The role of the human extrastriate visual cortex in mirror symmetry discrimination: A TMS-adaptation study

When do we find a third neural response to visual symmetry?

The human visual system is tuned to symmetry, and the neural response to visual symmetry has been well studied. One line of research measures an Event Related Potential (ERP) component called the Sustained Posterior Negativity (SPN). Amplitude is more negative at posterior electrodes when participants see symmetrical patterns compared to asymmetrical patterns. Source localization confirms that the SPN is generated by two dipoles in the left and right extrastriate cortex, in line with fMRI results. However, exploratory analysis by Tyson-Carr, Bertamini, Rampone, and Makin (2021) found a third symmetry response located approximately in the posterior cingulate peaking at around 600 msec. The third symmetry response was only generated in conditions where symmetry was 1) task relevant and 2) salient. We tested whether these findings are reliable by running source localization analysis on all suitable datasets from the complete Liverpool SPN catalogue (an online repository of all 40 SPN projects with 2215 participants https://osf.io/2sncj/). We predicted that less variance would be explained by a two-dipole model in experiments where participants classified regularity (hypothesis 1), and, when the third dipole is present, amplitude would correlate with that of the sensor-level SPN (hypothesis 2). Hypothesis 1 was not supported, while hypothesis 2 was. We conclude that the bilateral extrastriate symmetry response is sometimes followed by a third activation near the posterior cingulate. However, this third symmetry response is not as predictable as we had assumed. One possibility is that it may sometimes be hidden from average waveforms by temporal inconsistency between trials. This may happen more in experiments with longer presentation durations.

Symmetry Detection in Autistic Adults Benefits from Local Processing in a Contour Integration Task

Symmetry studies in autism are inconclusive possibly due to different types of stimuli used which depend on either local or global cues. Therefore, this study compared symmetry detection between 20 autistic and 18 non-autistic adults matched on age, IQ, gender and handedness, using contour integration tasks containing open and closed contours that rely more on local or global processing respectively. Results showed that the autistic group performed equally well with both stimuli and outperformed the non-autistic group only for the open contours, possibly due to a different strategy used in detecting symmetry. However, there were no group differences for the closed contour. Results explain discrepant findings in previous symmetry studies suggesting that symmetry tasks that favour a local strategy may be advantageous for autistic individuals. Implications of the findings towards understanding visual sensory issues in this group are discussed.

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.

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.

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.

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.

Effects of online repetitive transcranial magnetic stimulation (rTMS) on cognitive processing: A meta-analysis and recommendations for future studies

Online repetitive transcranial magnetic stimulation (rTMS), applied while subjects are performing a task, is widely used to disrupt brain regions underlying cognition. However, online rTMS has also induced "paradoxical enhancement". Given the rapid proliferation of this approach, it is crucial to develop a better understanding of how online stimulation influences cognition, and the optimal parameters to achieve desired effects. To accomplish this goal, a quantitative meta-analysis was performed with random-effects models fitted to reaction time (RT) and accuracy data. The final dataset included 126 studies published between 1998 and 2016, with 244 total effects for reaction times, and 202 for accuracy. Meta-analytically, rTMS at 10 Hz and 20 Hz disrupted accuracy for attention, executive, language, memory, motor, and perception domains, while no effects were found with 1 Hz or 5 Hz. Stimulation applied at and 10 and 20 Hz slowed down RTs in attention and perception tasks. No performance enhancement was found. Meta-regression analysis showed that fMRI-guided targeting and short inter-trial intervals are associated with increased disruptive effects with rTMS.

Sustained response to symmetry in extrastriate areas after stimulus offset: An EEG study

Electrophysiological (EEG) studies of human perception have found that amplitude at posterior electrodes is more negative for symmetrical patterns compared to asymmetrical patterns. This negativity lasts for hundreds of milliseconds and it has been called sustained posterior negativity (SPN). Symmetry activates a network of visual areas, including the lateral occipital complex (LOC). The SPN is a response to presence of symmetry in the image. Given the sustained nature of this activation, in this study we tested the persistence of the SPN after stimulus offset. Two shapes were presented (for 0.5 s each) with a 1 s blank interval in between. We observed a sustained response after stimulus offset, irrespective of whether the task required processing of shape information. This supports the idea that the response to symmetry is generated by information in the image, independently of task, and that it is sustained over approximately one second post stimulus onset.

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.

Temporal dynamics of mirror-symmetry perception

Recent studies have suggested that temporal dynamics rather than symmetrical motion-direction contribute to mirror-symmetry perception. Here we investigate temporal aspects of symmetry perception and implicitly, its temporal flexibility and limitations, by examining how symmetrical pattern elements are combined over time. Stimuli were dynamic dot-patterns consisting of either an on-going alternation of two images (sustained stimulus presentation) or just two images each presented once (transient stimulus presentation) containing different amounts of symmetry about the vertical axis. We varied the duration of the two images under five temporal-arrangement conditions: (a) whole patterns in which a symmetric pattern alternated with a noise pattern; (b) delayed halves—the halves of the symmetric and noise patterns were presented with temporal delay; (c) matched-pairs—two alternating images each containing equal amounts of symmetrical matched-pairs; (d) delayed matched-pairs—the same as arrangement (c), but with matched-pairs presented with delay; and (e) static—both images presented simultaneously as one. We found increased sensitivity in sustained compared to transient stimulus presentations and with synchronous compared to delayed matched-pairs stimuli. For the delayed conditions, sensitivity decreased gradually with longer image durations (>60 ms), prominently for the transient stimulus presentations. We conclude that spatial correlations across-the-symmetry-midline can be integrated over time (∼120 ms), and symmetry mechanisms can tolerate temporal delays between symmetric dot-pairs of up to ∼60 ms.

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.

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.

Luminance-polarity distribution across the symmetry axis affects the electrophysiological response to symmetry

Electrophysiological studies of symmetry have found a difference wave termed the Sustained Posterior Negativity (SPN) related to the presence of symmetry. Yet the extent to which the SPN is modulated by luminance-polarity and colour content is unknown. Here we examine how luminance-polarity distribution across the symmetry axis, grouping by luminance polarity, and the number of colours in the stimuli, modulate the SPN. Stimuli were dot patterns arranged either symmetrically or quasi-randomly. There were several arrangements: 'segregated'-symmetric dots were of one polarity and randomly-positioned dots were of the other; 'unsegregated'-symmetric dots were of both polarities in equal proportions; 'anti-symmetric'-dots were of opposite polarity across the symmetry axis; 'polarity-grouped anti-symmetric'-this is the same as anti-symmetric but with half the pattern of one polarity and the other half of opposite polarity; multi-colour symmetric patterns made of two, three to four colours. We found that the SPN is: (i) reduced by the amount of position-symmetry, (ii) sensitive to luminance-polarity mismatch across the symmetry axis, and (iii) not modulated by the number of colours in the stimuli. Our results show that the sustained nature of the SPN coincides with the late onset of a topographic microstate sensitive to symmetry. These findings emphasise the importance of not only position symmetry, but also luminance polarity matching across the symmetry axis.

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.

Electrophysiological responses to symmetry presented in the left or in the right visual hemifield

Symmetry is a highly salient feature in the visual world, abundant in both man-made and natural objects. In particular, humans find reflectional symmetry most salient. Electrophysiological work on symmetry perception has identified a difference wave known as the Sustained Posterior Negativity (SPN) originating from extrastriate areas. Amplitude is more negative for symmetrical than random patterns, from around 200 msec after stimulus onset. For the first time, we report responses to patterns presented exclusively in one hemifield. Participants were presented with reflection or random dot patterns to the left and right of fixation (3.2°). They judged whether the patterns were light red or dark red in colour. In Experiment 1, the pair always included one symmetrical and one random pattern. In Experiments 2 and 3 we varied the information presented contralaterally. The SPN was generated separately in each hemisphere in response to what was presented in the contralateral visual hemifield (a lateralised SPN). We conclude that a symmetry-sensitive network of extrastriate areas can be activated independently in each cerebral hemisphere.

Perceptual Learning at a Conceptual Level

Humans can learn to abstract and conceptualize the shared visual features defining an object category in object learning. Therefore, learning is generalizable to transformations of familiar objects and even to new objects that differ in other physical properties. In contrast, visual perceptual learning (VPL), improvement in discriminating fine differences of a basic visual feature through training, is commonly regarded as specific and low-level learning because the improvement often disappears when the trained stimulus is simply relocated or rotated in the visual field. Such location and orientation specificity is taken as evidence for neural plasticity in primary visual cortex (V1) or improved readout of V1 signals. However, new training methods have shown complete VPL transfer across stimulus locations and orientations, suggesting the involvement of high-level cognitive processes. Here we report that VPL bears similar properties of object learning. Specifically, we found that orientation discrimination learning is completely transferrable between luminance gratings initially encoded in V1 and bilaterally symmetric dot patterns encoded in higher visual cortex. Similarly, motion direction discrimination learning is transferable between first- and second-order motion signals. These results suggest that VPL can take place at a conceptual level and generalize to stimuli with different physical properties. Our findings thus reconcile perceptual and object learning into a unified framework.

SIGNIFICANCE STATEMENT

Training in object recognition can produce a learning effect that is applicable to new viewing conditions or even to new objects with different physical properties. However, perceptual learning has long been regarded as a low-level form of learning because of its specificity to the trained stimulus conditions. Here we demonstrate with new training tactics that visual perceptual learning is completely transferrable between distinct physical stimuli. This finding indicates that perceptual learning also operates at a conceptual level in a stimulus-invariant manner.

A TMS study on the contribution of visual area V5 to the perception of implied motion in art and its appreciation

Over the last decade, researchers have sought to understand the brain mechanisms involved in the appreciation of art. Previous studies reported an increased activity in sensory processing regions for artworks that participants find more appealing. Here we investigated the intriguing possibility that activity in cortical area V5-a region in the occipital cortex mediating physical and implied motion detection-is related not only to the generation of a sense of motion from visual cues used in artworks, but also to the appreciation of those artworks. Art-naïve participants viewed a series of paintings and quickly judged whether or not the paintings conveyed a sense of motion, and whether or not they liked them. Triple-pulse TMS applied over V5 while viewing the paintings significantly decreased the perceived sense of motion, and also significantly reduced liking of abstract (but not representational) paintings. Our data demonstrate that V5 is involved in extracting motion information even when the objects whose motion is implied are pictorial representations (as opposed to photographs or film frames), and even in the absence of any figurative content. Moreover, our study suggests that, in the case of untrained people, V5 activity plays a causal role in the appreciation of abstract but not of representational art.

Scaling of the extrastriate neural response to symmetry

Neuroimaging work has shown that visual symmetry activates extrastriate brain areas, most consistently the lateral occipital complex (LOC). LOC activation increases with proportion of symmetrical dots (pSymm) in a degraded display. In the current work, we recorded a posterior ERP called the sustained posterior negativity (SPN), which is relatively negative for symmetrical compared to random patterns. We predicted that SPN would also scale with pSymm, because it is probably generated by the LOC. Twenty-four participants viewed dot patterns with different levels of regularity: 0% regularity (full random configuration) 20%, 40%, 60%, 80%, and 100% (full reflection symmetry). Participants judged if the pattern contained "some regularity" or "no regularity". As expected, the SPN amplitude increased with pSymm, while the latency and duration was the same in all conditions. The SPN was independent of the participant's decision, and it was present on some trials where people reported 'no-regularity'. We conclude that the SPN is generated at an intermediate stage of visual processing, probably in the LOC, where perceptual goodness is represented. This comes after initial visual analysis, but before subsequent decision stages, which apply a threshold to the analog LOC response.

The causal role of the occipital face area (OFA) and lateral occipital (LO) cortex in symmetry perception

Symmetry is an important cue in face and object perception. Here we used fMRI-guided transcranial magnetic stimulation (TMS) to shed light on the role of the occipital face area (OFA), a key region in face processing, and the lateral occipital (LO) cortex, a key area in object processing, in symmetry detection. In the first experiment, we applied TMS over the rightOFA, its left homolog (leftOFA), rightLO, and vertex (baseline) while participants were discriminating between symmetric and asymmetric dot patterns. Stimulation of rightOFA and rightLO impaired performance, causally implicating these two regions in detection of symmetry in low-level dot configurations. TMS over rightLO but not rightOFA also significantly impaired detection of nonsymmetric shapes defined by collinear Gabor patches, demonstrating that rightOFA responds to symmetry but not to all cues mediating figure-ground segregation. The second experiment showed a causal role for rightOFA but not rightLO in facial symmetry detection. Overall, our results demonstrate that both the rightOFA and rightLO are sensitive to symmetry in dot patterns, whereas only rightOFA is causally involved in facial symmetry detection.

Right-lateralized alpha desynchronization during regularity discrimination: hemispheric specialization or directed spatial attention?

When actively classifying abstract patterns according to their regularity, alpha desynchronization (ERD) becomes right lateralized over posterior brain areas. This could reflect temporary enhancement of contralateral visual inputs and specifically a shift of attention to the left, or right hemisphere specialization for regularity discrimination. This study tested these competing hypotheses. Twenty-four participants discriminated between dot patterns containing a reflection or a translation. The direction of the transformation, which matched one half onto the other half, was either vertical or horizontal. The strategy of shifting attention to one side of the patterns would not produce lateralized ERD in the horizontal condition. However, right-lateralized ERD was found in all conditions, regardless of orientation. We conclude that right hemisphere networks that incorporate the early posterior regions are specialized for regularity discrimination.

Conditions for view invariance in the neural response to visual symmetry

Symmetry detection is slow when patterns are distorted by perspective, perhaps due to a time-consuming normalization process, or because discrimination relies on remaining weaker regularities in the retinal image. Participants viewed symmetrical or random dot patterns, either in a frontoparallel or slanted plane (±50°). One group performed a color discrimination task, while another performed a regularity discrimination task. We measured a symmetry-related event-related potential (ERP), beginning around 300 ms. During color discrimination, the ERP was reduced for slanted patterns, indexing only the remaining retinal structure. During regularity discrimination, the same ERP was view invariant, and identical for frontoparallel or slanted presentation. We conclude that normalization occurs rapidly during active symmetry discrimination, while symmetry-sensitive networks respond only to regularity in the retinal image when people are attending to other features.

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

Despite the fact that bilateral mirror symmetry is an important characteristic of the visual world, few studies have investigated its neural basis. Here we addressed this issue by investigating whether the object-selective lateral occipital (LO) cortex, a key brain region in object and shape processing, is causally involved in bilateral symmetry detection. Participants were asked to discriminate between symmetric and asymmetric dot patterns, while fMRI-guided repetitive TMS was delivered online over either the left LO, the right LO or two control sites in the occipital cortex. We found that the application of TMS over both right and left LO impaired symmetry judgments, with disruption being greater following right LO than left LO TMS, indicative of right hemisphere lateralization in symmetry processing. TMS over LO bilaterally also affected a visual contour detection task, with no evidence for hemispheric difference in this task. Overall, our results demonstrates that LO bilaterally plays a causal role in symmetry detection possibly due to symmetry acting as a strong cue in Gestalt processes mediating object recognition.

The mirror neuron system in post-stroke rehabilitation

Different treatments for stroke patients have been proposed; among them the mirror therapy and motion imagery lead to functional recovery by providing a cortical reorganization. Up today the basic concepts of the current literature on mirror neurons and the major findings regarding the use of mirror therapy and motor imagery as potential tools to promote reorganization and functional recovery in post-stroke patients. Bibliographic research was conducted based on publications over the past thirteen years written in English in the databases Scielo, Pubmed/MEDLINE, ISI Web of Knowledge. The studies showed how the interaction among vision, proprioception and motor commands promotes the recruitment of mirror neurons, thus providing cortical reorganization and functional recovery of post-stroke patients. We conclude that the experimental advances on Mirror Neurons will bring new rational therapeutic approaches to post-stroke rehabilitation.

Event-related potential study on image-symmetry discrimination in the human brain

The human visual system seems to have a highly perceptual sensitivity to symmetry. However, where and when the discrimination of symmetrical properties begins in the context of visual information processing is largely unclear. This study investigates event-related potential (ERP) patterns in humans when perceiving symmetry-varied complex object images. ERP responses were derived from electroencephalography (EEG) data recorded from eight healthy subjects using 128-channel scalp electrodes. Visual stimulation was provided using gray-scaled photographs of a car with six different viewpoints, hence disrupting the vertical symmetry, where one of the stimuli was intentionally made symmetric by mirroring the image about its center vertical axis. The results show that discrimination of image symmetry is revealed by potential deflection in early ERP components recorded at occipito-temporal sites and can be significantly observed around 220 ms after stimulus onset.

Memory and incidental learning for visual frozen noise sequences

Five experiments explored short-term memory and incidental learning for random visual spatio-temporal sequences. In each experiment, human observers saw samples of 8 Hz temporally-modulated 1D or 2D contrast noise sequences whose members were either uncorrelated across an entire 1-s long stimulus sequence, or comprised two frozen noise sequences that repeated identically between a stimulus' first and second 500 ms halves ("Repeated" noise). Presented with randomly intermixed stimuli of both types, observers judged whether each sequence repeated or not. Additionally, a particular exemplar of Repeated noise (a frozen or "Fixed Repeated" noise) was interspersed multiple times within a block of trials. As previously shown with auditory frozen noise stimuli (Agus, Thorpe, & Pressnitzer, 2010) recognition performance (d') increased with successive presentations of a Fixed Repeated stimulus, and exceeded performance with regular Repeated noise. However, unlike the case with auditory stimuli, learning of random visual stimuli was slow and gradual, rather than fast and abrupt. Reverse correlation revealed that contrasts occupying particular temporal positions within a sequence had disproportionately heavy weight in observers' judgments. A subsequent experiment suggested that this result arose from observers' uncertainty about the temporal mid-point of the noise sequences. Additionally, discrimination performance fell dramatically when a sequence of contrast values was repeated, but in reverse ("mirror image") order. This poor performance with temporal mirror images is strikingly different from vision's exquisite sensitivity to spatial mirror images.

Electrophysiological responses to visuospatial regularity

Humans are quicker to detect reflectional than rotational or translational symmetry, despite the fact that these patterns are equally regular. We were interested in the neural correlates of these perceptual effects. Participants viewed random, reflection, rotation, and translation patterns while we recorded EEG from the scalp. Half the participants classified the pattern regularity overtly, the other half did not explicitly attend to pattern regularity but reported rare oddball trials, where two squares were embedded among the dots. The amplitude of a symmetry-related ERP known as the sustained posterior negativity was most pronounced for reflection, then rotation and translation. We suggest that reflectional symmetry, despite its biological significance, may not be processed by unique visual mechanisms, but instead it could be a preferred stimulus for a more general regularity-sensitive network in the extrastriate visual cortex.

Combined neurostimulation and neuroimaging in cognitive neuroscience: past, present, and future

Modern neurostimulation approaches in humans provide controlled inputs into the operations of cortical regions, with highly specific behavioral consequences. This enables causal structure–function inferences, and in combination with neuroimaging, has provided novel insights into the basic mechanisms of action of neurostimulation on distributed networks. For example, more recent work has established the capacity of transcranial magnetic stimulation (TMS) to probe causal interregional influences, and their interaction with cognitive state changes. Combinations of neurostimulation and neuroimaging now face the challenge of integrating the known physiological effects of neurostimulation with theoretical and biological models of cognition, for example, when theoretical stalemates between opposing cognitive theories need to be resolved. This will be driven by novel developments, including biologically informed computational network analyses for predicting the impact of neurostimulation on brain networks, as well as novel neuroimaging and neurostimulation techniques. Such future developments may offer an expanded set of tools with which to investigate structure–function relationships, and to formulate and reconceptualize testable hypotheses about complex neural network interactions and their causal roles in cognition.