What our eyes see does not always correspond to what we think we see. Sometimes we are deceived, or even deceived, by our eyes. Or sometimes our brains play games with us. A well-known example of such an optical illusion is the inverted T illusion or the Horizontal-Vertical Illusion (HVI).
What is an inverted T illusion?
A subject is instructed to draw an inverted T, in such a way that both lines of the T are the same length. The subject will almost never succeed in this. Stupidity of the subject? No, this is called the inverted T illusion or HVI.
HVI stands for Horizontal-Vertical Illusion, which means an inverted T illusion. People cannot draw an inverted T in which both legs are the same length: we will all draw the horizontal line longer when drawing. How does this happen? Well, we see the vertical line perceptually longer than the horizontal, and we want to compensate for this by drawing the horizontal line longer. So that’s actually where we make the mistake.
In fact, you might as well ask the subject which of the lines in the T is the longest, the horizontal or vertical? Almost all subjects will answer the vertical line, even if the two lines are perfectly the same length.
In this case it is said that perception of the whole causes differences in the sub-components.
The explanation of this illusion please?
Several individuals have conducted studies to explain this phenomenon. Many theories have been put forward regarding the explanation, but no conclusive explanation has been found. There have been a few good attempts:
Sense of gravity
Some researchers try to explain this illusion in terms of our sense of gravity. This means that we would overestimate lines that run parallel to our sense of gravity (i.e. the vertical). The comparison is often made with stairs: it takes much more effort to climb the stairs than to just keep walking straight ahead. As a result, the vertical distance is systematically estimated to be larger than the horizontal. But no one has believed this statement for a long time.
Functioning of eyes and brain
A second possible explanation is sought in the functioning of our eyes and brain. Are there perhaps more receptors on our eye retina for the vertical than for the horizontal dimension? Or are there more cells that are sensitive to vertical features? We can thus find a number of neuroatomic properties that could contribute to this. But this theory was also never proven.
Structure of perceptual field
Perhaps the structure of our perceptual field is the cause of this illusion. In any case, our visual field has a greater range in width than in height. Our field of view is not fixed like a photograph, but can constantly change. If this were the illusion, we should be able to demonstrate it with the following experiments:
- Looking at the T with one eye (this narrows the field of view, but does not change its length). In that case, the HVI would have to disappear, or at least be reduced. Initially this hypothesis was confirmed, but later experiments refuted it. So we cannot explain it with certainty.
- We can show an illuminated T in a pitch-dark room: in this case there is no field of view, just a T, so the illusion should disappear. This was also first confirmed, but later debunked.
- Various other ways to explain the HVI with this theory have been tested, but the research results were never conclusive enough to accept this theory as an explanation.
Conclusion: we cannot say with certainty that this theory explains the HVI either.
Depth
The fourth explanation appeals to the third dimension to explain this illusion, namely depth. It is assumed that the horizontal line does not suggest depth, but the vertical line subconsciously does. In that case, the top of the vertical line would be further away, so to speak. This causes us to overestimate the length of the vertical line segment. But again: there is no evidence for this either.
Eye scan
Finally, we can look for a fifth explanation in the system of eye scanning. We unconsciously move our eyes constantly in different directions, scanning what we see, as it were. There is no systematic movement in young children, but this changes from the moment these children learn to read. In our Western languages we read everything (words, sentences, picture stories, books) from left to right. As a result, the scanning with our eyes would also happen from left to right (our so-called preferred direction). If this were true, this could be proven in two ways:
- conduct research into the HVI in young children who cannot yet read.
- conduct cross-cultural research, for example in China or Japan, where they do not read from left to right and therefore no HVI should be present.
But point 1 is partially refuted by the fact that experiments show that the illusion does not increase with age, but is strongest at the age of 3 years. Point 2 also does not provide clear results, but that may be due to several factors.
Decision
So far, no conclusive explanation has been found for this illusion. In general, the fifth theory is accepted as the most true, but in that case this theory is also seriously incomplete. So for the time being we will not be able to explain this illusion, and we will just have to enjoy the games that our eyes occasionally dare to play with us.