The aim of Experiment 2 was to determine the effect of inversion on ability to discriminate normal from Thatcherized images when only the mouth or only the eye region was shown.
The ability to discriminate differences based on the eye region is shown in Figure 3. Figure 3. Experiment 2 : Discrimination of the eye region with A same identity or B different identity images presented in an upright or inverted orientation. Pairs of eye regions could both be normal top , both be Thatcherized middle or be normal and Thatcherized bottom.
Figure 3 Experiment 2 : Discrimination of the eye region with A same identity or B different identity images presented in an upright or inverted orientation. The interaction was due to lower accuracy for the normal-Thatcherized condition In contrast, there was no significant difference between the normal-Thatcherized Next, we measured RT to each condition.
The ability to discriminate differences in the mouth region is shown in Figure 4. Figure 4. Experiment 2: Discrimination of the mouth region with A same identity or B different identity images presented in an upright or inverted orientation. Pairs of mouth regions could both be normal top , both be Thatcherized middle or be normal and Thatcherized bottom.
Participants were asked to report whether the mouth images were identical or different. Figure 4 Experiment 2: Discrimination of the mouth region with A same identity or B different identity images presented in an upright or inverted orientation. Accuracy for the same identity images is shown in Figure 4C. The significant interaction was due to the lower proportion of correct responses to normal-Thatcherized Accuracy for the different identity images is shown in Figure 4D.
The effect of Condition was due to a higher number of correct responses in the normal-Thatcherized Next, we determined the effect of Condition and Orientation on RT values. Reaction Time for the same Identity conditions is shown in Figure 4E. Reaction time for the different identity images is shown in Figure 4F. The key finding across each experiment was that inversion severely disrupted ability to discriminate normal from Thatcherized images of the same face or of the eye or mouth regions from the same face.
So, we determined the low-level differences between the image properties of normal and Thatcherized images created from the same face. First, we calculated the mean absolute difference in gray value across corresponding pixels in pairs of images from the same identity Figure 5A.
Next, we measured the correlation of gray values from corresponding pixels in the same image pairs Figure 5B. These analyses were performed on the whole face as used in Experiment 1 and on the eye and mouth regions Experiment 2.
This was due to a progressive increase in the mean difference for gray values for each pixel between images from the whole face pixel diff: 5. These results highlight that the reduced ability to discriminate normal from Thatcherized images when they were inverted was evident despite substantial low-level differences in the images.
Figure 5. Mean differences and correlations across normal and Thatcherized versions of images of the same identity. Errors represent one standard error. Figure 5 Mean differences and correlations across normal and Thatcherized versions of images of the same identity.
Configural processing is thought to be essential to perceiving the grotesque expression, and its disruption leads to the expression not being seen correctly when the image is upside down.
The aim of this study was to explore the role of spatial configuration in the Thatcher illusion. Participants judged whether simultaneously presented images were identical, or different in any way. We found that participants were easily able to discriminate a normal face from a Thatcherized version of the same face when the images were presented upright.
However, when the images were inverted, performance fell below chance level because participants simply failed to notice the difference between the images. This simple perceptual test offers strong evidence of how poorly the inverted Thatcherized expression is perceived.
To determine whether the illusion could be explained by a disruption to the overall facial configuration, we measured performance when only the eye region or only the mouth region of each image was visible. Again, participants were easily able to discriminate a normal from a Thatcherized version of the same image when upright.
However, when the images were inverted, participants were at chance levels. It is important to note that local information about the orientation of the eye or mouth regions is evident in these images, for example, the position of eyebrows or the shape of the visible part of the jaw.
However, when only the mouth region or eye region is shown, any second-order configural information about the spatial relationships between facial features is entirely absent. Our results suggest that previous attempts to explain the Thatcher illusion have been mistaken in ignoring the possibility that inversion disrupts feature processing. Instead, locally-inverted facial features mouth or eyes are themselves perceived as being abnormal, if they are interpreted as being in an upright orientation.
However, when the image is interpreted as inverted, the precision with which the features are encoded is diminished and the features do not look grotesque. As we show that these effects can be found for the face as a whole and for the isolated mouth and eye regions, this effect cannot be explained by a disruption to second-order configural processing.
Rather, our analysis of the Thatcher illusion shows that it depends primarily on sophisticated perceptual encoding of local face regions that are taken to be upright by the perceptual system. When the perceptual system interprets the features as being inverted, it is less able to encode them accurately.
A further remarkable aspect of the Thatcher illusion is that the low-level differences between a normal and Thatcherized image are identical in the upright and inverted orientations. So, it seems odd that when participants were asked only to make a simple visual discrimination between images based on any differences whatsoever, they failed to get above chance with the inverted images.
Nevertheless, it appears that these cues are sufficient to provide the critical orientation cues that influence our perception of the facial features. The magnitude of the inversion effect was lower in Experiment 2 , but this was presumably because the low-level image differences as a proportion of the whole image were greater in this Experiment 2 see Figure 5.
Our findings suggest that low-level image discrimination of faces can be influenced by the context in which the face is perceived. This fits with recent studies that have demonstrated how the global properties of natural images including faces can influence low-level feature detectors Neri, , It is possible that the inability to detect image differences may reflect feedback from higher to lower visual regions.
The Thatcher illusion also demonstrates a degree of independence between the processing of facial identity and expression. The identity of a Thatcherized face can still be recognized when the face is upside down, albeit with some difficulty, whereas the ability to perceive the grotesque facial expression is completely lost.
Inversion appears to be having a differential effect on the processing of facial expression and identity. In a recent study Psalta et al. This was reflected by an increased response in the STS when there was a change in the image from a normal to a Thatcherized face. However, there was no increase in response from a normal to a Thatcherized face when the faces were inverted. In conclusion, our results show that the inability to detect the grotesque expression in the inverted Thatcher illusion can be explained by a reduced sensitivity to inverted facial features.
This interpretation contrasts with previous work that has suggested that the Thatcher illusion reflects configural processing. We do not, of course, deny other clear evidence that configural processing plays a role in face perception and that it is disrupted by inversion. However, we suggest that the explanation of the Thatcher illusion lies with the orientation-specific encoding of local expressive features eyes and mouth.
The authors thank Lilyana Chukova and Lauren Hogan for their help in this study. Commercial relationships: none. Corresponding author: Tim Andrews. But it implies that we can't just mentally rotate an entire face to work out the arrangement of features. That's a bit odd, as we obviously don't have a problem rotating individual parts of the face: the smile on a normal non-Thatcherized face looks fine when the whole face is upside-down. It's worth pointing out that the illusion may not be specifically to do with faces, but with things that we are used to seeing one way up.
On this note, experiments have been done with upside-down writing, and it's interesting that the effect also works if more weakly for pictures of women wearing bikinis [8]. Of course, we see faces all the time, and bikinis quite rarely, so it's not surprising the 'bikini illusion' is weaker, but I've been quite convinced with the examples I've seen.
It may not be just humans either. Researchers have presented rhesus monkeys with faces of other, familiar monkeys that have been Thatcherized. The Thatcherized monkey faces don't look too bad to humans, either way up, and they aren't that distracting to monkeys when shown upside-down, either. But they are distracting when shown right way up - a sort of monkey-Thatcher effect [9]. There have also been tests done on people who can't identify faces but can recognise facial emotions - a condition known as prosopagnosia.
Prosopagnosiacs are actually quicker at spotting upside-down Thatcherized faces. Indeed, as you gradually rotate a Thatcherized face until it is upside-down, they don't seem to have that switch that normal people have from instant spotting of the effect, to needing much longer to work it out [10].
For example, recent research has highlighted the importance of an effect that we used to judge 3D. Faces are usually lit from above, and when the eyes and lips are upside-down, the reversal of shading makes them look particularly weird [11]. The explanations above seem a bit convoluted; my instinct is that they aren't phrased in a way that really captures the way the brain actually works - so I don't find them altogether satisfactory. However, it's possible that advances in our understanding of the brain will allow us to look at the neural pathways involved in the Thatcher illusion.
Coupled with recent research on how to get computers to recognise faces, I expect this to produce a much deeper understanding of the Thatcher illusion in the future. The Thatcher illusion, like so much of science, was accidentally discovered by someone experimenting for different reason. It's called this because psychologist Peter Thompson was distorting an easily available famous face a left-over election campaign poster of Margaret Thatcher in order to show his students a quite different visual effect.
The full details are described in ' The Thatcher illusion 28 years on There's a great optical illusions website which has a rotatable online version of the original Thatcher image. Also see the BBC news coverage. Book your free tickets here.
Will the internet ever develop its own artificial intelligence? When you look in a mirror, why are left and right reversed? Search term:. Read more. Both humans and monkeys notice changes in the orientation of the eyes in a face only when the face is upright, and not when it is upside down.
A new study , published earlier this month in the Journal of Neuroscience, has now gone further to determine the potential neural correlates of the Thatcher illusion in the primate visual system. At the same time, they presented the primates with upside-down and right-side-up Thatcherized and non-Thatcherized faces. The neural recordings showed that the responses of neurons in the macaque middle lateral face patch ML were consistent with the perception of the Thatcher illusion, but those of neurons in the anterior lateral face patch AL were not.
In particular, the ML neurons were highly sensitive to eye orientation in upright faces but not in inverted faces, a pattern of response that did not occur in face-selective neurons from other regions. The team concluded that ML is the potential neural seat for the perception of the Thatcher illusion. The views expressed are those of the author s and are not necessarily those of Scientific American. Macknik and Sandra Blakeslee. The Thatcher illusion was an important demonstration, because it was one of the first to highlight some of the underlying mechanisms by which our brains process information about faces.
By and large, faces are made up of the same, consistent features — two eyes, a nose, a mouth, some ears, and so on. One way in which our brains could process faces is to analyse them as a collection of these separate, individual features. If that were the case though, we might expect to be easier to pick out any discrepancies in an upside-down face.
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