The causal role of the lateral occipital complex in visual mirror symmetry detection and grouping: an fMRI-guided TMS study

Event related potentials (ERP) reveal a robust response to visual symmetry in unattended visual regions

Visual symmetry at fixation generates a bilateral Event Related Potential (ERP) called the Sustained Posterior Negativity (SPN). Symmetry presented in the left visual hemifield generates a contralateral SPN over the right hemisphere and vice versa. The current study examined whether the contralateral SPN is modulated by the focus of spatial attention. On each trial there were two dot patterns, one to the left of fixation, and one to the right of fixation. A central arrow cue pointed to one of the patterns and participants discriminated its regularity (symmetry or random). We compared contralateral SPN amplitude generated by symmetry at attended and unattended spatial locations. While the response to attended symmetry was slightly enhanced, the response to unattended symmetry was still substantial. Although visual symmetry detection is a computational challenge, we conclude that the brain processes visual symmetry in unattended parts of the visual field.

Functional relevance of the extrastriate body area for visual and haptic object recognition: a preregistered fMRI-guided TMS study

The extrastriate body area (EBA) is a region in the lateral occipito-temporal cortex (LOTC), which is sensitive to perceived body parts. Neuroimaging studies suggested that EBA is related to body and tool processing, regardless of the sensory modalities. However, how essential this region is for visual tool processing and nonvisual object processing remains a matter of controversy. In this preregistered fMRI-guided repetitive transcranial magnetic stimulation (rTMS) study, we examined the causal involvement of EBA in multisensory body and tool recognition. Participants used either vision or haptics to identify 3 object categories: hands, teapots (tools), and cars (control objects). Continuous theta-burst stimulation (cTBS) was applied over left EBA, right EBA, or vertex (control site). Performance for visually perceived hands and teapots (relative to cars) was more strongly disrupted by cTBS over left EBA than over the vertex, whereas no such object-specific effect was observed in haptics. The simulation of the induced electric fields confirmed that the cTBS affected regions including EBA. These results indicate that the LOTC is functionally relevant for visual hand and tool processing, whereas the rTMS over EBA may differently affect object recognition between the 2 sensory modalities.

Recognition and Processing of Visual Information after Neuronavigated Transcranial Magnetic Stimulation Session

Background: Transcranial magnetic stimulation (TMS) is a method of noninvasive and painless stimulation of the nervous system, which is based on Faraday’s law of electromagnetic induction. Over the past twenty years, the TMS technique has been deployed as a tool for the diagnosis and therapy of neurodegenerative diseases, as well as in the treatment of mental disorders (e.g., depression). Methods: We tested the inhibitory effects of repetitive TMS (rTMS) on reaction times to militarily relevant visual stimuli amidst distractors and on accompanying blood oxygenation level dependent (BOLD) signal functional magnetic resonance imaging (fMRI) in 20 healthy people. rTMS was applied over the visual cortices, V1, on both hemispheres with the inhibitory theta burst paradigm with the intensity of 70% of the active motor threshold fMRI in 20 healthy people. Results: Analysis of the reaction time to visual stimuli after using TMS to the V1 visual cortex revealed an increase in the number of incorrect recognitions, and the reaction time was from 843 to 910 ms. In the subgroup of participants (n = 15), after the stimulation, there were significant reductions of BOLD signal in blood flow within V1 cortices. Conclusions: The studies of reaction times after the rTMS revealed the inhibitory effect of rTMS on the reaction times and recognition performance of significant (military) objects in the visual field.

Symmetry as a grouping cue for numerosity perception

To estimate the number of objects in an image, each element needs to be segregated as a single unit. Several principles guide the process of element identification, one of the strongest being symmetry. In the current study, we investigated how symmetry affects the ability to rapidly estimate the number of objects (numerosity). Participants judged the numerosity of asymmetric or symmetric arrays of various numerosities. The results show that the numerosity of symmetrical arrays was significantly underestimated at low numerosities, but the effect was greatly reduced at higher numerosities. Adding an additional axis of symmetry (double symmetry) further reduced perceived numerosity. The magnitude of the symmetry-driven underestimation was inversely correlated with autistic personality traits, consistent with previous work associating autistic traits with perceptual grouping. Overall, these results support the idea that perceived numerosity relies on object segmentation and grouping cues, with symmetry playing a key role.

Overlapping Neural Responses to Reflectional Symmetry and Glass Patterns Revealed by an ERP Priming Paradigm

The extrastriate visual cortex is activated by visual regularity and generates an ERP known as the sustained posterior negativity (SPN). Spatial filter models offer a biologically plausible account of regularity detection based on the spectral properties of an image. These models are specific to reflection and therefore imply that reflectional symmetry and Glass patterns are coded by different neural populations. We utilised the SPN priming effect to probe representational overlap between reflection and Glass patterns. For each trial, participants were presented with a rapid succession of three patterns. In the Repeated condition, three reflections or three Glass patterns were presented. In the Changing condition, patterns alternated between reflection and Glass patterns. An increase in SPN amplitude (priming) was observed in both the Repeated and Changing conditions. Results indicate a greater representational overlap in the brain between reflection and Glass patterns than predicted by spatial filter models.

Haemodynamic Signatures of Temporal Integration of Visual Mirror Symmetry

EEG, fMRI and TMS studies have implicated the extra-striate cortex, including the Lateral Occipital Cortex (LOC), in the processing of visual mirror symmetries. Recent research has found that the sustained posterior negativity (SPN), a symmetry specific electrophysiological response identified in the region of the LOC, is generated when temporally displaced asymmetric components are integrated into a symmetric whole. We aim to expand on this finding using dynamic dot-patterns with systematically increased intra-pair temporal delay to map the limits of temporal integration of visual mirror symmetry. To achieve this, we used functional near-infrared spectroscopy (fNIRS) which measures the changes in the haemodynamic response to stimulation using near infrared light. We show that a symmetry specific haemodynamic response can be identified following temporal integration of otherwise meaningless dot-patterns, and the magnitude of this response scales with the duration of temporal delay. These results contribute to our understanding of when and where mirror symmetry is processed in the visual system. Furthermore, we highlight fNIRS as a promising but so far underutilised method of studying the haemodynamics of mid-level visual processes in the brain.

Continuous and intermittent theta burst stimulation to the visual cortex do not alter GABA and glutamate concentrations measured by magnetic resonance spectroscopy

BACKGROUND

Theta burst stimulation (TBS), a form of repetitive transcranial magnetic stimulation (rTMS), uses repeated high-frequency bursts to non-invasively modulate neural processes in the brain. An intermittent TBS (iTBS) protocol is generally considered "excitatory," while continuous TBS (cTBS) is considered "inhibitory." However, the majority of work that has led to these effects being associated with the respective protocols has been done in the motor cortex, and it is well established that TMS can have variable effects across the brain.

OBJECTIVES AND METHOD

We investigated the effects of iTBS and cTBS to the primary visual cortex (V1) on composite levels of gamma-aminobutyric acid + co-edited macromolecules (GABA+) and glutamate + glutamine (Glx) since these are key inhibitory and excitatory neurotransmitters, respectively. Participants received a single session of cTBS, iTBS, or sham TBS to V1. GABA+ and Glx were quantified in vivo at the stimulation site using spectral-edited proton magnetic resonance spectroscopy (¹ H-MRS) at 3T. Baseline pre-TBS GABA+ and Glx levels were compared to immediate post-TBS and 1 h post-TBS levels.

RESULTS

There were no significant changes in GABA+ or Glx following either of the TBS conditions. Visual cortical excitability, measured using phosphene thresholds, remained unchanged following both cTBS and iTBS conditions. There was no relationship between excitability thresholds and GABA+ or Glx levels. However, TBS did alter the relationship between GABA+ and Glx for up to 1 h following stimulation.

CONCLUSIONS

These findings demonstrate that a single session of TBS to the visual cortex can be used without significant effects on the tonic levels of these key neurotransmitters; and add to our understanding that TBS has differential effects at visual, motor, and frontal cortices.

Electrophysiological evidence of the amodal representation of symmetry in extrastriate areas

Extrastriate visual areas are strongly activated by image symmetry. Less is known about symmetry representation at object-level rather than image-level. Here we investigated electrophysiological responses to symmetry, generated by amodal completion of partially-occluded polygon shapes. We used a similar paradigm in four experiments (N = 112). A fully-visible abstract shape (either symmetric or asymmetric) was presented for 250 ms (t0). A large rectangle covered it entirely for 250 ms (t1) and then moved to one side to reveal one half of the shape hidden behind (t2, 1000 ms). Note that at t2 no symmetry could be extracted from retinal image information. In half of the trials the shape was the same as previously presented, in the other trials it was replaced by a novel shape. Participants matched shapes similarity (Exp. 1 and Exp. 2), or their colour (Exp. 3) or the orientation of a triangle superimposed to the shapes (Exp. 4). The fully-visible shapes (t0-t1) elicited automatic symmetry-specific ERP responses in all experiments. Importantly, there was an exposure-dependent symmetry-response to the occluded shapes that were recognised as previously seen (t2). Exp. 2 and Exp.4 confirmed this second ERP (t2) did not reflect a reinforcement of a residual carry-over response from t0. We conclude that the extrastriate symmetry-network can achieve amodal representation of symmetry from occluded objects that have been previously experienced as wholes.

Neural correlates of local parallelism during naturalistic vision

Human observers can rapidly perceive complex real-world scenes. Grouping visual elements into meaningful units is an integral part of this process. Yet, so far, the neural underpinnings of perceptual grouping have only been studied with simple lab stimuli. We here uncover the neural mechanisms of one important perceptual grouping cue, local parallelism. Using a new, image-computable algorithm for detecting local symmetry in line drawings and photographs, we manipulated the local parallelism content of real-world scenes. We decoded scene categories from patterns of brain activity obtained via functional magnetic resonance imaging (fMRI) in 38 human observers while they viewed the manipulated scenes. Decoding was significantly more accurate for scenes containing strong local parallelism compared to weak local parallelism in the parahippocampal place area (PPA), indicating a central role of parallelism in scene perception. To investigate the origin of the parallelism signal we performed a model-based fMRI analysis of the public BOLD5000 dataset, looking for voxels whose activation time course matches that of the locally parallel content of the 4916 photographs viewed by the participants in the experiment. We found a strong relationship with average local symmetry in visual areas V1-4, PPA, and retrosplenial cortex (RSC). Notably, the parallelism-related signal peaked first in V4, suggesting V4 as the site for extracting paralleism from the visual input. We conclude that local parallelism is a perceptual grouping cue that influences neuronal activity throughout the visual hierarchy, presumably starting at V4. Parallelism plays a key role in the representation of scene categories in PPA.

The chronometry of symmetry detection in the lateral occipital (LO) cortex

The lateral occipital cortex (LO) has been shown to code the presence of both vertical and horizontal visual symmetry in dot patterns. However, the specific time window at which LO is causally involved in symmetry encoding has not been investigated. This was assessed using a chronometric transcranial magnetic stimulation (TMS) approach. Participants were presented with a series of dot configurations and instructed to judge whether they were symmetric along the vertical axis or not while receiving a double pulse of TMS over either the right LO (rLO) or the vertex (baseline) at different time windows (ranging from 50 ms to 290 ms from stimulus onset). We found that TMS delivered over the rLO significantly decreased participants' accuracy in discriminating symmetric from non-symmetric patterns when TMS was applied between 130 ms and 250 ms from stimulus onset, suggesting that LO is causally involved in symmetry perception within this time window. These findings confirm and extend prior neuroimaging and ERP evidence by demonstrating not only that LO is causally involved in symmetry encoding but also that its contribution occurs in a relatively large temporal window, at least in tasks requiring fast discrimination of mirror symmetry in briefly (75 ms) presented patterns as in our study.

Symmetry Processing in the Macaque Visual Cortex

Symmetry is a highly salient feature of the natural world that is perceived by many species. In humans, the cerebral areas processing symmetry are now well identified from neuroimaging measurements. Macaque could constitute a good animal model to explore the underlying neural mechanisms, but a previous comparative study concluded that functional magnetic resonance imaging responses to mirror symmetry in this species were weaker than those observed in humans. Here, we re-examined symmetry processing in macaques from a broader perspective, using both rotation and reflection symmetry embedded in regular textures. Highly consistent responses to symmetry were found in a large network of areas (notably in areas V3 and V4), in line with what was reported in humans under identical experimental conditions. Our results suggest that the cortical networks that process symmetry in humans and macaques are potentially more similar than previously reported and point toward macaque as a relevant model for understanding symmetry processing.

Spinning objects and partial occlusion: Smart neural responses to symmetry

In humans, extrastriate visual areas are strongly activated by symmetry. However, perfect symmetry is rare in natural visual images. Recent findings showed that when parts of a symmetric shape are presented at different points in time the process relies on a perceptual memory buffer. Does this temporal integration need a retinotopic reference frame? For the first time we tested integration of parts both in the temporal and spatial domain, using a non-retinotopic frame of reference. In Experiment 1, an irregular polygonal shape (either symmetric or asymmetric) was partly occluded by a rectangle for 500 ms (T1). The rectangle moved to the opposite side to reveal the other half of the shape, whilst occluding the previously visible half (T2). The reference frame for the object was static: the two parts stimulated retinotopically corresponding receptive fields (revealed over time). A symmetry-specific ERP response from ~300 ms after T2 was observed. In Experiment 2 dynamic occlusion was combined with an additional step at T2: the new half-shape and occluder were rotated by 90°. Therefore, there was a moving frame of reference and the retinal correspondence between the two parts was disrupted. A weaker but significant symmetry-specific response was recorded. This result extends previous findings: global symmetry representation can be achieved in extrastriate areas non-retinotopically, through integration in both temporal and spatial domain.

Probing the role of the right inferior frontal gyrus during Pain-Related empathy processing: Evidence from fMRI and TMS

Recent studies have suggested that the right inferior frontal gyrus (rIFG) may be involved in pain-related empathy. To verify the role of the rIFG, we performed a functional magnetic resonance imaging (fMRI) experiment to replicate previous research and further designed a noninvasive repetitive transcranial magnetic stimulation (rTMS) experiment to probe the causal role of the rIFG in pain-related empathy processing. We assigned 74 volunteers (37 females) to three groups. Group 1 (n = 26) performed a task in which participants were required to perceive pain in others (task of pain: TP) and we used fMRI to observe the activity of the rIFG during pain-related empathy processing. Then, we applied online rTMS to the rIFG and the vertex site (as reference site) to observe the performance of Group 2 (n = 24; performing TP) and Group 3 (n = 24; performing a control task of identifying body parts; task of body: TB). fMRI experiment demonstrated stronger activation in the rIFG than in the vertex during the perception of pain in others (p < .0001, Bonferroni-corrected). rTMS experiment indicated that when the rIFG was temporarily disrupted, participants perceived pain in others significantly more slowly (p < .0001, Bonferroni-corrected) than when the vertex was disrupted. Our results provide evidence that the rIFG is involved in pain-related empathy processing, which yields insights into how the brain perceives pain in others.

Right lateralized alpha desynchronization increases with the proportion of symmetry in the stimulus

Research into the neural basis of symmetry perception has intensified in the last two decades; however, the functional role of neural oscillations remains unclear. In previous work Makin et al. (2014, Journal of Vision, 14, 1-12) and Wright et al. (2015, Psychophysiology, 52, 638-647) examined occipital alpha event-related desynchronization (alpha ERD). It was concluded that alpha ERD is right lateralized during active regularity discrimination but not during a secondary task. Furthermore, alpha ERD was unaffected by stimulus properties, such as the type of regularity. These conclusions are refuted by new time-frequency analysis on an electroencephalography (EEG) data set first introduced by Makin et al. (2020, Journal of Cognitive Neuroscience, 32, 353-366). We compared alpha ERD across five tasks. First, we found that right lateralization of alpha ERD was evident in all tasks, not just active regularity discrimination. This was caused by hemispheric differences in alpha power during prestimulus baseline (left < right), which equalized after stimulus onset (left = right). Second, we found that Alpha ERD increased with the proportion of symmetric elements in the image (PSYMM). Sensitivity to PSYMM was stronger on the right. These findings suggest that known extrastriate symmetry activations are accompanied by reduced alpha power.

Transcranial magnetic stimulation over the right dorsolateral prefrontal cortex modulates visuospatial distractor suppression

Visual selective attention allows us to filter relevant inputs from irrelevant inputs during visual processing. In contrast to rich research exploring how the brain facilitates task‐relevant inputs, less is known about how the brain suppresses irrelevant inputs. In this study, we used transcranial magnetic stimulation (TMS) to investigate the causal role of the right dorsolateral prefrontal cortex (DLPFC), a crucial brain area for attentional control, in distractor suppression. Specifically, 10‐Hz repetitive TMS (rTMS) was applied to the right DLPFC and Vertex at the stimuli onset (stimuli‐onset TMS) or 500 ms prior to the stimuli onset (prestimuli TMS). In a variant of the Posner cueing task, participants were instructed to identify the shape of a white target while ignoring a white or colored distractor whose location was either cued in advance or uncued. As anticipated, either the location cue or the colored distractor led to faster responses. Notably, the location cueing effect was eliminated by stimuli‐onset TMS to the right DLPFC, but not by prestimuli TMS. Further analyses showed that stimuli‐onset TMS quickened responses to uncued trials, and this TMS effect was derived from the inhibition at the distractor in both visual fields. In addition, TMS over the right DLPFC had no specific effect on the colored distractor compared to the white one. Considered collectively, these findings indicate that the DLPFC plays a crucial role in visuospatial distractor suppression and acts upon stimuli presentation. Besides, it seems the DLPFC contributes more to location‐based distractor suppression than to color‐based one.

Symmetry Modulates the Amplitude Spectrum Slope Effect on Visual Preference

Within the spectrum of a natural image, the amplitude of modulation decreases with spatial frequency. The speed of such an amplitude decrease, or the amplitude spectrum slope, of an image affects the perceived aesthetic value. Additionally, a human observer would consider a symmetric image more appealing than they would an asymmetric one. We investigated how these two factors jointly affect aesthetic preferences by manipulating both the amplitude spectrum slope and the symmetric level of images to assess their effects on aesthetic preference on a 6-point Likert scale. Our results showed that the preference ratings increased with the symmetry level but had an inverted U-shaped relation to amplitude spectrum slope. In addition, a strong interaction existed between symmetry level and amplitude spectrum slope on preference rating, in that symmetry can amplify the amplitude spectrum slope’s effects. A quadratic function of the spectrum slope can describe such effects. That is, preference is an inverted U-shaped function of spectrum slope whose intercept is determined by the number of symmetry axes. The modulation depth of the quadratic function manifests the interaction between the two factors.

A generalized workflow for conducting electric field-optimized, fMRI-guided, transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a noninvasive method to stimulate the cerebral cortex that has applications in psychiatry, such as in the treatment of depression and anxiety. Although many TMS targeting methods that use figure-8 coils exist, many do not account for individual differences in anatomy or are not generalizable across target sites. This protocol combines functional magnetic resonance imaging (fMRI) and iterative electric-field (E-field) modeling in a generalized approach to subject-specific TMS targeting that is capable of optimizing the stimulation site and TMS coil orientation. To apply this protocol, the user should (i) operationally define a region of interest (ROI), (ii) generate the head model from the structural MRI data, (iii) preprocess the functional MRI data, (iv) identify the single-subject stimulation site within the ROI, and (iv) conduct E-field modeling to identify the optimal coil orientation. In comparison with standard targeting methods, this approach demonstrates (i) reduced variability in the stimulation site across subjects, (ii) reduced scalp-to-cortical-target distance, and (iii) reduced variability in optimal coil orientation. Execution of this protocol requires intermediate-level skills in structural and functional MRI processing. This protocol takes ~24 h to complete and demonstrates how constrained fMRI targeting combined with iterative E-field modeling can be used as a general method to optimize both the TMS coil site and its orientation.

Cross-modal involvement of the primary somatosensory cortex in visual working memory: A repetitive TMS study

Recent literature suggests that the primary somatosensory cortex (S1), once thought to be a low-level area only modality-specific, is also involved in higher-level, cross-modal, cognitive functions. In particular, electrophysiological studies have highlighted that the cross-modal activation of this area may also extend to visual Working Memory (WM), being part of a mnemonic network specific for the temporary storage and manipulation of visual information concerning bodies and body-related actions. However, the causal recruitment of S1 in the WM network remains speculation. In the present study, by taking advantage of repetitive Transcranial Magnetic Stimulation (rTMS), we look for causal evidence that S1 is implicated in the retention of visual stimuli that are salient for this cortical area. To this purpose, in a first experiment, high-frequency (10 Hz) rTMS was delivered over S1 of the right hemisphere, and over two control sites, the right lateral occipital cortex (LOC) and the right dorsolateral prefrontal cortex (dlPFC), during the maintenance phase of a high-load delayed match-to-sample task in which body-related visual stimuli (non-symbolic hand gestures) have to be retained. In a second experiment, the specificity of S1 recruitment was deepened by using a version of the delayed match-to-sample task in which visual stimuli depict geometrical shapes (non-body related stimuli). Results show that rTMS perturbation of S1 activity leads to an enhancement of participants' performance that is selective for body-related visual stimuli; instead, the stimulation of the right LOC and dlPFC does not affect the temporary storage of body-related visual stimuli. These findings suggest that S1 may be recruited in visual WM when information to store (and recall) is salient for this area, corroborating models which suggest the existence of a dedicated mnemonic system for body-related information in which also somatosensory cortices play a key role, likely thanks to their cross-modal (visuo-tactile) properties.

The response to symmetry in extrastriate areas and its time course are modulated by selective attention

Neurophysiological studies have shown a strong activation in visual areas in response to symmetry. Electrophysiological (EEG) studies, in particular, have confirmed that amplitude at posterior electrodes is more negative for symmetrical compared to asymmetrical patterns. This response is present even when observers perform tasks that do not require processing of symmetry. In this sense the activation is automatic. In this study we test this automaticity more directly by presenting stimuli that contain both symmetry and asymmetry, as overlapping patterns of dots of different colour (black and white). Observers were asked to respond to symmetry in only one of the two colours. If feature-based attention has no role the response should depend on properties of the image. If attention fully filters only the relevant colour the response should depend on properties of the relevant colour only. Neither of these models fully explained the data. We conclude that selective attention does modulate the neural response to symmetry, however we also found a significant contribution from the irrelevant pattern.

The Effect of Blindness on Spatial Asymmetries

The human cerebral cortex is asymmetrically organized with hemispheric lateralization pervading nearly all neural systems of the brain. Whether the lack of normal visual development affects hemispheric specialization subserving the deployment of visuospatial attention asymmetries is controversial. In principle, indeed, the lack of early visual experience may affect the lateralization of spatial functions, and the blind may rely on a different sensory input compared to the sighted. In this review article, we thus present a current state-of-the-art synthesis of empirical evidence concerning the effects of visual deprivation on the lateralization of various spatial processes (i.e., including line bisection, mirror symmetry, and localization tasks). Overall, the evidence reviewed indicates that spatial processes are supported by a right hemispheric network in the blind, hence, analogously to the sighted. Such a right-hemisphere dominance, however, seems more accentuated in the blind as compared to the sighted as indexed by the greater leftward bias shown in different spatial tasks. This is possibly the result of the more pronounced involvement of the right parietal cortex during spatial tasks in blind individuals compared to the sighted, as well as of the additional recruitment of the right occipital cortex, which would reflect the cross-modal plastic phenomena that largely characterize the blind brain.

Electrophysiological priming effects confirm that the extrastriate symmetry network is not gated by luminance polarity

It is known that the extrastriate cortex is activated by visual symmetry. This activation generates an ERP component called the Sustained Posterior Negativity (SPN). SPN amplitude increases (i.e., becomes more negative) with repeated presentations. We exploited this SPN priming effect to test whether the extrastriate symmetry response is gated by element luminance polarity. On each trial, participants observed three stimuli (patterns of dots) in rapid succession (500 ms. with 200 ms. gaps). The patterns were either symmetrical or random. The dot elements were either black or white on a grey background. The triplet sequences either showed repeated luminance (black > black > black, or white > white > white) or changing luminance (black > white > black, or white > black > white). As predicted, SPN priming was comparable in repeated and changing luminance conditions. Therefore, symmetry with black elements is not processed independently from symmetry with white elements. Source waveform analysis confirmed that this priming happened within the extrastriate symmetry network. We conclude that the network pools information across luminance polarity channels.

The extrastriate symmetry response can be elicited by flowers and landscapes as well as abstract shapes

Previous research has investigated the neural response to visual symmetry. It is well established that symmetry activates a network of extrastriate visual regions, including V4 and the Lateral Occipital Complex. This symmetry response generates an event-related potential called the sustained posterior negativity (SPN). However, previous work has used abstract stimuli, typically dot patterns or shapes. We tested the generality of the SPN. We confirmed that the SPN wave was present and of similar amplitude for symmetrical shapes, flowers and landscapes, whether participants were responding either to image symmetry or to image color. We conclude that the extrastriate symmetry response can be generated by any two-dimensional image and is similar in different stimulus domains.

Electrophysiological responses to regularity show specificity to global form: The case of Glass patterns

The holographic weight of evidence model (van der Helm & Leeuwenberg, J Math Psychol, 35, 1991, 151; van der Helm & Leeuwenberg, Psychol Rev, 103, 1996, 429) estimates that the perceptual goodness of moiré structures (Glass patterns), irrespective of their global form, is comparable to that of reflection symmetry. However, both behavioural and neuroscience evidences suggest that certain Glass forms (i.e. circular and radial structures) are perceptually more salient than others (i.e. translation structures) and may recruit different perceptual mechanisms. In this study, we tested whether brain responses for circular, radial and translation Glass patterns are comparable to the response for onefold bilateral reflection symmetry. We recorded an event‐related potential (ERP), called the sustained posterior negativity (SPN), which has been shown to index perceptual goodness of a range of regularities. We found that circular and radial Glass patterns generated a comparable SPN amplitude to onefold reflection symmetry (in line with the prediction of the holographic model), starting approx. 180 ms after stimulus onset. Conversely, the SPN response to translation Glass patterns had a longer latency (approx. 400 ms). These results show that Glass patterns are a special case of visual regularity, and perceptual goodness may not be fully explained by the holographic identities that constitute it. Specialised processing mechanisms might exist in the regularity‐sensitive extrastriate areas, which are tuned to global form configurations.

The Formation of Symmetrical Gestalts Is Task-Independent, but Can Be Enhanced by Active Regularity Discrimination

The brain can organize elements into perceptually meaningful gestalts. Visual symmetry is a useful tool to study gestalt formation, and we know that there are symmetry-sensitive regions in the extrastriate cortex. However, it is unclear whether symmetrical gestalt formation happens automatically, whatever the participant's current task is. Does the visual brain always organize and interpret the retinal image when possible, or only when necessary? To test this, we recorded an ERP called the sustained posterior negativity (SPN). SPN amplitude increases with the proportion of symmetry in symmetry + noise displays. We compared the SPN across five tasks with different cognitive and perceptual demands. Contrary to our predictions, the SPN was the same across four of the five tasks but selectively enhanced during active regularity discrimination. Furthermore, during regularity discrimination, the SPN was present on hit trials and false alarm trials but absent on miss and correct rejection trials. We conclude that gestalt formation is automatic and task-independent, although it occasionally fails on miss trials. However, it can be enhanced by attention to visual regularity.

Symmetric patterns with different luminance polarity (anti-symmetry) generate an automatic response in extrastriate cortex

People can quickly detect bilateral reflection in an image. This is true when elements of the same luminance are matched on either side of the axis (symmetry) and when they have opposite luminance polarity (anti-symmetry). Using electroencephalography, we measured the well-established sustained posterior negativity (SPN) response to symmetry and anti-symmetry. In one task, participants judged the presence or absence of regularity (Regularity Discrimination Task). In another, they judged the presence or absence of rare colored oddball trials (Colored Oddball Task). Previous work has concluded that anti-symmetry is only detected indirectly, through serial visual search of element locations. This selective attention account predicts that the anti-symmetry SPN should be abolished in the Colored Oddball Task because there is no need to search for anti-symmetry. However, this prediction was not confirmed: The symmetry and anti-symmetry SPN waves were not modulated by task. We conclude that at least some forms of anti-symmetry can be extracted from the image automatically, in much the same way as symmetry. This is an important consideration for models of symmetry perception, which must be flexible enough to accommodate opposite luminance polarity, while also accounting for the fact anti-symmetry is often perceptually weaker than symmetry.

Stimulus information supporting bilateral symmetry perception

A classification image (a psychophysical reverse-correlation) method was used to investigate what stimulus regions and information the visual system uses for bilateral symmetry perception. The stimuli were symmetric random-dot patterns with either low or high dot density. First, the spatial integration region supporting symmetry perception was estimated, by analyzing the trial-to-trial correlation between the spatial location of symmetric dots and the corresponding response. It was observed that the integration region was rather compact (3 deg² with dense stimulus), vertically elongated and located near to the axis of symmetry. The size of the area was dependent on the pattern density, being larger with low-density stimulus. Next, the resolution of the symmetry matching was probed by estimating how close to the perfect symmetry the dots in two stimulus parts must be to be perceived as symmetric (classification image for symmetry tolerance). Dot pairings up to 6 arc min off from the mirror symmetry correlated with symmetry response, suggesting that the process underlying symmetry matching has large tolerance and low resolution. Outside the integration region, the symmetry tolerance classification image weights were essentially zero, suggesting that the lack of symmetry integration there is not a byproduct of high tolerance.

Category-selective areas in human visual cortex exhibit preferences for stimulus depth

Multiple regions in the human brain are dedicated to accomplish the feat of object recognition; yet our brains must also compute the 2D and 3D locations of the objects we encounter in order to make sense of our visual environments. A number of studies have explored how various object category-selective regions are sensitive to and have preferences for specific 2D spatial locations in addition to processing their preferred-stimulus categories, but there is no survey of how these regions respond to depth information. In a blocked functional MRI experiment, subjects viewed a series of category-specific (i.e., faces, objects, scenes) and unspecific (e.g., random moving dots) stimuli with red/green anaglyph glasses. Critically, these stimuli were presented at different depth planes such that they appeared in front of, behind, or at the same (i.e., middle) depth plane as the fixation point (Experiment 1) or simultaneously in front of and behind fixation (i.e., mixed depth; Experiment 2). Comparisons of mean response magnitudes between back, middle, and front depth planes reveal that face and object regions OFA and LOC exhibit a preference for front depths, and motion area MT+ exhibits a strong linear preference for front, followed by middle, followed by back depth planes. In contrast, scene-selective regions PPA and OPA prefer front and/or back depth planes (relative to middle). Moreover, the occipital place area demonstrates a strong preference for "mixed" depth above and beyond back alone, raising potential implications about its particular role in scene perception. Crucially, the observed depth preferences in nearly all areas were evoked irrespective of the semantic stimulus category being viewed. These results reveal that the object category-selective regions may play a role in processing or incorporating depth information that is orthogonal to their primary processing of object category information.

Hemispheric asymmetry of liking for representational and abstract paintings

Although the neural correlates of the appreciation of aesthetic qualities have been the target of much research in the past decade, few experiments have explored the hemispheric asymmetries in underlying processes. In this study, we used a divided visual field paradigm to test for hemispheric asymmetries in men and women's preference for abstract and representational artworks. Both male and female participants liked representational paintings more when presented in the right visual field, whereas preference for abstract paintings was unaffected by presentation hemifield. We hypothesize that this result reflects a facilitation of the sort of visual processes relevant to laypeople's liking for art-specifically, local processing of highly informative object features-when artworks are presented in the right visual field, given the left hemisphere's advantage in processing such features.

Recurrent Processing of Contour Integration in the Human Visual Cortex as Revealed By fMRI-Guided TMS

Contour integration is a critical step in visual perception because it groups discretely local elements into perceptually global contours. Previous investigations have suggested that striate and extrastriate visual areas are involved in this mid-level processing of visual perception. However, the temporal dynamics of these areas in the human brain during contour integration is less understood. The present study used functional magnetic resonance imaging-guided transcranial magnetic stimulation (TMS) to briefly disrupt 1 of 2 visual areas (V1/V2 and V3B) and examined the causal contributions of these areas to contour detection. The results demonstrated that the earliest critical time window at which behavioral detection performance was impaired by TMS pluses differed between V1/V2 and V3B. The first critical window of V3B (90-110 ms after stimulus onset) was earlier than that of V1/V2 (120-140 ms after stimulus onset), thus indicating that feedback connection from higher to lower area was necessary for complete contour integration. These results suggested that the fine processing of contour-related information in V1/V2 follows the generation of a coarse template in the higher visual areas, such as V3B. Our findings provide direct causal evidence that a recurrent mechanism is necessary for the integration of contours from cluttered background in the human brain.

Emergence of symmetry selectivity in the visual areas of the human brain: fMRI responses to symmetry presented in both frontoparallel and slanted planes

Symmetry is effortlessly perceived by humans across changes in viewing geometry. Here, we re-examined the network subserving symmetry processing in the context of up-to-date retinotopic definitions of visual areas. Responses in object selective cortex, as defined by functional localizers, were also examined. We further examined responses to both frontoparallel and slanted symmetry while manipulating attention both toward and away from symmetry. Symmetry-specific responses first emerge in V3 and continue across all downstream areas examined. Of the retinotopic areas, ventral occipital VO1 showed the strongest symmetry response, which was similar in magnitude to the responses observed in object selective cortex. Neural responses were found to increase with both the coherence and folds of symmetry. Compared to passive viewing, drawing attention to symmetry generally increased neural responses and the correspondence of these neural responses with psychophysical performance. Examining symmetry on the slanted plane found responses to again emerge in V3, continue through downstream visual cortex, and be strongest in VO1 and LOB. Both slanted and frontoparallel symmetry evoked similar activity when participants performed a symmetry-related task. However, when a symmetry-unrelated task was performed, fMRI responses to slanted symmetry were reduced relative to their frontoparallel counterparts. These task-related changes provide a neural signature that suggests slant has to be computed ahead of symmetry being appropriately extracted, known as the "normalization" account of symmetry processing. Specifically, our results suggest that normalization occurs naturally when attention is directed toward symmetry and orientation, but becomes interrupted when attention is directed away from these features.

TMS over right OFA affects individuation of faces but not of exemplars of objects

In addition to its well-documented role in processing of faces, the occipital face area in the right hemisphere (rOFA) may also play a role in identifying specific individuals within a class of objects. Here we explored this issue by using fMRI-guided TMS. In a first experiment, participants had to judge whether two sequentially presented images of faces or objects represented exactly the same exemplar or two different exemplars of the same class, while receiving online TMS over either the rOFA, the right lateral occipital cortex (rLO) or the Vertex (control). We found that, relative to Vertex, stimulation of rOFA impaired individuation of faces only, with no effect on objects; in contrast, TMS over rLO reduced individuation of objects but not of faces. In a second control experiment participants judged whether a picture representing a fragment of a stimulus belonged or not to the subsequently presented image of a whole stimulus (part-whole matching task). Our results showed that rOFA stimulation selectively disrupted performance with faces, whereas performance with objects (but not with faces) was selectively affected by TMS over rLO. Overall, our findings suggest that rOFA does not contribute to discriminate between exemplars of non-face objects.

Evaluating the Role of the Dorsolateral Prefrontal Cortex and Posterior Parietal Cortex in Memory-Guided Attention With Repetitive Transcranial Magnetic Stimulation

The contents of working memory (WM) can affect the subsequent visual search performance, resulting in either beneficial or cost effects, when the visual search target is included in or spatially dissociated from the memorized contents, respectively. The right dorsolateral prefrontal cortex (rDLPFC) and the right posterior parietal cortex (rPPC) have been suggested to be associated with the congruence/incongruence effects of the WM content and the visual search target. Thus, in the present study, we investigated the role of the dorsolateral prefrontal cortex and the PPC in controlling the interaction between WM and attention during a visual search, using repetitive transcranial magnetic stimulation (rTMS). Subjects maintained a color in WM while performing a search task. The color cue contained the target (valid), the distractor (invalid) or did not reappear in the search display (neutral). Concurrent stimulation with the search onset showed that relative to rTMS over the vertex, rTMS over rPPC and rDLPFC further decreased the search reaction time, when the memory cue contained the search target. The results suggest that the rDLPFC and the rPPC are critical for controlling WM biases in human visual attention.

Measuring Integration Processes in Visual Symmetry with Frequency-Tagged EEG

Symmetry is a highly salient feature of the natural world which requires integration of visual features over space. The aim of the current work is to isolate dynamic neural correlates of symmetry-specific integration processes. We measured steady-state visual evoked potentials (SSVEP) as participants viewed symmetric patterns comprised of distinct spatial regions presented at two different frequencies (f₁ and f₂). We measured intermodulation components, shown to reflect non-linear processing at the neural level, indicating integration of spatially separated parts of the pattern. We generated a wallpaper pattern containing two reflection symmetry axes by tiling the plane with a two-fold reflection symmetric unit-pattern and split each unit-pattern diagonally into separate parts which could be presented at different frequencies. We compared SSVEPs measured for wallpapers and control patterns for which both images were equal in terms of translation and rotation symmetry but reflection symmetry could only emerge for the wallpaper pattern through integration of the image-pairs. We found that low-frequency intermodulation components differed between the wallpaper and control stimuli, indicating the presence of integration mechanisms specific to reflection symmetry. These results showed that spatial integration specific to symmetry perception can be isolated through a combination of stimulus design and the frequency tagging approach.

The neural basis of visual symmetry and its role in mid- and high-level visual processing

Symmetry is an important and prominent feature of the visual world. It has been studied as a basis for image segmentation and perceptual organization, but it also plays a role in higher level processes, such as face and object perception. Over the past decade, there has been progress in the study of the neural mechanisms of symmetry perception in humans and other animals. There is extended activity in the ventral stream, including the lateral occipital complex (LOC) and VO1; this activity starts in V3 and it occurs independently of the task (automatic response). Additionally, when the task requires processing of symmetry, the activation may emerge for objects that are symmetrical, even though they do not project a symmetrical image. There is also some evidence of hemispheric lateralization, especially for the LOC. We review the studies on the cortical basis of visual symmetry processing and its links to encoding of other aspects of the visual world, such as faces and objects.

Supramodal agnosia for oblique mirror orientation in patients with periventricular leukomalacia

Periventricular leukomalacia (PVL) is characterized by focal necrosis at the level of the periventricular white matter, often observed in preterm infants. PVL is frequently associated with motor impairment and with visual deficits affecting primary stages of visual processes as well as higher visual cognitive abilities. Here we describe six PVL subjects, with normal verbal IQ, showing orientation perception deficits in both the haptic and visual domains. Subjects were asked to compare the orientation of two stimuli presented simultaneously or sequentially, using both a two alternative forced choice (2AFC) orientation-discrimination and a matching procedure. Visual stimuli were oriented gratings or bars or collinear short lines embedded within a random pattern. Haptic stimuli comprised two rotatable wooden sticks. PVL patients performed at chance in discriminating the oblique orientation, both for visual and haptic stimuli. Moreover when asked to reproduce the oblique orientation, they often oriented the stimulus along the symmetric mirror orientation. The deficit generalized to stimuli varying in many low level features, was invariant for spatiotopic object orientation, and also occurred for sequential presentations. The deficit was specific to oblique orientations, and not for horizontal or vertical stimuli. These findings show that PVL can affect a specific network involved with the supramodal perception of mirror symmetry orientation.

The dorsomedial prefrontal cortex mediates the interaction between moral and aesthetic valuation: a TMS study on the beauty-is-good stereotype

Attractive individuals are perceived as possessing more positive personal traits than unattractive individuals. This reliance on aesthetic features to infer moral character suggests a close link between aesthetic and moral valuation. Here we aimed to investigate the neural underpinnings of the interaction between aesthetic and moral valuation by combining transcranial magnetic stimulation (TMS) with a priming paradigm designed to assess the Beauty-is-Good stereotype. Participants evaluated the trustworthiness of a series of faces (targets), each of which was preceded by an adjective describing desirable, undesirable, or neutral aesthetic qualities (primes). TMS was applied between prime and target to interfere with activity in two regions known to be involved in aesthetic and moral valuation: the dorsomedial prefrontal cortex (dmPFC, a core region in social cognition) and the dorsolateral prefrontal cortex (dlPFC, critical in decision making). Our results showed that when TMS was applied over vertex (control) and over the dlPFC, participants judged faces as more trustworthy when preceded by positive than by negative aesthetic primes (as also shown in two behavioral experiments). However, when TMS was applied over the dmPFC, primes had no effect on trustworthiness judgments. A second Experiment corroborated this finding. Our results suggest that mPFC plays a causal role linking moral and aesthetic valuation.

Symmetric Objects Become Special in Perception Because of Generic Computations in Neurons

Symmetry is a salient visual property: It is easy to detect and influences perceptual phenomena from segmentation to recognition. Yet researchers know little about its neural basis. Using recordings from single neurons in monkey IT cortex, we asked whether symmetry—being an emergent property—induces nonlinear interactions between object parts. Remarkably, we found no such deviation: Whole-object responses were always the sum of responses to the object’s parts, regardless of symmetry. The only defining characteristic of symmetric objects was that they were more distinctive compared with asymmetric objects. This was a consequence of neurons preferring the same part across locations within an object. Just as mixing diverse paints produces a homogeneous overall color, adding heterogeneous parts within an asymmetric object renders it indistinct. In contrast, adding identical parts within a symmetric object renders it distinct. This distinctiveness systematically predicted human symmetry judgments, and it explains many previous observations about symmetry perception. Thus, symmetry becomes special in perception despite being driven by generic computations at the level of single neurons.

Dynamics of perceptual decisions about symmetry in visual cortex

Neuroimaging studies have identified multiple extra-striate visual areas that are sensitive to symmetry in planar images (Kohler et al., 2016; Sasaki et al., 2005). Here, we investigated which of these areas are directly involved in perceptual decisions about symmetry, by recording high-density EEG in participants (n = 25) who made rapid judgments about whether an exemplar image contained rotation symmetry or not. Stimulus-locked sensor-level analysis revealed symmetry-specific activity that increased with increasing order of rotation symmetry. Response-locked analysis identified activity occurring between 600 and 200 ms before the button-press, that was directly related to perceptual decision making. We then used fMRI-informed EEG source imaging to characterize the dynamics of symmetry-specific activity within an extended network of areas in visual cortex. The most consistent cortical source of the stimulus-locked activity was VO1, a topographically organized area in ventral visual cortex, that was highly sensitive to symmetry in a previous study (Kohler et al., 2016). Importantly, VO1 activity also contained a strong decision-related component, suggesting that this area plays a crucial role in perceptual decisions about symmetry. Other candidate areas, such as lateral occipital cortex, had weak stimulus-locked symmetry responses and no evidence of correlation with response timing.

An Exploratory TMS Study on Prefrontal Lateralization in Valence Categorization of Facial Expressions

Abstract. Converging neuroimaging and patient data suggest that the dorsolateral prefrontal cortex (DLPFC) is involved in emotional processing. However, it is still not clear whether the DLPFC in the left and right hemisphere is differentially involved in emotion recognition depending on the emotion considered. Here we used transcranial magnetic stimulation (TMS) to shed light on the possible causal role of the left and right DLPFC in encoding valence of positive and negative emotional facial expressions. Participants were required to indicate whether a series of faces displayed a positive or negative expression, while TMS was delivered over the right DLPFC, the left DLPFC, and a control site (vertex). Interfering with activity in both the left and right DLPFC delayed valence categorization (compared to control stimulation) to a similar extent irrespective of emotion type. Overall, we failed to demonstrate any valence-related lateralization in the DLPFC by using TMS. Possible methodological limitations are discussed.