Biomedical subjects

F Aboitiz

Publications and source records attributed to F Aboitiz.

At least 19 recordsLinked to original sources

PubMed1997

The evolutionary origin of the language areas in the human brain. A neuroanatomical perspective.

The capacity to learn syntactic rules is a hallmark of the human species, but whether this has been acquired by the process of natural selection has been the subject of controversy. Furthermore, the cortical localization of linguistic capacities has prompted some authors to suggest a modular representation of language in the brain. In this paper, we rather propose that the neural device involved in language is embedded into a large-scale neurocognitive network comprising widespread connections between the temporal, parietal and frontal (especially prefrontal) cortices. This network is involved in the temporal organization of behavior and motor sequences, and in working (active) memory, a sort of short-term memory that participates in immediate cognitive processing. In human evolution, a precondition for language was the establishment of strong cortico-cortical interactions in the postrolandic cortex that enabled the development of multimodal associations. Wernicke's area originated as a converging place in which such associations (concepts) acquired a phonological correlate. We postulate that these phonological representations projected into inferoparietal areas, which were connected to the incipient Broca's area, thus forming a working memory circuit for processing and learning complex vocalizations. As a result of selective pressure for learning capacity and memory storage, this device yielded a sophisticated system able to generate complicated utterances (precursors of syntax) as it became increasingly connected with other brain regions, especially in the prefrontal cortex. This view argues for a gradual origin of the neural substrate for language as required by natural selection.

Animals↗

PubMed1996

Age-related changes in fibre composition of the human corpus callosum: sex differences.

We found positive correlations between the number of myelinated callosal fibres > 1 micron in diameter and age in humans. The relatively abundant axons with diameters between 1 and 3 microns correlated with age only in females, while the scarce fibres > 3 microns in diameter correlated significantly with age only in males. When analysing different callosal segments, it was found that in the midbody (but not in the splenium) of females the number of fibres > 3 microns also increased with age. In males, the relationship between these large diameter fibres and age disappeared after dividing the callosum into distinct segments. There may, therefore, be sex differences in the course of callosal fibre growth and myelination during the normal lifespan.

Adult↗

PubMed1996

Does bigger mean better? Evolutionary determinants of brain size and structure.

Current perspectives on brain evolution relate brain size variability to two main parameters: a scaling factor that corresponds to overall body size and an ecological factor associated with behavioral capacity. I suggest in this paper that in evolution body weight and ecological conditions have different effects on brain structure, resulting in distinct differences in neural architecture, even if both factors may produce brain size increases. There are two postulated modalities of brain growth, one passive that lags behind increases in body size, and one active that relates to selection of specific behavioral abilities and hence increased processing capacity. These two modes of growth differ in three main aspects: (i) cellular and connectional rearrangements are modest in passive brain growth while they are conspicuous in active growth, corresponding to increases in processing capacity; (ii) passive brain growth follows a rather conservative allometric rule between brain components, while active growth usually affects only a few brain parts, thereby producing much steeper allometric relations between these parts and sometimes also in brain/body relations; and (iii) passive growth may either affect early periods of ontogenic brain development or produce a generalized increase in cell proliferation in later periods. On the other hand, active growth is restricted to relatively late developmental periods. Finally, an evolutionary scenario for the active mode is proposed where phylogenetic selection of an increased number of cells in particular brain regions occurs in order to facilitate neural reorganization and to increase the specificity of connections. This view emphasizes the role of connectional modifications in increasing brain capacity and contrasts with current ideas of a unitary process of phylogenetic brain growth, where a larger brain size per se produces better processing capacity, regardless of the causal factor behind it.

Animals↗

PubMed1995

Working memory networks and the origin of language areas in the human brain.

Temporoparietal-prefrontal working memory networks are proposed as fundamental in the evolutionary origin of the language regions. Having a primordial capacity to name objects or situations, primitive hominids may have strongly benefited from the possibility to recall past events from memory, in order to refer to them through vocal communication. Working memory cortical networks are related to these types of tasks, and are arranged quite similarly to the language networks in the brain. It is possible that the language areas and their connections arose as a local specialization of these large-scale cortical networks, that developed as neural strategies to recall past events to be shared in community. The origin of syntax may have taken place after these networks were sufficiently stabilized, and (at least originally) may have been related to aspects of vocal motor control, involving the progressive differentiation of the anterior language areas and their connections.

Biological Evolution↗

PubMed1995

Homology in the evolution of the cerebral hemispheres. The case of reptilian dorsal ventricular ridge and its possible correspondence with mammalian neocortex.

The present paper reviews some issues related to the evolutionary origin of distinct components of the cerebral hemispheres in vertebrates, which entails the problem of biological homology between anatomical structures. Considering that the term homology is essentially a comparative concept, making emphasis on structural correspondences between organs or body parts, I use the term evolutionary, or phylogenetic homology to denote a common evolutionary origin of two characters. In particular, the controversy of a possible phylogenetic homology between reptilian dorsal ventricular ridge (DVR) and parts of mammalian neocortex is analyzed in some detail. Although it is likely that DVR is a derived character of reptiles while neocortex is a derived character of mammals, the two structures might still originate from the same primordial anlage in the common ancestor. One main problem in the comparison of telencephalic components between reptiles and mammals is that the protrusion of reptilian DVR into the lateral ventricle causes a distortion of the topographic relations in the hemisphere. In order to determine possible homologues of DVR, it is necessary to establish clear-cut telencephalic landmarks. Since lateral cortex is similarly localized in reptiles and mammals, it is suggested that the embryonic position and timing of development of reptilian DVR in relation to lateral cortex may give special insight on the phylogenetic origins of the former. If, as implied by the work of early authors, DVR arose in evolution through an extension of the embryonic period of neuronal proliferation and migration, it may be considered as a genuine novelty in brain evolution. It is also proposed that, regardless of whether DVR and extrastriate neocortex can or cannot be considered phylogenetic homologues, some of the integrative functions performed by them might have a common evolutionary origin, that became localized in reptilian DVR and in mammalian extrastriate neocortex.

Animals↗

PubMed1995

Sexual dimorphism in interhemispheric relations: anatomical-behavioral convergence.

An embryogenetic hypothesis states that hemispheric specialization is inversely related to callosal connectivity (Geschwind and Galaburda, 1985). We tested this hypothesis (i) anatomically by relating postmortem planum temporale asymmetry to regional callosal morphology and (ii) behaviorally by relating the right visual field advantage in a lateralized lexical decision task with associative primes to regional callosal morphometry using magnetic resonance imaging (MRI). The postmortem study showed a significant negative correlation between planum temporale asymmetry and the number of small diameter fibers in the isthmus of the corpus callosum, but only for males. The MRI study showed a significant negative correlation between the right visual hemifield advantage for associated words and the cross section size of the isthmus of the corpus callosum, but again only in males. There was no sex difference in either the anatomical asymmetry, the behavioral asymmetry, or the callosal morphology. These convergent results suggest that there is a sexual dimorphism in interhemispheric relations in humans.

Adult↗

PubMed1995

The anatomical substrates for language and hemispheric specialization.

Three main lines of investigation are discussed in this paper: (1) the comparison between the anatomical arrangement of the language areas and the large-scale neurocognitive cortical networks partly involved in active or working memory; (2) the relations between hemispheric specialization and the development of interhemispheric communication; and (3) the analysis of individual differences in brain organization for language. The hypothesis and evidence presented stem from work being performed in our laboratories.

Cerebral Cortex↗

PubMed1995

Evolutionary origins of the reptilian brain: the question of putative homologues of dorsal ventricular ridge. An overview and proposal.

The reptilian brain is characterized by a structure that bulges into the lateral ventricle, called dorsal ventricular ridge (DVR). The DVR was originally considered to be a part of the basal ganglia, although more recent studies indicate that it may correspond to the dorsal part of the hemisphere. The anterior portion of the DVR has several connectional and functional similarities with parts of the mammalian neocortex, for which reason it has been claimed that the two structures can be considered as homologues. In this article I review the evidence supporting and refuting homology of the DVR with different telencephalic structures of mammals, concluding that it is still early to unequivocally ascribe structural correspondences between the different components in the two vertebrate classes. However, a way out of the problem is suggested by comparing the embryonic position of DVR with that of lateral cortex in the reptilian hemisphere. The lateral cortex is considered to be quite comparable in reptiles and mammals, and hence may be a good marker for the original position of the DVR. If the DVR originates dorsal to lateral cortex, it may be considered comparable to parts of the mammalian neocortex, while if it develops in its same position or ventral to it, it may not correspond to the neocortex. Early embryological work indicated that the DVR develops in the same position as the lateral cortex, but arises as a late migration wave, after cells destined to lateral cortex are generated. In other words, instead of being interposed between dorsal and lateral cortices, the DVR may originate in a position overlapping with lateral cortex. If this alternative turns out to be the case, it may imply that the DVR arose de novo, through an extension of the ancestral period of neuroblast proliferation. As a consequence, there may be no structures comparable to it in other vertebrate classes. Finally, it is also proposed that, regardless of whether the DVR and the extrastriate neocortex can or cannot be considered phylogenetic homologues, some of the integrative functions performed by them might have a common evolutionary origin, that became localized in the reptilian DVR and in the mammalian extrastriate neocortex.

Animals↗

PubMed1994

The evolution of brain size and organization in vertebrates. A program for research.

In vertebrates, brain size variability relates to two main parameters: body size and ecological factors (in particular diet and foraging strategy). It has been considered by many authors that evolutionary brain growth is a unitary phenomenon whose main effect is to increase processing capacity. Alternatively, in this paper it is considered that brain growth is significantly associated with higher processing capacity only when it occurs associated with ecological circumstances (selection of behavioral or perceptual skills). This process is referred to as "active" growth. When the brain scales on body size, there is little change in processing capacity, and this will be referred to as "passive" growth. I propose that these two modes of phylogenetic brain growth relate to different developmental/evolutionary processes and are distinguishable at the level of adult and developing structure. Shortly, growth due to selection of behavioral capacities is associated with more differentiated brains in terms of number of areas, connectional rearrangements and cell types. Growth due to scaling of body mass produces little brain rearrangements, and many of those that occur relate to the maintenance of functions in a larger brain. In addition, active selection of brain size is triggered by plastic, ontogenic rearrangements of connectivity in the organisms, while passive growth produces the minor rearrangements that take place. Finally, I propose a research program oriented to test this model by separating the effects of body size and ecological variables in brain organization across species.

Adaptation, Physiological↗

PubMed1993

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Animals↗

PubMed1993

Further comments on the evolutionary origin of the mammalian brain.

This paper is an extension of a previous report on the origin of the mammalian neocortex. Two main aspects are elaborated. The first is the evolution of visual projections from the midbrain to the telencephalon, featuring the encephalization of visual functions. Associated to this, the progressive fusion of the two main visual systems (thalamofugal and tectofugal) in the mammalian telencephalon (striate and extrastriate cortex, respectively) is viewed in the context of increasing cortico-cortical connectivity in the evolution of the mammalian brain. In addition, the issue of a presumed homology between mammalian extrastriate cortex and reptilian anterior dorsal ventricular ridge (ADVR) is reviewed in some detail, and it is concluded that extrastriate cortex is a derived character of mammals while ADVR is a derived character of reptiles and birds. It is not likely that ADVR is ancestral to extrastriate cortex. The second aspect under analysis is the origin of the inverted (inside-out) lamination pattern of mammalian neocortex that differs from the outside-in pattern of reptilian cortex. Furthermore, mammals have developed a transient embryonic cell layer (the subplate zone) that serves as a waiting compartment for thalamic and cortico-cortical axons while their prospective target cells end their migration process to reach their final positions. It is postulated that both, inverted lamination and the subplate zone arose in evolution as successive and complementary strategies to maximize synaptic contacts between thalamic afferents and the new cortical cell types (belonging to prospective granular and supragranular layers) that were being originated at that moment.

Animals↗

PubMed1992

Fiber composition of the human corpus callosum.

The densities of fibers of different sizes were calculated in ten regions of the corpus callosum of twenty human brains (ten females, ten males). Light microscopic examination revealed a consistent pattern of regional differentiation of fiber types in the corpus callosum. Thin fibers are most dense in the anterior corpus callosum (genu), and decrease in density posteriorly towards the posterior midbody, where they reach a minimum. Towards the posterior corpus callosum (splenium), the density of thin fibers increases again, but in the posterior pole of the callosum the density decreases locally. Large-diameter fibers show a pattern complementary to that of thin fibers, having a peak of density in the posterior midbody and a local increase of density in the posterior pole of the corpus callosum. Across subjects, the overall density of callosal fibers had no significant correlation with callosal area and an increased callosal area indicated an increased total number of fibers crossing through. Considering different fiber sizes, this was only true for small diameter fibers, whose large majority is believed to interconnect association cortex. No sex differences in fiber composition of the corpus callosum were found.

Adult↗

PubMed1992

Individual differences in brain asymmetries and fiber composition in the human corpus callosum.

There have been several recent reports concerning individual differences in the gross morphometry of the human corpus callosum. However, no studies exist on individual differences in the fiber composition of the corpus callosum. Here we report for the first time the relation of fiber composition in specific callosal segments (as seen in light microscopy) to anatomical asymmetries in language-gifted cortex, as a function of sex. We found a significant negative correlation between Sylvian fissure asymmetries and the total numbers of fibers in the isthmus of males, and in the anterior splenium of females. In addition, a population of relatively large fibers (between 1 micron and 3 microns in diameter) in the isthmus showed a strong negative correlation with perisylvian asymmetries only in males. These findings suggest a sex-dependent, pathway-specific decrease in interhemispheric connectivity with increasing lateralization.

Adult↗

PubMed1992

The origin of the mammalian brain as a case of evolutionary irreversibility.

The origin of the mammalian neocortex is usually considered as an improvement in the design of the brain. I suggest that the mammalian neocortex arose as a consequence of contingent adaptations in which there was no specific selection for more elaborate cognitive abilities. This perspective differs from the current view of brain evolution as a progressive phenomenon towards increased intelligence.

Animals↗

PubMed1992

Mechanisms of adaptive evolution. Darwinism and Lamarckism restated.

This article discusses the conceptual basis of the different mechanisms of adaptive evolution. It is argued that only two such mechanisms may conceivably exist, Lamarckism and Darwinism. Darwinism is the fundamental process generating the diversity of species. Some aspects of the gene-centered approach to Darwinism are questioned, since they do not account for the generation of biological diversity. Diversity in biological design must be explained in relation to the diversity of interactions of organisms (or other higher-level units) with their environment. This aspect is usually overlooked in gene-centered views of evolution. A variant of the gene-selectionist approach has been proposed to account for the spread of cultural traits in human societies. Alternatively, I argue that social evolution is rather driven by what I call pseudo-Lamarckian inheritance. Finally, I argue that Lamarckian and pseudo-Lamarckian inheritance are just special cases of faithful replication which are found in the development of some higher-order units, such as multicellular organisms and human societies.

Adaptation, Physiological↗

PubMed1992

Morphometry of the Sylvian fissure and the corpus callosum, with emphasis on sex differences.

The relationship between anatomical asymmetries in the perisylvian region and the sizes of different regions of the corpus callosum was investigated post-mortem in 40 brains of right-handed hospital admissions (20 males, 20 females) with no cortical involvement. There were no sex differences either in anatomical asymmetries or in regional size of the callosum. There was a negative correlation between the absolute value of Sylvian fissure (planum temporale) asymmetries and the size of the isthmus in males but not in females. Further, there was a significant negative correlation between the size of the Sylvian fissure (or planum temporale) and the size of the callosal mid-body in males but not in females. There was no correlation between the asymmetry of the planum temporale magnitude of left-right and total size of the planum (left+right). These findings constrain theories about the ontogenesis of hemispheric specialization through changes in callosal connectivity and about sex differences in interhemispheric organization.

Adult↗

PubMed1992

The evolutionary origin of the mammalian cerebral cortex.

The origin of the mammalian neocortex in usually considered as an improvement in the structure of the brain. Alternatively, I suggest that the mammalian neocortex arose as a consequence of contingent adaptations in which there was no specific selection for more elaborate cognitive abilities. In primitive mammals, the adaptation to nocturnal life produced a reduction of the optic tectum (superior colliculus). In addition, the development of the olfactory system triggered the development of the cerebral cortex. It is proposed that, since both the optic tectum and the cerebral cortex are laminar structures, the growing cortex replaced the tectum in many integratory functions. When mammals reinvaded diurnal niches, the optic tectum did not redevelop, and the cerebral cortex remained the main integratory and perceptual system. This is a case of irreversible reduction of an organ. In reptiles and especially in birds, although there was also an increase in brain size (associated with higher cognitive capacities), the optic tectum grew in size and complexity and the forebrain grew largely as a nonlaminar structure (except the Wulst in birds). Therefore, the origin of the cerebral cortex resulted from the combination of adaptations to nocturnality and the development of olfactory-driven behavior, and its origin is not directly related to higher cognitive capacities.

Animals↗

PubMed1992

Brain connections: interhemispheric fiber systems and anatomical brain asymmetries in humans.

The present review summarizes some results of a research program oriented to determine the anatomical substrates of interhemispheric communication in humans, as seen in postmortem material. One main finding is a sensible pattern of histological differentiation along the corpus callosum, indicating specific properties of interhemispheric conduction for axonal fibers involved in different brain functions. Callosal regions that connect primary and secondary sensory and motor areas are characterized by a large proportion of fast-conducting, large-diameter fibers, while regions connecting the so-called association areas and prefrontal areas bear a high density of slow-conducting, lightly myelinated and thin fibers. These findings are interpreted in a functional context, suggesting that the fast-conducting fibers connecting sensory and motor areas contribute to fuse the two hemirepresentations in each hemisphere. It has also been determined that an increased callosal area indicates an increased number of callosal fibers, a finding that validates previous morphometric studies done in several laboratories. No sex differences in callosal size, shape, or in callosal fiber composition were found. Finally, an inverse relation was found between the anatomical asymmetries in the size of the Sylvian fissure and the size and number of fibers in specific segments of the corpus callosum. There were sex differences in terms of the particular callosal regions showing a significant correlation with asymmetries, and in terms of the fiber types that were correlated with asymmetries.

Animals↗