Thatcher effect: what is this optical illusion?

The Thatcher effect tells us about the way the brain processes faces.

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

One of the ones that most test 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.

What is the Thatcher effect?

To speak of the Thatcher effect is to speak of one of the best known optical illusions.. This is a phenomenon by which, if we modify the image of a human face, turning it 180º (i.e., from top to bottom), but keeping both eyes and mouth in 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 without problems the face, if it is of someone famous or known.

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 the opposite position, then it causes a powerful effect of rejection in the person who is watching it, realizing immediately that there is something disturbing in the image, which is not as it should be a normal face.

But why is it called the Thatcher effect, or Thatcher illusion? The explanation is very simple. When Peter Thompson, a professor of psychology, was making the experiments modifying faces from photographs for a study on perceptionBy chance, he discovered this curious phenomenon, 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 on the Internet images of different celebrities altered with this effect to surprise the people who observe them with this peculiar alteration of perception.

Causes

We already know what the Thatcher effect consists of. Now let's 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 as we have evolved.and that we have acquired evolutionarily. We have two systems of visual perception to recognize elements in general.

One of them identifies objects (and faces) as a whole, based on the outline of all their parts. Once identified, what our brain does is to compare it with the mental database we have and thus we can identify it, if we 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 upside down, the first system stops working, the first system stops working, since the inverted arrangement of the photograph makes it impossible for us to identify the image in this way.. This is when the second system comes into play, which analyzes the elements (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 upside down, individually they do not present anomalies, so they are integrated into a single image, thus facilitating our brain to identify it as a normal face, only upside down.

As soon as we turn the image and put it in its usual position, this time with the eyes and mouth upside down, the first identification system is activated again and sets off the 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 it is that if we have the image with the elements of the Thatcher effect applied (mouth and eyes upside down), in normal position, and we start to rotate it very slowly, there comes an exact point in which we stop perceiving the anomaly, managing to trick our brain again.and we manage to trick our brain again.

Prosopagnosia

We have seen that the Thatcher effect is possible because of the way our brain system works to identify faces. But what happens then with people who have this function altered? This pathology exists, and it is known as Prosopagnosia. The impossibility of recognizing faces, as well as other varied perceptual alterations, have been explored in 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 loved ones are not affected by the Thatcher effect.The Thatcher effect is not present in them, because the recognition and comparison system we mentioned before does not work in them, and therefore they realize much earlier that there are flipped elements than a person who is not affected by this pathology.

In the previous point we commented that, if the modified image was turned slowly, from its normal arrangement to the flipped position, there was a moment, halfway, when the Thatcher effect suddenly appeared, and the sensation of strange elements in front of the mouth and eyes was no longer present. However, people with prosopagnosia do not experience this phenomenon, and they can continue to turn the image completely upside down without feeling the Thatcher effect.

Animals

But is the Thatcher effect a phenomenon exclusive to humans? We might think so, given that face recognition is a more developed ability 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 macaques). (specifically with chimpanzees and rhesus macaques) and the results are conclusive: they also fall into the Thatcher effect.

When presented with images of faces of individuals of their own species, with the parts of the mouth and eyes upside down from their usual position, no variations in attentional responses were noted with respect to those without the elements of the Thatcher effect, which already suggested that, in fact, they were not noticing the parts that had been turned upside down.

However, when the images were turned upside down and the eyes and mouth were inverted, there was a greater attention to these images, which showed that they somehow perceived the anomaly, something that was not happening in the first phase of the study, when the photos were presented upside down.

This leads the researchers to believe that, in fact, the face-recognition mechanism is not the same as in the first phase of the study, the face-recognition mechanism is not unique to humansas demonstrated in the Thatche effect experiments, but that this mechanism had to have originated in a species prior to both our own and 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, researchers launched a series of studies to see how far it could go, what limits could be placed on this alteration of perception and whether it would also work with elements that were not human faces, and even not only with static figures but also with animations representing 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 turned over are the pieces of a woman's bikini. 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 particularly good at recognizing faces, much more so than with any other element, which 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 any other element instead.

Bibliographical 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.
• Thompson, P. (1980). Margaret Thatcher: a new illusion. Perception.