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16 October 2023

An Optical Illusion to Fool the Brain

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We live, move and (inter)act in a three-dimensional world. However, our eyes, or more precisely the retina, is a flat (actually curved) screen: our visual perception, the information we receive through our sense of sight, is a two-dimensional image that the brain has to process in order to infer depth and thus transform it into a three-dimensional representation of our surroundings.

But how does it do this? How does the brain process this flat image to give it depth? By using a series of clues or pieces of information contained in the image. One of these is perspective—the way the apparent size of an object decreases in relation to a vanishing point. Other clues to how far away an object is are the contrast between it and the background, and the sharpness or definition of its boundaries, two factors that are lost or dissipate with distance. These and other visual cues provide valuable information about relative depth, i.e. whether one object is closer or further away than another. And therefore the size of one compared to the other.

A more challenging problem is to determine the absolute size of objects—their real dimensions. One of the resources the brain uses to estimate this is to compare the “target” object with other objects in the scene whose size we are familiar with: a bus, a person, a traffic light, a swing… But this is not always possible, especially in an unfamiliar environment. In these circumstances, the brain needs a Plan B, as a recent study by researchers at the University of York has shown experimentally.

Brainteaser 1: Would you ride in that train?

One of the experiments the volunteers in the study were asked to perform was to determine which of the trains shown was the biggest. More specifically, which of the photographs depicted real trains and which showed model trains.


 (Don’t) lose focus

This Plan B method for determining the true scale of different objects—perhaps the most important one—is the degree of blurriness at the top and bottom of the image: when a given object at a certain distance is in focus, anything in front of or behind the plane of focus will be slightly out of focus. This applies to both nearer and more distant objects (in a typical scene, closer objects are located at the bottom and more distant objects are at the top). And the more out of focus they are, the further they are from the plane of focus. 

This is something that is easy to visualise with our mobile phone cameras: when we take a photo and focus on a person’s face, the rest of the image looks less sharp. And if we focus on another person closer or further away, the sharpness of the rest of the image will change, including the first person’s face, which will now no longer look so perfectly sharp. This degree of blur, combined with other cues, allows the brain to infer the actual size of things. 

But, as the saying goes, “every law has a loophole”. In more academic terms, the fact that the degree of blurriness at the top and bottom of the image is the basic visual cue also means that our eyes can deceive our brain. Or to be more precise (and fairer), the brain can be fooled relatively easily by the eyes. Simply changing the blurriness of the edges of the image is enough to trick the brain

Brainteaser 2: The trickster outwitted?

This effect is known as “tilt-shift miniaturisation” also known as the “diorama effect”, which is an optical illusion created by increasing the blurriness at the top and bottom of the image through digital processing, makes a life-size scene look like a miniature.


An in-depth explanation

The tilt-shift effect is explained by the fact that the smaller the distance to the object in focus, but also the smaller the actual size of the object in focus (largely because we need to get very close to it to see it clearly), the shallower the depth of field. This is the space that remains in focus (or so little out of focus that it is barely noticeable) in front of and behind the object that is the focus of our attention, a consequence of the way the eye contracts to focus. So, by blurring the top and bottom of a real-world image, it appears to be much closer to us, and thus becomes a miniature model “in our brain’s eye”.

Brainteaser 3: Seeing is believing?

One of the conclusions drawn by the researchers responsible for the study is that this limitation may have implications for many aspects of our daily lives, such as driving a car, but also for other less everyday matters, such as the reliability of witness testimony or eyewitness identification in accidents, for example.


Miguel Barral 

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