Essentially, these represent the significant groups of voxels showing significant functional lateralisation in Neurosynth. The preliminary step that included the embedding in the first two dimensions Fig.
A t -ratio test, i. The same analysis was used to explore other dimensions and revealed three additional triangles and a fourth vertex given by decision-making. Low-dimensional structure of functional lateralisation. Embedded in two-dimensional a and three-dimensional b space according to similarity in their lateralisation profile MATLAB interactive 3D file available as Supplementary Data 1.
See Supplementary Figures 1 and 2 for the spatial embedding of all Neurosynth terms. Here, to provide a graphical summary of all results reported in the section, we plotted the significantly lateralised components maps, named by the highest-loading terms Supplementary Table 3 , in place of the actual Neurosynth terms Supplementary Table 1. The archetype maps corresponding to the symbolic communication axis was characterised by a left dominant activation of the dorsal and ventral posterior part of the frontal lobe, including Broca area and the presupplementary motor area, the posterior part of the temporal lobe, including Wernicke area and the Visual Word Form Area i.
Right dominant activations were located in the posterior lobe of the cerebellum, including area Crus II Fig. Archetypes of functional lateralisation.
Upper panel corresponds to the lateral view, middle panel to the medial view and lower panel to the cerebellum view lateral and posterior views of the reconstructed pattern of activations. Right dominant activations included frontal eye field, intraparietal region, and ventral frontal regions, frontal eye field, presupplementary motor area, basal forebrain and anterior cerebellum i.
The archetype emotion map involved the left anterior cingulate cortex, the basolateral complex of the right amygdala, the posterior part of the right inferior frontal gyrus, the right intraparietal sulcus and the posterior part of the right temporal lobe Fig. Finally, the decision-making archetype map involved mostly the right prefrontal cortex i.
Given that the microscopic diffusion of water molecules in the brain is easier along rather than across axons, tractography derived from diffusion-weighted magnetic resonance imaging allows for peering into the structural organisation of brain connectivity Fig. Lateralisation and inter-hemispheric connectivity. In the following, we analysed the relationship between functional lateralisation and corpus callosum connectivity measures by contrasting the connectivity profiles of lateralised and non-lateralised regions see Methods for the definition of non-lateralised regions.
Two measures of connectivity were employed, both computed by averaging across participants in the HCP sample. The first measure, axonal water fraction 28 , is microscopic and is estimated in the midsection of the corpus callosum crossed by streamlines originating from voxels of a selected cortical region.
The second measure is macroscopic and estimates the replicability of connections 29 , 30 calculated as the proportion of participants in which a voxel is connected to the corpus callosum, which we will refer to as probability of connection for shortness.
By sampling voxels from lateralised and non-lateralised regions, in each hemisphere separately, we first constructed the distribution of the differences in the axonal water fraction between lateralised and non-lateralised regions. Figure 3b indicates that the axonal water fraction was consistently lower for corpus callosum voxels projecting onto lateralised regions when compared to non-lateralised voxels.
Additionally, the plots suggested a slightly lower axonal water fraction for left hemisphere regions as compared to the right hemisphere. Next, we constructed an analogous distribution for the probability of connection. Figure 3c demonstrates that lateralised regions when compared to non-lateralised voxels did not differ in this macrostructural measure of connectivity.
The previous analysis failed to reveal a categorical difference between lateralised and non-lateralised regions in macroscopic measure of connectivity.
However, the degree of functional hemispheric dominance see Methods for the definition of this measure can vary —from a unilateral to a relatively asymmetric pattern of activity. In the latter case, both hemispheres are involved in a function, but one is more active than the other.
Therefore, we explored whether a proportional relationship existed between the degree of functional lateralisation and the probability of corpus callosum connectivity. As the overall level of activation of two homotopic areas in the left and the right hemispheres may have an influence on its corpus callosum connections, we duplicated the same analysis after regressing out the left and right hemispheres average level of activity for every functionally lateralised voxel.
Additional supplementary analyses indicated that there was no relationship between the difference in corpus callosum connectivity of lateralised and non-lateralised voxels and their distance from the midsection of the corpus callosum Supplementary Figure 3.
In the present study, we provide for the first time a comprehensive mapping of the functional brain architecture of lateralised cognitive functions. Additionally, lateralised regions, as compared to non-lateralised regions, were connected to regions of the corpus callosum with reduced microstructural connectivity. Finally, within the pool of lateralised regions, corpus callosum macrostructural connectivity was proportionally associated with the degree of hemispheric functional dominance.
The meta-analysis of task-related activation maps in relation to cognitive terms replicated several known functional lateralisation profiles Figs. However, it did show a strong negative weighting on the language component, corroborating previous reports of a balance between language and attention in similar brain regions Lateralised maps in the left and right cortex had counterparts in the contralateral cerebellum in agreement with a role of the cerebellum in supporting cognition 36 , 37 , and the known anatomical organisation of cortico-pontine-cerebellar-thalamic pathways Even the phylogenetical organisation of functional regions in cerebellar networks was respected For instance, area Crus II, part of the neo-cerebellum, connected with frontal regions involved in language, showed a significant right lateralisation for the language component.
These findings support the validity and the anatomical precision of the functional lateralisation maps based on fMRI meta-analyses also see for comparison our meta-analytic results with a task-based fMRI results on finger tapping in Supplementary Figure 4. The overall functional lateralisation of the brain could be summarised with a low-dimensional architecture defined by spatial patterns of activity and groups of cognitive terms. The association of different terms along the different axis defined domains of function that are not trivially associated.
As recently shown, attention and motor deficits co-vary after focal lesions The triangular organisation of this morphospace may be significant in relation to the theory of Pareto optimality.
In evolutionary biology 27 , the theory posits that in complex systems e. Pareto optimality distributions in human cognition and behaviour have been recently reported in association with the ability to inhibit immediate reward for larger delayed rewards, a trait associated with numerous other cognitive, behaviour, health and socioeconomic variables The low-dimensional structure of lateralised functions is consistent with previous findings that reported a low-dimensional structure of functional networks 46 and of cognitive performance in both healthy controls 47 and patients Accordingly, individual performances or deficits are not task-specific but instead shared across a range of cognitive tasks.
For example, in stroke patients, two axes of behavioural deficits, one related to language and the other to attention-motor functions, occur Our result suggests that, at least in stroke, two supplementary axes of deficits might exist along the emotional and decision-making dimensions and that these dimensions tend to be under-represented by the standard behavioural and cognitive examinations.
Additionally, the similarity between the grand-scale organisation of functional lateralisation in healthy controls and behavioural deficits in stroke points to the importance of inter-hemispheric connection for recovery from stroke as shown recently by several studies 9 , The distribution of the probability of connection of the corpus callosum onto the brain surface matched the previous atlases that were derived from inter-hemispheric homotopic functional connectivity analyses Extra conduction time and energy consumption are required to integrate information across hemispheres.
Therefore, the role of inter-hemispheric connectivity for functional lateralisation has long been debated in the literature The current study presents a comprehensive demonstration that functional lateralisation is linked to a decrease of callosal function 51 i.
The alternative hypothesis that functional lateralisation depends on a competition between the hemispheres that inhibits each other via the corpus callosum, hence predicting stronger connectivity in lateralised regions, is not supported. Notably, reduced inter-hemispheric communication may improve processing time of lateralised functions, but it may lead to a decreased capacity to recover after a brain injury. This is an issue that deserves further studies as recent studies indicate a proportional recovery similar for different functions motor, vision, visuospatial attention, language, memory It is important to stress that several factors limited the interpretation of the findings.
For instance, while the meta-analytic approach has the power to summarise thousands of task-related fMRI findings, it is limited by publication biases which prevent a generalisation of the current findings to all brain functions Additionally, the experimental paradigms probing brain function may systematically use the same or similar material which may have biased some of the asymmetries reported. For example, processes such as emotion are frequently assessed using emotional faces that typically involve the right hemisphere more than the left hemisphere 55 , This appeared to have had a limited effect on our results, because the maps driving the emotion axis did not involve the face fusiform area that is specialised in face perception However, we cannot rule out the possibility that biases in label selection by the experimenters that ran the studies housed in Neurosynth may in part affect our findings.
Another issue concerns whether the left lateralisation of some functions, such as finger tapping, movement and touch, could be related to the laterality of stimulus presentation or response.
While we cannot rule out this possibility, lesion studies indicate that apraxia, a deficit of motor planning and control, occurs more frequently and severely after left hemisphere damage 41 , Moreover, the effect of the laterality of stimulus presentation or response is often counterbalanced by the use of both hands or mask out using control tasks.
In addition, we found an agreement between the foci of lateralised response in left SMA and left thalamus identified in our meta-analysis, and the results of a finger tapping task in a functional MRI study of right-handed participants that controlled for the laterality of the manual response Supplementary Figure 4. A third limitation, which is not specific to the current study, is that fMRI signal on the medial wall can be blurred at the acquisition stage, due to voxel size and spatial smoothing applied to the fMRI data as a standard and typically compulsory preprocessing step.
This problem can limit the ability to detect lateralised regions along the medial wall of the brain or in regions close to the midline. Even though we observed several lateralised regions on the medial walls of the brain, it is not possible to estimate how many putatively lateralised regions were lost due to limited spatial resolution. Finally, the limitation of the connectivity analyses derived from diffusion-weighted imaging 58 also prevented us from investigating with confidence the distinct contribution of homotopic and heterotopic areas to the functional lateralisation as well as smaller inter-hemispheric connections such as the anterior commissure, hippocampal commissure, massa intermedia i.
The advent of new diffusion imaging methods 60 , as well as post-mortem investigations 61 , might circumvent this bias in the future. It also reveals some of its mechanisms, such as the relationship between functional lateralisation and the strength of communication between the hemispheres.
The similarity between the current findings and recent work on neurological symptoms give rise to new hypotheses on the mechanisms that support brain recovery after a brain lesion. In this study we used a meta-analytic approach to the functional MRI studies described by Yarkoni et al. We downloaded the Neurosynth database that contained reversed unthresholded functional maps and the details of 11, literature sources as of the 25th of September Two researchers V.
The selection procedure consisted of two stages. During the first stage, the judges made their selection independently. Brain anatomical e. For the remaining terms, the judges made their decision together. In the end, cognitive terms were selected for the study. In the present analysis, we corrected for the anatomical differences between the left and the right hemispheres to focus on the functional asymmetries. The estimated transformation between non-symmetrical and symmetrical MNI spaces were then applied to all functional maps.
The following steps were performed to obtain lateralisation indices for each functional map following their co-registration with the symmetrical template. We then flipped the left hemisphere maps and subtracted them from unflipped right hemisphere maps in order to obtain laterality indices LI maps see 24 for a similar approach.
Positive and negative values in these maps would signify a higher meta-analytic evidence for, respectively, right and left lateralisation of the function associated with a term. The scanning parameters have previously been described in Vu et al. In brief, each diffusion-weighted imaging consisted of a total of near-axial slices acquired with an acceleration factor of 3 ref.
At each slice location, diffusion-weighted images were acquired with 65 uniformly distributed gradients in multiple Q-space shells 65 and 6 images with no diffusion gradient applied. The default HCP preprocessing pipeline v3. In short, the susceptibility-induced off-resonance field was estimated from pairs of images with diffusion gradient applied with distortions going in opposite directions 68 and corrected for the whole diffusion-weighted dataset using TOPUP A damped Richardson-Lucy algorithm was applied for spherical deconvolutions Two hundred algorithm iterations were run.
Second, a study-specific template of streamline density volumes was generated using the Greedy symmetric diffeomorphic normalisation GreedySyN pipeline distributed with ANTs. This provided an average template of the streamline density volumes for all subjects. This step produced a streamline density template in the MNI space. Third, individual streamline density volumes were registered to the streamline density template in the MNI space template and the same transformation was applied to the individual whole-brain streamline tractography using the trackmath tool distributed with the software package Tract Querier 75 , and to the axonal water fraction maps, using ANTs GreedySyn.
This step produced a whole-brain streamline tractography and axonal water fraction maps in the standard MNI space. In these analyses, completed in two steps, we thought to identify the regions with significant functional lateralisation. See Supplementary Figure 5. In the first step, we addressed the redundancy while preserving the richness of the Neurosynth data. For instance, many selected terms were related as singular and plural forms of the same word e.
Following a standard principal component analysis, involving the eigendecomposition of the covariance matrix, extracted orthogonal components with eigenvalues more than the grand average were submitted to the varimax-rotation procedure using Kaiser normalisation criterion 79 , with a maximum of iterations for convergence.
This accounted for The distribution of loadings along varimax-rotated principal components is typically skewed and only a few items receive large loadings. Britannica, T. Editors of Encyclopaedia , January Broca area. Encyclopedia Britannica. Editors of Encyclopaedia , March Wernicke area. Clements, A. Sex differences in cerebral laterality of language and visuospatial processing. Brain and Language, 98 2 , Gazzaniga, M. Facial recognition and brain asymmetries: Clues to underlying mechanisms.
Olulade, O. The neural basis of language development: Changes in lateralization over age. Proceedings of the National Academy of Sciences, 38 , Paradiso, S. Cerebral blood flow changes associated with attribution of emotional valence to pleasant, unpleasant, and neutral visual stimuli in a PET study of normal subjects. American Journal of Psychiatry, 10 , Reber, J. Sex differences in the functional lateralization of emotion and decision making in the human brain.
Journal of Neuroscience Research, 95 , Silberman, E. Hemispheric lateralization of functions related to emotion. Brain and Cognition, 5 3 , Sperry, R.
Split-brain approach to learning problems. The neu. Starkstein, S. Mood changes after right-hemisphere lesions. The British Journal of Psychiatry, 1 , Szaflarski, J. Cerebral asymmetry itself is pervasive in the animal kingdom Rogers et al.
A general left-hemispheric bias for action dynamics exists in many species, including marine animals and some primates MacNeilage, Conversely, a right-hemisphere dominance for emotion seems to be present in all primates so far investigated, suggesting an evolutionary continuity going back at least 30 to 40 million years Lindell, The sense of a trade-off is also suggested by the fact that cerebral and behavioral asymmetries are seldom if ever universal, unlike asymmetries of the internal organs in which the vast majority of individuals show the same asymmetries.
Where a given direction of asymmetry is the norm, the proportion of individuals exhibiting the asymmetry lies within the range of about 65—90 percent—a range that seems to apply across the animal kingdom Ghirlanda and Vallortigara, , with human handedness and cerebral asymmetry at the top of the range. In contrast, the asymmetries of the internal organs are remarkably consistent, with only about one in 10, people showing reversal, a condition known as situs inversus Torgersen, In the brain, the relative demands of symmetry and asymmetry may therefore be labile, and there may even be population-level advantages in variation.
Perhaps the inclusion of a minority of left-handers led to an advantage in warfare or in some sports, but only so long as they remained a minority. There is some evidence that mixed handers are more creative than right- or left-handers handers Shobe et al.
Cerebral asymmetry for language is often linked to handedness. For example, Bruner suggested that the functional difference between the hands could be extrapolated to the cerebral hemispheres, with the right hemisphere holding the context while the left provides the operation, the actual output.
More generally, the link between handedness and brain asymmetry may have come about in the evolution of complex manual activities such as the manufacture and use of tools, or more directly through gestural communication itself. Indeed there are some compelling reasons to suppose that language evolved from manual gestures rather than from primate calls e.
For example, it has proven virtually impossible to teach great apes anything resembling vocal language, but gestural forms of communication with language-like properties seems to come about quite naturally in chimpanzees, bonobos, and gorillas Savage-Rumbaugh et al. Again, signed languages are purely gestural, with no functional acoustic component, yet carry all the hallmarks of true language Emmorey, Nevertheless handedness itself is actually rather poorly correlated with cerebral asymmetry for language.
Some 95—99 percent of right-handers are left-hemispheric for language, but so are some 70—80 percent of left-handers Corballis et al. Different aspects of hemispheric asymmetry are also poorly correlated; one study, for example, shows zero correlation between left-hemispheric dominance for language and right-hemispheric dominance for spatial attention Badzakova-Trajkov et al.
In another study of brain activity recorded in participants at rest, factor analysis of asymmetries at different sites indicated four independent lateralized networks, two favoring the left hemisphere and two the right Liu et al. Such findings suggest that cerebral asymmetry is not due to some all-encompassing gradient, but depends on multiple influences. Indeed, attempts to locate a single laterality gene have largely failed, and it has been suggested that as many as 40 different genes may be involved McManus et al.
Factor analyses of task-evoked brain activity also suggest independent circuits. In one study fMRI responses to word generation, processing of faces making emotional expressions, and the landmark test a measure of spatial attention yielded three orthogonal factors Badzakova-Trajkov et al.
Another was linked to spatial attention, with right-hemispheric activation predominantly in the pars opercularis, the inferior and superior parietal lobules, and the supramarginal gyrus. The third was also a right-hemispheric circuit linked to face processing, with activation predominantly in the pars triangularis, the fusiform gyrus, and the middle temporal gyrus. Independent circuits also seem to exist within the left hemisphere. Gonzalez et al. There may be a closer relation, though, between pantomime and language.
Vingerhoets et al. Eighty percent of the participants in each group were left-handed, leaving some question as to whether the relation would hold among right-handers. In a sample of right-handers, Xu et al. Again, though, factor analysis opens the possibility of a more comprehensive account. One was clearly language related, loading on activity in language areas when the participants undertook either of the language tasks.
Another, loading on parietal and frontal areas, was activated by observation of actions and was strongly associated with handedness. The third involved frontal and temporal areas partly overlapping with the language circuit, although uncorrelated with it. This circuit was also associated with action observation, but was independent of the handedness circuit. It was the least lateralized and may well be the residue of the original mirror system, dedicated to simple acts such as grasping and reaching Marangon et al.
It is unlikely that new circuits in the brain emerge de novo , but are formed from ancestral systems. This can occur in several ways: through expansion and fissioning of an ancestral system into separate systems, through copying and differential modification of an existing circuits, or sometimes through modified circuits fusing to create new functions. These processes in turn can involve the splitting of genes, rather than the emergence of new genes Oakley and Rivera, The evolution of new and more specialized circuits may also have increased pressure to lateralization, enabling more efficient packaging and less redundancy and competition.
Such pressure may have been especially intense in hominin evolution, as our forebears adapted to increased social and environmental complexity. In this last example given above, the three circuits may well have derived from the primate mirror system, which responds both when a monkey makes an intentional movement such as grasping a piece of food, and when it observes another individual making the same movement Rizzolatti and Sinigaglia, This ancestral system seems to provide an ideal platform for the evolution of language, since it relates the perception of action to its production, and indeed can be taken as further support for the idea that language evolved from manual gestures Rizzolatti and Arbib, ; Corballis, The mirror property also provides the basis for mutual understanding between speaker and listener or signer and watcher , an understanding that goes beyond the words themselves and is indeed necessary for effective communication Sperber and Wilson, Within the left hemisphere, then, the language circuit may have fissioned from the ancestral mirror system, and may have been the first new circuit to form, since it was the most lateralized of the three.
The circuit linked to handedness may have split off as an adaptation to the use and manufacture of tools, in which handedness is most strongly expressed. And as suggested above, the third and least lateralized circuit, which was independent of handedness, may be the residue of the ancestral mirror system.
A similar fissioning may explain the lateralized representation of reading. Behrmann and Plaut document evidence that the fusiform gyrus in the primate brain is specialized for face recognition. In humans, the emergence of literacy resulted in a split into a right-hemispheric system for face recognition and a left-hemispheric one for the recognition of printed words, at least among people who have learned to read literacy is still not universal. This would also have created the asymmetry required for mirror-image discrimination, as in the distinct recognition of letters like b and d or words like was and saw.
It is perhaps not so much a question of recycling, though, as one of the invasion of cortical territory initially dedicated to one function by a related but more specialized function. This complementarity probably goes beyond the fusiform area.
In the analysis by Badzakova-Trajkov et al. Some asymmetries, then, probably arise as a secondary consequence of an asymmetry emerging in one hemisphere, so the other hemisphere assumes dominance over a function that was previously bilateral. Although this scenario suggests that complementarity can arise in the emergence of asymmetries, it does not support the global view of the so-called dual brain, in which each cerebral hemisphere is assumed to represent complementary but global aspects of human cognition, variously characterized as linear, analytical and rational on the left, and divergent, holistic and intuitive on the right e.
This view has persisted to a remarkable degree in modern scholarship e. Left-brained adj : 1 Having the left brain dominant. Right-brained adj : 1 Having the right brain dominant. It has become clear that cerebral asymmetries are more complex and multidimensional, both in terms of their circuitry and their genetic underpinnings. Moreover, cerebral asymmetries are never absolute; even in a strongly left-lateralized function such as language, the right hemisphere makes a significant contribution e.
Definition of lateralization. Examples of lateralization in a Sentence Recent Examples on the Web The type of lateralization most familiar to people is undoubtedly handedness. First Known Use of lateralization , in the meaning defined above.
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