Neuronal subtypes and anatomic asymmetry: changes in neuronal number and cell-packing density

Semantic priming and neurobiology in schizophrenia: A theoretical review

In this theoretical review we bridge the cognitive and neurobiological sciences to shed light on the neurocognitive foundations of the semantic priming effect in schizophrenia. We review and theoretically evaluate the neurotransmitter systems (dopaminergic, GABAergic and glutamatergic) and neurobiological underpinnings of behavioural and electrophysiological (N400) semantic priming in the pathology, and the main hypotheses on their geneses: a disinhibition of the semantic spread of activation, a disorganised semantic storage or noisy lexical-semantic associations, a psychomotor artefact, an artefact of relatedness proportions, or an inability to mobilise contextual information. We further assess the literature on the endophenotype of Formal Thought Disorder from multiple standpoints, ranging from neurophysiology to cognition: considerations are weaved on neuronal (PV basket cell, SST, VIP) and receptor deficits (DRD1, NMDA), neurotransmitter imbalances (dopamine), cortical and dopaminergic lateralisation, inter alia. In conclusion, we put forth novel postulates on the underlying causes of controlled hypopriming, automatic hyperpriming, N400 reversals (larger amplitudes for close associations), indirect versus direct hyperpriming, and the endophenotype of lexical-semantic disturbances in schizophrenia.

The left human speech-processing cortex is thinner but longer than the right

We present histological data from 21 post-mortem, adult human cases that indicate the neocortex on the left planum temporale (secondary auditory cortex) is thinner but longer than that on the right side. The volumes of the left and right regions are approximately equal. Thus, the left planum temporale cortex is long and thin and the right short and thick. The present data fit excellently with previous studies of the volume, surface area, cytoarchitectonics, and neuronal structures of these areas. From these studies we suggest that the hemispheric differences arise from a so-called "balloon model" of cortical development. In this the cortex is extended and stretched by white matter growth. The stretching is greater on the left side, leaving greater distances between neuronal columns and more tangentially (to the pial surface) oriented dendrites on that side. This difference in fine structure can result in more independent activity of individual columns on the left, and could be an anatomical factor in the usual dominance of the left hemisphere for speech perception (Seldon, 1982, 1985).

Phonetically irregular word pronunciation and cortical thickness in the adult brain

Accurate pronunciation of phonetically irregular words (exception words) requires prior exposure to unique relationships between orthographic and phonemic features. Whether such word knowledge is accompanied by structural variation in areas associated with orthographic-to-phonemic transformations has not been investigated. We used high-resolution MRI to determine whether performance on a visual word-reading test composed of phonetically irregular words, the Wechsler Test of Adult Reading (WTAR), is associated with regional variations in cortical structure. A sample of 60 right-handed, neurologically intact individuals were administered the WTAR and underwent 3T volumetric MRI. Using quantitative, surface-based image analysis, cortical thickness was estimated at each vertex on the cortical mantle and correlated with WTAR scores while controlling for age. Higher scores on the WTAR were associated with thicker cortex in bilateral anterior superior temporal gyrus, bilateral angular gyrus/posterior superior temporal gyrus, and left hemisphere intraparietal sulcus. Higher scores were also associated with thinner cortex in left hemisphere posterior fusiform gyrus and central sulcus, bilateral inferior frontal gyrus, and right hemisphere lingual gyrus and supramarginal gyrus. These results suggest that the ability to correctly pronounce phonetically irregular words is associated with structural variations in cortical areas that are commonly activated in functional neuroimaging studies of word reading, including areas associated with grapheme-to-phonemic conversion.

The genetic control of neocortex volume and covariation with neocortical gene expression in mice

Background

The size of the cerebral cortex varies widely within human populations, and a large portion of this variance is modulated by genetic factors. The discovery and characterization of these genes and their variants can contribute to an understanding of individual differences in brain development, behavior, and disease susceptibility. Here we use unbiased stereological techniques to map quantitative trait loci (QTLs) that modulate the volume of neocortex.

Results

We estimated volumes bilaterally in an expanded set of BXD recombinant inbred strains (n = 56 strains and 223 animals) taken from the Mouse Brain Library . We generated matched microarray data for the cerebral cortex in the same large panel of strains and in parental neonates to efficiently nominate and evaluate candidate genes. Volume of the neocortex varies widely, and is a heritable trait. Genome-wide mapping of this trait revealed two QTLs – one on chromosome (Chr) 6 at 88 ± 5 Mb and another at Chr 11 (41 ± 8 Mb). We generated both neonatal and adult neocortical gene expression databases using microarray technology. Using these databases in combination with other bioinformatic tools we have identified positional candidates on these QTL intervals.

Conclusion

This study is the first to use the expanded set of BXD strains to map neocortical volume, and we found that normal variation of this trait is, at least in part, genetically modulated. These results provide a baseline from which to assess the genetic contribution to regional variation in neocortical volume, as well as other neuroanatomic phenotypes that may contribute to variation in regional volume, such as proliferation, death, and number and packing density of neurons

Genetic modulation of striatal volume by loci on Chrs 6 and 17 in BXD recombinant inbred mice

Natural variation in the absolute and relative size of different parts of the human brain is substantial, with a range that often exceeds a factor of 2. Much of this variation is generated by the cumulative effects of sets of unknown gene variants that modulate the proliferation, growth and death of neurons and glial cells. Discovering and testing the functions of these genes should contribute significantly to our understanding of differences in brain development, behavior and disease susceptibility. We have exploited a large population of genetically well-characterized strains of mice (BXD recombinant inbred strains) to map gene variants that influence the volume of the dorsal striatum (caudate-putamen without nucleus accumbens). We used unbiased methods to estimate volumes bilaterally in a sex-balanced sample taken from the Mouse Brain Library (www.mbl.org). We generated a matched microarray data set to efficiently evaluate candidate genes (www.genenetwork.org). As in humans, volume of the striatum is highly heritable, with greater than twofold differences among strains. We mapped a locus that modulates striatal volume on chromosome (Chr) 6 at 88 +/- 5 Mb. We also uncovered an epistatic interaction between loci on Chr 6 and Chr 17 that modulates striatal volume. Using bioinformatic tools and the corresponding expression database, we have identified positional candidates in these quantitative trait locus intervals.

Histological asymmetries of primary motor cortex predict handedness in chimpanzees (Pan troglodytes)

Like humans, chimpanzees display robust and consistent hand preferences during the performance of certain tasks. Although correlations have been demonstrated between gross anatomic measures of primary motor cortex asymmetry and handedness in captive chimpanzees, the relationship between histological architecture and behavioral lateralization has not yet been investigated. Therefore, we examined interhemispheric asymmetry of several different microstructural characteristics of the primary motor cortex in the region of hand representation from 18 chimpanzees tested on a coordinated bimanual task before death. At the population level our data showed leftward bias for higher layer II/III neuron density. Of note, however, there was no population-level asymmetry in the areal fraction of Nissl-stained cell bodies, a finding that differs from previous studies of this cortical region in humans. Nonetheless, we found that asymmetry in the density of layer II/III parvalbumin-immunoreactive interneurons was the best predictor of individual hand preference. These results suggest that histological asymmetries are related to handedness in chimpanzees, while overall patterns of asymmetry at the population level might differ from humans.

Correlated activity of sensorimotor cortex neurons in the left and right hemispheres of the rabbit brain in immobilization catatonia

Spike sequences extracted from multineuron activity from neurons in the sensorimotor cortex, and recorded simultaneously in the left and right hemispheres of the brains of rabbits in the state of immobilization catatonia ("animal hypnosis") and on recovery of animals from this state were analyzed. Cross-correlation analysis of spike flows revealed a temporal relationship between the appearance of neuron spikes in the left and right hemispheres; these were regarded as the mutual influences of these neurons on each other. The intensity of the influences of left hemisphere neurons on cells in the right brain was shown to change significantly in relation to baseline measures at all stages of the experiment and at all of the time points studied. The intensity of the influences of neurons in the right hemisphere on cells in the left hemisphere changed significantly only after animals recovered from the state of immobilization and over much more restricted time periods.

Structure of dependent relationships between neurons in the sensorimotor cortex of the left and right hemispheres in rabbits in immobilization catatonia

Dependence in the activity of sensorimotor cortex neurons recorded simultaneously in the left and right hemispheres was detected in rabbits in baseline conditions, during the state of immobilization ("animal hypnosis"), and recovery of animals from this state. In baseline conditions, the total percentage of dependent relationships between close-lying (within 50 microm) neurons in the left hemisphere was significantly smaller than in the right hemisphere and did not change either in the state of immobilization or on recovery from it. The total percentage of dependent relationships between close-lying neurons in the right hemisphere decreased significantly during immobilization and returned to baseline levels on recovery from this state. The percentage of dependent relationships between distant (500 microm) neurons in immobilization, conversely, showed no change in the cortex of the right hemisphere, though it changed significantly in the cortex of the left hemisphere, returning to baseline values when the rabbits recovered from this state. Further analysis showed that this cortical interhemisphere asymmetry was based on the asymmetrical activity of individual neurons and small neuronal populations. Thus, changes in the structure of dependent relationships between neurons in microareas of the cortex of the left and macroareas of the cortex of the right hemisphere could be in different directions, while changes in microareas of the right hemisphere and macroareas of the left hemisphere were synergistic. Thus, asymmetry was detected at different levels of neuronal combinations (neuron pairs, micro- and macrogroups of neurons), which suggests mosaicism in neuron structure, which ultimately leads to overall functional asymmetry in "animal hypnosis." Some changes in the structure of dependent relationships between sensorimotor cortex neurons arising in "animal hypnosis" persisted or even became more marked after recovery of animals from this state.

White matter hemisphere asymmetries in healthy subjects and in schizophrenia: a diffusion tensor MRI study

Hemisphere asymmetry was explored in normal healthy subjects and in patients with schizophrenia using a novel voxel-based tensor analysis applied to fractional anisotropy (FA) of the diffusion tensor. Our voxel-based approach, which requires precise spatial normalization to remove the misalignment of fiber tracts, includes generating a symmetrical group average template of the diffusion tensor by applying nonlinear elastic warping of the demons algorithm. We then normalized all 32 diffusion tensor MRIs from healthy subjects and 23 from schizophrenic subjects to the symmetrical average template. For each brain, six channels of tensor component images and one T2-weighted image were used for registration to match tensor orientation and shape between images. A statistical evaluation of white matter asymmetry was then conducted on the normalized FA images and their flipped images. In controls, we found left-higher-than-right anisotropic asymmetry in the anterior part of the corpus callosum, cingulum bundle, the optic radiation, and the superior cerebellar peduncle, and right-higher-than-left anisotropic asymmetry in the anterior limb of the internal capsule and the anterior limb's prefrontal regions, in the uncinate fasciculus, and in the superior longitudinal fasciculus. In patients, the asymmetry was lower, although still present, in the cingulum bundle and the anterior corpus callosum, and not found in the anterior limb of the internal capsule, the uncinate fasciculus, and the superior cerebellar peduncle compared to healthy subjects. These findings of anisotropic asymmetry pattern differences between healthy controls and patients with schizophrenia are likely related to neurodevelopmental abnormalities in schizophrenia.

Birthdates of neurons in induced microgyria

Freezing injury to the cortical plate of the newborn rat results in the formation of a focal region of cerebrocortical microdysgenesis resembling, in many ways, human 4-layered microgyria. Previous research has shown that neurons born during embryonic day (E) 20 migrate through the initial damage and take their place in the cell-dense layer of the microgyric lesion. The current study was conducted to determine: (1) whether neurons generated earlier in development would be found in microgyric cortex; and (2) whether the freezing injury would stimulate production of neurons postnatally. Rat pups from mothers who were injected with S-phase markers on E15, E17, E19, and E21 were subjected to freezing injury of the cortex to induce microgyria on postnatal day (P) 1. Other pups received a freezing lesion and then pulse or cumulative injections of S-phase markers for the next 72 h. Neurons born on E17 and E19 were found scattered throughout the cell-dense layer of the microgyric cortex. Early (E15) generated neurons were nearly absent in the microgyric cortex, and there was no evidence of postnatal induction of cortical neurogenesis. These results are considered in light of recent work demonstrating postnatal neocortical neurogenesis in response to early neocortical injury.

Structural asymmetries in the human forebrain and the forebrain of non-human primates and rats

Possible asymmetries of the following structures were studied: volumes of total human hemispheres, cortex and white matter volumes in post-mortem- (unknown handedness) and living brains (male right-handers); volumes of the rat primary visual cortex, its mon- and binocular subfields, its layer iv and the density of myelinated fibres in layer iv; transmitter receptor densities (NMDA, AMPA, kainate and GABAA receptors) in sensorimotor regions of the rat cortex; volume of the motor cortex and the 3D-extent of the central sulcus in the post-mortem- (unknown handedness) and living human brain (male right-handers); petalia of the hemispheres in human (male right- and left-handers) and chimpanzee brains. Histological, MRI and receptor autoradiographic techniques were used. With the notable exceptions of the transmitter receptors and the total primary visual cortex in rats and the hemispheres in chimpanzees, which do not show any significant directional asymmetry, all other parameters studied are asymmetrically distributed between the right- and left hemispheres. The regional distribution pattern and the degree of asymmetry of frontal and occipital petalia in living human brains differ between right- and left-handers.

Cellular, morphometric, ontogenetic and connectional substrates of anatomical asymmetry

Although anatomical cerebral asymmetry appears in all animals that have been examined, its link to functional lateralization is not clear. In an attempt to further elucidate this relationship between structure and function, we have compared, in rats and humans, brains that have asymmetric architectonic areas to those that are symmetric. We have found that (1) asymmetry is the result of the production of a small side rather than the production of a large side; (2) architectonic asymmetry is the result of changes in the total numbers of neurons rather than cell-packing density; (3) events occurring early in corticogenesis--specifically during the period of progenitor cell proliferation and/or death--are important for the formation of asymmetric cortical areas; and (4) symmetric brains have relatively greater numbers of callosal fibers and more patches of termination than their asymmetric counterparts. These results, taken together, suggest that if anatomic asymmetry underlies functional lateralization, it may have more to do with the different organization of symmetric and asymmetric brains, rather than simply which hemisphere (or brain region) is larger.