Symmetry is what we see at a glance; based on the fact that there is no reason for any difference. And based also on the face of man; Whence it happens that symmetry is only wanted in breadth, not in height or depth. (Pascal, 2003)

Verse

Verse Tyger, Tyger burning bright, In the forests of the night: What immortal hand or eye, Dare frame thy fearful symmetry? (Blake, 1969)

Symmetry is often considered a visual primitive by scholars such as the French philosopher Blaise Pascal, who suggested that there is no need to be particularly attentive to perceive it (Pascal, 2003). Symmetry is also important for Gestalt psychology, which formulated one of the laws of perception in their framework regarding symmetry (Wagemans et al., 2012).

What Is Symmetry and Where It Can Be Found

Symmetry is a natural occurring phenomenon in living organisms, structures as well as in various human artefacts or creations. Sometimes symmetry occurs out of necessity or by design, sometimes it appears as having a solely aesthetic aim. Naturally occurring symmetry fascinates humans and other species, for example in the cases of flowers or butterflies, identical twins, the reflection of a landscape in a lake, symmetrical patterns in tiles, rhymes, or music.

When it occurs in landscapes such as Mount Fuji or in crystal structures it seems to bare a mystical drive expressed by William Blake in his poem The Tyger. Reading a poem, we observe symmetry in how the text graphically appears on page: number of verses, repetitions of letters, words, but we perceive its symmetry even stronger when we hear it. Listening to how sounds are arranged in time through rhyme-generating rhythms, we observe accents and find sense in the poem built of words by the poet through his nuanced play between symmetry and asymmetry.

Visual symmetry is probably the most observable for humans, but other forms of symmetry exist, such as symmetry in sound, speech and in any type of structure. In movement, symmetry is also important as the complexity of it generates synchrony, which might be seen as a type of symmetry that encompasses the concept of time. Synchrony exerts fascination when it occurs in nature where it is present in waves, birds dancing and swarming together and in human activities such as dance, figure skating, majorettes’ baton twirling, or synchronised swimming, which became an Olympic Sport in 1984 under the name of Artistic Swimming. Positive states are a common feature in movement mirroring (Papasteri et al., 2020; Tomescu et al., 2022) and the perception of symmetrical shapes (Winkielman & Cacioppo, 2001).

In general, all oscillatory behaviours are symmetrical or can be described based on their similarity towards symmetry. In general terms we can think of symmetry as any kind of repetition of a form, pattern, behaviour and even state, such as in human relationships where it is suggested both on an individual and group level in the idea of symmetric or unsymmetric relations. In such a context, a symmetrical individual relationship could be the example of a reciprocal romantic relationship and an example of an asymmetrical individual and collective relationship is that of exploitation, such as slavery. In both cases it is similarly striking that symmetry would be a preferable social relationship as opposed to an asymmetrical one.

This overview will focus on the status of research on symmetry in neuroaesthetics giving special attention to the contributions from the fields of visual perception and visual arts. We will outline the importance and relevance of research on symmetry in neuroaesthetics and adjacent fields, its methods, main results and future research directions.

Box 1: Gestalt Psychology on Symmetry

Gestalt Psychology is a school of psychology that originated in Austria and Germany at the beginning of the XX century, which stated that psychological phenomena are not to be researched through their individual components but as a whole. The central aspect of their school of thought is that in perception, and therefore psychological process, one cannot simply add elements, as the products resulting are more than the constituent parts. Such an example is given in their proposed law on symmetry which states that “when two symmetrical elements are unconnected the mind perceptually connects them to form a coherent shape”. A simple example would be when one plays with parentheses and other typographical symbols:

(((…….)))(*∨*) (°∨°) (▰˘◡˘▰) ◎[▪‿▪]◎ ≧◡≦ ((.).*.∨..)▰ (≦) (*∨ (°°V) ◡ (▰˘ ∨ ≧◡ ◎[▪[▪‿≧◎

As you can observe, you will tend to see the sets of parentheses as whole images, especially if they are arranged to evoke a face and not just as individual signs, as you would tend to perceive them were they not symmetrically grouped.

Box 2: What Is a Primitive?

In neuroscience, researchers refer to primitives as building blocks of more complex processing. Primitives are generally considered shared but are not necessarily the same across taxa in the phylogenetic tree. Visual processing is based on first level processing of different components such as contrast, line orientation, edges, or movement—properties often referred to as primitives. Some researchers also put forward symmetry as a primitive of visual processing (Olivers & Van Der Helm, 1998).

Primitives are present in every perceptual modality. It is proposed that planning movement is based on motor primitives. The concept is used also in biolinguistics, in semantic and concept research. More recently, researchers such as David J. Anderson have suggested that primitives count as units in emotional processing. Primitives are also important for fields like computer vision and artificial neural networks in which they are used as basic properties in processing (Anderson & Adolphs, 2014).

Symmetry in Nature

Symmetry can be found in nature at all levels of organisation in living forms and chemical structures. One major example of perfect symmetry is the structure of benzene, an organic chemical compound with six carbon and six hydrogen atoms (C6H6). The carbon atoms are placed in a planar regular hexagonal structure with sixfold rotational symmetry. Crystalline solids like quartz, sugar or diamonds have symmetrical structures based on translation, reflection and inversion. Snowflakes also have sixfold rotational symmetry with complex patterns emerging on the hexagonal structure based on the exact temperature and conditions of humidity in the moment each water molecule crystallises. Johannes Kepler was one of the first to ask: “There must be a cause why snow has the shape of a six-cornered starlet”. Kepler wrote in De nive sexangular: “It cannot be chance. Why always six?” (Kepler, 1611). Snowflake symmetry was documented in China as early as the second century BC (Ball, 2011) (Fig. 1).

Fig. 1
9 photos of forms of symmetry in 3 rows, a, b, and c. Images 1 and 3 of row a, a floral pattern. Image 1 of row c displays the same pattern as wallpaper on the ceiling. 2 a, a butterfly. 1 and 3 b, 2 lizards. 2 b, hibiscus flower. 2 and 3 c, bags with colored liquid arranged as wall hanging.

Various types of symmetry in nature and human-made objects. A and C first rows floral pattern for fabric designed by William Morris, one of the most important figures of Arts and Crafts movement. We can observe radial symmetry in the flowers, bilateral symmetry in the butterfly and lizard but also repetition symmetry in lamp decoration as well as translational symmetry and mirror symmetry in the arrangement of photos in the collage. In row C between the two images on the right from the BioArt installation Watch me we can observe scaled symmetry. Using building blocks generated in repetition is highly efficient since all steps are clearly planned (Berceanu & Comănescu, 2022)

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Symmetry is important for living organisms like plants and animals but also bacteria, viruses and fungi, with most multicellular organisms exhibiting a form of symmetry. Biological symmetry is not perfect but apparent. Most often in organisms one speaks about radial symmetry, or bilateral symmetry which can be seen as radial symmetry on one axis. Radial symmetry is present in plants (for instance cacti) and it can have a fixed number of symmetry axes (up to 12 for icosahedral symmetry) or numerous axes in the case of spherical symmetry of green algae volvox. In plants there can be several types of symmetry in the same plant: the root and the branches of a tree have significant radial symmetry; leaves have apparent bilateral symmetry, while most flowers have high rotational symmetry. Some of these types of symmetry are linked to a growth axis of symmetry (such as branches around the tree trunk or leaves on the stem). In flower symmetry a special class of genes, CYCLOIDEA, has a prominent role and it is suggested that flower symmetry develops selectively based on pollinator preferences.

Main Types of Symmetry

Mirror symmetry: identical parts across an axis, generally vertical or horizontal. This type of symmetry is sometimes referred to as reflectional symmetry.

Rotational symmetry: an object or pattern is repetitive in a plane in relationship with a fixed point. This type of symmetry can be found in a plane as well as in three dimensions.

Translational symmetry: Objects are identical after a translation (all points are identical in two or multiple objects and all are separated by identical distances).

Besides those main types of symmetry there is also curved symmetry, across a regularly curved axis, scaled symmetry when objects are proportionally reduced as in fractals, helical symmetry, or glide symmetry.

In animals, it is proposed that symmetry is connected mainly to locomotory aspects, and it is important to note that while the exterior aspect of most animals has bilateral symmetry it is not the case for most internal organs. While an animal has a symmetrical number of legs and some of the internal organs, such as kidneys or lungs, they have just one liver, digestive tract, or heart. We find a complex interplay between symmetry and asymmetry in the organisation of the brain, where we can find a strong bilateral organisation, but also important asymmetries at anatomical and functional levels, such as speech. Mammals have complex types of symmetries and asymmetries, with 152 genes involved in the determination of mouse bilateral symmetry, according to MGI (Mouse Genome Informatics group).

If an organism presents a type of symmetry, it is essential for the organism’s survival and health. Symmetric cellular division, when one cell divides into two identical ones, provides the building blocks for tissular development, which is a strong example of using identical units for a large structure, a principle widely used in human activities. Detecting symmetry is important for animals, as it can provide information about the health of conspecifics or the presence of another animal.

Symmetry in Human Activities

It is suggested that we can identify the use of symmetry in human artefacts as early as 1.4 million years ago. The oldest stone tools date back 2.5 million years, biface stone tools appeared around 1.4 million years ago, probably under the hands of homo erectus. Although symmetry in those tools is very rudimentary it is proposed that it is voluntary and that special sets of cognitive and motor skills are needed, especially when it comes to Olduvai discoid artefacts. Highly symmetrical tools appeared 500,000 years ago, and they have congruent dimensions and present bi and three-dimensional symmetry. It is put forward that Neanderthal and Palaeolithic man wore necklaces made from similar parts, which could have strong symmetrical aspects. Diverse populations across the globe, such as the Urubu Ka’apor in Amazonia, the African Massai, the Huli in Papua New Guinea or the Ainu in Japan decorate their bodies with jewellery, paint, tattoos scars and clothes with strong symmetrical patterns. Folk costumes from central European and Balkan regions present striking intricate symmetrical patterns in most of the pieces worn, with special care being noticeable in some of the more elaborate items, such as in the Romanian peasant blouse, “ie”.

Symmetry is very present in human culture and relates to the repetition of patterns, sometimes in mechanical production. In architecture, symmetry can arise at a granular level, from the materials used, due to physics or due to architectural conception or world view. Romanic or Gothic arches are made of similar or identical stone and use mirroring symmetry to provide maximum resistance with the smallest quantity of material, but both styles abound in symmetry at levels where there would be no structural need. We can find cathedrals with a strong bilateral symmetry plan, as is the case of Notre-Dame of Paris, but in other cases, such as the Chartres Cathedral, the two steeples are very different from each other.

We can find amazing examples of highly planned symmetry in architecture around the world, starting with the Pyramids, Greek Temples, Pagodas, Indian temples or the Taj Mahal. Symmetry represents power, harmony and authority. Although there are also contemporary examples of architecture with strong symmetry, such as the Sagrada Familia or, more recently, The House of the Parliament in Bucharest, they tend to be marginal.

Symmetry is very present in decorative arts where repetition of patterns often plays central roles, with geometrical mosaics of great complexity, such as the Alhambra or Samarkand mosques. Tapestry, carpets, woodworking, pottery, porcelain, glass, or stained glass provide techniques for the use of symmetrical shapes and motives. The arts and crafts movement brought an intricate symmetry, patterns inspired by nature, using it as a solution for art in serial production, which is more and more prevalent in the industrial world.

In painting, highly symmetrical compositions are mostly found with a symmetrical balance, in which the midline of the painting is an axis of symmetry, but generally the sides are balanced and not identical. This type of symmetry, which is not exact, but balances proportions, is very much in line with the meaning of the Greek word summetria, with measure or well-proportioned σῠν- (sun-, ‘with’) + μέτρον (métron, ‘measure’) + -ος. Admittedly, examples of paintings using forms of perfect symmetry do exist, with examples in the work of Escher, Magritte or Delvaux, generally producing an eerie feeling of an alternate reality. In Renaissance paintings, but not only, symmetry appears from the relations between architectural backgrounds, occasionally highly symmetric, found in masterpieces such as The Last Supper of Leonardo, Rafael’s School of Athens or Perugino and Christ Giving the Keys of the Kingdom to St. Peter. Compositions based on symmetry are strongly represented in Christian religious scenes where the crucified Christ represents a strong axis of symmetry for a singular painting, or a set of paintings forming a unitary composition in the case of polyptychs.

Symmetry is also linked to the perception of action and time. In novels and scripts, we can find strong symmetries between the start and the ending of a plot, notably with Chekhov, who plays a lot with “asymmetrical symmetries”. Gustav Freytag’s pyramid of dramatic structure, or the Hero’s Journey, developed by Joseph Campbell, propose symmetric ascents and descents into action. Symmetry is also perceived across the domain of time. Mircea Eliade brought forward a symmetrical perspective on time passage through his proposal of the myth of the eternal return implicit or explicit actions instate symmetry by turning the past into a mythical golden age to be atained in the present. At a more profane level, symmetry in behaviour in the time axis is very present, humans repeating identical behaviours over time. Sometimes repetition of action can be seen as a coronation and culminant accomplishment, sometimes it can be seen as stereotypical such as in Ionesco’s Bald Soprano and other similar works, where symmetry is a sign of general lack of meaning, denoting mere mechanical repetition.

Symmetrical movements are present in nature, especially in swarming and mating rituals culminating in complex synchronous choreographies as the ones performed by Western or Clark’s Grebes. Dance often relies on synchrony creating complex moving images formed by even hundreds of dancers moving in synchrony. Examples range from old folk dances in tribes such as Ainu but also in ballet, hip hop, musical and so on. Army march is performed also in synchrony. Mirroring and imitation are at the heart of learning. Mirroring of movements constructing therefore symmetrical patterns were found to decrease self-awareness, discontinuity of mind with corelations in dynamics of EEG microstate EEG generating positive affective states and increasing salivary oxytocin in women (Papasteri et al., 2020; Tomescu et al., 2022) (Fig. 2).

Fig. 2
A photograph of women engaged in a mirror task, while facing each other. On the left, a woman stands and raises her left hand, while on the right, another woman stands and raises her right hand.

Performing the mirroring task

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Performing the mirroring task. The trainer shows simple movements the participant subject follows the movements synchronously generating symmetrical movements (Fig. 3).

Fig. 3
2 violin graphs illustrate data points plotted against various categories, including discontinuity, others, self, planning, sleepiness, comfort, somatic awareness, health, visual, and verbal, for both pre and post-conditions in the titled categories of symmetrical movement mirroring and control.

Effects of performing synchronous symmetrical movements in a dyad, reproduced with modifications and permissions from (Tomescu et al., 2022). After the mirroring task, subjects reported less fragmented thoughts, fewer thoughts about themselves and their bodies. The study reported significant changes induced by mirroring in four different factor categories: discontinuity of mind, self-related thoughts, sleepiness and somatic awareness. No significant changes were detected after the CTRL conditions

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The ARSQ questionnaire quantifies resting state mind-wandering fluctuations in four dimensions: dynamics of thoughts: discontinuity of mind,; content of thoughts: other-related thoughts, or thinking about other people, self-related thoughts, or thinking primarily about own person, planning, or thoughts about the future, and health-related thoughts about general well-being or pain; physiological state: sleepiness and comfort quantify the level of relaxation during resting state, and somatic awareness reports thoughts about interoceptive bodily states such as breathing; modality of mind-wandering: visual imagery versus verbal, or thoughts formulated in words (Diaz et al., 2014).

Symmetry as Subject of Analyses and Research

Symmetry is an object of interest for both top-down and bottom-up approaches to aesthetics. Immanuel Kant considered that: “All stiff regularity (such as borders on mathematical regularity) is inherently repugnant to taste”, but he continues “We do not grow to hate the very sight of it” (Kant, 2007). Furthermore, he describes the aesthetic pleasure of observing an ordered pepper garden in Sumatra in opposition to a chaotic repetitive jungle in nature. While he thought of symmetry as dull (Gombrich, 1988), art historian Ernest Gombrich also proposed that it relates to fixity, while asymmetry would be connected to the representation of motion and dynamism (Gombrich, 1984). He also underlined the importance of the opposition between order and chaos alongside a universal human impulse to seek order and rhythm in space and time (Gombrich, 1984).

Why Is Important to Study Symmetry for Neuroaesthetics?

Symmetry has been present in multiple forms in artistic productions since its beginnings all over the globe. Processing symmetry is important for both humans and other animals. Different levels of preference for symmetry have evolved in the phylogenetic tree, providing opportunities of research at genetic, functional, and subjective levels. The study of the impact of symmetry on aesthetic experience provides the unique opportunity of developing a holistic perspective on aesthetic experience grounded in the evolution from simple to complex processing. Symmetry is a measurable variable, making it an ideal subject of study in an experimental field. The special position that the perception of symmetry has in the phylogenetic tree might offer a window for understanding the development of aesthetic experience, hence building a unified theory of aesthetics.

Scientific Approaches in the Study of Symmetry Preference and Neural Processing of Symmetry

The first observations on symmetry perception came from the Austrian philosopher and physicist Ernst Mach, known for his contributions in the domains of shockwaves and the speed of sound, as well as the Mach-bands, the illusion provided to the human eye by exaggeration of contrast at edges of similar grey tones. He observed that vertical symmetry is more salient than other types and proposed that this relates to the overall symmetrical left–right organisation of sight and its equivalent in the central nervous system (Mach, 1959).

Psychology and empirical aesthetics were concerned with symmetry work very early on, as early as the beginning of the twentieth century, providing important data regarding aesthetic preference for symmetry. In her PhD work later published in the influential book “Psychology of Beauty”, psychologist and suffragette Ethel Puffer observed in her study that symmetry composition is one of the preferred strategies for generating a pleasant composition, and these results were also confirmed more than one hundred years later (Hübner & Thömmes, 2019). Besides empirical aesthetics and neuroaesthetics, other disciplines also focus on symmetry and are very informative for these fields, even if their focus is on the aesthetic experience. Important knowledge as to how brains process symmetry comes from research in visual processing or computational neuroscience with artificial neural networks that provide mathematical tools for the investigation of brain function. Extensive research was done in all those fields on the recurrence of special issues of volumes (C. Tyler, 2003; C. W. Tyler, 1995) on these topics, expressing its importance, and the journal Symmetry covering its occurrence in all aspects of natural sciences, including in the field of aesthetics.

Although not universally observed, symmetry preference appears as general in cross-cultural studies along other features of images such as regularity, contrast and curvature, while the complexity effect vary culturally (Che et al., 2018). Common preference for symmetrical patterns was observed among subjects in the USA and Nigeria and common higher beauty ratings for symmetrical patterns were reported for British and Egyptian subjects (Che et al., 2018).

The preference for symmetry seems to be dependent on several factors, starting with the type of symmetry, of which lateral and radial symmetry are preferred. Preference for symmetry was found in the case of shapes and faces, but in the same study no perfect symmetry images were preferred for natural landscapes (Bertamini, Rampone, Makin, et al., 2019). Level of expertise is also diversifying the effect of symmetry on preference, with experts in the arts reported to prefer asymmetrical compositions (Gartus et al., 2020; Mcmanus, 2005).

Gender differences were also reported relating to symmetry, which was significant in the preferences of male subjects (n = 40) but not of women (n = 40) in a study regarding preference on abstract and real-world objects (Shepherd & Bar, 2011). The women’s preference for symmetrical male faces was reported (Gangestad & Thornhill, 1998) with hormonal levels and menstrual cycle variations moderating the preference of women for the scent of shirts worn by symmetrical men (Gangestad & Thornhill, 1998; Garver-Apgar et al., 2008; Little et al., 2007).

It was widely proposed that preference for symmetry is an evolutionary adaptation facilitating the selection of healthy mates based on the link between symmetry and health at genetic and infectious levels with the objective of ensuring stronger health for offspring. Preference for partner symmetry is a strong argument for this theory, but a lot of researchers consider it insufficient to explain aesthetic preference, a process of a higher order.

An important proposal explaining the preference for symmetry is based on the concept of fluency. Perceptual fluency was defined as the subjective ease with which an incoming stimulus can be processed (Reber, 2002). Fluency of perception is proposed to have a strong influence on aesthetic preferences. Studies show that the more fluent an object is perceived as being, the more positive is the aesthetic response, with support coming from subjective rating but also confirmed by physiological data (Reber & Zupanek, 2002; Winkielman & Cacioppo, 2001). It is easier to process symmetrical than asymmetrical stimuli, with vertical symmetry detected faster than horizontal or rotational symmetry, as reaction time studies showed (Reber, 2002).

Ramachandran and Hirstein ask the question “Why is it useful to detect symmetry?”. Stating that a principle in neuroaesthetics should be clearly grounded on clear answers to the questions: “what?” “how?” and “why?”. Their proposal for the salience of symmetry is the need to detect prey, predators and mates from the surrounding environment, all of them having symmetrical bodies, as opposed to an asymmetrical surrounding. This view is in line with the grouping property proposed by Gestalt psychologists to symmetry. Preference for abstract symmetry is proposed to be an evolutionary consequence of the peak shift phenomenon (Ramachandran & Seckel, 2011).

Peak Shift

This was originally observed in seagull chicks by biologist Nikolaas Tinbergen, whose work was influential in the development of ethology thanks to his discoveries on social behaviour patterns. He studied pecking in seagull chicks, which have a strong pecking behaviour when they see the mother’s beak. Seagull’s beaks have a red spot, which is the centre point of chicken pecking, producing the regurgitation of food from the mother into the chick’s beak. Tinberg observed that not only was pecking behaviour present when chicks were presented with beaks alone, but it would increase in intensity if seagull chicks are presented a stick with three stripes. Ramachandran proposes that this would be an “ultrabeak” which provokes a stronger response in the chicken due to more fluent processing of the stimulus due to the overlapping of its simple futures on the minimum requirements for recognition of a possible feeding source. He proposes that peak shift is one of the neural processing mechanisms in the development of the production and understanding of the abstract art. In his view, artists are developing works which, based on peak shifts, produce stronger responses than the real stimuli. He also proposes that peak shift is central to aesthetic experience (Ramachandran & Seckel, 2011). Peak shift could be a possible explanation for the fascination exerted by mandalas or kaleidoscopes, a display of symmetry never found in nature, which produces strong positive states in the viewer.

Symmetry is processed very fast by humans, being detected even during exposures to stimuli of under 50ms. Although there is still a lot to be understood on the neural processing of symmetry, some aspects are considered well established in the field. Neurofunctional techniques are concentrated on establishing the relationship between the observed behavioural salience of symmetrical stimuli and its neural processing.

Through fMRI studies the location of the neural response to symmetry is widespread through the visual cortex, within the ventral areas of higher visual perception V3A, V4, V7 and lateral occipital complex (LOC). The level of BOLD fMRI signal in these regions correlated with the level of symmetry in the stimuli (Sasaki et al., 2005). This network responds to symmetry automatically, without the need for the subject’s attention. No single area for symmetry detection was identified as being dedicated to symmetry processing, but it is an extended network responding differently than from a quantitative level for different aspects of symmetry, such as texture or noise. For the observed aspects it is proposed that symmetry detection is part of the basic perception of shapes and forms (Bertamini et al., 2018).

The neural substrate for processing symmetry, beauty and complexity was targeted in an fMRI study (Jacobsen et al., 2006). Participants in the experiment were asked to judge a set of specially designed levels of beauty, symmetry and complexity. Symmetrical patterns were reported as more beautiful than nonsymmetrical ones. Some areas responded specifically to a task of aesthetic judgement: the medial frontal cortex, the precuneus and the ventral prefrontal cortex, others were involved in both tasks of judgments of symmetry and beauty: the left parietal cortex (the intraparietal sulcus) was engaged by both symmetry and beauty. Specific to symmetry processing, authors reported parietal and premotor areas involved in spatial processing. The beauty and complexity of the images evoked activity in the orbito-frontal cortex, an area confirmed to be implicated in the processing of beauty (Ishizu & Zeki, 2011).

Visual processing is performed in the visual cortex, bilaterally from the stream of information originating in the eyes, arriving there through the lateral geniculate nucleus of the thalamus. The visual cortex is situated in the occipital lobe. The visual cortex performs different tasks for distinct aspects of visual information in specialized areas. Visual area one (V1) is the site of first level processing. V1 has a retinotopic organization, with each neuron responding to a specific site within the retina, with neurons in the upper side responding strongly to the lower half of the retina (below the centre), and the lower side to the upper half of visual field. V1 creates a map of edges, generating a salience indication for other more specialized processing areas. From V1, two pathways divide, the ventral area which processes meaning of visual stimuli and the dorsal area which processes context of visual stimuli and integration with motor control. V2 has also ventral and dorsal areas and it is the visual association area. V3 covers motion and motion patterns. Aspects of visual processing are very specialized for some of the subregions such as middle temporal visual area for moving stimuli, V8 for colours (Goodale, 2004). The Lateral Occipital Complex (LOC) is an functional area critical for shape perception and is composed of the lateral occipital cortex (LO) and the posterior fusiform gyrus (pFs) (Margalit et al., 2016).

Some of the most interesting data on neural processing of symmetry was produced in the Bertamini Lab, which showed that symmetry not only sets a sustained posterior negativity in the occipital area, but that this is independent of instructions to detect symmetry of other features of the image and that it lasted for one second after the offset of the stimulus. Sustained posterior negativity (SPN) is a sustained negative amplitude at posterior electrodes which lasts for hundreds of milliseconds when comparing symmetrical patterns to asymmetrical patterns (Bertamini, Rampone, Oulton, et al., 2019). During the pandemic, the lab made a lot of its resources available online, including all SPN datasets 6674 SPNs from 2215 participants in the compiled a Complete Liverpool SPN catalogue, on open science framework and a data visualisation app (https://www.bertamini.org/lab/SPNcatalogue.html).

On the side of neuroprocessing, one important question to be answered in the future is if symmetry is processed at the level of single neurons or if it is coded by a population. One study recording single units in macaque monkeys found that neurons in the inferior temporal area showed whole-object responses as the sum of responses to the object’s parts, regardless of symmetry. The only defining characteristic of symmetric objects observed was a more distinctive response when compared to asymmetric objects because of neurons preferring the same part across locations within an object (Pramod & Arun, 2018). The author proposes that the neural response to symmetry is driven by generic computations at the level of single neurons (Pramod & Arun, 2018). Even if IT was not reported for humans, the results are nonetheless informative for human perception of symmetry since common aspects and differences were reported. V3A, V4d and IT were found to be activated by symmetrical stimuli combined with the human data from the same study, suggesting that neural mechanisms tuned to visual symmetry are present in nonhuman primates, although they are less developed than in humans (Sasaki et al., 2005).

At many levels of the phylogenetic tree, animals exhibit preference for symmetrical stimuli in mating and non-mating contexts. Pigeons can be trained to discriminate against various symmetrical patterns (Delius & Nowak, 1982) as well as chickens (Mascalzoni et al., 2011). Bumblebees, even if they have not previously seen flowers, prefer symmetrical ones (Rodríguez et al., 2004). Preference for mating partners with more symmetry is present in earwigs (Radesäter & Halldórsdóttir, 1993; Swaddle & Cuthill, 1994) zebrafish and humans, with attractiveness and declared partner preference increasing alongside the level of symmetry (Rhodes et al., 1998).

Developmental studies are very informative for the emergence of aesthetic preference. Studies on the emergence of symmetry preference on 4-month-old infants observed no preference for symmetry, and processing of vertically symmetrical patterns was more efficient than horizontally symmetrical or asymmetrical ones. In line with this observation, at 12 months, infants prefer vertical symmetry to horizontal symmetry and asymmetry. Vertical symmetry recognition is proposed as innate or maturing very quickly but it is suggested that the preference for symmetry develops later (Ferdinandsen & Gross, 1981).

One study assessed self-reported aesthetic preferences between symmetrical and asymmetrical visual patterns of four-year-old children and adults. The study also measured their spontaneous attentional preferences between the patterns. Children watched longer symmetrical patterns when compared to similar asymmetrical patterns, but they did not explicitly report a preference for those patterns. The authors theorise that: “These findings suggest that the human’s aesthetic preferences have high postnatal plasticity, calling into question theories that symmetry is a “core feature” mediating people’s aesthetic experience throughout life. The findings also call into question the assumption, common to many studies of human infants, that attentional choices reflect subjective preferences or values” (Huang et al., 2018).

Another study had the objective to develop the understanding of the cognitive development of aesthetic preference using the development of symmetry preference as a model. In a game, 4-year-old children were exposed to either symmetric or asymmetric non-figurative forms. Results showed that „The group of children who received exposure to symmetric patterns showed aesthetic preference to the exposed patterns, while no preference was found in the group that received exposure to asymmetric patterns.” Symmetric objects were recognised and remembered better by children in a recognition test, indicating stronger encoding for symmetrical objects. Authors suggest that an “emerging perceptual sensitivity to ‘good features’ such as symmetry, provides the prior cognitive prerequisites, allowing visual perceptual exposure to nourish the eventual formation of aesthetic preference. Thus, the preferences for aesthetic appreciation are likely the outcome of the interplay between biological and ecological adaptation” (Huang et al., 2020).

Both studies could be considered strong developmental arguments for the theory proposing the emergence of aesthetical preference for symmetry arising from its faster coding and easier processing, generating fluency. This is also sustained by data from the field of computational neuroscience from a fascinating study. In their seminal study Enquist and Arak trained a neural network to recognise patterns generated by coloured squares in a 5×5 grid irrespective of the rotational degree of the grid to the retina. The neural network developed a preference for patterns presenting discrete symmetries with high contrast. To explain the observed phenomenon, Enquist and Arak propose: “that symmetry preferences may arise as a by-product of the need to recognise objects irrespective of their position and orientation in the visual field. The existence of sensory biases for symmetry may have been exploited independently by natural selection acting on biological signals and by human artistic innovation. This may account for the observed convergence on symmetrical forms in nature and decorative art” (Enquist & Arak, 1994).

Point to Take Forward

Symmetry is an important constant in aesthetic experience. Studying symmetry provides a unique window for understanding how aesthetic preference emerges from neural processing across taxa in the phylogenetic tree. Multiple proposals are made to explain the complexity of the impact of symmetry for the aesthetic experience, which appears dependent on age, gender, experience and other factors. Symmetry is processed in similar brain regions in humans and monkeys. No specific region for symmetry processing is reported. In several higher order visual cortical increased bold fMRI signal is observed when subjects are presented symmetrical stimuli. It is currently suggested that neural response to symmetry is driven by generic computations at the level of single neurons. The emergence of preferences for symmetry is proposed to be an evolutionary adaptation in mate choice with symmetry used as a proxy for gene quality and other aspects of health, linked to the detection of prey and predators or determined by less effort being required for the processing of symmetrical shapes and objects. Future research using different approaches such as functional analysis or single unit neural EEG recordings could shine light on the proposed theories. Processing of symmetry in movement, and therefore imitation, might share a common processing path with shape symmetry processing with important implications for understating vicarious learning. Further cross-cultural studies could strongly contribute to establishing the nuanced impact of symmetry on aesthetic preference. Understating the impact of specialisation in the arts on the preferences for symmetry and complexity could greatly contribute to the understating of aesthetic experience in general. Widely present in non-specialised subjects, the preference for symmetry appears diminished in specialised subjects, understating this apparent paradoxical behaviour could be informative for other aspects of aesthetic preference.

Symmetry preference is a constant even nowadays in art production and preference. The film director Wes Anderson uses symmetrical shots in feature films and animation in highly aestheticized shots which are his highly acclaimed trademark. Trends on Tik-Tok and other social media often use elaborate action and spatial symmetries in memes. Studying symmetry within the neuroaesthetics framework has a very important potential in contributing to understanding subjective human experience and its impact on cultural and artistic artefacts and phenomena.