Perceptual similarity and the neural correlates of geometrical illusions in human brain structure

Depth inversion illusion and its relationship to positive symptoms in clinical and non-clinical voice hearers

Among people with schizophrenia (PSZ), positive symptoms such as hallucinations and delusions are often conceptualised as resulting from abnormal top-down modulation of sensory information. PSZ often exhibit reduced susceptibility to visual illusions compared to healthy control subjects (HCS), suggesting that top-down impairments yield enhanced perception of stimuli that would otherwise be distorted by contextualising visual elements. However, it remains unknown whether resistance to illusions is uniquely associated with positive symptoms, or if it is associated with some other aspect of serious mental illness. To examine this question, 77 PSZ, 50 HCS, and 40 individuals who hear voices and hold unusual beliefs but do not have a psychiatric illness (nonclinical voice hearers; NCVH) completed a hollow mask illusion task. HCS reported experiencing the illusion significantly more often than PSZ and more often than NCVH at the trend level, whereas the latter two groups did not differ from one another. Additionally, there was no consistent association between illusion perception and symptom severity for either PSZ or NCVH. We interpret these results to indicate that resistance to visual illusions may mark a vulnerability for experiencing voices and holding unusual beliefs; however, it may not be associated with the severity of these symptoms.

The influence of body orientation on length judgements

Perceiving the size of a visual object requires the combination of various sources of visual information. A recent paper by Kim et al. (Body Orientation Affects the Perceived Size of Objects. Perception 2022, 51: 25-36) concluded that body orientation played a substantial role. The present paper aims to answer the question of whether the reported effect of body orientation on visuo-haptic size matching was due to effects on the visual or the haptic judgements of size. To do so, we used a within-participant design combining an experiment using visuo-haptic size matching with two experiments that assessed the visual and haptic size-percept using free magnitude estimation. Our experiments produced a systematic visuo-haptic mismatch, but the sign of the mismatch was opposite to that of the original study. Moreover, our study did not reveal a systematic effect of body orientation on this mismatch. Thirdly, we found that the mismatch we determined from participants matching a visual and haptic percept was considerably smaller than the mismatch we derived from their visual and haptic size estimates. In summary, our results emphasise that conclusions about the perceived size of objects are very sensitive to details of the experimental approach.

Deceptive illusory cues can influence orthogonally directed manual length estimations

We examined participants' abilities to manually estimate one of two perpendicular line segment lengths using curved point-to-point movements. Configurations involved symmetrical, unsymmetrical, and no bisection in upright and rotated orientation alterations to vertical-horizontal (V-H) illusions, where people often perceive longer vertical than horizontal segments for equal segment lengths. Participants used two orthogonally directed movements for length estimations: positively proportional (POS) - where greater fingertip displacement involved longer length estimation between configuration intersection start position and fingertip end, and negatively proportional (NEG) - where greater fingertip displacement from the screen edge start position toward configuration intersection involved a shorter length estimation between configuration intersection and fingertip end. Length estimations followed most standard perceptual aspects of the V-H illusion for POS estimations, yet differed between upright and rotated orientations for the symmetrical configuration. NEG estimations revealed no illusory influences. Use of allocentric programming likely accompanied POS estimations to explain V-H illusory influences on perceptuomotor control.

Ebbinghaus, Müller-Lyer, and Ponzo: Three examples of bidirectional space-time interference

Previous studies have shown interference between illusory size and perceived duration. The present study replicated this space-time interference in three classic visual-spatial illusions, the Ebbinghaus, the Müller-Lyer, and the Ponzo illusion. The results showed bidirectional interference between illusory size and duration for all three illusions. That is, subjectively larger stimuli were judged to be presented longer, and stimuli that were presented longer were judged to be larger. Thus, cross-dimensional interference between illusory size and duration appears to be a robust phenomenon and to generalize across a wide range of visual size illusions. This space-time interference most likely arises at the memory level and supports the theoretical notion of a common representational metric for space and time.

Response to geometrical visual illusions in non-human animals: a meta-analysis

Visual illusions have been studied in many non-human species, spanning a wide range of biological and methodological variables. While early reviews have proved useful in providing an overview of the field, they have not been accompanied by quantitative analysis to systematically evaluate the contribution of biological and methodological moderators on the proportion of illusory choice. In the current meta-analytical study, we confirm that geometrical visual illusion perception is a general phenomenon among non-human animals. Additionally, we found that studies testing birds report stronger illusion perception compared to other classes, as do those on animals with lateral-positioned eyes compared to animals with forward-facing eyes. In terms of methodological choices, we found a positive correlation between the number of trials during training or testing and the effect sizes, while studies with larger samples report smaller effect sizes. Despite studies that trained animals with artificial stimuli showing larger effect sizes compared with those using spontaneous testing with naturalistic stimuli, like food, we found more recent studies prefer spontaneous choice over training. We discuss the challenges and bottlenecks in this area of study, which, if addressed, could lead to more successful advances in the future.

The BTPI: An online battery for measuring susceptibility to visual illusions

Visual illusions provide a powerful tool for probing the mechanisms that underlie perception. While most previous studies of visual illusions focused on average group-level performance, less attention has been devoted to individual differences in susceptibility to illusions. Unlike in other perceptual domains, in which there are established, validated tools to measure individual differences, such tools are not yet available in the domain of visual illusions. Here, we describe the development and validation of the BTPI (Ben-Gurion University Test for Perceptual Illusions), a new online battery designed to measure susceptibility to the influence of three prominent size illusions: the Ebbinghaus, the Ponzo, and the height-width illusions. The BTPI also measures perceptual resolution, reflected by the just noticeable difference (JND), to detect size differences in the context of each illusion. In Experiment 1 (N = 143), we examined performance in typical self-paced tasks, whereas in Experiment 2 (N = 69), we employed a fixed presentation duration paradigm. High test-retest reliability scores were found for all illusions, with little evidence for intercorrelations between different illusions. In addition, lower perceptual resolution (larger JND) was associated with a larger susceptibility to the illusory effect. The computerized task battery and analysis codes are freely available online.

Similarity in activity and laterality patterns in the angular gyrus during autobiographical memory retrieval and self-referential processing

Long-term memory is arguably one of the key cognitive functions. At the neural level, the lateral parietal cortex and the angular gyrus, particularly in the left hemisphere, exhibit strong activations during autobiographical and episodic memory retrieval. In a separate sub-field, left-lateralized activations of the angular gyrus are also found during self-referential processing, defined as higher activity when a trait term is judged by participants as being related to them vs. related to someone else. The question is whether episodic/autobiographical memory retrieval and self-referential processing effects are related. In the present study, thirty participants participated in the fMRI study with two separate experiments: autobiographical memory retrieval (Experiment 1) and self-referential processing (Experiment 2). In a series of analyses, including the most critical spatial correlation analysis between experiments, we found neural similarity between autobiographical memory retrieval and self-referential processing. Given that self-referential processing was identified in a selective way, the most plausible interpretation of our findings is that self-referential processing might partly explain the activation of the left angular gyrus during autobiographical memory retrieval. Our results are in line with the seminal view of Endel Tulving that the sense of self is a fundamental attribute of long-term memory recollection. However, it should be emphasized that: a) our results do not imply that the left angular gyrus is not involved in the retrieval of episodic memory details; and b) given that our experiment included an autobiographical memory task, generalization of our results to the episodic memory laboratory tasks has yet to be tested.

Effects of surrounding frames and inner targeted line segments on event-related potentials in geometric-optical illusions

OBJECTIVE

The Sander illusion and the horizontal-vertical (H-V) illusion are both size and orientation geometric-optical illusions. The Sander geometric figures can be simply regarded as being made up of surrounding frames and inner targeted line segments. Similarly, H-V illusory geometric figures are made up of the targeted line segments. The role of surrounding frames and inner targeted line segments in the perception and cognition of geometric-optical illusions is not well understood.

METHODS

The time course of event-related potentials (ERP) and the ERP-based standardized low-resolution electromagnetic tomography (sLORETA) source localization were investigated in the Sander illusion and the H-V illusion, which had the same length as the targeted line segments, respectively. The P1, N1, P2, N2 and P3 components of the ERP were focused and measured.

RESULTS

The ERP results demonstrated that the existence of surrounding frames in the Sander illusions-induced significant alterations in the P1, N1, P2, N2 and P3 components, compared with the H-V illusion without surrounding frames. In the Sander illusion, different tilted line segments and surrounding frames resulted in significant differences in the P2, N2 and P3 components. The sLORETA results also demonstrated brain activities of source localization as a function of the surrounding frames and the tilted inner line segments.

CONCLUSIONS

These findings implicate that the perceptual and cognitive processes of the geometric-optical illusions are correlated to the surrounding frames/background, as well as the orientation/direction of inner targeted line segments in geometric figures.

Functional near-infrared spectroscopy imaging of the prefrontal cortex during a naturalistic comedy movie

Naturalistic stimulation (i.e., movies and auditory narratives of some minutes' length) has been a powerful approach to bringing more real-life experiences into laboratory experiments. Data-driven, intersubject correlation (ISC) analysis permits examining to what extent activity in a specific brain region correlates across participants during exposure to a naturalistic stimulus, as well as testing whether neural activity correlates with behavioral measures. Notably, most of the previous research with naturalistic stimuli was conducted using functional fMRI (fMRI). Here, we tested whether a naturalistic approach and the ISC are feasible using functional near-infrared spectroscopy (fNIRS) - the imaging method particularly suited for populations of patients and children. Fifty-three healthy adult participants watched twice a 3-min segment of a Charlie Chaplin movie while we recorded the brain activity on the surface of their prefrontal cortex using fNIRS. In addition, an independent group of 18 participants used a continuous scoring procedure to rate the extent to which they felt that different parts of the movie fragment were funny. Our two findings were as follows. First, we found higher-than-zero ISC in fNIRS signals in the prefrontal cortex lobes, a result that was particularly high in the oxygenated channels during the first repetition of the movie. Second, we found a significant negative correlation between oxygenated brain signals and ratings of the movie's humorousness. In a series of control analyses we demonstrated that this latter correlation could not be explained by various non-humor-related movie sensory properties (e.g., auditory volume and image brightness). The key overall outcome of the present study is that fNIRS in combination with the naturalistic paradigms and the ISC might be a sensitive and powerful research method to explore cognitive processing. Our results also suggest a potential role of the prefrontal cortex in humor appreciation.

The Prospects of Utilizing Geometrical Visual Illusions as Tools for Neuroscience

Geometrical visual illusions have long been used as tools in neuroscience. Most commonly, researchers have taken illusions as a given and attempted to explain phenomenal impressions in terms of known neural mechanisms. In a psychophysical approach to this topic, it is customary to modify stimuli until conditions for which illusions are enhanced, attenuated, or annihilated have been found. Additionally, the focus is not exclusively on response bias but equally on sensitivity, because observers may fall prey to an illusion but at the same time be able to discriminate between stimuli perfectly. For the T-figure, the length of the undivided line is usually overestimated relative to the length of the divided line, and evidence has accrued that suggests that the illusion may be due to the processing of the figure as a coherent unit (a “T-schema”). Dissecting the T or tilting its lines influenced the amount of illusion, suggesting that interactions between orientation-sensitive and end-inhibited neurons are at work. Examples of cognate research with the Ponzo, Ebbinghaus, and Müller-Lyer illusions are also discussed.

Footpad Monitoring: Reliability of an Automated System to Assess Footpad Dermatitis in Turkeys (Meleagris gallopavo) During Slaughter

Footpad dermatitis (FPD) is an indicator of animal welfare in turkeys, giving evidence of the animals' physical integrity and providing information on husbandry management. Automated systems for assessing FPD at slaughter can present a useful tool for objective data collection. However, using automated systems requires that they reliably assess the incidence. In this study, the feet of turkeys were scored for FPD by both an automated camera system and a human observer, using a five-scale score. The observer reliability between both was calculated (Krippendorff's alpha). The results were not acceptable, with an agreement coefficient of 0.44 in the initial situation. Therefore, pictures of 3,000 feet scored by the automated system were evaluated systematically to detect deficiencies. The reference area (metatarsal footpad) was not detected correctly in 55.0% of the feet, and false detections of the alteration on the footpad (FPD) were found in 32.9% of the feet. In 41.3% of the feet, the foot was not presented straight to the camera. According to these results, the algorithm of the automated system was modified, aiming to improve color detection and the distinction of the metatarsal footpad from the background. Pictures of the feet, now scored by the modified algorithm, were evaluated again. Observer reliability could be improved (Krippendorff's alpha = 0.61). However, detection of the metatarsal footpad (50.9% incorrect detections) and alterations (27.0% incorrect detections) remained a problem. We found that the performance of the camera system was affected by the angle at which the foot was presented to the camera (skew/straight; p < 0.05). Furthermore, the laterality of the foot (left/right) was found to have a significant effect (p < 0.001). We propose that the latter depends on the slaughter process. This study also highlights a high variability in observer reliability of human observers. Depending on the respective target parameter, the reliability coefficient (Krippendorff's alpha) ranged from 0.21 to 0.82. This stresses the importance of finding an objective alternative. Therefore, it was concluded that the automated detection system could be appropriate to reliably assess FPD at the slaughterhouse. However, there is still room to improve the existing method, especially when using FPD as a welfare indicator.

Study of Geometric Illusory Visual Perception – A New Perspective in the Functional Evaluation of Children With Strabismus

Despite the various perceptual-motor deficits documented in strabismus, there is a paucity of studies evaluating visual illusions in patients with strabismus. The aim of this study was to examine how the illusionary perception occurs in children/adolescents (10–15 years old) with strabismus with referral for surgery to correct ocular deviations. A controlled cross-sectional study was carried out in which 45 participants with strabismus and 62 healthy volunteers aged 10–15 years were evaluated. The behavioral response to three geometric illusions [Vertical-Horizontal illusion, Müller-Lyer illusion (Bretano version) and Ponzo illusion] and respective neutral stimuli (non-illusory images) regarding the estimation of image size and response time were measured using the Method of Adjustment. To analyze the influence of secondary factors: type of ocular deviation (convergent, divergent or associated with vertical deviation); amount of eye deviation; presence of amblyopia and stereopsis, a one-way ANOVA was performed. Among the tested illusions, children with strabismus showed greater susceptibility (p = 0.006) and response time (p = 0.004) to Ponzo’s illusory images. Children with strabismus and preserved stereopsis, on the other hand, showed similar susceptibility and response time to control group patients to the Ponzo illusion (p < 0.005). Patients with amblyopia showed overcorrection in the estimate of non-illusory Ponzo images (p = 0.046). Children with horizontal ocular deviation (esotropia or exotropia) associated with vertical deviation (hypertropia, DVD and/or alphabetical anisotropy) showed higher susceptibility to vertical adjustment images for the Müller-Lyer illusion (Brentano version) (p = 0.017). Individuals with strabismus tended to overcorrect the length of the straight-line segment adjusted for non-illusory images when testing non-illusory images in the Müller-Lyer test (Brentano version) (p = 0.009), as well as for the neutral images in the Vertical-Horizontal test (p = 0.000). The findings indicated impairment in the perception of geometric illusions and neutral figures, especially for the Ponzo illusion test by children with strabismus. As the behavioral response to illusory images may indirectly reflect the visual and morphofunctional alterations present in these individuals, we suggest that the investigation of visual illusory perception can be used as a new research strategy in the field of investigating the visual function in strabismus.

An auditory time perception illusion analogous to the visual Müller-Lyer illusion

According to the centroid hypothesis, the visual Müller-Lyer-type illusions in which subjects misperceive lines or gaps as longer or shorter depending on surrounding distracters result from the pooling of positioning neural signals such that the perceived object is shifted towards these distracters. However, it is uncertain if this type of pooling is a more general principal that influences perceptions in other sensory modalities, including time perception based on auditory signals. In this study, I show that by applying the principles of the centroid hypothesis, an audial time duration illusion can be constructed. The perception of two sequential time intervals, which were defined by three short tone signals, was distorted by placing distracting white noise sounds near each signal. Misperception magnitude, which peaked at 31%, changed with the time interval between the tone signals and distracters; the relationship between the target-distracter distance and the illusion strength closely paralleled with that of a Müller-Lyer-type illusion, whereby the visual objects were analogically arranged in space rather than time. These results demonstrate that even if signals and distracters are distinguishable, the neural mechanisms for estimating time duration utilize coarser sampling to preserve processing resources at the expense of high accuracy. I hypothesize that systems that are dedicated to visual length and time duration estimations are based on similar perceptual magnitude evaluation algorithms. Moreover, this signal pooling principle may be applicable to other perceptual modalities across different species.

Dogs (Canis lupus familiaris) are susceptible to the Kanizsa's triangle illusion

The ability to complete partially missing contours is widespread across the animal kingdom, but whether this extends to dogs is still unknown. To address this gap in knowledge, we assessed dogs' susceptibility to one of the most common contour illusions, the Kanizsa's triangle. Six dogs were trained to discriminate a triangle from other geometrical figures using a two-alternative conditioned discrimination task. Once the learning criterion was reached, dogs were presented with the Kanizsa's triangle and a control stimulus, where inducers were rotated around their centre, so as to disrupt what would be perceived as a triangle by a human observer. As a group, dogs chose the illusory triangle significantly more often than control stimuli. At the individual level, susceptibility to the illusion was shown by five out of six dogs. This is the first study where dogs as a group show susceptibility to a visual illusion in the same manner as humans. Moreover, the analyses revealed a negative effect of age on susceptibility, an effect that was also found in humans. Altogether, this suggests that the underling perceptual mechanisms are similar between dogs and humans, and in sharp contrast with other categories of visual illusions to which the susceptibility of dogs has been previously assessed.

Dynamic presentation boosts the Ebbinghaus illusion but reduces the Müller-Lyer and orientation contrast illusions

Mruczek et al. (2015) showed that a moving version of the Ebbinghaus illusion almost doubles in strength compared to the standard version. In their stimulus, the size of the surrounding inducers was modulated between large and small and the whole stimulus was made to drift during the surround modulation. We first replicated the original dynamic Ebbinghaus illusion and then explored dynamic presentations for other simultaneous contrast and geometric illusions. We found no increase in illusion strength in any that we sampled. Here we report the results for the Müller-Lyer illusion and the orientation contrast illusion. Surprisingly, when these two illusions were presented dynamically, their effects were greatly reduced for the Müller-Lyer illusion and eliminated for the orientation contrast illusion.

First approach validating a scoring system for foot-pad dermatitis in broiler chickens developed for application in practice

Measuring the severity of foot-pad dermatitis is an accepted tool monitoring the quality of animal husbandry and welfare. Up to now, a variety of scoring schemes have been used, most of them based on visual evaluation. However, a standardisation and validation of scoring systems is beneficial, not only to compare different studies, but to provide objective indicators in poultry welfare. In this study, we validated one visual scoring system, widely used in Northern Germany, by using additional information of histological measurements. Therefore, feet of broiler chickens (ROSS 308) from one flock were visually scored at the slaughter plant (4-point score). Ten feet per score level (n = 40) were sampled and analysed macroscopically and microscopically. Data were analysed using cluster analysis, providing a classification based on these histopathological findings. Validity of the visual scoring system was analysed by (1) testing the interobserver reliability between different observers and (2) by comparing both, visual and cluster classification types using the McNemar's test. In a last step Kendall tau correlations were calculated in order to find suitable parameters to judge the severity in a visual score more reliably. Results could show that most agreement was found for the score levels 1 and 2, whereas results for score levels 3 and 4 were more divergent. These results were found in both, interobserver reliability and comparison of classification types (visual vs. cluster). Results revealed interaction effects of classification type and scoring level for the width of ulcers (p = 0.0044) and the size of the lesion (p = 0.0081). In the cluster classification, higher values in both, width of ulcer and size of lesion could be found in score level 3. Furthermore, a positive correlation of the size of lesion with the depth of the ulcer was found (0.73). In conclusion, we found that histological findings coincided well with the less severe visual scores (1; 2), whereas the differentiation between the severe scores (3; 4) seemed to be less valid. For practical purposes we therefore recommend keeping visual scoring systems simple. Furthermore, as the correlation coefficient between both was quite high, the size of the lesion might serve as an indirect indicator of the depth.

Individual differences in visual science: What can be learned and what is good experimental practice?

We all pass out our lives in private perceptual worlds. The differences in our sensory and perceptual experiences often go unnoticed until there emerges a variation (such as 'The Dress') that is large enough to generate different descriptions in the coarse coinage of our shared language. In this essay, we illustrate how individual differences contribute to a richer understanding of visual perception, but we also indicate some potential pitfalls that face the investigator who ventures into the field.