RESUMO
Until fairly late in the nineteenth century, it was held that the brain was bilaterally symmetrical. With the discovery of left-brain dominance for language, the so-called "laws of symmetry" were revoked, and asymmetry was then seen as critical to the human condition, with the left hemisphere, in particular, assuming superordinate properties. I trace this idea from the early discoveries of the late nineteenth century through the split-brain studies of the 1960s, and beyond. Although the idea has persisted, the evidence has revealed widespread cerebral asymmetries in nonhuman animals, and even language and its asymmetries are increasingly understood to have evolved gradually, rather than in a single speciation event. The left hemisphere nevertheless seemed to take over a role previously taken by other structures, such as the pineal gland and the hippocampus minor, in a determined effort to place humans on a pedestal above all other species.
Assuntos
Encéfalo , Lateralidade Funcional , Animais , Mapeamento Encefálico , Dominância Cerebral , Humanos , IdiomaRESUMO
It is commonly assumed that cerebral asymmetry is unidimensional, but evidence increasingly suggests that different brain circuits are independently lateralized. This might explain why the search for a laterality gene has provided multiple candidates, each with weak linkage. An alternative possibility is that there is a single genetically invariant source of lateralization, perhaps cytoplasmic, and subject to many influences, some genetic, some epigenetic, and some random. This could further explain why laterality is associated with a variety of disorders, such as dyslexia, schizophrenia, stress disorders, and depression.
Assuntos
Dislexia , Esquizofrenia , Encéfalo , Lateralidade Funcional , HumanosRESUMO
Recently, the discussion regarding the consequences of cutting the corpus callosum ("split-brain") has regained momentum (Corballis, Corballis, Berlucchi, & Marzi, Brain, 141(6), e46, 2018; Pinto et al., Brain, 140(5), 1231-1237, 2017a; Pinto, Lamme, & de Haan, Brain, 140(11), e68, 2017; Volz & Gazzaniga, Brain, 140(7), 2051-2060, 2017; Volz, Hillyard, Miller, & Gazzaniga, Brain, 141(3), e15, 2018). This collective review paper aims to summarize the empirical common ground, to delineate the different interpretations, and to identify the remaining questions. In short, callosotomy leads to a broad breakdown of functional integration ranging from perception to attention. However, the breakdown is not absolute as several processes, such as action control, seem to remain unified. Disagreement exists about the responsible mechanisms for this remaining unity. The main issue concerns the first-person perspective of a split-brain patient. Does a split-brain harbor a split consciousness or is consciousness unified? The current consensus is that the body of evidence is insufficient to answer this question, and different suggestions are made with respect to how future studies might address this paucity. In addition, it is suggested that the answers might not be a simple yes or no but that intermediate conceptualizations need to be considered.
Assuntos
Estado de Consciência/fisiologia , Procedimento de Encéfalo Dividido , Atenção , Corpo Caloso/fisiopatologia , HumanosRESUMO
During the last decade, the ontogeny of tool making has received growing attention in the literature on tool-related behaviors. However, the cognitive demands underlying tool making are still not clearly understood. In this cross-sectional study of 52 Turkish preschool children from 3 to 6â¯years of age, the roles of executive function (response inhibition), ability to form hierarchical representations (hierarchical structuring), and social learning were investigated with the hook task previously used with children and animals. In this task, children needed to bend a pipe cleaner to fetch a small bucket with a sticker out of a tall jar. This study replicated earlier findings that preschoolers have great difficulty in tool innovation. However, social learning facilitates tool making, especially after 5â¯years of age. Capacities to form hierarchical representations and to inhibit prepotent responses were significant positive predictors of tool making after social learning.
Assuntos
Cognição/fisiologia , Função Executiva/fisiologia , Inibição Psicológica , Aprendizado Social , Atenção/fisiologia , Criança , Pré-Escolar , Compreensão/fisiologia , Estudos Transversais , Feminino , Humanos , MasculinoRESUMO
Cognition is heavily grounded in space. As animals that move in space, we travel both physically and mentally in space and time, reliving past events, imagining future ones, and even constructing imaginary scenarios that play out in stories. Mental exploration of space is extraordinarily flexible, allowing us to zoom, adopt different vantage points, mentally rotate, and attach objects and sense impressions to create events, whether remembered, planned, or simply invented. The properties of spatiotemporal cognition depend on a hippocampal-entorhinal circuit of place cells, grid cells and border cells, with combinations of grid-cell modules generating a vast number of potential spatial remappings. The generativity of language, often considered one of its defining properties, may therefore derive not from the nature of language itself, but rather from the generativity of spatiotemporal scenarios, with language having evolved as a means of sharing them. Much our understanding of the hippocampal-entorhinal circuit is derived from neurophysiological recording in the rat brain, implying that the spatiotemporal cognition underpinning language has a long evolutionary history.
Assuntos
Cognição/fisiologia , Hipocampo/fisiologia , Imaginação/fisiologia , Idioma , Animais , Humanos , Modelos Neurológicos , Percepção Espacial/fisiologiaRESUMO
Handedness and brain asymmetry are widely regarded as unique to humans, and associated with complementary functions such as a left-brain specialization for language and logic and a right-brain specialization for creativity and intuition. In fact, asymmetries are widespread among animals, and support the gradual evolution of asymmetrical functions such as language and tool use. Handedness and brain asymmetry are inborn and under partial genetic control, although the gene or genes responsible are not well established. Cognitive and emotional difficulties are sometimes associated with departures from the "norm" of right-handedness and left-brain language dominance, more often with the absence of these asymmetries than their reversal.
Assuntos
Córtex Cerebral/fisiologia , Lateralidade Funcional/genética , Fala/fisiologia , Animais , Córtex Cerebral/anatomia & histologia , Córtex Cerebral/patologia , Dislexia/genética , Dislexia/metabolismo , Dislexia/fisiopatologia , Fatores de Transcrição Forkhead/genética , Expressão Gênica , Humanos , Padrões de Herança , Idioma , Proteínas de Membrana/genética , Proteínas do Tecido Nervoso/genética , Polimorfismo de Nucleotídeo Único , Pró-Proteína Convertases/genética , Esquizofrenia/genética , Esquizofrenia/metabolismo , Esquizofrenia/fisiopatologia , Serina Endopeptidases/genéticaRESUMO
Many children pass through a mirror stage in reading, where they write individual letters or digits in mirror and find it difficult to correctly utilize letters that are mirror images of one another (e.g., b and d). This phenomenon is thought to reflect the fact that the brain does not naturally discriminate left from right. Indeed, it has been argued that reading acquisition involves the inhibition of this default process. In the current study, we tested the ability of literate pigeons, which had learned to discriminate between 30 and 62 words from 7832 nonwords, to discriminate between words and their mirror counterparts. Subjects were sensitive to the left-right orientation of the individual letters, but not the order of letters within a word. This finding may reflect the fact that, in the absence of human-unique top-down processes, the inhibition of mirror generalization may be limited.
Assuntos
Columbidae/fisiologia , Reconhecimento Visual de Modelos , Leitura , Animais , Comportamento Animal , Condicionamento OperanteRESUMO
Hemispheric asymmetry is commonly viewed as a dual system, unique to humans, with the two sides of the human brain in complementary roles. To the contrary, modern research shows that cerebral and behavioral asymmetries are widespread in the animal kingdom, and that the concept of duality is an oversimplification. The brain has many networks serving different functions; these are differentially lateralized, and involve many genes. Unlike the asymmetries of the internal organs, brain asymmetry is variable, with a significant minority of the population showing reversed asymmetries or the absence of asymmetry. This variability may underlie the divisions of labor and the specializations that sustain social life. (JINS, 2017, 23, 710-718).
Assuntos
Encéfalo/fisiologia , Lateralidade Funcional/fisiologia , Humanos , IndividualidadeRESUMO
Information regarding anatomical connectivity in the human brain can be gathered using diffusion tensor imaging (DTI). Fractional anisotropy (FA) is the most commonly derived value, and reflects how strongly directional are the underlying tracts. Differences in FA are thus associated with differences in the underlying microstructure of the brain. The relationships between these differences in microstructure and functional differences in corresponding regions have also been examined. Previous studies have found an effect of handedness on functional lateralization in the brain and corresponding microstructural differences. Here, using tract-based spatial statistics to analyse DTI-derived FA values, we further investigated the structural white matter architecture in the brains of right- and left-handed males. We found significantly higher FA values for left-handed, relatively to right-handed, individuals, in all major lobes, and in the corpus callosum. In support of previous suggestions, we find that there is a difference in the microstructure of white matter in left- and right-handed males that could underpin reduced lateralization of function in left-handed individuals.
Assuntos
Encéfalo/diagnóstico por imagem , Lateralidade Funcional , Substância Branca/diagnóstico por imagem , Adulto , Imagem de Tensor de Difusão , Humanos , Processamento de Imagem Assistida por Computador , Masculino , Vias Neurais/diagnóstico por imagem , Adulto JovemRESUMO
In 46 right-handers and 46 left-handers, we used functional magnetic resonance imaging to record activity in the frontal lobes while they generated words, the temporal lobe while they made synonym judgments, and the parietal lobe while they watched videos of manual actions. In each case we also recorded activity in the cerebellum. Laterality indices showed a significant left-hemispheric bias in each cortical lobe and a right-hemispheric bias in the cerebellum for the 2 language tasks, but not during action observation. Cerebellar asymmetry also correlated negatively with frontal and temporal asymmetry, reflecting contralateral connections, but not with parietal asymmetry. A factor analysis of the inter-correlations among laterality indices revealed 2 factors, implying independent lateralized networks, with cerebellar asymmetry strongly linked to a language network in frontal and temporal cortices, and handedness strongly linked to an action-observation network in the parietal lobe.
RESUMO
Phil Bryden was a seminal figure in the development of the field of cerebral lateralization in the last half of the twentieth century, and a founding editor of this journal. Here his founding co-editors reminisce about their friend and colleague, and reflect on his wide-ranging influence in the field and in their own careers.
RESUMO
The role of the hippocampus in imagining the future has been of considerable interest. Preferential right hippocampal engagement is observed for imagined future events relative to remembered past events, and patients with hippocampal damage are impaired when imagining detailed future events. However, some patients with hippocampal damage are not impaired at imagining, suggesting that there are conditions in which the hippocampus may not be necessary for episodic simulation. Given the known hippocampal role in memory encoding, the hippocampal activity associated with imagining may reflect the encoding of simulations rather than event construction per se. The present functional (f)MRI study investigated this possibility. Participants imagined future events in response to person, place, and object cues. A postscan cued-recall test probing memory for detail sets classified future events as either successfully encoded or not. A contrast of successfully versus unsuccessfully encoded events revealed anterior and posterior right hippocampal clusters. When imagined events were successfully encoded, both anterior and posterior hippocampus showed common functional connectivity to a network including parahippocampal gyrus, medial parietal and cingulate cortex, and medial prefrontal cortex. However, when encoding was unsuccessful, only the anterior hippocampus, and not the posterior, exhibited this pattern of connectivity. These findings demonstrate that right hippocampal activity observed during future simulation may reflect the encoding of the simulations into memory. This function is not essential for constructing coherent scenarios and may explain why some patients with hippocampal damage are still able to imagine the future.
Assuntos
Previsões , Hipocampo/fisiologia , Imaginação/fisiologia , Adolescente , Adulto , Mapeamento Encefálico , Sinais (Psicologia) , Humanos , Imageamento por Ressonância Magnética/métodos , Masculino , Memória/fisiologia , Adulto JovemRESUMO
We used diffusion tensor imaging to assess callosal morphology in 35 pairs of monozygotic twins, of which 17 pairs were concordant for handedness and 18 pairs were discordant for handedness. Functional hemispheric language dominance was established for each twin member using fMRI, resulting in 26 twin pairs concordant and 9 twin pairs discordant for language dominance. On the basis of genetic models of handedness and language dominance, which assume one "right shift" (RS) gene with two alleles, an RS+ allele biasing toward right-handedness and left cerebral language dominance and an RS- allele leaving both asymmetries to chance, all twins were classified according to their putative genotypes, and the possible effects of the gene on callosal morphology was assessed. Whereas callosal size was under a high genetic control that was independent of handedness and language dominance, twin pairs with a high probability of carrying the putative RS+ allele showed a connectivity pattern characterized by a genetically controlled, low anisotropic diffusion over the whole corpus callosum. In contrast, the high connectivity pattern exhibited by twin pairs more likely to lack the RS+ allele was under significantly less genetic influence. The data suggest that handedness and hemispheric dominance for speech production might be at least partly dependent on genetically controlled processes of axonal pruning in the corpus callosum.
Assuntos
Corpo Caloso/fisiologia , Dominância Cerebral/fisiologia , Lateralidade Funcional/fisiologia , Idioma , Gêmeos Monozigóticos/genética , Adolescente , Adulto , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Desempenho Psicomotor/fisiologia , Adulto JovemRESUMO
We investigated whether abnormalities in the structural organization of the corpus callosum in the presence of curvilinear lipoma are associated with increased facilitation of response time to bilateral stimuli, an effect known as the redundancy gain (RG). A patient (A.J.) with a curvilinear lipoma of the corpus callosum, his genetically-identical twin, and age-matched control participants made speeded responses to luminant stimuli. Structural organization of callosal regions was assessed with diffusion-tensor imaging. A.J. was found to have reduced structural integrity in the splenium of the corpus callosum and produced a large RG suggestive of neural summation.
Assuntos
Neoplasias Encefálicas/diagnóstico , Neoplasias Encefálicas/fisiopatologia , Corpo Caloso/patologia , Lateralidade Funcional , Lipoma/diagnóstico , Lipoma/fisiopatologia , Adulto , Análise de Variância , Estudos de Casos e Controles , Corpo Caloso/fisiopatologia , Imagem de Tensor de Difusão/métodos , Feminino , Humanos , Imageamento Tridimensional , Masculino , Estimulação Luminosa , Desempenho Psicomotor , Tempo de ReaçãoRESUMO
Left-hemispheric dominance for language and right-hemispheric dominance for spatial processing are distinctive characteristics of the human brain. However, variations of these hemispheric asymmetries have been observed, with a minority showing crowding of both functions to the same hemisphere or even a mirror reversal of the typical lateralization pattern. Here, we used diffusion tensor imaging and functional magnetic imaging to investigate the role of the corpus callosum in participants with atypical hemispheric dominance. The corpus callosum was segmented according to the projection site of the underlying fibre tracts. Analyses of the microstructure of the identified callosal segments revealed that atypical hemispheric dominance for language was associated with high anisotropic diffusion through the corpus callosum as a whole. This effect was most evident in participants with crowding of both functions to the right. The enhanced anisotropic diffusion in atypical hemispheric dominance implies that in these individuals the two hemispheres are more heavily interconnected.
Assuntos
Mapeamento Encefálico , Corpo Caloso/fisiologia , Lateralidade Funcional/fisiologia , Imagem de Tensor de Difusão , Feminino , Humanos , Interpretação de Imagem Assistida por Computador , Imageamento por Ressonância Magnética , Masculino , Vias Neurais/fisiologia , Adulto JovemRESUMO
Previous research has revealed a strong right bias in allocation of attention in split brain subjects, suggesting that a pathological attention bias occurs not only after unilateral (usually right-hemispheric) damage but also after functional disconnection of intact right-hemispheric areas involved in allocation of attention from those in the left hemisphere. Here, we investigated the laterality bias in spatial attention, as measured with the greyscales task, in two split-brain subjects (D.D.C. and D.D.V.) who had undergone complete callosotomy. The greyscales task requires participants to judge the darker (or brighter) of two left-right mirror-reversed luminance gradients under conditions of free viewing, and offers an efficient means of quantifying pathological attentional biases in patients with unilateral lesions. As predicted, the results of the two split-brain subjects revealed a pathological rightward bias in allocation of attention, suggesting strong dependence on a single hemisphere (the left) in spatial attention, which is opposite to what one expects from people with intact commissures, and is remarkable in that it occurs in free viewing. In that sense both split-brain patients are behaving as though the brain is indeed split, especially in D.D.C. who had undergone partial resection of the anterior commissure in addition to complete callosotomy, whereas the anterior commissure is still intact in D.D.V. The findings support the view that the commissural pathways play a significant role in integration of attentional processes across cerebral hemispheres.
Assuntos
Viés de Atenção , Procedimento de Encéfalo Dividido , Viés , Encéfalo/diagnóstico por imagem , Lateralidade Funcional , Humanos , Percepção EspacialRESUMO
The idea that the mind might be composed of distinct conscious entities goes back at least to the mid-19th century, and was at first based on the bilateral symmetry of the brain, with each side seemingly a mirror-image replica of the other. This led to early speculation as to whether section of the forebrain commissures might lead to separate, independent consciousnesses. This was not put to the test until the 1960s, first in commissurotomized cats and monkeys, and then in humans who had undergone commissurotomy for the relief of intractable epilepsy. Initial results did indeed suggest independent consciousness in each separated hemisphere, but later findings have also revealed a degree of mental unity, especially in some perceptual functions and in motor control. Some of these findings might be interpreted in terms of subcortical connections or external cross-cuing, and also address questions about the nature of consciousness in a more concrete way.
Assuntos
Encéfalo , Procedimento de Encéfalo Dividido , Estado de ConsciênciaRESUMO
Euan Macphail's work and ideas captured a pivotal time in the late 20th century when behavioral laws were considered to apply equally across vertebrates, implying equal intelligence, but it was also a time when behaviorism was challenged by the view that language was unique to humans, and bestowed a superior mental status. Subsequent work suggests greater continuity between humans and their forebears, challenging the Chomskyan assumption that language evolved in a single step ("the great leap forward") in humans. Language is now understood to be based on an amalgam of cognitive functions, including mental time travel, theory of mind, and what may be more broadly defined as imagination. These functions probably evolved gradually in hominin evolution and are present in varying degrees in non-human species. The blending of language into cognition provides for both interspecies differences in mental function, and continuity between humans and other species. What does seem to be special to humans is the ability to communicate the contents of imagination, although even this is not absolute, and is perhaps less adaptive than we like to think.