Human Eyes Can 'Trick' The Brain Into Seeing Things Differently, Study Suggests

The fact that the eyes may trick the mind has an important implication in driving, in the criminal justice system, and security issues such as drone sightings.

Human eyes can 'trick' the brain into seeing things differently, resulting in inaccurate assumptions about the size, shape, structure and colour of objects, among other things, a new study has found. If one's eyes trick them while driving, the chances of accidents may increase. Similarly, eyewitnesses to crimes may end up giving a wrong description of what they saw if their eyes tricked them into seeing something else. 

Therefore, the fact that the eyes may trick the mind has an important implication in driving, in the criminal justice system, and security issues such as drone sightings. Researchers from the University of York and Aston University recently conducted a study in which they showed participants photographs of full-scale railway scenes. The lower and upper parts of these images had been blurred. They were also shown images of small-scale railway models which had not been blurred. 

The study describing the findings was recently published in the journal PLoS One. 

How the study was conducted

The researchers asked the participants to compare each image and decide which picture had the real full-scale railway scene. According to the study, the participants perceived that the blurred images, which were those of the real full-scale scenes, were smaller than the small-scale models. This means that the participants' eyes had tricked them into thinking that the images of the small-scale models represented the real railway scenes. 

How visual perception works

Visual perception makes us build inside our mind a three-dimensional representation of the world around us from our flat retinal images, the study said. The retinal images provide us with a lot of details, but cannot tell us about the scale, or the absolute depth and size of the real objects. 

For instance, a perfect scale model has pictorial depths that are identical to the depths in the real scene. 

Why were the images blurred?

The researchers investigated blur gradients which represent the limited depth of field available for any optical device, such as the eye. The blur gradients can help estimate visual scale because the mind does not perceive everything the eyes see in a real scene. Also, the eyes do not register everything in front of them. 

In a statement released by the University of York, Daniel Baker, one of the authors on the paper, said in order for people to determine the real size of objects they see around them, their visual system needs to estimate the distance to the object. 

How are humans fooled in their estimates of object size?

It is believed that to arrive at an understanding of absolute size, a visual system takes into account the parts of the image that are blurred out, similar to the out-of-focus areas that a camera produces, and this gives the brain the knowledge of spatial scale. 

Daniel said, however, the new study shows that humans can be fooled in their estimates of object size, and that photographers take advantage of this using a technique called 'tilt-shift miniaturisation', which can make life-size objects appear to be scale models. 

The study found that human vision makes use of tilt-shift miniaturisation when making forced-choice judgements about scale. 

Human visual system is flexible

The authors concluded that the human visual system is highly flexible, and is sometimes capable of perceiving size accurately by exploiting 'defocus blur', a type of blur that results from improper depth of focus. However, at other times, humans are incapable of making accurate perceptions of size due to external influences, and also because of a failure to perceive real-world object size.