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Experiment 1 method and stimuli A) Trial structure in normal and oddball trials. All trials involved a sequence of three 500 ms presentations, separated by 200 ms gaps. Participants responded by pressing buttons for "All patterns" (all three presentations involved black and white patterns, as in the top row) or "Blank in the middle" (the second item in the sequence was blank, as in the lower row). B) Different triplets used. 24 participants were presented with complex patterns (left column) and another 24 were presented with simple patterns (right column). Individual patterns shown here are examples, in the real experiment each trial used different patterns. The coloured frames are matched with the colour of ERP waves in subsequent figures.
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![Example Sustained Posterior Negativity from Palumbo et al. [6]
A) Grand...](publication/353151678/figure/fig1/AS:11431281274649163@1725039469003/Example-Sustained-Posterior-Negativity-from-Palumbo-et-al-6-A-Grand-average-ERP-waves_Q320.jpg)
An Event Related Potential (ERP) component called the Sustained Posterior Negativity (SPN) is generated by regular visual patterns (e.g. vertical reflectional symmetry, horizontal reflectional symmetry or rotational symmetry). Behavioural studies suggest symmetry becomes increasingly salient when the exemplars update rapidly. In line with this, Exp...
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... These observations suggest RefOrtho distractors would not be processed at all, and therefore Ref (Rand) and Ref(RefOrtho) should produce the same SPN. However, SPN priming studies indicate neural overlap between horizontal and vertical reflection (Makin, Tyson-Carr, Derpsch, Rampone, & Bertamini, 2021), and Treder, van der Vloed, and van der Helm (2011) found comparable behavioral results. These observations suggest that task irrelevant RefOrtho may be processed automatically, and therefore Ref(RefOtho) would produce a larger SPN that Ref(Rand). ...
Previous work has found that feature attention can modulate electrophysiological responses to visual symmetry. In the current study, participants observed spatially overlapping clouds of black and white dots. They discriminated vertical symmetry from asymmetry in the target dots (e.g., black or white) and ignored the regularity of the distractor dots (e.g., white or black). We measured an electroencephalography component called the sustained posterior negativity (SPN), which is known to be generated by visual symmetry. There were five conditions with different combinations of target and distractor regularity. As well as replicating previous results, we found that an orthogonal axes of reflection in the distractor dots had no effect on SPN amplitude. We conclude that the visual system can processes reflectional symmetry in independent axis-orientation specific channels.
... The SPN is very robust to experimental manipulations of task, but not completely indifferent to them [149,[157][158][159]. Makin et al. [141] found that the parametric SPN response was comparable across five tasks but selectively enhanced when participants attended to regularity ( Figure 4). ...
... SPN priming does not transfer between hemispheres, or between reflections with unpredictably changing orientations. However, it does transfer between horizontal and vertical reflection, and between vertical reflection and 90-degree rotation [159]. Furthermore, SPN priming also transfers from black to white exemplars [184], and between symmetry and Glass patterns [185]. ...
This review of symmetry perception has six parts. Psychophysical studies have investigated symmetry perception for over 100 years (part 1). Neuroscientific studies on symmetry perception have accumulated in the last 20 years. Functional MRI and EEG experiments have conclusively shown that regular visual arrangements, such as reflectional symmetry, Glass patterns, and the 17 wallpaper groups all activate the extrastriate visual cortex. This activation generates an event-related potential (ERP) called sustained posterior negativity (SPN). SPN amplitude scales with the degree of regularity in the display, and the SPN is generated whether participants attend to symmetry or not (part 2). It is likely that some forms of symmetry are detected automatically, unconsciously, and pre-attentively (part 3). It might be that the brain is hardwired to detect reflectional symmetry (part 4), and this could contribute to its aesthetic appeal (part 5). Visual symmetry and fractal geometry are prominent in hallucinations induced by the psychedelic drug N,N-dimethyltryptamine (DMT), and visual flicker (part 6). Integrating what we know about symmetry processing with features of induced hallucinations is a new frontier in neuroscience. We propose that the extrastriate cortex can generate aesthetically fascinating symmetrical representations spontaneously, in the absence of external symmetrical stimuli.
... This effect in an ERP signal is also observed when a 2D rotational-symmetry is seen (Makin et al., 2012(Makin et al., , 2013. This ERP effect of 2D rotational-symmetry is weaker than the effect of 2D mirror-symmetry with a single symmetry axis when the rotational-symmetry is with 2-folds (a rotation of 180 degrees, Makin et al., 2013;Makin et al., 2012) but the effect of the rotational-symmetry can be stronger when the rotational-symmetry is with 4-folds (a rotation of 90 degrees, Makin et al., 2021). The importance of the extrastriate cortex for the perception of 2D symmetry has also been suggested in studies using fMRI (Keefe et al., 2018;Sasaki et al., 2005;Tyler et al., 2005; see also Kohler et al., 2016) and in studies using TMS Cattaneo et al., 2011). ...
... It simply means that the visual system is much less sensitive to 3D rotational-symmetry than to 3D mirrorsymmetry. Note that this reduced sensitivity can be compensated if the number of folds of the 3D rotational-symmetry is sufficiently large (Sawada & Zaidi, 2018; see also Makin et al., 2021) or if there are some other types of 3D information. These comparisons, between our psychophysical results and the geometrical properties of 3D symmetry, and the differences observed between the two types of 3D symmetry, suggest that the human visual system has evolved to favour the perception of 3D mirrorsymmetry. ...
Detecting 3D symmetry is important for the human visual system because many objects in our everyday life are 3D symmetrical. Many are 3D mirror-symmetrical and others are 3D rotational-symmetrical. But note that their retinal images are 2D symmetrical only in degenerate views. It has been suggested that a human observer can detect 3D mirror-symmetry even from a 2D retinal image of a 3D mirror-symmetrical pair of contours. There are model-based invariants of the 3D mirror-symmetrical pair of contours in the retinal image and there are additional invariant features when the contours are individually planar. There are also model-based invariants of a 3D rotational-symmetrical pair of contours. These invariant features of 3D mirror-symmetry and rotational-symmetry are analogous to one another but the features of 3D rotational-symmetry are computationally more difficult than the features of 3D mirror-symmetry. Experiment 1 showed that only 3D mirror-symmetry could be detected reliably while the detection of 3D rotational-symmetry was close to chance-level. Experiment 2 showed that the detection of 3D mirror-symmetry is partly based on the model-based invariants of 3D mirror-symmetry and the planarity of the contours. These results show that the visual system has evolved to favour the perception of 3D mirror-symmetry.
... Utilising this priming paradigm, SPN priming effects have been reported. Makin et al. [44] recently demonstrated that the repeated presentation of reflectional symmetry leads to an increase in SPN amplitude. They termed this effect SPN priming. ...
... Each comprised 100 dots. Temporal parameters were the same as that which was used in the work by Makin et al. [44]. In each trial, we presented three dot patterns. ...
... . Three windows were chosen for statistical analysis of SPN priming: the first window = 250-600 ms, the second window = 950-1300 ms, and the third window = 1650-2000 ms. These windows were defined a priori based on previous SPN priming research [24,44]. These windows were selected in order to capture the primary time window where the SPN was present following each stimulus presentation. ...
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.
... The Sustained Posterior Negativity (SPN) is a response occurring approximately 300 ms after stimulus onset [15,16]. Building on earlier fMRI and TMS studies, EEG sourcelocalisation techniques have shown that the SPN is generated in the extra-striate cortex, generally in the vicinity of the LOC [17,18]. ...
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.
... To avoid confusion, we should mention a phenomenon called SPN priming (i.e. increase in SPN amplitude with presentation of symmetrical images in succession [86][87][88] ). This form of past-history effect reflects interdependency of responses to fully-visible image symmetries. ...
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.
Symmetry is a salient visual feature in the natural world, yet the perception of symmetry may be influenced by how natural lighting conditions (e.g., shading) fall on the object relative to its symmetry axis. Here, we investigate how symmetry detection may interact with luminance polarity grouping, and whether this modulates neural responses to symmetry, as evidenced by the Sustained Posterior Negativity (SPN) component of Event-Related Potentials (ERPs). Stimuli were dot patterns arranged either symmetrically (reflection, rotation, translation) or quasi-randomly, and by luminance polarity about a grouping axis (i.e., black dots on one side and white dots on the other). We varied the relative angular separation between the symmetry and polarity-grouping axes: 0, 30, 60, 90 deg. Participants performed a two interval-forced-choice (2IFC) task indicating which interval contained the symmetrical pattern. We found that accuracy for the 0 deg polarity-grouped condition was higher compared to the single-polarity condition for rotation and translation (but not reflection symmetry), and higher than all other angular difference (30, 60, 90) conditions for all symmetry types. The SPN was found to be separated topographically into an early and late component, with the early SPN being sensitive to luminance polarity grouping at parietal-occipital electrodes, and the late SPN sensitive to symmetry over central electrodes. The increase in relative angular differences between luminance polarity and symmetry axes highlighted changes between cardinal (0, 90 deg) and other (30, 60 deg) angles. Critically, we found a polarity-grouping effect in the SPN time window for noise only patterns, which was related to symmetry type, suggesting a task/ symmetry pattern influence on SPN processes. We conclude that luminance polarity grouping can facilitate symmetry perception when symmetry is not readily salient, as evidenced by polarity sensitivity of early SPN, yet it can also inhibit neural and behavioral responses when luminance polarity and symmetry axes are not aligned.
