Seven strange quirks of human vision

It might be the best-studied of all our senses (and see our collection in this month's issue), but surprises about the way our vision works just keep on coming. Recent research has startling and also salutary lessons about how we see.

1. Your brain makes up a lot of what you 'see'

Whether you're walking around, or sitting at a desk, you no doubt feel that you can see pretty clearly all around you. Yes, so you might be looking ahead as you walk through a park, say, but you can see a rich world of grass, trees and people to either side of you. Well, you might be seeing it – but that doesn't mean it's all actually there. As research published in Psychological Science in 2016 revealed, your brain uses information from the clearly-focused central region of the visual field to fill in detail in the relatively data-poor periphery. In fact, as the lead author, Marte Otten at the University of Amsterdam, commented at the time, 'Our findings show that, under the right circumstances, a large part of the periphery may be a visual illusion'. It's what's known as a 'uniformity illusion', and you can try it for yourself at www.uniformillusion.com.

However, not all peripheral vision is the same… work published the following year suggests that we all have 'good' and 'bad' regions. Some of the participants in this study, led by John Greenwood at UCL, had sharper left-side than right-side peripheral vision, for example.

And this could have real-world effects, says Greenwood – someone searching a room for missing keys, for example, could fail to spot those keys if they're on their 'bad' side.

2. It's easy to make someone hallucinate (without drugs)

If you have a willing partner to hand, you can try one hallucination-inducing technique right now. All you have to do, according to research led by Giovanni Caputo and published in Psychiatry Research, is to sit in chairs placed opposite each other, about a metre apart, in a fairly dimly lit room, and look into each other's eyes. When the 20 young adults in the study tried this for 10 minutes, they reported all kinds of odd experiences. Along with altered perceptions of sounds and time, 90 per cent saw some deformed facial features, while three-quarters said they saw a monster. A state of dissociation might explain these results, the researchers suggest.

Another study, published last year by Anupama Nair and David Brang, found that it's possible to induce synaesthesia-type visual sensations in regular people. For this work, participants sat with their eyes closed in a darkened room for five minutes. Then they were given a simple mental imagery task, designed purely to raise activity levels in the visual cortex. During this task, occasional, unpredictably-timed beeps were played. Overall, nearly 60 per cent reported at least one sound-induced visual perception, such as seeing colourful small circles, flashes or sparkles of light or visual pulses.

It's known that the visual cortex processes auditory data, too. Short-term visual deprivation can, it seems, make it more likely that sounds will actually trigger visual hallucinations.

3. Time stops when you blink

For about 10 per cent of your waking hours, your eyes are actually shut, because of blinking. Your brain stitches together retinal information from around these gaps, giving you the impression that your vision is uninterrupted. However, these brief but regular dark moments do have an impact: a study led by Ayelet Landau suggests they cause your sense of the passing of time to stop temporarily. This work, published in Psychological Science in 2019 and covered by me on the Research Digest, shows that processing in the visual cortex influences our perceptions of time. It's possible that when we're engaged in tasks that stimulate plenty of other senses (when we're outside, say, rather than sitting at a desk, staring at a screen, reading about psychological research…), this effect could be counteracted.

4. You can see infrared and ultraviolet

A few animals, such as pit vipers and common vampire bats, are infamous for being able to sense the infrared body heat of their prey. This is not a human ability. In fact, we're not meant to be able to detect infrared at all. However, as an initially serendipitous discovery by researchers at the University of Washington reveals, there are very particular circumstances in which it becomes possible. While working with an infrared laser, which is meant to be invisible to the human eye, researchers at the university reported seeing occasional green flashes. Vladimir Kefalov and colleagues investigated. They found that if two long-wavelength infrared photons of light hit a light-sensitive molecule in a cone cell in the retina in rapid succession, they have the same energy impact as a shorter, visible photon – and this triggers a green perception.

If such infrared vision is very rare, there are anecdotal reports that seeing very short, ultraviolet wavelengths of light – an ability normally associated with birds and butterflies, for instance – is much more achievable. The cornea and lens absorb this light, so, it's true, we normally can't see it. However, people who have had their lenses removed – often because of cataracts – have reported seeing (in blueish-purple) everything from patterns on flowers to ultraviolet light beams from counterfeit banknote detectors.

5. Childhood experience can create strange visual categories

It probably wouldn't surprise you to learn that you have dedicated circuits of neurons in your visual cortex dedicated to processing particular categories of visual stimulus – such as one for faces, and another for words. But, it seems, these category regions are established through experience in childhood, and as long as your youthful experience with a given type of visual stimulus is intense enough, your visual cortex may well set up a dedicated response unit to handle it on into adulthood. The main recent evidence for this comes from a study by Jesse Gomez and his team, showing that adults who played Pokémon extensively as kids had a visual region that was more active when viewing Pokémon characters.

6. When you're afraid, you literally see things differently

According to a popular model of perception, what you see is generally a function of data coming from the retina to your brain and what your brain expects to see. Depending on the situation, sometimes the retinal data will be given more weight, but other times the brain will rely more on expectation in generating your visual perceptions. We all have a tendency to err on the side of caution – given the potential risk vs benefit, if you're walking through snake-infested territory, say, it's safer to see a long shape on the ground as a snake rather than a stick.

But work published in Nature Communications in 2017 provided evidence that when our heart is beating rapidly – when we're afraid (and perhaps also angry) – we're especially likely to see threats when there are none.

In this study, racial biases also came into play. The team – Ruben Azevedo, Sarah Garfinkel, Hugo Critchley and Manos Tsakiris – showed images of either black or white individuals holding either a gun or a mobile phone. When the images were presented on the heartbeat, rather than between heartbeats, they were 10 per cent more likely to see the object as a gun when it was held by a black person. The researchers think this effect might explain at least some shootings of black unarmed men by police officers.

Another study, published a year earlier by Maria Lojowska's team, found that when we're afraid, we perceive some aspects of the world more clearly, but at the cost of ignoring much of the detail.

7. How to mess with your colleagues' minds

Many optical illusions are presented in 2D, on paper, or on a computer screen. But in 2018, a mind-bending 3D chair illusion showed itself in the office of Nick Scott-Samuel at the University of Bristol. The set of stackable chairs seem to be arranged in an impossible position – a little like an Escher drawing, only stranger. This is because, Scott-Samuel and his colleagues report, the brain gets confused about apparent depth cues when viewing the chairs at an angle. Just four chairs are apparently enough to achieve the effect. So next time you're early for a meeting, you know what to do…