A graph that is made by perceiving it

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).

The Einstein Monroe was created by Dr. Aude Oliva at MIT for the March 31st 2007 issue of New Scientist magazine

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.

What triggers that feeling of being watched?

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.

Statistical fallacy impairs post-publication mood

banksyNo 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)

UPDATE 5/11/15: It’s been retracted

Actually, still no good explanation of ‘that dress’

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?

How muggers size up your walk

The way people move can influence the likelihood of an attack by a stranger. The good news, though, is that altering this can reduce the chances of being targeted.

How you move gives a lot away. Maybe too much, if the wrong person is watching. We think, for instance, that the way people walk can influence the likelihood of an attack by a stranger. But we also think that their walking style can be altered to reduce the chances of being targeted.

A small number of criminals commit most of the crimes, and the crimes they commit are spread unevenly over the population: some unfortunate individuals seem to be picked out repeatedly by those intent on violent assault. Back in the 1980s, two psychologists from New York, Betty Grayson and Morris Stein, set out to find out what criminals look for in potential victims. They filmed short clips of members of the public walking along New York’s streets, and then took those clips to a large East Coast prison. They showed the tapes to 53 violent inmates with convictions for crimes on strangers, ranging from assault to murder, and asked them how easy each person would be to attack.

The prisoners made very different judgements about these notional victims. Some were consistently rated as easier to attack, as an “easy rip-off”. There were some expected differences, in that women were rated as easier to attack than men, on average, and older people as easier targets than the young. But even among those you’d expect to be least easy to assault, the subgroup of young men, there were some individuals who over half the prisoners rated at the top end of the “ease of assault” scale (a 1, 2 or 3, on the 10 point scale).

The researchers then asked professional dancers to analyse the clips using a system called Laban movement analysis – a system used by dancers, actors and others to describe and record human movement in detail. They rated the movements of people identified as victims as subtly less coordinated than those of non-victims.

Although Professors Grayson and Stein identified movement as the critical variable in criminals’ predatory decisions, their study had the obvious flaw that their films contained lots of other potentially relevant information: the clothes the people wore, for example, or the way they held their heads. Two decades later, a research group led by Lucy Johnston of the University of Canterbury, in New Zealand, performed a more robust test of the idea.

The group used a technique called the point light walker. This is a video recording of a person made by attaching lights or reflective markers to their joints while they wear a black body suit. When played back you can see pure movement shown in the way their joints move, without being able to see any of their features or even the limbs that connect their joints.

Research with point light walkers has shown that we can read characteristics from joint motion, such as gender or mood. This makes sense, if you think for a moment of times you’ve recognised a person from a distance, long before you were able to make out their face. Using this technique, the researchers showed that even when all other information was removed, some individuals still get picked out as more likely to be victims of assault than others, meaning these judgements must be based on how they move.

Walk this way

But the most impressive part of Johnston’s investigations came next, when she asked whether it was possible to change the way we walk so as to appear less vulnerable. A first group of volunteers were filmed walking before and after doing a short self defence course. Using the point-light technique, their walking styles were rated by volunteers (not prisoners) for vulnerability. Perhaps surprisingly, the self-defence training didn’t affect the walkers’ ratings.

In a second experiment, recruits were given training in how to walk, specifically focusing on the aspects which the researchers knew affected how vulnerable they appeared: factors affecting the synchrony and energy of their movement. This led to a significant drop in all the recruits’ vulnerability ratings, which was still in place when they were re-tested a month later.

There is school of thought that the brain only exists to control movement. So perhaps we shouldn’t be surprised that how we move can give a lot away. It’s also not surprising that other people are able to read our movements, whether it is in judging whether we will win a music competition, or whether we are bluffing at poker. You see how someone moves before you can see their expression, hear what they are saying or smell them. Movements are the first signs of others’ thoughts, so we’ve evolved to be good (and quick) at reading them.

The point light walker research a great example of a research journey that goes from a statistical observation, through street-level investigations and the use of complex lab techniques, and then applies the hard won knowledge for good: showing how the vulnerable can take steps to reduce their appearance of vulnerability.

My BBC Future column from Tuesday. The original is here. Thanks to Lucy Johnston for answering some of my queries. Sadly, and suprisingly to me, she’s no longer pursuing this line of research.

A taxonomy of ayahuasca hallucinations

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.

Photographing hallucinations

BMJ Case Reports has a paper that describes two patients with Parkinson’s disease who experienced hallucinations that transferred onto photos they took to try and prove they were real.

This is ‘Patient 1’ from the case report:

Patient 1 was first evaluated at age 66, having been diagnosed with PD [Parkinson’s Disease] at age 58… She complained of daytime and night-time visual hallucinations for the past one year. Most of the time she did not have insight about them. She described seeing three children playing in her neighbour’s yard and a brunette woman sleeping under the covers in one of the beds in her house. She also saw images of different people sitting quietly in her living room. Most of her visual hallucinations subsided in open and brightly lit spaces but were, nevertheless, troublesome. In one instance, she saw a man covered in blood, holding a child and called 911.

Her husband, in an attempt to prove to her that these were hallucinations, took pictures of the neighbour’s yard and the bed in their house. Surprisingly, when shown these photos, the patient continued to identify the same children playing in the yard and the same brunette woman sleeping under the covers. This perception was present every time the patient looked at these photos. Within 6 months of stopping ropinirole and titrating quetiapine to 75 mg every night at bedtime the hallucinations were less severe and shorter in duration, but the patient continued to see them in the photos.


Link to locked article in BMJ Case Reports.

Hallucinating sheet music

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 retrospective editing of consciousness

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.

Hallucinations caused by lightning

A 23-year-old mountain climber was hit by a lightning bolt and awoke in hospital to find herself experiencing bizarre hallucinations.

The case, reported in BMJ Case Reports, describes how the healthy young woman was mountaineering with her climbing partner when they heard heard cracking thunder and were thrown to the ground by a massive shockwave.

The air rescue team took her to hospital and she was put in a drug induced coma for three days as she was disoriented and extremely agitated.

When she awoke, her world was somewhat different.

In the evening, still awake and 6 h after extubation, strange phenomena occurred. These exclusively visual sensations consisted of unknown people, animals and objects acting in different scenes, as if in a movie. None of the persons or scenes was familiar to her and she was severely frightened by their occurrence. For example, an old lady was sitting on a ribbed radiator, who then became thinner and thinner, finally vanishing through the slots of the radiator. Later, on her left side a cowboy riding on a horse came from the distance. As he approached her, he tried to shoot her, making her feel defenceless because she could not move or shout for help.

In another scene, two male doctors, one fair and one dark haired, and a woman, all with strange metal glasses and unnatural brownish-red faces, were tanning in front of a sunbed, then having sexual intercourse and afterwards trying to draw blood from her. These formed hallucinations, partially with delusional character, were in the whole visual field and constantly present for approximately 20 h. At the time of appearance, the patient was not sure whether they were real or unreal, but did not report them for fear that she might be considered insane. However, as she was still frightened after cessation of the hallucinations, she insisted on being transferred to her hometown hospital. Over the next few days, she had increasingly better insight and later forgot about this episode.

Her brain scan showed damage to the occipital lobes, the areas at the back of the brain that are largely taken up with the visual cortex that deal with the early stages of visual perception.

Luckily, the patient survived without sustaining any serious brain damage although the article mentions that because the occipital lobe has so many blood vessels “it could be particularly vulnerable to lightning damage”.

Link to case report on lightning-strike hallucinations.

Deaf police to monitor security cameras in Mexico

Deaf police officers have been recruited to monitor security cameras in the Mexican city of Oaxaca because of their ‘heightened visual abilities’.

There’s a brief and somewhat clunky English-language news article from the local paper that describes the project:

Ignacio Villalobos Carranza, Deputy Secretary for the Ministry of Public Security of Oaxaca, said most of the monitoring of the 230 cameras is done by law enforcement officials that are hearing or speech impaired. He noted these police officers have a very strong deaf and visual sense and can better detect what is happening in different places where the cameras are located; they can often remotely read the conversations of people, to the benefit of this security system that operates 24 hours a day.

The ability to lip read conversations is a fantastic advantage, but the project raises the question of whether deaf people would actually be better at security monitoring in general.

As far as I know, there are no studies comparing hearing and deaf people on specific monitoring tasks but there is evidence that deaf people have certain advantages in visual attention.

This isn’t vision in general, such as having sharper visual acuity – where there seems to be no difference, but there is good evidence that deaf people are better at noticing things in the periphery of vision and detecting movement.

This potentially makes them perfect for the job and likely better than their hearing colleagues.

So the project turns out to be a targeted way not of recruiting ‘disabled people’ into the workforce, but of recruiting the ‘super able’. In fact, turning the whole idea of disability on its head.

There’s also a Spanish-language video report from BBC Mundo if you want more información.

Link to brief new article on the project.
Link to Spanish-language video report from BBC Mundo.

Hallucinating body flowers

A curious and kaleidoscopic case of hallucinations reported in the latest journal Arquivos de Neuro-Psiquiatria:

A 95-year-old woman, with four years of schooling, had a seven-year history of DI [delusional infestation]. In the beginning, there were itching and prickling sensations on arms and head. Subsequently, she felt small worms, with different shapes and colors, crawling through her skin or swirling around her body.

After two years, she began to see small pumpkins and flowers coming out of her body and lettuce crawling on the table. She complained of water trickling out of walls and forming puddles on the ground. Occasionally, she saw small children walking on the walls and also worms on the floor and walls.

Sometimes, the parasites set fire to small objects. She became upset with her family and physicians who did not believe her.

The belief that you are infested with hallucinatory parasites is more typically called delusional parasitosis but it is usually not linked to the florid circus of hallucinations reported here, which are more typical of Charles Bonnet syndrome.

Link to case report in Arquivos de Neuro-Psiquiatria.

Snakes on a brain

The latest Journal of Neuroscience features a study on the neuroscience behind Akiyoshi Kitaoka’s famouse Rotating Snakes illusion and to celebrate they’re made a ‘Rotating Brain’ illusion for the front cover.

This type of illusion, often called a peripheral drift illusion, was thought to occur due to slow drifting eye movements but this new study suggests that it is more likely to be explained by rapid but tiny eye movements called saccades.

Brain-shaped version of Akiyoshi Kitaoka’s “Rotating Snakes” illusion. In its usual presentation, the image consists of concentric circles of stepwise luminance gradients with curved edges, which produces a strong illusion of rotation in most observers. New evidence suggests this illusion is produced by transient oculomotor events such as microsaccades, saccades, and blinks, rather than continuous drift.

Despite the fantastic cover I expect the journal to outdo itself next time and have both an article explaining the neuroscience Brocken spectre as well as an image you can hide up a mountanside to create 20 metre tall ghost-like figures.

Link to study (via

Transplanted corneas are a window to the soul

A fascinating note on the social meaning of eyes and why people are much more reluctant to donate the cornea after death than other bodily organs.

From a recent article in the journal Transplanatation:

At the time that a patient is diagnosed as brain dead, a substantial proportion of families who give consent to heart and kidney donation specifically refuse eye donation. This in part may relate to the failure of those involved in transplantation medicine and public education to fully appreciate the different meanings attached to the body of a recently deceased person.

Medicine and science have long understood the body as a “machine.” This view has fitted with medical notions of transplantation, with donors being a source of biologic “goods.” However, even a cursory glance at the rituals surrounding death makes it apparent that there is more to a dead body than simply its biologic parts; in death, bodies continue as the physical substrate of relationships. Of all the organs, it is the eyes that are identified as the site of sentience, and there is a long tradition of visual primacy and visual symbolism in virtually all aspects of culture.

It therefore seems likely that of all the body parts, it is the eyes that are most central to social relationships. A request to donate the eyes therefore is unlikely to be heard simply in medical terms as a request to donate a “superfluous” body part for the benefit of another. That the eyes are not simply biologic provides one explanation for both the lower rates of corneal donation, compared with that of other organs, and the lack of adequate corneal donation to meet demand.

What’s interesting is that the operation to remove the cornea does no visible damage to the donor. It’s just the idea of the thing that puts people off.

Link to locked article on eyes, meaning and cornea transplants.