Variation in the cortical area map of C57BL/6J and DBA/2J inbred mice predicts strain identity

Gene and stress history interplay in emergence of PTSD-like features

Systematically distinguishing genetic liability from other contributing factors is critical for designing a preventive strategy for post-traumatic stress disorder (PTSD). To address this issue, we investigated a murine model exposing C57BL/6j, DBA/2j and BALB/cj mice to repeated stress via exposure to conspecific aggressors (Agg-E). Naïve mice from each strain were subjected to the proximity of aggressor (Agg) mice for 6h using a 'cage-within-a-cage' paradigm, which was repeated for 5 or 10 days with intermittent and unpredictable direct contact with Agg mice. During the Agg-E stress, DBA/2j developed a different strategy to evade Agg mice, which potentially contributed to its phenotypic resilience to Agg-E stress. Although Agg mice inflicted C57BL/6j and BALB/cj with equivalent numbers of strikes, BALB/cj displayed a distinct behavioral phenotype with delayed exhibition of a number of PTSD-like features. By contrast, C57BL/6j mice displayed unique vulnerability to Agg-E stress induced myocardopathy, possibly attributable to their particular susceptibility to hypoxia. A group of genes (Bdnf, Ngf, Zwint, Cckbr, Slc6a4, Fkbp5) linked to PTSD and synaptic plasticity were significantly altered in C57BL/6j and BALB/cj Agg-E mice. Contributions of Agg-E stress history and genotypic heterogeneity emerged as the key mediators of PTSD-like features. Linking genetic components to specific phenotypic and pathological features could have potential clinical implications.

Mapping arealisation of the visual cortex of non-primate species: lessons for development and evolution

The integration of the visual stimulus takes place at the level of the neocortex, organized in anatomically distinct and functionally unique areas. Primates, including humans, are heavily dependent on vision, with approximately 50% of their neocortical surface dedicated to visual processing and possess many more visual areas than any other mammal, making them the model of choice to study visual cortical arealisation. However, in order to identify the mechanisms responsible for patterning the developing neocortex, specifying area identity as well as elucidate events that have enabled the evolution of the complex primate visual cortex, it is essential to gain access to the cortical maps of alternative species. To this end, species including the mouse have driven the identification of cellular markers, which possess an area-specific expression profile, the development of new tools to label connections and technological advance in imaging techniques enabling monitoring of cortical activity in a behaving animal. In this review we present non-primate species that have contributed to elucidating the evolution and development of the visual cortex. We describe the current understanding of the mechanisms supporting the establishment of areal borders during development, mainly gained in the mouse thanks to the availability of genetically modified lines but also the limitations of the mouse model and the need for alternate species.

Evolutionary development of neural systems in vertebrates and beyond

The emerging field of "neuro-evo-devo" is beginning to reveal how the molecular and neural substrates that underlie brain function are based on variations in evolutionarily ancient and conserved neurochemical and neural circuit themes. Comparative work across bilaterians is reviewed to highlight how early neural patterning specifies modularity of the embryonic brain, which lays a foundation on which manipulation of neurogenesis creates adjustments in brain size. Small variation within these developmental mechanisms contributes to the evolution of brain diversity. Comparing the specification and spatial distribution of neural phenotypes across bilaterians has also suggested some major brain evolution trends, although much more work on profiling neural connections with neurochemical specificity across a wide diversity of organisms is needed. These comparative approaches investigating the evolution of brain form and function hold great promise for facilitating a mechanistic understanding of how variation in brain morphology, neural phenotypes, and neural networks influences brain function and behavioral diversity across organisms.

A segmentation protocol and MRI atlas of the C57BL/6J mouse neocortex

The neocortex is the largest component of the mammalian cerebral cortex. It integrates sensory inputs with experiences and memory to produce sophisticated responses to an organism's internal and external environment. While areal patterning of the mouse neocortex has been mapped using histological techniques, the neocortex has not been comprehensively segmented in magnetic resonance images. This study presents a method for systematic segmentation of the C57BL/6J mouse neocortex. We created a minimum deformation atlas, which was hierarchically segmented into 74 neocortical and cortical-related regions, making it the most detailed atlas of the mouse neocortex currently available. In addition, we provide mean volumes and relative intensities for each structure as well as a nomenclature comparison between the two most cited histological atlases of the mouse brain. This MR atlas is available for download, and it should enable researchers to perform automated segmentation in genetic models of cortical disorders.

Sensory deprivation during early development causes an increased exploratory behavior in a whisker-dependent decision task

Stimulation of sensory pathways is important for the normal development of cortical sensory areas, and impairments in the normal development can have long-lasting effect on animal's behavior. In particular, disturbances that occur early in development can cause permanent changes in brain structure and function. The behavioral effect of early sensory deprivation was studied in the mouse whisker system using a protocol to induce a 1-week sensory deprivation immediately after birth. Only two rows of whiskers were spared (C and D rows), and the rest were deprived, to create a situation where an unbalanced sensory input, rather than a complete loss of input, causes a reorganization of the sensory map. Sensory deprivation increased the barrel size ratio of the spared CD rows compared with the deprived AB rows; thus, the map reorganization is likely due, at least in part, to a rewiring of thalamocortical projections. The behavioral effect of such a map reorganization was investigated in the gap-crossing task, where the animals used a whisker that was spared during the sensory deprivation. Animals that had been sensory deprived performed equally well with the control animals in the gap-crossing task, but were more active in exploring the gap area and consequently made more approaches to the gap - approaches that on average were of shorter duration. A restricted sensory deprivation of only some whiskers, although it does not seem to affect the overall performance of the animals, does have an effect on their behavioral strategy on executing the gap-crossing task.

Association of common genetic variants in GPCPD1 with scaling of visual cortical surface area in humans

Visual cortical surface area varies two- to threefold between human individuals, is highly heritable, and has been correlated with visual acuity and visual perception. However, it is still largely unknown what specific genetic and environmental factors contribute to normal variation in the area of visual cortex. To identify SNPs associated with the proportional surface area of visual cortex, we performed a genome-wide association study followed by replication in two independent cohorts. We identified one SNP (rs6116869) that replicated in both cohorts and had genome-wide significant association (P(combined) = 3.2 × 10(-8)). Furthermore, a metaanalysis of imputed SNPs in this genomic region identified a more significantly associated SNP (rs238295; P = 6.5 × 10(-9)) that was in strong linkage disequilibrium with rs6116869. These SNPs are located within 4 kb of the 5' UTR of GPCPD1, glycerophosphocholine phosphodiesterase GDE1 homolog (Saccharomyces cerevisiae), which in humans, is more highly expressed in occipital cortex compared with the remainder of cortex than 99.9% of genes genome-wide. Based on these findings, we conclude that this common genetic variation contributes to the proportional area of human visual cortex. We suggest that identifying genes that contribute to normal cortical architecture provides a first step to understanding genetic mechanisms that underlie visual perception.

Cytoarchitectonic and chemoarchitectonic characterization of the prefrontal cortical areas in the mouse

This study describes cytoarchitectonic criteria to define the prefrontal cortical areas in the mouse brain (C57BL/6 strain). Currently, well-illustrated mouse brain stereotaxic atlases are available, which, however, do not provide a description of the distinctive cytoarchitectonic characteristics of individual prefrontal areas. Such a description is of importance for stereological, neuronal tracing, and physiological, molecular and neuroimaging studies in which a precise parcellation of the prefrontal cortex (PFC) is required. The present study describes and illustrates: the medial prefrontal areas, i.e., the infralimbic, prelimbic, dorsal and ventral anterior cingulate and Fr2 area; areas of the lateral PFC, i.e., the dorsal agranular insular cortical areas and areas of the ventral PFC, i.e., the lateral, ventrolateral, ventral and medial orbital areas. Each cytoarchitectonically defined boundary is corroborated by one or more chemoarchitectonic stainings, i.e., acetylcholine esterase, SMI32, SMI311, dopamine, parvalbumin, calbindin and myelin staining.

Comparison of C57BL/6 and DBA/2 mice in food motivation and satiety

Demand functions describe the relationship between the consumption of a commodity and its mean or unit price. In the first experiment, we analyzed food demand in two strains of mice (C57BL/6 and DBA/2) that differ on several behavioral dimensions, but have not been examined extensively for differences in feeding and meal patterns. Mice worked for food pellets in a continuous access closed economy in which total intake and meal patterns could be measured. A series of fixed (FUP), variable (VUP), and progressive (PUP) unit price schedules were imposed. Under all schedules, DBA/2 mice consumed significantly more food than C57BL/6, a difference that was not attributable to disparity in body weight or weight gain. The higher intake of DBA/2 mice was due predominantly to larger meal size compared with C57BL/6, with no strain difference in meal frequency. In a second experiment, strain differences in meal size were not found to correlate with anorectic sensitivity to cholecystokinin (CCK) administration, or with c-Fos expression induced by CCK in PVN, AP and NTS. Thus, DBA/2 mice were motivated to sustain a higher daily food intake and meal size than C57BL/6 under the range of demand costs employed in the present work, but this strain difference is unlikely to be due to CCK action or responsiveness.

What are the Effects of Severe Visual Impairment on the Cortical Organization and Connectivity of Primary Visual Cortex?

The organization and connections of the primary visual area (V1) were examined in mice that lacked functional rods (Gnat−/−), but had normal cone function. Because mice are nocturnal and rely almost exclusively on rod vision for normal behaviors, the Gnat−/− mice used in the present study are considered functionally blind. Our goal was to determine if visual cortex is reorganized in these mice, and to examine the neuroanatomical connections that may subserve reorganization. We found that most neurons in V1 responded to auditory, or some combination of auditory, somatosensory, and/or visual stimulation. We also determined that cortical connections of V1 in Gnat−/− mice were similar to those in normal animals, but even in normal animals, there is sparse input from auditory cortex (AC) to V1. An important observation was that most of the subcortical inputs to V1 were from thalamic nuclei that normally project to V1 such as the lateral geniculate (LG), lateral posterior (LP), and lateral dorsal (LD) nuclei. However, V1 also received some abnormal subcortical inputs from the anterior thalamic nuclei, the ventral posterior, the ventral lateral and the posterior nuclei. While the vision generated from the small number of cones appears to be sufficient to maintain most of the patterns of normal connectivity, the sparse abnormal thalamic inputs to VI, existing inputs from AC, and possibly abnormal inputs to LG and LP may be responsible for generating the alterations in the functional organization of V1.

Selective vulnerability of neurons in primary cultures and in neurodegenerative diseases

Primary neuronal cultures are commonly used to dissect the molecular and cellular mechanisms that underlie human brain diseases. Neurons dissociated from an embryonic brain and grown in culture dishes are almost by definition different from those residing inside a living brain. Not only are the individual cells stripped of their normal chemical and physical contacts, but the cellular composition of the cultures (the ratio of cell types) can be affected by many intrinsic and extrinsic factors, including brain region, neuronal birthday, gender, genetic background and in vitro age. Changes in any of these factors may have a strong impact on the outcome of the experiment. In a recent study, Romito-DiGiacomo et al. /54/ demonstrated that when neurons were harvested from murine embryonic cortex, the typical protocol favored cells that were just finishing cell division at the time of harvest. By taking advantage of the fact that the date of the final cell division (birthday) of a neuron correlates with its final position in the cortical plate they were able to assay deeper layer neurons (layers V-VI) separately from the more superficial layers (layers II-III). They reported that while the superficial cells were sensitive to the toxic effect of beta-amyloid, the deeper layer neurons were virtually resistant to death in its presence. The findings recapitulate selective vulnerability in the neocortex of Alzheimer's disease. This is a beautiful example of how to turn the apparent weakness of primary cultures into strength through experimental design and data interpretation. Selective vulnerability is a common feature of neurodegenerative disease, thus it is critical to use the right primary culture. Do you know what is in your culture?

Effects of bilateral enucleation on the size of visual and nonvisual areas of the brain

Alterations in the activity of one sensory system can affect the development of cortical and subcortical structures in all sensory systems. In this study, we characterize the changes that occur in visual and nonvisual areas of the brain following bilateral enucleation in short-tailed opossums. We demonstrate that bilateral enucleation early in development can significantly decrease brain size. This change is driven primarily by a decrease in the size of the thalamus, midbrain, and hindbrain, rather than a decrease in the size of the cortical hemispheres. We also found a significant decrease in the size of the lateral geniculate nucleus in bilaterally enucleated animals. Although the overall size of the neocortex was the same, the percentage of neocortex devoted to visual areas V1 (primary visual area) and caudotemporal area were significantly smaller in bilaterally enucleated opossums and the percentage of neocortex devoted to the primary somatosensory area (S1) was significantly larger, although S1 did not change in size to the same extent as V1. Our data suggest that during development the relative activity patterns between sensory systems, which are driven by activity from unique sets of sensory receptor arrays, play a major role in determining the relative size and organization of cortical and subcortical areas.

Genetic regulation of arealization of the neocortex

Arealization of the neocortex is controlled by a regulatory hierarchy beginning with morphogens secreted from patterning centers positioned at the perimeter of the dorsal telencephalon. These morphogens act in part to establish within cortical progenitors the differential expression of transcription factors that specify their area identity, which is inherited by their neuronal progeny, providing the genetic framework for area patterning. The two patterning centers most directly implicated in arealization are the commissural plate, which expresses fibroblast growth factors, and the cortical hem, which expresses bone morphogenetic proteins and vertebrate orthologs of Drosophila wingless, the Wnts. A third, albeit putative, patterning center is the antihem, identified by its expression of multiple signaling molecules. We describe recent findings on roles for these patterning centers in arealization. We also present the most recent evidence on functions of the four transcription factors, Emx2, COUP-TFI, Pax6, and Sp8, thus far implicated in arealization. We also describe screens for candidate target genes of these transcription factors, or other genes potentially involved in arealization. We conclude with an assessment of a forward genetics approach for identifying genes involved in determining area size based in part on quantitative trait locus mapping, and the implications for significant differences between individuals in area size on behavioral performance.

Post-traumatic stress behavioural responses in inbred mouse strains: can genetic predisposition explain phenotypic vulnerability?

Clinical studies of twin pairs and families of post-traumatic stress disorder (PTSD) patients raise questions as to possible genetic predisposition to PTSD. Studies using isogenic animal populations exposed to a stress paradigm could elucidate the relative contributions of genotype and environment to endophenotypic expression. The prevalence of individuals displaying severely compromised behavioural responses to predator scent stress (PSS) was assessed in six inbred strains of mice in an animal model of PTSD that classifies individuals into groups according to the degree of their behavioural response. The choice of strains was based on the frequent use of these mice in transgenic research. The prevalence of extreme behavioural response in the elevated plus maze and the acoustic startle response paradigms, performed in sequence, was assessed at baseline and 7 d after PSS exposure between and within strains, and compared to differences in circulating corticosterone levels. Narrow-sense trait heritability was determined by comparing the between-strain variance to the total variance. Although strain-specific differences in anxiety-like behaviours were demonstrated, the results revealed a significant degree of individual variability in response patterns within each of the inbred strains, yielding a baseline heritability factor for anxiety-like behaviours of 30%, but only 10% for response to stress exposure. Baseline anxiety-like behaviours were found not to be predictive of post-exposure behavioural responses. The response of the individual to stress is multifactorial and environmental factors play a predominant role in characterizing the individual response to stress exposure, although there are significant genetic underpinnings.

Genetic analysis of posterior medial barrel subfield (PMBSF) size in somatosensory cortex (SI) in recombinant inbred strains of mice

BACKGROUND

Quantitative trait locus (QTL) mapping is an important tool for identifying potential candidate genes linked to complex traits. QTL mapping has been used to identify genes associated with cytoarchitecture, cell number, brain size, and brain volume. Previously, QTL mapping was utilized to examine variation of barrel field size in the somatosensory cortex in a limited number of recombinant inbred (RI) strains of mice. In order to further elucidate the underlying natural variation in mouse primary somatosensory cortex, we measured the size of the posterior medial barrel subfield (PMBSF), associated with the representation of the large mystacial vibrissae, in an expanded sample set that included 42 BXD RI strains, two parental strains (C57BL/6J and DBA/2J), and one F1 strain (B6D2F1). Cytochrome oxidase labeling was used to visualize barrels within the PMBSF.

RESULTS

We observed a 33% difference between the largest and smallest BXD RI strains with continuous variation in-between. Using QTL linkage analysis from WebQTL, we generated linkage maps of raw total PMBSF and brain weight adjusted total PMBSF areas. After removing the effects of brain weight, we detected a suggestive QTL (likelihood ratio statistic [LRS]: 14.20) on the proximal arm of chromosome 4. Candidate genes under the suggestive QTL peak for PMBSF area were selected based on the number of single nucleotide polymorphisms (SNPs) present and the biological relevance of each gene. Among the candidate genes are Car8 and Rab2. More importantly, mRNA expression profiles obtained using GeneNetwork indicated a strong correlation between total PMBSF area and two genes (Adcy1 and Gap43) known to be important in mouse cortex development. GAP43 has been shown to be critical during neurodevelopment of the somatosensory cortex, while knockout Adcy1 mice have disrupted barrel field patterns.

CONCLUSION

We detected a novel suggestive QTL on chromosome 4 that is linked to PMBSF size. The present study is an important step towards identifying genes underlying the size and possible development of cortical structures.

Area patterning of the mammalian cortex

Here we describe mechanisms regulating area patterning of developing mammalian neocortex, referred to as arealization. Current findings indicate an interplay between intrinsic genetic mechanisms and extrinsic information relayed to cortex by thalamocortical input. Intrinsic mechanisms are based on morphogens and signaling molecules secreted by patterning centers, positioned at the perimeter of dorsal telencephalon, that generate across nascent cortex the graded expression of transcription factors in cortical progenitors. Two major patterning centers are the commissural plate, which expresses Fgf8 and Fgf17, and the cortical hem, which expresses Bmps and Wnts. Four transcription factors, COUP-TFI, Emx2, Pax6, and Sp8, with graded expression across the embryonic cortical axes, are shown to determine sizes and positions of cortical areas by specifying or repressing area identities within cortical progenitors. They also interact to modify their expression, as well as expression of Fgf8. We review these mechanisms of arealization and discuss models and concepts of cortical area patterning.

Screening mouse vision with intrinsic signal optical imaging

The introduction of forward genetic screens in the mouse asks for techniques that make rapid screening of visual function possible. Transcranial imaging of intrinsic signal is suitable for this purpose and could detect the effects of retinal degeneration, and the increased predominance of the contralateral eye in albino animals. We quantified visual response properties of the cortex by introducing a normalization method to reduce the impact of biological noise. In addition, the presentation of a 'reset'-stimulus shortly after the probing stimulus at a different visual location could reduce the interstimulus time necessary for the decay of the response. Applying these novel methods, we found that acuity of C57Bl/6J mice rises from 0.35 cycles per degree (cpd) at postnatal day 25 to 0.56 cpd in adults. Temporal resolution was lower in adults than in juvenile animals. There was no patchy organization of spatial or temporal frequency preference at the intrinsic signal resolution. Monocular deprivation, a model for amblyopia and critical period plasticity, led to a loss in acuity and a shift towards the nondeprived eye in juvenile animals. Short deprivation did not lead to increased acuity of the nondeprived eye. In adults, a small ocular dominance shift was detectable with urethane anaesthesia. This was not observed when the combination of the opiate fentanyl, fluanisone with a benzodiazepine was used, adding evidence to the hypothesis that enhancing GABA(A)-receptor function masks an adult shift. Together, these novel applications confirm that noninvasive screening of many functional properties of the visual cortex is possible.

Cortical area size dictates performance at modality-specific behaviors

The mammalian neocortex is organized into unique areas that serve functions such as sensory perception and modality-specific behaviors. The sizes of primary cortical areas vary across species, and also within a species, raising the question of whether area size dictates behavioral performance. We show that adult mice genetically engineered to overexpress the transcription factor EMX2 in embryonic cortical progenitor cells, resulting in reductions in sizes of somatosensory and motor areas, exhibit significant deficiencies at tactile and motor behaviors. Even increasing the size of sensorimotor areas by decreasing cortical EMX2 levels can lead to diminished sensorimotor behaviors. Genetic crosses that retain ectopic Emx2 transgene expression subcortically but restore cortical Emx2 expression to wild-type levels also restore cortical areas to wild-type sizes and in parallel restore tactile and motor behaviors to wild-type performance. These findings show that area size can dictate performance at modality-specific behaviors and suggest that areas have an optimal size, influenced by parameters of its neural system, for maximum behavioral performance. This study underscores the importance of establishing during embryonic development appropriate levels of regulatory proteins that determine area sizes, thereby influencing behavior later in life.

Phenotypic diversity is the cornerstone of evolution: variation in cortical field size within short-tailed opossums

Natural selection operates on phenotypic variation that exists within a population. Variable aspects of cortical organization, such as the size and connections of a cortical field, can generate differences in behavior, which is a target of natural selection. Yet studies pertaining to within-species variability in cortical organization are limited. In the present investigation, we examined variation in brain size, cortical sheet size, and primary sensory cortical field sizes in the adult short-tailed opossum (Monodelphis domestica). Within individuals, we found no significant difference between the right and left hemispheres in the overall size of the dorsolateral cortex or in primary cortical field sizes. Between individuals, we found relatively little intraspecies variation in brain weight, brain volume, and cortical sheet area for the dorsolateral neocortex and pyriform cortex; however, we observed a large degree of variability in body weight and primary sensory cortical field size, as defined by myeloarchitecture. Further, we found that the size of each cortical field correlated with the size of the other cortical fields as well as with the total size of the dorsolateral cortex. Here we discuss the possible sources of variation and examine the relationship between cortical field size and sensory processing abilities and behaviors across species. Since behavior is the target of natural selection, variation in cortical field size across individuals may supply the raw material necessary for cortical field evolution.

Genetic and Structural Analysis of the Basolateral Amygdala Complex in BXD Recombinant Inbred Mice

The amygdala integrates and coordinates emotional and autonomic responses. The genetics that underlie variation in amygdala structure may be coupled to variation in levels of aggression, fear, anxiety, and affiliated behaviors. We systematically quantified the volume and cell populations of the basolateral amygdala complex (BLAc) across 35 BXD recombinant inbred (RI) lines, the parental strains--C57BL/6J (B6) and DBA/2J (D2)--and F1 hybrids (n cases=199, bilateral analysis). Neuron number and volume vary 1.7- to 2-fold among strains (e.g., neuron number ranged from 88,000 to 170,000). Glial and endothelial populations ranged more widely (5- to 8-fold), in part because of higher technical error. A quantitative trait locus (QTL) for the BLAc size is located on chromosome (Chr) 8 near the Large gene. This locus may also influence volume of other regions including hippocampus and cerebellum. Cell populations in the BLAc appear to be modulated more weakly by loci on Chrs 11 and 13. Candidate genes were selected on the basis of correlation with BLAc traits, chromosomal location, single nucleotide polymorphism (SNP) density, and expression patterns in the Allen Brain Atlas. Neurod2, a gene shown to be significant for the formation of the BLAc by knockout studies, is among the candidates genes. Other candidates include Large, and Thra. Responses to drugs of abuse and locomotor activity were the most notable behavioral correlates of the BLAc traits.

Geometric morphometrics defines shape differences in the cortical area map of C57BL/6J and DBA/2J inbred mice

Background

We previously described planar areal differences in adult mouse visual, somatosensory, and neocortex that collectively discriminated C57BL/6J and DBA/2J inbred strain identity. Here we use a novel application of established methods of two-dimensional geometric morphometrics to examine shape differences in the cortical area maps of these inbred strains.

Results

We used Procrustes superimposition to align a reliable set of landmarks in the plane of the cortical sheet from tangential sections stained for the cytochrome oxidase enzyme. Procrustes superimposition translates landmark configurations to a common origin, scales them to a common size, and rotates them to minimize an estimate of error. Remaining variation represents shape differences. We compared the variation in shape between C57BL/6J and DBA/2J relative to that within each strain using a permutation test of Goodall's F statistic. Significant differences in shape in the posterior medial barrel subfield (PMBSF), as well as differences in shape across primary sensory areas, characterize the cortical area maps of these common inbred, isogenic strains.

Conclusion

C57BL/6J and DBA/2J have markedly different cortical area maps, in both size and shape. These differences suggest polymorphism in genetic factors underlying cortical specification, even between common isogenic strains. Comparing cortical phenotypes between normally varying inbred mice or between genetically modified mice can identify genetic contributions to cortical specification. Geometric morphometric analysis of shape represents an additional quantitative tool for the study of cortical development, regardless of whether it is studied from phenotype to gene or gene to phenotype.

Multisensory plasticity in congenitally deaf mice: how are cortical areas functionally specified?

The neocortex of congenitally deaf mice was examined using electrophysiological recording techniques combined with cortical myeloarchitecture. Our results indicate that relative activity patterns across sensory systems during development contribute to modality assignment of cortical fields as well as the size of cortical fields. In congenitally deaf mice, "auditory cortex" contained neurons that responded to somatosensory, visual, or both somatosensory and visual stimulation; the primary visual area contained a larger proportion of neurons that responded to somatosensory stimulation than in normal animals, and the primary visual area had significantly increased in size. Thus, cortical architecture and functional specification were de-correlated. When results are considered in the light of molecular studies and studies in which peripheral activity is altered in development, it becomes clear that similar types of changes to the neocortex, such as alterations in cortical field size, can be achieved in more than one way in the developing and evolving neocortex.

The evolution of the neocortex in mammals: how is phenotypic diversity generated?

Evolution of the mammalian neocortex is difficult to examine directly. For this reason, comparative studies and developmental studies are the best way of gaining insight into the evolutionary process. Comparative studies indicate that neocortical evolution is constrained, and that the types of systems-level modifications made to the neocortex are limited. Developmental studies of gene expression suggest that genetic contingencies set up aspects of cortical organization and connectivity, and that the complex spatial and temporal interactions of genes constrain development and evolution. Although genes obviously contribute to phenotypic variability, variability can also be achieved through alterations in the sensory receptor arrays, or changes in sensory driven activity. The intracellular mechanisms that enable phenotypic variability might evolve, but often the phenotypic characteristic in question is context-dependent.