Why does the thatcher illusion arise

Abstract Face recognition in humans is a complex cognitive skill that requires sensitivity to unique configurations of eyes, mouth, and other facial features. Publication types Research Support, N. Moreover, given the status of the Thatcher illusion as the prime exemplar of the effect of configurality on face processing, any evidence that it does not result from configural processing would require us to ask fundamental questions about configural face processing in general.

In the redundant-targets paradigm, performance is compared in three conditions: A here defined as inverted eyes , B here defined as inverted mouths , and A plus B here defined as inverted eyes and mouths. First, RTs are compared in order to determine whether there is a gain for the redundant-target condition as compared with the single-target conditions. If such a redundancy gain is found, the critical issue is whether this gain arises from supercapacity processing.

Supercapacity arises when the work done in the A-plus-B condition is greater than the sum of the work done in the A and B conditions. The work done is calculated from the RT distributions, as explained later. All four measures were used to provide converging evidence for or against the hypothesis that supercapacity processing of full Thatcherisation eyes and mouth inverted will occur relative to partial Thatcherisation either the eyes or the mouth inverted.

Average RTs alone cannot be used to calculate capacity, and therefore cannot address the question of configurality in the detection of Thatcherisation.

The formulae for calculating the capacity coefficient, the Miller inequality, and the Grice inequality are presented in the following sections, together with discussion of possible outcomes.

Formally, it is the probability that a response is made at a given time point probability density function of task completion over time divided by the probability that a response has not yet been made up to and including a given time point the survivor function Footnote 1. This measure is calculated using Eq. This cumulative hazard function is calculated as the negative log of the survivor function.

The Miller inequality is given in Eq. A violation of the Miller inequality implies supercapacity. That is, at time t , the probability that a response has not occurred for the full-Thatcher condition is lower than the summed probabilities for the partial-Thatcher conditions, and this difference at least 1.

Footnote 2 In other words, responses occur faster for the full-Thatcher condition than they do for either of the partial-Thatcher conditions. The Grice inequality Eq. As well as measuring processing capacity, the Grice inequality can also be used to determine a redundancy gain in a finer-grained way than can be achieved with mean RTs. A violation of the Grice inequality implies limited capacity.

In other words, a violation occurs at time t if the probability of not having responded is lower in one of the partial-Thatcher conditions than in the full-Thatcher condition. Put simply, a violation occurs when responses are faster in at least one of partial-Thatcher conditions than in the full-Thatcher condition. A violation therefore provides evidence for limited capacity, in which there is no capacity for processing the extra information in the full-Thatcher condition, and responses are actually slower than the faster of the two partial-Thatcher conditions.

Unlike the capacity coefficient and the Miller inequality, the Grice inequality is not reliant on a comparison between the full-Thatcher and both partial-Thatcher conditions.

Rather, it compares the full-Thatcher condition with the faster of the partial-Thatcher conditions, regardless of whether one partial-Thatcher condition was relatively fast and the other relatively slow. The aim of Experiment 1 was to test whether inverted mouths and eyes, presented in an upright face context, produce supercapacity processing, or whether responses to full-Thatcher faces are equivalent to the combination of responses to inverted eyes and inverted mouths in partial-Thatcher faces.

Inverted faces were not tested, as we did not hypothesise supercapacity processing for inverted faces. A group of 12 undergraduate students participated in Experiment 1 for course credit. The participants had a mean age of Two of the participants were male, ten were right-handed, and all had self-reported normal or corrected-to-normal vision.

Responses were made using the left and right mouse buttons, with the buttons counterbalanced across participants. The participants were seated approximately 57 cm from the screen. Sixteen greyscale female faces were obtained from the Stirling Picture Database.

These 16 faces were used to create four sets of stimuli: no stimulus manipulation typical faces , only the eyes or only the mouths inverted partial-Thatcher faces , and both the eyes and mouths inverted full-Thatcher faces see Fig.

Footnote 3 In total, there were 64 face stimuli. The faces were manipulated in Adobe Photoshop with the blur tool used to remove high-contrast edges caused by manipulating the images. Whole images were blurred using Gaussian blur with a radius of one pixel. The stimuli subtended 6. Example stimuli from Experiments 1 and 2, with a typical face top left , eye partial-Thatcher top right , mouth partial-Thatcher bottom left and full-Thatcher bottom right.

Note the example of the face image is not one of the stimuli used in the experiment, but an illustrative example of the Thatcher illusion as instantiated in this study. In order to address the hypothesis that detection of the Thatcherised features in upright faces would be marked by supercapacity processing, only upright faces were presented. Participants were presented with a single, typical partial- or full-Thatcher face.

The stimulus was displayed until response. The experiment comprised trials, divided into four blocks. Each Thatcherised face was presented twice in each block, to give 32 eye, 32 mouth, and 32 eye-and-mouth trials. Each unchanged face was presented six times in each block to give 96 typical trials.

In total, each participant viewed eye, mouth, and eye-and-mouth Thatcher trials, and typical trials. The participants had a short, self-paced break between blocks. Face identity and type of manipulation were randomised within all blocks. However, the trial proportions chosen may increase the chance of finding a redundancy gain by artificially inhibiting responses to one of the partial-Thatcher faces.

Such inhibition can occur for two reasons. Firstly, given no inversion for one feature e. This probability is lower than the overall probability of an inversion. It is worth noting that the same contingencies are not implied by the presence of an inversion: Inversion of one feature e. For example, if the participant attends to the eye location, an eye inversion would result in faster responses than would a mouth inversion. This difference would not affect the responses to full-Thatcher trials but would slow the mouth-inversion trials relative to the other trial types, causing an overall slowing for single-inversion trials.

The participants were not informed about these probabilities of co-occurrence. Before assessing processing efficiency, analyses were conducted on the performance data RT and accuracy in order to establish whether a redundancy gain occurred for detecting grotesqueness in the full-Thatcher condition as compared with the partial-Thatcher conditions. These analyses were conducted both for individual participants and over all participants.

Secondly, given a redundancy gain, the measures of processing efficiency were calculated in order to establish the capacity levels underlying the redundancy gain. All of the analyses were conducted only on Thatcher trials, in which a feature inversion had occurred, in order to compare between the partial- and full-Thatcher conditions. All post-hoc t tests were Bonferroni corrected by adjusting the p value.

To test for an overall redundancy gain, the effect of the number of feature changes partial vs. In order to minimise positive skew in the RT data, and hence to eliminate the need for outlier trimming, reciprocal RTs were analysed Ratcliff, using the mean reciprocal RT over correct trials.

Nontransformed mean RTs were also analysed see note 5 for the results. This transformation was required to normalise the binomial distribution of the accuracy data. Footnote 4. Mean response times RTs, in milliseconds and error rates percentages from Experiment 1 black and Experiment 2 grey. Error bars represent one standard error. For each participant, a separate one-way independent-samples ANOVA Footnote 5 was conducted for the Feature Change factor, with three levels eye and mouth vs.

Follow-up t tests were conducted on the ten participants showing a significant effect of feature change. The RT analyses revealed a significant redundancy gain at the level of the group, and at the level of the individual for five participants. We wished to determine whether or not this redundancy gain arose due to supercapacity processing.

Mean-RT analyses alone cannot be used to answer questions regarding processing capacity, particularly when capacity changes over time, and therefore the measures of processing capacity defined earlier were employed.

The three panels in Fig. Given that the three methods are nonparametric, no transformation of RTs was required before analysis. RTs for incorrect trials were removed before analysis, and for each incorrect-trial RT, the most similar correct-trial RT was also removed from the same condition. This removal of twin RTs is standard practice and is necessary because there were two possible responses odd and normal , so for every incorrect trial that could be considered a guess, there was also a correct trial that could be considered a guess Eriksen, Footnote 6.

Processing efficiency data from Experiment 1 , presented at the level of the group. Black lines represent the mean values for the group of participants exhibiting a redundancy gain, and grey lines represent the mean values for the group who did not. The capacity coefficient means were calculated by lining up the earliest response of each participant before averaging across relative time bins.

Processing efficiency data from Experiment 1. Each line represents the efficiency measure for one participant over time. Black lines represent values from those participants who exhibited a redundancy gain, and grey lines represent values from those who did not.

In sum, there was only modest evidence for supercapacity processing in five out of 12 participants, and for most of these the evidence was limited to very early time points. Those participants demonstrating the strongest violations were those that exhibited a redundancy gain black lines, Fig. There was some evidence of extreme limited capacity from the Grice inequality for some participants, with violations at some time points values below zero. The participants demonstrating violations were ones who did not exhibit a redundancy gain grey lines, Fig.

All but one of these participants exhibited a redundancy gain black lines, Fig. For mid-to-late time points, the capacity coefficients were between 0 and 1, indicating limited to unlimited capacity, but no evidence of supercapacity processing. The analysis revealed a redundancy gain at the level of the mean, as well as redundancy gains at the level of the individual for five of the 12 participants.

For most participants, the capacity data are inconsistent with interdependencies between features that serve to boost performance. Therefore, the results demonstrate that supercapacity is not necessary, nor automatic, for detection of the Thatcher illusion, and some participants were able to complete the task with limited capacity.

However, there were some participants, particularly those demonstrating a redundancy gain, for whom there was some evidence of processing reaching supercapacity, with nonadditive processing of eyes and mouths.

The results suggest that the discrimination between Thatcher and typical faces does not rely on supercapacity processing and can occur using a limited to an unlimited capacity system.

There is little evidence that the redundancy gain arises from supercapacity, and no evidence that supercapacity is necessary for a redundancy gain, especially at mid-to-late time points. No extra work is necessarily achieved in the full-Thatcher condition, as compared with the total work across both partial-Thatcher conditions. In addition, there is little evidence that the redundancy gain, when present, emerges because of positive interactions between features.

These results could be explained by a race to completion of two independently processed features. In these tasks, our reasoning was that participants sought evidence of inversion and, hence, oddity. Allen, E. Data visualization in the neurosciences: overcoming the curse of dimensionality.

Neuron 74, — Bartlett, J. Inversion and configuration of faces. Calder, A. Calder, G. Rhodes, M. Johnson, J. Haxby Oxford: Oxford University Press , — Carlin, J. A head view-invariant representation of gaze direction in anterior superior temporal sulcus.

Direction-sensitive codes for observed head turns in human superior temporal sulcus. Cortex 22, — Engell, A. Facial expression and gaze-direction in human superior temporal sulcus. Neuropsychologia 45, — Gelman, A. The top row appears as two regular upside-down Thatchers, but the bottom row appears like a Thatcher on the left and a creepy monster on the right.

In reality, the left column contains normal faces albeit the upper face is upside down , but the right column features composites of Thatcher with only the eyes and mouths flipped vertically.

The lower right Thatcher is disturbing because the eyes and mouth are upside down, and therefore all wrong, despite the fact that the face as a whole is right side up. 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.