Thatcher effect: what is this optical illusion?

We have all seen an optical illusion at one time or another and have marveled at discovering its curious effects on our perception.

One of the most testing our ability to discern between the real and the unreal is the one that uses the so-called thatcher effect. We will explore the origin of this optical illusion and what are the keys to produce this distortion when we see it.

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What is the Thatcher effect?

Talking about the Thatcher effect is talking about one of the best known optical illusions. It is a phenomenon whereby, if we modify the image of a human face, turning it 180º (that is, from top to bottom), but keeping both the eyes and the mouth in a normal position, the person who sees it is not able to appreciate anything strange in the image (or detects something strange, but does not know what), recognizing the face without problems, if it is someone famous or acquaintance.

The curious thing is that when the photograph is rotated and put back in its standard position, leaving, this time, both eyes and mouth in their opposite position, then it does cause a powerful rejection effect on the person who is looking at it, realizing immediately that there is something disturbing in the image, that it is not as it should be a face normal.

But why is it called the Thatcher effect, or the Thatcher illusion? The explanation is very simple. When Peter Thompson, Professor of Psychology, was doing experiments modifying faces from photographs for a study on perception, discovered this curious phenomenon by chance, and one of the first photographs he used was that of, at that time, the Prime Minister of the United Kingdom, who was none other than Margaret Thatcher.

In any case, the Thatcher effect is one of the most popular optical illusions, and it is very common to see images of different celebrities altered with this effect to surprise the people who observe them with this peculiar alteration of the perception.

Causes

We already know what the Thatcher effect consists of. Now we are going to delve into the processes that allow this optical illusion to take place. The key to this whole matter would reside in the mechanisms that our brain uses to identify faces, and that we have been acquiring evolutionarily. We have two visual perception systems to recognize elements in general.

One of them identifies objects (and faces) as a whole, based on the scheme that all its parts make up. Once identified, what our brain does is compare it with the mental database we have and thus we are able to identify it, if we even know it. The other, on the contrary, would focus on each independent element of the object (or face), trying to identify the global image through its small parts.

In the case of the Thatcher effect, the key would be that, when we flip the image, the first system stops working, because the inverted layout of the photograph makes it impossible for us to identify the image in that way. This is when the second system comes into play, which analyzes the elements (the mouth, eyes, nose, hair, etc.) individually.

It is then when the optical illusion occurs, since, although some stimuli are in their normal position and others are turned, individually they do not present abnormalities, so they are integrated into a single image, thus making it easier for our brain to identify it as a normal face, only that mouth below.

As soon as we rotate the image and put it in its usual position, this time leaving the eyes and mouth upside down, the first is activated again. identification system and triggers alarms by immediately verifying that this image, as we are seeing it, is impossible. Something does not fit, and we are immediately aware of it, so the Thatcher effect disappears.

In addition, another curious effect occurs, and that is that if we have the image with the elements of the Thatcher effect applied (mouth and eyes upside down), in a normal position, and we begin to rotate it very slowly, there comes an exact point at which we stop perceiving the anomaly, managing to fool our brain again.

prosopagnosia

We have seen that the Thatcher effect is possible due to the way our brain system works to be able to identify faces. But what then happens to people who have this altered function? This pathology exists, and is known as Prosopagnosia. The impossibility of recognizing faces, as well as other highly varied perceptual alterations, have been explored in the work of Oliver Sacks, The Man Who Mistook His Wife for a Hat.

It has been proven that people who suffer from prosopagnosia and therefore do not recognize the faces of even their most loved ones are not affected by the Thatcher effect, since the system of recognition and comparison that we mentioned before does not work in them, and therefore realize long before there are flipped items that a person who is not affected by it pathology.

In the previous point we commented that, if the modified image was rotated slowly, from its normal position towards the flipped position, there was a moment, halfway, when the Thatcher effect suddenly appeared, ceasing to have that sensation of foreign elements before the mouth and the fingers. eyes. However, people with prosopagnosia do not experience this phenomenon, and they can continue to rotate the image until it is completely flipped without feeling the Thatcher effect.

• You may be interested in: "Prosopagnosia, the inability to recognize human faces"

Animals

But is the Thatcher effect a phenomenon unique to humans? We might think so, since face recognition is a more developed skill in our species than in any other, but the truth is that no, it is not exclusive to humans. Different studies have been carried out with different types of primates. (specifically with chimpanzees and rhesus macaques) and the results are conclusive: they also fall under the Thatcher effect.

When presented with images of the faces of individuals of their own species, with the mouth and eye parts reversed from their usual position, no variations were noted in the attentional responses with respect to those without the elements of the Thatcher effect, which already presaged that, indeed, they were not realizing the parts that had been turned around.

However, when the images were turned upside down and placed straight, the eyes and mouth being then inverted, a greater attention was produced towards those images. images, which showed that they somehow perceived the anomaly, something that was not happening in the first phase of the study, when the photos of the reverse.

This leads researchers to believe that, in reality, the face recognition mechanism is not exclusive to the human being, as demonstrated in the Thatche effect experiments, but that said mechanism had to be originated in a species prior to both the ours as well as that of these primates, which would be an ancestor of all of them, which is why we would both have inherited this ability, among others.

other experiments

Once the Thatcher effect and its mechanisms were discovered, the researchers launched a whole series of studies to see how far where its scope reached, what were the limits that could be placed on this alteration of perception and if it would also work with elements that were not human faces, and even not only with static figures but with animations that represented the movements of People and animals.

In fact, the most varied versions have been made, some of them rotating letters and words in images with texts, and others in which what is flipped are the pieces of the bikini of a women. The general conclusions that have been obtained with all these experiments is that the characteristics of the Thatcher effect can be extrapolated to other elements that are not faces, but the intensity of the effect obtained will always be less than in the original example.

This is probably because we are especially good at recognizing faces, much more so than with any other element, that is why we have a specific perception system for it, as we have already described at the beginning of this article. Therefore, it is not surprising that the Thatcher effect is much more noticeable when we work with human faces than if we use anything else instead.

Bibliographic references:

• Psalta, L., Young, A.W., Thompson, P., Andrews, T.J. (2013). The Thatcher illusion reveals orientation dependence in brain regions involved in processing facial expressions. Psychological Science.
• Psalta, L., Young, A.W., Thompson, P., Andrews, T.J. (2014). Orientation-sensitivity to facial features explains the Thatcher illusion. Journal of vision.
• Snowden, R., Snowden, R.J., Thompson, P., Troscianko, T. (2012). Basic vision: an introduction to visual perception. Oxford.
• Thomson, P. (1980). Margaret Thatcher: a new illusion. Perception.