for multisensory integration, which is necessary for both the design of ideal sensory aids and optimal training for low-vision individuals."

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