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The term laterality refers to the preference most humans show for one side of their body over the other. Examples include left-handedness/right-handedness and left/right-footedness; it may also refer to the primary use of the left or right hemisphere in the brain. It may also apply to animals or plants. The majority of tests have been conducted on humans, specifically to determine the effects on language.

Human

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Most humans are right-handed. Many are also right-sided in general (that is, they prefer to use their right eye, right foot and right ear if forced to make a choice between the two). The reasons for this are not fully understood, but it is thought that because the left cerebral hemisphere of the brain controls the right side of the body, the right side is generally stronger; it is suggested that the left cerebral hemisphere is dominant over the right in most humans because in 90–92% of all humans, the left hemisphere is the language hemisphere.

Human cultures are predominantly right-handed, and so the right-sided trend may be socially as well as biologically enforced. This is quite apparent from a quick survey of languages. The English word left comes from the Anglo-Saxon word lyft, which means 'weak' or 'useless'. Similarly, the French word for left, gauche, is also used to mean 'awkward' or 'tactless', and sinistra, the Latin word from which the English word sinister was derived, means 'left'. Similarly, in many cultures the word for right also means 'correct'. The English word right comes from the Anglo-Saxon word riht, which also means 'straight' or 'correct'.

This linguistic and social bias is not restricted to European cultures: for example, Chinese characters are designed for right-handers to write, and no significant left-handed culture has ever been found in the world.

When a person is forced to use the hand opposite of the hand that they would naturally use, this is known as forced laterality, or more specifically forced dextrality. A study done by the Department of Neurology at Keele University, North Staffordshire Royal Infirmary suggests that forced dextrality may be part of the reason that the percentage of left-handed people decreases with the higher age groups, both because the effects of pressures toward right-handedness are cumulative over time (hence increasing with age for any given person subjected to them) and because the prevalence of such pressure is decreasing, such that fewer members of younger generations face any such pressure to begin with.[1]

Ambidexterity is when a person has approximately equal skill with both hands and/or both sides of the body. True ambidexterity is very rare. Although a small number of people can write competently with both hands and use both sides of their body well, even these people usually show preference for one side of their body over the other. However, this preference is not necessarily consistent for all activities. Some people may, for instance, use their right hand for writing, and their left hand for playing racket sports and eating[2] (see also: cross-dominance).

Also, it is not uncommon that people preferring to use the right hand prefer to use the left leg, e.g. when using a shovel, kicking a ball, or operating control pedals. In many cases, this may be because they are disposed for left-handedness but have been trained for right-handedness, which is usually attached to learning and behavioural disorders (term usually so called as "cross dominance").[3] In the sport of cricket, some players may find that they are more comfortable bowling with their left or right hand, but batting with the other hand.

Approximate statistics, complied in 1981, are given below:[4]

Laterality of motor and sensory control has been the subject of a recent intense study and review.[5] It turns out that the hemisphere of speech is the hemisphere of action in general and that the command hemisphere is located either in the right or the left hemisphere (never in both). Around 80% of people are left hemispheric for speech and the remainder are right hemispheric: ninety percent of right-handers are left hemispheric for speech, but only 50% of left-handers are right hemispheric for speech (the remainder are left hemispheric). The reaction time of the neurally dominant side of the body (the side opposite to the major hemisphere or the command center, as just defined) is shorter than that of the opposite side by an interval equal to the interhemispheric transfer time. Thus, one in five persons has a handedness that is the opposite for which they are wired (per laterality of command center or brainedness, as determined by reaction time study mentioned above).

Different expressions

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Board footedness
The stance in a boardsport is not necessarily the same as the normal-footedness of the person. In skateboarding and other board sports, a "goofy footed" stance is one with the right foot leading. A stance with the left foot forward is called "regular" or "normal" stance.
Jump and spin
Direction of rotation in figure skating jumps and spins is not necessarily the same as the footedness or the handedness of each person. A skater can jump and spin counter-clockwise (the most common direction), yet be left-footed and left-handed.
Ocular dominance
The eye preferred when binocular vision is not possible, as through a keyhole or monocular microscope.

Speech

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Cerebral dominance or specialization has been studied in relation to a variety of human functions. With speech in particular, many studies have been used as evidence that it is generally localized in the left hemisphere. Research comparing the effects of lesions in the two hemispheres, split-brain patients, and perceptual asymmetries have aided in the knowledge of speech lateralization. In one particular study, the left hemisphere's sensitivity to differences in rapidly changing sound cues was noted (Annett, 1991). This has real world implication, since very fine acoustic discriminations are needed to comprehend and produce speech signals. In an electrical stimulation demonstration performed by Ojemann and Mateer (1979), the exposed cortex was mapped revealing the same cortical sites were activated in phoneme discrimination and mouth movement sequences (Annett, 1991).

As suggested by Kimura (1975, 1982), left hemisphere speech lateralization might be based upon a preference for movement sequences as demonstrated by American Sign Language (ASL) studies. Since ASL requires intricate hand movements for language communication, it was proposed that skilled hand motions and speech require sequences of action over time. In deaf patients with a left hemispheric stroke and damage, noticeable losses in their abilities to sign were noted. These cases were compared to studies of normal speakers with dysphasias located at lesioned areas similar to the deaf patients. In the same study, deaf patients with right hemispheric lesions did not display any significant loss of signing nor any decreased capacity for motor sequencing (Annett, 1991).

One theory, known as the acoustic laterality theory, the physical properties of certain speech sounds are what determine laterality to the left hemisphere. Stop consonants, for example t, p, or k, leave a defined silent period at the end of words that can easily be distinguished. This theory postulates that changing sounds such as these are preferentially processed by the left hemisphere. As a result of the right ear being responsible for transmission to sounds to the left hemisphere, it is capable of perceiving these sounds with rapid changes. This right ear advantage in hearing and speech laterality was evidenced in dichotic listening studies. Magnetic imaging results from this study showed greater left hemisphere activation when actual words were presented as opposed to pseudowords.[6] Two important aspects of speech recognition are phonetic cues, such as format patterning, and prosody cues, such as intonation, accent, and emotional state of the speaker (Imaizumi, Koichi, Kiritani, Hosoi & Tonoike, 1998).

In a study done with both monolinguals and bilinguals, which took into account language experience, second language proficiency, and onset of bilingualism among other variables, researchers were able to demonstrate left hemispheric dominance. In addition, bilinguals that began speaking a second language early in life demonstrated bilateral hemispheric involvement. The findings of this study were able to predict differing patterns of cerebral language lateralization in adulthood (Hull & Vaid, 2006).

In other animals

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It has been shown that cerebral lateralization is a widespread phenomenon in the animal kingdom.[7] Functional and structural differences between left and right brain hemispheres can be found in many other vertebrates and also in invertebrates.[8]

It has been proposed that negative, withdrawal-associated emotions are processed predominantly by the right hemisphere, whereas the left hemisphere is largely responsible for processing positive, approach-related emotions. This has been called the "laterality-valence hypothesis".[9]

One sub-set of laterality in animals is limb dominance. Preferential limb use for specific tasks has been shown in species including chimpanzees, mice, bats, wallabies, parrots, chickens and toads.[8]

Another form of laterality is hemispheric dominance for processing conspecific vocalizations, reported for chimpanzees, sea lions, dogs, zebra finches and Bengalese finches.[8]

In mice

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In mice (Mus musculus), laterality in paw usage has been shown to be a learned behavior (rather than inherited),[10] due to which, in any population, half of the mice become left-handed while the other half becomes right-handed. The learning occurs by a gradual reinforcement of randomly occurring weak asymmetries in paw choice early in training, even when training in an unbiased world.[11][12] Meanwhile, reinforcement relies on short-term and long-term memory skills that are strain-dependent,[11][12] causing strains to differ in the degree of laterality of its individuals. Long-term memory of previously gained laterality in handedness due to training is heavily diminished in mice with absent corpus callosum and reduced hippocampal commissure.[13] Regardless of the amount of past training and consequent biasing of paw choice, there is a degree of randomness in paw choice that is not removed by training,[14] which may provide adaptability to changing environments.

In other mammals

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Domestic horses (Equus caballus) exhibit laterality in at least two areas of neural organization, i.e. sensory and motor. In thoroughbreds, the strength of motor laterality increases with age. Horses under 4 years old have a preference to initially use the right nostril during olfaction.[15] Along with olfaction, French horses have an eye laterality when looking at novel objects. There is a correlation between their score on an emotional index and eye preference; horses with higher emotionality are more likely to look with their left eye. The less emotive French saddlebreds glance at novel objects using the right eye, however, this tendency is absent in the trotters, although the emotive index is the same for both breeds.[16] Racehorses exhibit laterality in stride patterns as well. They use their preferred stride pattern at all times whether racing or not, unless they are forced to change it while turning, injured, or fatigued.[17]

Fearfulness is an undesirable trait in guide dogs, therefore, testing for laterality can be a useful predictor of a successful guide dog. Knowing a guide dog's laterality can also be useful for training because the dog may be better at walking to the left or the right of their blind owner.[18]

Domestic cats (Felis catus) show an individual handedness when reaching for static food. In one study, 46% preferred to use the right paw, 44% the left, and 10% were ambi-lateral; 60% used one paw 100% of the time. There was no difference between male and female cats in the proportions of left and right paw preferences. In moving-target reaching tests, cats have a left-sided behavioural asymmetry.[19] One study indicates that laterality in this species is strongly related to temperament. Furthermore, individuals with stronger paw preferences are rated as more confident, affectionate, active, and friendly.[20]

Chimpanzees show right-handedness in certain conditions. This is expressed at the population level for females, but not males. The complexity of the task has a dominant effect on handedness in chimps.[21]

Cattle use visual/brain lateralisation in their visual scanning of novel and familiar stimuli.[22] Domestic cattle prefer to view novel stimuli with the left eye, (similar to horses, Australian magpies, chicks, toads and fish) but use the right eye for viewing familiar stimuli.[23]

Schreibers' long-fingered bat is lateralized at the population level and shows a left-hand bias for climbing or grasping.[24]

In marsupials

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Marsupials are fundamentally different from other mammals in that they lack a corpus callosum.[25] However, wild kangaroos and other macropod marsupials have a left-hand preference for everyday tasks. Left-handedness is particularly apparent in the red kangaroo (Macropus rufus) and the eastern gray kangaroo (Macropus giganteus). The red-necked wallaby (Macropus rufogriseus) preferentially uses the left hand for behaviours that involve fine manipulation, but the right for behaviours that require more physical strength. There is less evidence for handedness in arboreal species.[26]

In birds

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Parrots tend to favor one foot when grasping objects (for example fruit when feeding). Some studies indicate that most parrots are left footed.[27]

The Australian magpie (Gymnorhina tibicen) uses both left-eye and right-eye laterality when performing anti-predator responses, which include mobbing. Prior to withdrawing from a potential predator, Australian magpies view the animal with the left eye (85%), but prior to approaching, the right eye is used (72%). The left eye is used prior to jumping (73%) and prior to circling (65%) the predator, as well as during circling (58%) and for high alert inspection of the predator (72%). The researchers commented that "mobbing and perhaps circling are agonistic responses controlled by the LE[left eye]/right hemisphere, as also seen in other species. Alert inspection involves detailed examination of the predator and likely high levels of fear, known to be right hemisphere function."[28]

Yellow-legged gull (Larus michahellis) chicks show laterality when reverting from a supine to prone posture, and also in pecking at a dummy parental bill to beg for food. Lateralization occurs at both the population and individual level in the reverting response and at the individual level in begging. Females have a leftward preference in the righting response, indicating this is sex dependent. Laterality in the begging response in chicks varies according to laying order and matches variation in egg androgens concentration.[29]

In fish

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Laterality determines the organisation of rainbowfish (Melanotaenia spp.) schools. These fish demonstrate an individual eye preference when examining their reflection in a mirror. Fish which show a right-eye preference in the mirror test prefer to be on the left side of the school. Conversely, fish that show a left-eye preference in the mirror test or were non-lateralised, prefer to be slightly to the right side of the school. The behaviour depends on the species and sex of the school.[30]

In amphibians

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Three species of toads, the common toad (Bufo bufo), green toad (Bufo viridis) and the cane toad (Bufo marinus) show stronger escape and defensive responses when a model predator was placed on the toad's left side compared to their right side.[31] Emei music frogs (Babina daunchina) have a right-ear preference for positive or neutral signals such as a conspecific's advertisement call and white noise, but a left-ear preference for negative signals such as predatory attack.[32]

In invertebrates

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The Mediterranean fruit fly (Ceratitis capitata) exhibits left-biased population-level lateralisation of aggressive displays (boxing with forelegs and wing strikes) with no sex-differences.[33] In ants, Temnothorax albipennis (rock ant) scouts show behavioural lateralization when exploring unknown nest sites, showing a population-level bias to prefer left turns. One possible reason for this is that its environment is partly maze-like and consistently turning in one direction is a good way to search and exit mazes without getting lost.[34] This turning bias is correlated with slight asymmetries in the ants' compound eyes (differential ommatidia count).[35]

See also

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References

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Laterality

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Laterality refers to the preferential dominance or specialization of one side of the body or over the other in performing biological functions, encompassing motor preferences like and cerebral asymmetries in cognitive processing. This phenomenon manifests across diverse , from to vertebrates, and is characterized by two main forms: individual lateralization, where functions are divided within an organism, and directional or population-level lateralization, where a majority of individuals share the same bias. In humans, laterality is most prominently observed in , with approximately 90% of the population exhibiting a right-hand preference for tasks such as writing and tool use, influenced by both genetic and prenatal environmental factors like thumb-sucking . lateralization complements this, with the left hemisphere typically specializing in , , and fine of the right side of the body, while the right hemisphere predominates in visuospatial tasks, emotional processing, and gross on the left side. For instance, 95-99% of right-handed individuals show left-hemisphere dominance for , though this is less consistent in left-handers, where bilateral or right-hemisphere involvement occurs in approximately 30% of cases. Evolutionarily, laterality enhances cognitive efficiency by enabling parallel processing of tasks, such as simultaneous predator vigilance and , without requiring larger sizes—a benefit documented in like chicks and where lateralized individuals outperform symmetric ones in survival scenarios. In vertebrates, including and , genetic mechanisms like Nodal signaling establish early left-right asymmetries in and , suggesting deep evolutionary roots that promote social cohesion and adaptive responses. Disruptions in lateralization have been linked to psychological disorders, underscoring its role in typical development and function.

Introduction and Definition

Core Concepts

Laterality refers to the preferential use and superior functioning of one side of the body or one over the other in biological systems, manifesting as asymmetric preferences in motor, sensory, or cognitive functions. This phenomenon contrasts with bilateral symmetry, where both sides are equivalently utilized, and instead promotes functional specialization that enhances efficiency in processing and responding to environmental demands. In essence, laterality enables parallel processing of distinct tasks, such as simultaneous monitoring of threats and , thereby optimizing cognitive capacity without redundancy. A key distinction exists between behavioral laterality, which involves observable asymmetries in actions like the preferential use of one hand for writing or , and neural laterality, which pertains to the hemispheric specialization of functions underlying these behaviors. Behavioral laterality is typically measured through everyday activities, whereas neural laterality reflects differential in regions, such as left-hemisphere dominance for analytical processing. This separation highlights how external preferences may stem from internal neural organization, though the two are closely correlated in most individuals. In humans, a prominent example of laterality is , with approximately 90% of the exhibiting right-hand preference, often linked to left-hemisphere . This right-handed bias correlates strongly with left-hemisphere dominance for language and speech, where about 96% of strongly right-handed individuals show left-hemispheric specialization for these functions. Such patterns underscore laterality's role in coordinating sensory-motor integration, with deviations like left-handedness occasionally associated with atypical hemispheric organization. Measurement of laterality, particularly , commonly employs standardized tools like the Edinburgh Handedness Inventory, a 10-item assessing preferences across activities such as writing, throwing, and using utensils to compute a laterality quotient ranging from strong left to strong right dominance.90067-4) This inventory provides a reliable, non-invasive method to quantify behavioral , facilitating research into its neural correlates without requiring advanced imaging. Other approaches include observational tasks or self-reports, ensuring consistent evaluation across populations.

Historical Background

The earliest documented observations of laterality appear in from around 3000 BCE, where tomb paintings and hieroglyphic depictions consistently show individuals performing tasks—such as writing, holding tools, or offering items—with their right hand, providing graphic evidence of predominant right-handedness in that society. In , further elaborated on bodily in his Metaphysics, positing that is an innate trait rather than a learned , with individuals naturally predisposed to favor one hand over the other, thus recognizing inherent functional differences between the sides of the body. The marked the onset of systematic scientific inquiry into laterality, driven by advances in and . In 1861, French surgeon examined the brain of patient Louis Victor Leborgne, known as "Tan," who suffered from but intact comprehension; the revealed a lesion in the left , leading Broca to conclude that is lateralized to the left hemisphere in most individuals. This discovery, later termed , established a foundational link between brain asymmetry and specific cognitive functions, shifting focus from mere behavioral preferences to underlying neural mechanisms. Twentieth-century research expanded these insights through experimental approaches. In the 1960s and 1970s, neurobiologist Roger Sperry conducted studies on patients who had undergone surgical severing of the to alleviate ; these experiments revealed that the disconnected hemispheres operate independently, with the left specializing in verbal and analytical tasks and the right in visuospatial and holistic processing, thereby confirming profound hemispheric specialization. Sperry's findings, which demonstrated how each hemisphere maintains its own sensory, motor, and cognitive domains without intercommunication, earned him the 1981 in or . Initially human-centric, laterality research underwent a in the 1980s toward comparative perspectives, as studies on non-human animals demonstrated that functional asymmetries are not unique to Homo sapiens. Fernando Nottebohm's 1977 work on songbirds showed population-level left-hemispheric dominance in vocal control, while Lesley Rogers' 1980 experiments with revealed right-eye (left-hemisphere) superiority in visual discrimination tasks, prompting a broader evolutionary examination of laterality across and refuting prior assumptions of human exceptionalism.

Laterality in Humans

Handedness

refers to the preferential use of one hand over the other for manual tasks, with right-handedness being the dominant form in human populations. Globally, approximately 85-90% of individuals are right-handed, while 10-15% are left-handed, though these rates exhibit slight variations across cultures and regions. In some non-Western and indigenous groups, such as certain Australian Aboriginal communities, left-handedness can reach up to 21%, potentially reflecting less cultural pressure to conform to right-hand use compared to industrialized societies. Overall, left-handedness prevalence ranges from about 5% to 27% in diverse populations, influenced by both biological and environmental factors. The determinants of handedness involve a combination of prenatal and postnatal influences. Prenatally, exposure to higher levels of testosterone has been linked to a right-hand bias, as proposed in models like the right-shift theory, where elevated fetal testosterone promotes stronger lateralization toward the right hand in . Studies indicate that increased prenatal testosterone correlates with reduced strength of handedness overall, but it contributes to the population-level skew toward right-handedness by enhancing hemispheric asymmetry. Postnatally, learning and environmental play a key role in solidifying hand preferences; for instance, repetitive use of the right hand in daily activities, such as tool manipulation or writing, strengthens the initial bias through motor practice and social modeling. This reinforcement can amplify innate tendencies, leading to consistent by . Assessment of handedness typically employs a mix of self-report and performance-based tasks to classify individuals as strongly right-handed, left-handed, or mixed-handed. The Edinburgh Handedness Inventory, a widely used , evaluates preferences across 10 activities like writing, throwing, and using utensils, scoring responses on a scale from -100 (strongly left-handed) to +100 (strongly right-handed) to quantify laterality strength. Performance tasks, such as pegboard tests, measure manual dexterity and speed; the Purdue Pegboard Test, for example, requires participants to place pins into holes using one hand or both, with faster completion times on the dominant hand indicating preference, and it distinguishes strong from mixed handedness by comparing assembly efficiency. The Grooved Pegboard Test similarly assesses fine motor skills by timing the insertion of pegs into keyed slots, providing objective data on hand asymmetry that correlates well with questionnaire results. Culturally, handedness has been shaped by historical biases against left-hand use, often leading to suppression. Until the mid-20th century, many schools in Western countries, including and the , enforced right-hand writing on left-handed children through physical correction or retraining, viewing left-handedness as a defect that could be "cured" to align with societal norms. This practice, rooted in associations of the left hand with uncleanliness or inferiority in various traditions, artificially reduced reported left-handedness rates in earlier generations. In modern times, greater acceptance has emerged, with reduced institutional pressure and accommodations like left-handed desks in schools promoting natural expression of .

Other Bodily and Sensory Preferences

Footedness refers to the preferential use of one foot over the other in tasks such as kicking or balancing. In humans, right-footedness predominates, with approximately 88% of individuals classified as right-footed based on meta-analytic data from 164 studies encompassing various assessment methods. Left-footedness occurs in about 12% of the , though this rate rises to around 60% among left-handers, indicating a substantial but not perfect alignment with manual laterality. Common assessment tasks include kicking a toward a target or pointing with the toes to select an object, which reveal functional preferences less influenced by cultural factors than . These preferences have implications for athletic performance, as footedness correlates with self-reported sporting abilities in activities requiring lower-body coordination. Eyedness, or , describes the tendency to favor one eye for tasks like sighting along a line. Right-eye dominance prevails in roughly 65-70% of the population, with left-eye dominance in about 35%, according to meta-analyses of behavioral and measures. This is assessed through sighting tests, such as aligning a distant object through a small hole in a card (binocular method, yielding ~71% right dominance), or sensory tests that induce blur in one eye to determine fixation preference ( method, ~54-61% right dominance). Unlike , eyedness shows only moderate concordance, with left-handers exhibiting left-eye dominance in about 57% of cases compared to 34% in right-handers, highlighting partial across sensory modalities. Earedness involves preferential processing of auditory stimuli by one ear, often evaluated via dichotic listening tasks where different sounds are presented simultaneously to each ear. A right-ear advantage for verbal material, such as syllables or words, is observed in approximately 70-82% of individuals, reflecting contralateral pathways to the language-dominant hemisphere. This preference is less pronounced for non-verbal sounds like music, where ear advantages may reverse. Right-handers typically show a stronger right-ear bias than left-handers, though the overall prevalence remains high across groups, underscoring auditory laterality's role in selective attention. Across these bodily and sensory preferences, moderate correlations exist, with overall right-sided alignment in 50-70% of cases depending on the modality pair; for instance, and show a of about 0.5, while eyedness and earedness exhibit weaker links to manual preference. Such partial consistencies suggest underlying neural mechanisms that are not fully unified, influencing outcomes in sports where multimodal coordination is key, like soccer or .

Language and Speech Lateralization

In humans, language processing exhibits a robust hemispheric asymmetry, with the left hemisphere typically dominant for both and comprehension. This pattern is observed in approximately 95% of right-handers and 70% of left-handers, as determined through various and studies. The prevalence of left-hemisphere dominance is notably higher among right-handers, reflecting a strong association between manual laterality and linguistic lateralization, though the underlying mechanisms remain under investigation. Central to this lateralization are key regions in the left hemisphere, including in the , which is primarily involved in and syntactic processing, and in the , responsible for language comprehension and semantic interpretation. These areas are interconnected by the arcuate fasciculus, a tract that facilitates the integration of phonological and articulatory information essential for fluent language use. (fMRI) studies provide strong evidence for this asymmetry, demonstrating greater activation in left-hemisphere regions during speech-related tasks such as verb generation or sentence comprehension, with lateralization indices often exceeding 0.7 in typical cases. Exceptions to this typical left-hemisphere dominance occur in a minority of individuals, where representation may be bilateral or even right-lateralized, particularly among left-handers. Bilateral activation patterns are observed in about 4% of right-handers and up to 30% of left-handers during fMRI tasks, potentially conferring resilience against unilateral brain damage but sometimes linked to subtle processing inefficiencies. Additionally, the right hemisphere plays a specialized role in processing prosody and emotional tone in speech, contributing to the interpretation of affective nuances that enhance communicative beyond literal meaning.

Laterality in Non-Human Animals

In Mammals

In mammals, laterality manifests primarily through motor preferences, such as or hand usage in manipulation tasks, and corresponding asymmetries, often paralleling patterns observed in human handedness but varying by and task. These asymmetries are studied to understand evolutionary conservation and functional specialization, with individual preferences common across populations, though population-level biases appear in specific contexts like tool use. Research emphasizes stable, task-dependent lateralization that aids in genetic and neurobiological modeling. In , particularly mice, preferences for food manipulation tasks reveal strong individual biases, with approximately 81% of mice and 84% of rats showing a for either the left or right , but no consistent population-level bias toward either side, as shown in meta-analyses of reaching behaviors. These findings have significant implications for genetic models, as laterality in mice is linked to variations in genes like the , enabling investigations into hemispheric specialization and disorders akin to human lateralization deficits. Among non-primate mammals, cats and dogs demonstrate or preferences in reaching tasks, with variability depending on the activity. In cats, about 78% of individuals display a consistent preference for food retrieval or stepping maneuvers, though population-level biases are weak and task-specific. Dogs similarly show individual lateralization in 68% of cases, with a population-level right- preference around 60% for fetching or toy-reaching tasks, influenced by factors like owner . These preferences highlight adaptive motor asymmetries in domestic species, aiding in emotional and cognitive assessments. Primates, especially chimpanzees, exhibit more pronounced laterality in tool-use contexts, with individual hand preferences but a clear population-level right-handedness. For nut-cracking, wild chimpanzees show a significant right-hand bias, with approximately 65% favoring the right hand across observed groups, contrasting with left biases in other tasks like termite-fishing. This pattern underscores laterality's role in complex manipulation, similar to tool behaviors. Brain correlates in mammals include asymmetries in regions homologous to human language areas, such as the (PT) in great apes. In chimpanzees, MRI studies reveal population-level leftward asymmetries in PT surface area (about 5% larger on the left) and volume (about 7% larger on the left), with stronger biases in right-handed individuals. These structural differences suggest conserved neural foundations for lateralized processing across .

In Birds

In birds, visuomotor biases are prominent, particularly in species like domestic chicks (Gallus gallus domesticus), where visual processing is lateralized due to largely non-overlapping visual fields and segregated pathways to the hemispheres. The right eye, projecting primarily to the left hemisphere, is preferentially used for discriminating and responding to familiar stimuli, such as food items or conspecifics, enabling efficient categorization and controlled behaviors. Conversely, the left eye, connected to the right hemisphere, shows a bias for detecting novel objects or environmental changes, facilitating rapid attention shifts and spatial processing. A 2025 study confirmed left-eye (right hemisphere) superiority in rapid threat detection, with binocular and right-eye chicks learning feed positions faster than left-eye ones, integrating motion cues for escape behaviors. This asymmetry enhances overall cognitive efficiency by allowing simultaneous monitoring of routine tasks with one hemisphere and vigilance for threats with the other. Feeding behaviors in birds also exhibit laterality, with foot use and head positioning showing population-level biases that aid in manipulation and . In parrots (Psittaciformes), is well-documented, with many displaying strong preferences for using one foot to hold food while the other provides balance; for instance, cockatoos often show up to 90% left-foot dominance for food-holding in tasks involving manipulation, though preferences vary by and can reach similar strengths for the right foot in others like certain macaws; recent 2025 observations link foot dominance in cockatoos to social hierarchy. Pigeons (Columba livia) demonstrate asymmetric head-turning during feeding, turning the head to position food objects in the preferred —typically the right eye for small seeds requiring fine , but the left eye for larger or novel items—to optimize and pecking accuracy, which correlates with faster consumption rates. These biases likely evolved to streamline while minimizing exposure to predators. Brain asymmetry in birds arises early in development, influenced by factors like embryonic exposure to hormones. In chick embryos, gradients of yolk-deposited testosterone contribute to hemispheric specialization, with higher levels promoting left-hemisphere dominance for analytical tasks and right-hemisphere superiority for holistic processing; experimental injections of testosterone have been shown to reverse or enhance visual discrimination asymmetries post-hatching. Songbirds, such as zebra finches (Taeniopygia guttata), exhibit pronounced lateralization in vocal learning circuits, with the left hemisphere dominating song production and , while the right hemisphere handles auditory perception and for tutor songs, mirroring human lateralization and supporting efficient learning during a critical sensory phase. Recent research highlights how these asymmetries aid in predator detection, with studies in demonstrating that chicks respond more rapidly to threatening stimuli viewed by the left eye, integrating motion cues for escape behaviors; for example, lateralized vigilance allows the right hemisphere to prioritize predator movements, improving survival in open environments. This visuospatial bias underscores the adaptive value of laterality in avian .

In Other Vertebrates and Invertebrates

In , laterality manifests prominently in detour tasks, where s navigate around barriers to approach or avoid stimuli, often showing biases that aid in predator avoidance. For instance, wild-caught (Danio rerio) often exhibit individual turning preferences in tasks, with approximately 27% showing a right in some Y-maze studies, supporting coordinated schooling behaviors by aligning group movements and reducing collision risks during evasion maneuvers. This directional preference supports coordinated schooling behaviors by aligning group movements and reducing collision risks during evasion maneuvers. Amphibians display asymmetric sensory preferences, particularly in visual processing for prey detection and threat assessment. In toads such as Bufo marinus, the right eye is preferentially used to guide predatory tongue strikes toward moving prey, reflecting a specialization in the left hemisphere for appetitive behaviors. Similarly, in music frogs (Babina daunchina), right-eye lateralization during predation involves distinct neural processing, with measures indicating structured variability in this bias. Frogs also exhibit asymmetric neural circuits, as evidenced by low-frequency electroencephalogram oscillations that govern left-eye dominance for predator monitoring, contrasting with right-eye use for foraging and highlighting hemispheric complementarity in survival tasks. Among reptiles, limb and sensory asymmetries support locomotion and chemosensory functions. Turtles demonstrate right-limb dominance in terrestrial locomotion, with individuals favoring the right forelimb for propulsion during straight-line walking, which may optimize stability on uneven substrates. In alligators (Alligator mississippiensis), brain asymmetry influences olfaction, with the right hemisphere mediating visually guided behaviors that integrate olfactory cues, as prenatal androgen exposure disrupts this lateralization and leads to indiscriminate eye use. Invertebrates exhibit functional asymmetries in appendages and sensory organs, often at the individual level but with population biases in social species. Many species, such as snapping (Alpheidae), specialize the right for feeding and defense, using it to prey or deliver strikes, while the left handles manipulation. Honeybees (Apis mellifera) show a right-antenna preference for detection and learning, with stronger lateralization in tasks involving rewarded scents processed via the right antennal lobe. Octopuses display arm specialization, where specific arms are designated for tasks like feeding or exploration, with the right third arm often favored for prey handling due to centralized neural control in the ventral lobes. These asymmetries contribute to population-level rightward tendencies in schooling fish, where aligned biases enhance group cohesion and anti-predator efficiency, and in social insects like honeybees, where right-antenna dominance facilitates collective odor-guided foraging without disrupting hive coordination.

Neural and Genetic Basis

Brain Asymmetry

Brain asymmetry manifests in both structural and functional differences between the left and right hemispheres, observable across species and essential for specialized cognitive processing. In humans, one of the most prominent structural asymmetries is found in the (PT), a region in the implicated in auditory processing. The left PT is typically larger than the right, with volume differences averaging around 30% relative to cortical volume in postmortem studies. This asymmetry is present in approximately 65-70% of individuals and is thought to support language-related functions, though it varies with factors like and . Another key structural feature is cerebral petalia, where the right protrudes anteriorly (right frontal petalia) and the left protrudes posteriorly (left occipital petalia), forming a "torque" pattern unique to humans. This configuration occurs in about 60-70% of human brains and is absent or inconsistent in non-human like chimpanzees. Functionally, the left hemisphere specializes in sequential, analytical processing, such as language production and temporal ordering of events, while the right hemisphere excels in holistic, synthetic processing, including spatial relations and facial recognition. Lesion studies provide strong evidence for this dichotomy: damage to left-hemisphere regions like impairs sequential speech output, leading to non-fluent , whereas right-hemisphere lesions disrupt holistic , resulting in or impaired recognition of emotional expressions. These specializations arise from differential neural connectivity and activation patterns, with the left favoring fine-grained, linear analysis and the right integrating global context. Similar asymmetries appear in non-human animals, highlighting evolutionary conservation. In songbirds like zebra finches, the caudomedial nidopallium (NCM), a secondary auditory area analogous to mammalian , exhibits hemispheric asymmetry in calbindin-positive neurons during song learning. Successful imitators show right-hemisphere dominance in NCM neuron distribution, correlating with better vocal copying of tutor songs (r = -0.76, p < 0.01). In fish, such as zebrafish, the dorsal habenula displays left-right asymmetry in subnuclei connectivity to the interpeduncular nucleus, influencing fear responses. The left habenula attenuates freezing behaviors during aversive conditioning, while disrupting this asymmetry shifts responses toward excessive flight or immobility, underscoring its role in modulating innate fear circuits. Imaging techniques like positron emission tomography (PET) and electroencephalography (EEG) have been instrumental in quantifying these asymmetries. PET measures regional cerebral blood flow (rCBF) to reveal functional lateralization, such as greater left-hemisphere activation during sequential tasks or rightward asymmetries in emotional processing, using statistical parametric mapping to detect significant differences (threshold T > 3.0, p < 0.05). EEG, particularly alpha-band (8-13 Hz) asymmetry, assesses cortical activation indirectly: reduced alpha power indicates higher activity, with frontal alpha asymmetry indexing approach-withdrawal motivation (e.g., greater left frontal activation for positive affect). These methods confirm activation differences, such as parietal alpha asymmetry in spatial tasks, and show high short-term reliability (intraclass correlation > 0.7) across electrodes.

Genetic and Developmental Factors

Laterality in humans and other organisms is influenced by a combination of genetic and developmental factors that establish asymmetric patterns early in embryonic life. Genetic contributions to , a key aspect of behavioral laterality, are polygenic, with twin and family studies estimating that additive genetic factors explain approximately 25% of the variance in handedness. Specific genes, such as LRRTM1 on chromosome 2p12, have been implicated in modulating handedness through paternal effects; a particular of LRRTM1 is associated with a modest increase in the likelihood of left-handedness or mixed-handedness, reflecting its role in neuronal connectivity and asymmetry. More recent genome-wide association studies (GWAS) have identified 48 common genetic variants associated with handedness, including loci involved in microtubule-related processes and neuronal development, which also correlate with cerebral asymmetries in regions such as the and anterior insula. These findings, from analyses of over 1.7 million individuals as of 2021, reinforce the polygenic basis and highlight shared genetic influences on behavioral and brain lateralization. Developmental processes during embryogenesis critically determine the left-right body axis through conserved signaling pathways. The Nodal signaling pathway, involving TGF-β family members, plays a pivotal role in this axis formation by generating an asymmetric morphogen gradient via leftward fluid flow at the embryonic node, which directs the expression of downstream genes like lefty and pitx2 on the left . Disruptions in this pathway, such as in inv/inv or iv/iv mutants with impaired nodal cilia , lead to randomized or reversed laterality, resulting in —a mirror-image reversal of visceral organs—in about 50% of affected individuals. Prenatal environmental factors further shape laterality preferences observable in utero. Ultrasound studies of human fetuses reveal a right-hand bias in thumb-sucking as early as 15 weeks gestation, with over 90% preferring the right thumb, a preference that persists to term and correlates with postnatal handedness. This early lateralization appears independent of fetal position and may reflect underlying genetic and developmental influences. Epigenetic modifications modulated by maternal factors can also alter laterality outcomes. Elevated maternal anxiety during early pregnancy (around 18 weeks gestation) is associated with increased odds of atypical handedness, such as mixed-handedness, in offspring (odds ratio 1.23), potentially mediated by stress-induced cortisol exposure that affects fetal neurodevelopment and gene expression patterns.

Evolutionary and Adaptive Perspectives

Evolutionary Origins

The origins of laterality trace back to the period, approximately 500 million years ago, where fossil evidence reveals asymmetric features in early mollusks, such as the coiled shells of primitive gastropods that exhibit consistent in their spiral patterns. These asymmetries in shell morphology represent one of the earliest documented instances of directional bias in bilaterian animals, predating more complex vertebrate forms. Complementing this paleontological record, the Nodal signaling pathway, which regulates left-right asymmetry in embryonic development, is genetically conserved across bilaterians, including mollusks like snails, indicating a deep phylogenetic root for molecular mechanisms of laterality that likely emerged around the same era. In vertebrate evolution, laterality manifests prominently through the rightward looping of the embryonic heart tube, a process conserved across species from early fish to mammals and occurring shortly after gastrulation in development. This looping establishes the basic left-right orientation of the cardiovascular system and is regulated by asymmetric gene expression, such as Nodal on the left side, which has been preserved since the divergence of vertebrates around 500 million years ago. Population-level biases in behavioral laterality, such as fin preferences in predation or navigation, began to emerge in ancient fish lineages during the Devonian period approximately 400 million years ago, coinciding with the diversification of jawed vertebrates and reflecting the stabilization of neural asymmetries. Within the human lineage, evidence of handedness bias appears in early hominins, with cut marks on bones and scratch patterns on teeth from specimens (dating to about 50,000–100,000 years ago) indicating a right-hand in approximately 90% of individuals, higher than the ~50% inferred from some earlier tool-use traces but still showing a directional trend. Earlier fossils, such as those from around 1.8 million years ago, reveal similar rightward striations on dental surfaces from tool manipulation, suggesting a gradual strengthening of population-level right-handedness from through species. This progression aligns with increasing reliance on bimanual tool use in hominid evolution. Theories of gene-culture coevolution explain the establishment of handedness norms in humans, positing that genetic predispositions for right-handedness interacted with cultural pressures, such as standardized tool-making and social imitation, to amplify population biases over time. This model integrates heritable factors with transmitted behaviors, accounting for the near-universal right-hand dominance observed in modern populations without invoking purely genetic .

Functional Advantages

Brain lateralization provides significant cognitive efficiency by enabling parallel processing across hemispheres, allowing specialized functions to occur simultaneously without interference. For instance, the left hemisphere often handles detail-oriented tasks such as language processing and fine motor control, while the right hemisphere manages holistic or big-picture functions like spatial awareness and emotional processing. This division reduces cognitive redundancy and enhances multitasking capabilities, as demonstrated in studies on domestic chicks where lateralized individuals could forage for food while vigilantly monitoring for predators, outperforming non-lateralized counterparts in dual-task scenarios. Such specialization increases overall brain capacity by allocating distinct neural resources, leading to faster reaction times and improved performance in complex environments. In terms of roles, lateralization confers adaptive advantages in predator detection and evasion across . In , a population-level bias toward left-eye use for vigilance allows the right hemisphere to process threats rapidly, reducing reaction times during predatory encounters and improving escape success compared to symmetric visual processing. Similarly, in humans, right-handed dominance facilitates specialized tool use, enhancing efficiency in manipulative tasks essential for , crafting, and resource exploitation, as evidenced by archaeological records showing consistent right-hand biases in tool production over . These asymmetries minimize processing delays in high-stakes situations, promoting individual fitness. At the population level, shared lateral biases foster social coordination and cultural efficiency. The prevalence of right-handedness, observed in approximately 90% of humans, enables standardized tool designs and collaborative activities, such as shared weaponry or implements, which streamline group interactions and reduce training costs for complex skills. Comparative studies further underscore these benefits, revealing that non-lateralized models or individuals exhibit reduced cognitive capacity, with slower task performance and diminished multitasking proficiency due to overlapping hemispheric functions. For example, experiments with induced symmetric brains in animals show 10-20% longer latencies in vigilance tasks, highlighting the scalable advantages of for both individual and collective survival.

Pathological and Clinical Aspects

Disorders Linked to Atypical Laterality

Atypical laterality has been implicated in several neurodevelopmental disorders, particularly those involving disruptions in cerebral asymmetry for and cognitive processing. In , a reading disorder characterized by difficulties in phonological processing, individuals often exhibit reduced left-hemisphere dominance for functions, leading to more symmetric or rightward-biased activation patterns during reading tasks. This atypical asymmetry is associated with a higher of non-right-handedness; meta-analyses indicate that approximately 11.2% of individuals with are left-handed compared to 5.8% in controls, suggesting a roughly doubled rate of left-handedness among those affected. Furthermore, up to 30% of left-handers show right-hemisphere dominance, compared to only 5% of right-handers, which may contribute to the ~10-15% higher of in left-handed populations relative to right-handers. Schizophrenia, a psychotic disorder marked by hallucinations, delusions, and cognitive impairments, is frequently linked to atypical cerebral laterality, including reduced asymmetry in and emotion processing regions. Meta-analyses reveal an excess of non-right-handedness in patients, with rates around 15-20% compared to 10% in the general , corresponding to approximately a 50% increased of the disorder among non-right-handers. This is attributed to disrupted neurodevelopmental processes affecting hemispheric specialization. Specifically, is associated with atypical right-hemisphere dominance for emotional processing, such as in facial , where patients show diminished right-lateralized activation and greater bilateral or leftward involvement, contributing to deficits in . Attention-deficit/hyperactivity disorder (ADHD) is also associated with atypical laterality, including higher rates of non-right-handedness (approximately 15-20%) and reduced cerebral asymmetry in attention and motor networks, potentially contributing to executive function deficits. In autism spectrum disorder (ASD), a condition involving challenges in social interaction and repetitive behaviors, laterality patterns often deviate from typical left-hemisphere dominance for and enhanced right-hemisphere involvement for visuospatial tasks. Functional imaging studies demonstrate enhanced right-hemisphere bias in visuospatial processing among individuals with ASD, with atypical asymmetries showing extreme rightward deviations in motor and perceptual networks, potentially linked to superior local detail processing but impaired global integration. Additionally, rare cases of —a complete reversal of visceral organ laterality—occur in ASD, particularly in association with Kartagener syndrome, a subtype of ; molecular studies identify shared genetic pathways between ASD, congenital heart defects, and left-right asymmetry disruptions like situs inversus. Atypical laterality also influences recovery outcomes following , a cerebrovascular event causing tissue damage. Left-handers, who more frequently exhibit bilateral representations for motor and functions, may experience better recovery compared to right-handers due to more distributed neural facilitating reorganization. This is evident in motor recovery, where bilateral hemispheric involvement in left-handers can result in more efficient compensatory mechanisms and functional gains post-, particularly for coordination.

Assessment and Interventions

Assessment of laterality encompasses a range of neuropsychological batteries and behavioral tasks designed to quantify hemispheric dominance for functions like and manual preference. The , an invasive procedure involving temporary anesthesia of one via intracarotid injection, remains the gold standard for determining lateralization in patients, allowing clinicians to evaluate contralateral function and predict surgical risks. (fMRI) provides a noninvasive complement, mapping networks through activation patterns during tasks such as verb generation or reading, with studies showing high concordance (up to 90%) with Wada results in frontal and temporal regions. For broader laterality evaluation, tools like the Florence Laterality Inventory assess preferences across hand, foot, eye, and ear modalities using a 16-item scale, offering reliable quantification of mixed or atypical patterns. Handedness assessment typically relies on self-report inventories and performance-based tasks to establish consistency and directionality. The Edinburgh Handedness Inventory, a seminal 10-item evaluating preferences for activities like writing, throwing, and using utensils, yields a laterality quotient ranging from -100 (strong left) to +100 (strong right), facilitating classification of individuals as right-, left-, or mixed-handed. Behavioral paradigms, such as dual-task protocols combining solving—a verbal task lateralized to the left hemisphere—with unimanual finger tapping, measure interference effects to infer cerebral asymmetry, with right-handers showing greater right-hand disruption during language processing. In clinical settings, these tools are integral to pre-surgical evaluation for , where fMRI-based mapping identifies language-dominant regions to minimize postoperative deficits, such as , in up to 65% of surgical candidates across centers. For instance, passive auditory fMRI paradigms enable safe lateralization assessment even in uncooperative patients, supporting tailored resection strategies. Interventions for atypical laterality focus primarily on behavioral strategies, as no pharmacological treatments exist to alter inherent asymmetries, with ongoing monitoring recommended for associated developmental challenges. In children with mixed-handedness, programs emphasize activities to foster motor consistency, such as unilateral threading, drawing, or tool use, which promote establishment of a dominant hand and enhance fine motor skills like . These therapies, often involving midline-crossing exercises, have been shown to reduce hand-switching and improve coordination in preschoolers, addressing delays linked to inconsistent laterality. For disorders involving atypical laterality, such as , early interventions include supportive monitoring alongside targeted reading programs, though remediation remains symptom-focused rather than asymmetry-corrective. Emerging technologies in 2025 leverage AI for proactive laterality profiling, particularly in early detection where atypical cerebral contributes to reading impairments. AI-powered handwriting tools, trained on kinematic from children's writing samples, enable early identification of dyslexic patterns reflective of motor and lateralization deficits, enabling interventions before formal diagnosis. Similarly, models applied to fMRI reveal reduced left-hemisphere dominance in dyslexic adults during tasks, paving the way for -informed screening protocols.

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