Category: Seeing

The effect varies for different people. Take a moment and look at this. Some people don’t see anything special: just a blue iris in a red eye.

For me though, there is an incredibly strong depth illusion – the blue and the red appear as if they are at different distances.

I can enhance the effect by blinking rapidly, turning the brightness up on my screen and viewing in a dark room. Sometimes it disappears for a few seconds before snapping back in. Because the colours appear at different depths they even appear to glide separately when I move my head from side to side, something which is obviously impossible for a static image.

The effect is called chromostereopsis and it is weirding me out, for several reasons.

The first is that I’d thought I’d seen all the illusions, and this one is completely new to me. Like, guys, did you all know and weren’t telling?

The second is that there are big individual differences in perception of the effect. This isn’t just in terms of strength, although obviously I’m one of those it hits hard. People also differ in which colour looks closer. For most people it is red, with blue looking deeper or further away. I’m in the minority, so if you’re like me this reverse of the image above should look more natural: the iris set deeper than the surrounding eye.

The third reason this effect is weird to me is that stereo-depth illusions usually require two images, separately presented to each eye. This is how 3D cinema works – you wear red-green or polarising classes and the 3D parts of the film present two superimposed images, each image filtered out by only one lens, putting slightly different images in each eye. Your visual system combines the image and ‘discovers’ depth information, adding to the 3D perception of the objects shown. The superimposed images are why the film looks funny if you take the glasses off.

Chromostereopic illusions are true stereo illusions – they require the information to be combined across both eyes. There are many depth illusions which aren’t stereo illusions, but this isn’t one of them. You can prove this to yourself by making the effect disappear by closing one eye. The image stays the same, but it has to go in both eyes for the illusion of depth to occur. You can also try and find someone who is “stereoblind” and show them the illusion. A small percentage of the population don’t combine information across both eyes, and so only perceive depth via the other, monocular, cues. Our visual system is so adept at doing this that many people live their whole lives without realising they are stereoblind (although I suspect they tend not to go into professions which require exact depth perception, like juggling).

The way chromostereopsis works is not entirely understood. Even the great Michael Bach, who wrote for the Mind Hacks book, describes the explanation for the phenomenon as ‘multi-varied and intricate‘. That red and blue are at opposite ends of the light spectrum has something to do with it, and the consequent fact that different wavelengths of light will be focussed differently on the back of the eyes. This may also be why some people report that their glasses intensify the effect. The luminance of the image and the background also seems to be important.

The use of colour has a long history in art, from stained glass windows to video games, and probably many visual artists have discovered chromostereopsis by instinct. One of my favourite real-world uses is the set of the panel show Have I Got News For You:

For more on Depth Illusions, see the Mind Hacks book, chapters 20,22,24 and 31. For how I made the images, see the colophon on my personal blog.

More on the science: Kitaoka, A. (2016). Chromostereopsis. in Ming Ronnier Luo (Ed.), Encyclopedia of Color Science and Technology, Vol.1, New York; Springer (pp. 114-125).

Corrected 2022-05-18 The coloured part of the eye is the iris, not – as I originally wrote – the pupil (which is the black centre part)

The contrast sensitivity function shows how our sensitivity to contrasts is affected by spatial frequency. You can test it using gratings of alternating light and darker shade. Ian Goodfellow has this neat observation:

It’s a graph that makes itself! The image is the raw data, and by interacting with your visual system, you perceive a discontinuity which illustrates the limits of your perception.

Spatial frequency means how often things change in space. High spatial frequency changes means lots of small detail.  Spatial frequency is surprisingly important to our visual system – lots of basic features of the visual world, like orientation or motion, are processed first according to which spatial frequency the information is available at.

Spatial frequency is behind the Einstein-Marilyn illusion, whereby you see Albert Einstein if the image is large or close up, and Marilyn Monroe if the image is small / seen from a distance (try it! You’ll have to walk away from your screen to see it change).

Depending on distance, different spatial frequencies are easier to see, and if those spatial frequencies encode different information then you can make a hybrid image which switches as you alter your distance from it.

Spatial frequency is also why, when you’re flying over the ocean, you can see waves which appear not to move. Although you vision is sensitive enough to see the wave, the motion sensitive part of your visual system isn’t as good at the fine spatial frequencies – which creates a natural illusion of static waves.

The contrast sensitivity image at the head of this post varies contrast top to bottom (low to high) and spatial frequency left to right (low to high). The point at which the bars stop looking distinct picks out a ridge which rises (to a maximum at about about 10 cycles per degrees of angle) and then drops off. Where this ridge is will vary depending on your particular visual system and what distance you view the image at. It is the ultimate individualised data visualisation – it picks out the particular sensitivity of your own visual system, in real time. It’s even interactive, instantly adjusting for momentary changes in parameters like brightness!

More on hybrid images (including some neat examples): Oliva, A., Torralba, A., & Schyns, P. G. (2006, July). Hybrid images. In ACM Transactions on Graphics (TOG) (Vol. 25, No. 3, pp. 527-532). ACM.

More on the visual system, including the contrast sensitivity function: Frisby, J. P., & Stone, J. V. (2010). Seeing: The computational approach to biological vision. The MIT Press.

You feel somebody is looking at you, but you don’t know why. The explanation lies in some intriguing neuroscience and the study of a strange form of brain injury.

Something makes you turn and see someone watching you. Perhaps on a busy train, or at night, or when you’re strolling through the park. How did you know you were being watched? It can feel like an intuition which is separate from your senses, but really it demonstrates that your senses – particularly vision – can work in mysterious ways.

Intuitively, many of us might imagine that when you look at something with your eyes, signals travel to your visual cortex and then you have the conscious experience of seeing it, but the reality is far weirder.

Once information leaves our eyes it travels to at least 10 distinct brain areas, each with their own specialised functions. Many of us have heard of the visual cortex, a large region at the back of the brain which gets most attention from neuroscientists. The visual cortex supports our conscious vision, processing colour and fine detail to help produce the rich impression of the world we enjoy. But other parts of our brain are also processing different pieces of information, and these can be working away even when we don’t – or can’t – consciously perceive something.

The survivors of neural injury can cast some light on these mechanisms. When an accident damages the visual cortex, your vision is affected. If you lose all of your visual cortex you will lose all conscious vision, becoming what neurologists call ‘cortically blind’. But, unlike if you lose your eyes, cortically blind is only mostly blind – the non-cortical visual areas can still operate. Although you can’t have the subjective impression of seeing anything without a visual cortex, you can respond to things captured by your eyes that are processed by these other brain areas.

In 1974 a researcher called Larry Weiskrantz coined the term ‘blindsight’ for the phenomenon of patients who were still able to respond to visual stimuli despite losing all conscious vision due to destruction of the visual cortex. Patients like this can’t read or watch films or anything requiring processing of detail, but they are – if asked to guess – able to locate bright lights in front of them better than mere chance. Although they don’t feel like they can see anything, their ‘guesses’ have a surprising accuracy. Other visual brain areas are able to detect the light and provide information on the location, despite the lack of a visual cortex. Other studies show that people with this condition can detect emotions on faces and looming movements.

More recently, a dramatic study with a blindsight patient has shown how we might be able feel that we are being looked at, without even consciously seeing the watchers’ face. Alan J Pegna at Geneva University Hospital, Switzerland, and team worked with a man called TD (patients are always referred to by initials only in scientific studies, to preserve anonymity). TD is a doctor who suffered a stroke which destroyed his visual cortex, leaving him cortically blind.

People with this condition are rare, so TD has taken part in a string of studies to investigate exactly what someone can and can’t do without a visual cortex. The study involved looking at pictures of faces which had their eyes directed forward, looking directly at the viewer, or which had their eyes averted to the side, looking away from the viewer. TD did this task in an fMRI scanner which measured brain activity during the task, and also tried to guess which kind of face he was seeing. Obviously for anyone with normal vision, this task would be trivial – you would have a clear conscious visual impression of the face you were looking at at any one time, but recall that TD has no conscious visual impression. He feels blind.

The scanning results showed that our brains can be sensitive to what our conscious awareness isn’t. An area called the amygdala, thought to be responsible for processing emotions and information about faces, was more active when TD was looking at the faces with direct, rather than averted, gaze. When TD was being watched, his amygdala responded, even though he didn’t know it. (Interestingly, TD’s guesses as to where he was being watched weren’t above chance, and the researchers put this down to his reluctance to guess.)

Cortical, conscious vision, is still king. If you want to recognise individuals, watch films or read articles like this you are relying on your visual cortex. But research like this shows that certain functions are simpler and maybe more fundamental to survival, and exist separately from our conscious visual awareness.

Specifically, this study showed that we can detect that people are looking at us within our field of view – perhaps in the corner of our eye – even if we haven’t consciously noticed. It shows the brain basis for that subtle feeling that tells us we are being watched.

So when you’re walking that dark road and turn and notice someone standing there, or look up on the train to see someone staring at you, it may be your nonconscious visual system monitoring your environment while you’re conscious attention was on something else. It may not be supernatural, but it certainly shows the brain works in mysterious ways.

This is my BBC Future column from last week. The original is here.

No scientific paper is perfect, but a recent result on the affect of mood on colour perception is getting a particularly rough ride post-publication. Thorstenson and colleagues published their paper this summer in Psychological Science, claiming that people who were sad had impaired colour perception along the blue-yellow colour axis but not along the red-green colour axis. Pubpeer – a site where scholars can anonymously discuss papers after publication – has a critique of the paper, which observes that the paper commits a known flaw in its analysis.

The flaw, anonymous comments suggest, is that a difference between the two types of colour perception is claimed, but this isn’t actually tested by the paper – instead it shows that mood significantly affects blue-yellow perception, but does not significantly affect red-green perception. If there is enough evidence that one effect is significant, but not enough evidence for the second being significant, that doesn’t mean that the two effects are different from each other. Analogously, if you can prove that one suspect was present at a crime scene, but can’t prove the other was, that doesn’t mean that you have proved that the two suspects were in different places.

This mistake in analysis  – which is far from unique to this paper – is discussed in a classic 2011 paper by Nieuwenhuis and colleagues: Erroneous analyses of interactions in neuroscience: a problem of significance. At the time of writing the sentiment on Pubpeer is that the paper should be retracted – in effect striking it from the scientific record.

With commentary like this, you can see why Pubpeer has previously been the target of legal action by aggrieved researchers who feel the site unfairly maligns their work.

(h/t to Daniël Lakens and jjodx on twitter)

• Original paper: Thorstenson, C. A., Pazda, A. D., & Elliot, A. J. (2015). Sadness impairs color perception. Psychological science
• Pubpeer comments
• Nieuwenhuis, S., Forstmann, B. U., & Wagenmakers, E. J. (2011). Erroneous analyses of interactions in neuroscience: a problem of significance. Nature neuroscience, 14(9), 1105-1107.

UPDATE 5/11/15: It’s been retracted

The last time I almost went blind staring at “that dress” was thanks to Liz Hurley and on this occasion I find myself equally unsatisfied.

I’ll spare you the introduction about the amazing blue/black or white/gold dress. But what’s left me rather disappointed are the numerous ‘science of the dress’ articles that have appeared everywhere and say they’ve explained the effect through colour constancy.

Firstly, this doesn’t explain what we want to know – which is why people differ in their perceptions, and secondly, I don’t think colour constancy is a good explanation on its own.

To explain a little, colour constancy is an effect of the human visual system where colours are perceived as being different depending on their context as the brain adjusts for things like assumed lighting and surroundings. Here’s a good and topical example from XKCD. The dress colours are the same in both pictures but the seem different because the background colour is different.

An important feature of the visual system is that the experience of colour is not a direct result of the wavelength of the light being emitted by the surface. The brain modifies the experiences to try and ensure that things appear the same colour in different lighting because if we just went off wavelength everything would wildly change colour as it moved through a world which is lit unevenly and has different colour light sources.

Visual illusions take advantange of this and there are plenty of examples where you can see that even completely physically identical colours can be perceived as markedly different shades if the image suggests one is in shadow and the other in direct light, for example.

Firstly, this isn’t an explanation of why people differ in perceiving the dress. In fact, all of the ‘science explanations’ have simply recounted how perceived colours can change but not the most important thing which is why people are having two stable but contradictory experiences.

Colour constancy works on everyone with normal colour vision. If you take the panels from the XKCD cartoon, people don’t markedly disagree about what the perceived colours are. The effect of each image is very reliable between individuals.

That’s not the case with the dress. Also, if you say context makes a difference, changing the surroundings of the dress should change the colours. It doesn’t.

Some have argued that individual assumptions about lighting in the picture are what’s making the difference. In other words, the context is the unconscious assumptions people make about lighting in the picture.

But if this is the case, this still isn’t an explanation because it doesn’t tell us why people have different assumptions. Psychologists called these top-down effects or, if we’re going to get Bayesian, perceptual priors.

75% of people in this BuzzFeed poll said they saw white/gold, 25% said they saw blue/black, and a small minority of people say they’ve seen the picture ‘flip’ between the two perceptions. How come?

And there’s actually a good test of the colour constancy or any other other ‘implicit interpretation’ explanation. You should be able to create images that alter the visual system’s assumptions and make perception of the dress reliably flip between white/gold and blue/black, as with the XKCD cartoon.

So, any vision scientists out there who can come up with a good explanation of why people differ in their perceptions? Psychophysicists, have I gone wildly off track?

A wonderful list categorising hallucinations experienced by the Cashinahua people of Peru after drinking the hallucinogenic brew ayahuasca.

1. Brightly colored, large snakes
2. Jaguars and ocelots
3. Spirits, both of ayahuasca and others
4. Large trees, often falling trees
5. Lakes, frequently filled with anacondas and alligators
6. Cashinahua villages and those of other Indians
7. Traders and their goods
8. Gardens

It was reported by the anthropologist Ken Kensinger in a chapter in the book Hallucinogens and Shamanism.

It reminded me of writer Jorge Luis Borges’ whimsical classification system for animals.

Oliver Sacks has just published an article on ‘Hallucinations of musical notation’ in the neurology journal Brain that recounts eight cases of illusory sheet music escaping into the world.

The article makes the interesting point that the hallucinated musical notation is almost always nonsensical – either unreadable or not describing any listenable music – as described in this case study.

Arthur S., a surgeon and amateur pianist, was losing vision from macular degeneration. In 2007, he started ‘seeing’ musical notation for the first time. Its appearance was extremely realistic, the staves and clefs boldly printed on a white background ‘just like a sheet of real music’, and Dr. S. wondered for a moment whether some part of his brain was now generating his own original music. But when he looked more closely, he realized that the score was unreadable and unplayable. It was inordinately complicated, with four or six staves, impossibly complex chords with six or more notes on a single stem, and horizontal rows of multiple flats and sharps. It was, he said, ‘a potpourri of musical notation without any meaning’. He would see a page of this pseudo-music for a few seconds, and then it would suddenly disappear, replaced by another, equally nonsensical page. These hallucinations were sometimes intrusive and might cover a page he was trying to read or a letter he was trying to write.

Though Dr. S. has been unable to read real musical scores for some years, he wonders, as did Mrs. J., whether his lifelong immersion in music and musical scores might have determined the form of his hallucinations.

Sadly, the article is locked behind a paywall. However you can always request it via the #icanhazpdf hashtag on twitter .
 

Link to locked article on ‘Hallucinations of musical notation’.

A new study has found that conscious experience can be altered retrospectively, so that experience of visual information can be changed almost half a second later by manipulating where our attention is drawn.

The research, led by cognitive scientist Claire Sergent, involved asking people to stare at a centre point of a screen with two empty circles either side.

At some point, one of the two circles would fill with randomly oriented stripes for just 50ms (one twentieth of a second) and afterwards the participants were asked to say which direction the stripes were pointing in.

Crucially however, each time this happened, one of the two circles would dim either before or after the stripes appeared.

This would happen at different times – from 400ms before the stripes appeared, up to 400ms after the stripes appeared, and the dimmed circle might appear on the matching side to the stripes or on the opposite side.

Dimming one of the circles grabs your attention. It makes you instantly focus more on whichever side of space it happens.

For example, if the left-hand circle dims, it grabs your attention, and if the stripes then appear on the left, you’re more likely to make a correct judgement about which direction they’re pointing because you’re already focused on this area. But if the stripes subsequently appear on the other side, you’re distracted and you do worse.

The key discovery from this experiment was that this also happens if the dimmed circle appears after the stripes. Up to 400ms seconds after.

In other words, you perceive the original visual details that would otherwise have escaped consciousness if your attention is drawn to the area after the picture disappears. It’s like a retrospective editing of consciousness by post-event attention.

This suggests that consciousness isn’t ‘filtered’ sensory information, but an active ‘conclusion’ drawn from information distributed across senses, space and time.
 

Link to locked scientific study.
Link to open-access commentary from same journal.