Impaired detection of variable duration embedded tones in ectopic NZB/BINJ mice

Therapeutic Interventions in Rat Models of Preterm Hypoxic Ischemic Injury: Effects of Hypothermia, Caffeine, and the Influence of Sex

Infants born prematurely have an increased risk of experiencing brain injury, specifically injury caused by Hypoxia Ischemia (HI). There is no approved treatment for preterm infants, in contrast to term infants that experience Hypoxic Ischemic Encephalopathy (HIE) and can be treated with hypothermia. Given this increased risk and lack of approved treatment, it is imperative to explore and model potential treatments in animal models of preterm injury. Hypothermia is one potential treatment, though cooling to current clinical standards has been found to be detrimental for preterm infants. However, mild hypothermia may prove useful. Caffeine is another treatment that is already used in preterm infants to treat apnea of prematurity, and has shown neuroprotective effects. Both of these treatments show sex differences in behavioral outcomes and neuroprotective effects, which are critical to explore when working to translate from animal to human. The effects and research history of hypothermia, caffeine and how sex affects these treatment outcomes will be explored further in this review article.

Neurodevelopmental malformations of the cerebellum and neocortex in the Shank3 and Cntnap2 mouse models of autism

There are many mouse models of autism with broad use in neuroscience research. Genetic background can be a major contributor to the phenotype observed in any mouse model of disease, including genetic models of autism. C57BL/6 mice display spontaneous glio-neuronal heterotopia in the cerebellar vermis and neocortex which may also exist in mouse models of autism created on this background. In the present report, we document the presence of cerebellar and neocortical heterotopia in heterozygous and KO Shank3 and Cntnap2 mice which are due to the C57BL/6 genotype and discuss the role these malformations may play in research using these genetic models of autism.

Intravital Imaging of Neocortical Heterotopia Reveals Aberrant Axonal Pathfinding and Myelination around Ectopic Neurons

Neocortical heterotopia consist of ectopic neuronal clusters that are frequently found in individuals with cognitive disability and epilepsy. However, their pathogenesis remains poorly understood due in part to a lack of tractable animal models. We have developed an inducible model of focal cortical heterotopia that enables their precise spatiotemporal control and high-resolution optical imaging in live mice. Here, we report that heterotopia are associated with striking patterns of circumferentially projecting axons and increased myelination around neuronal clusters. Despite their aberrant axonal patterns, in vivo calcium imaging revealed that heterotopic neurons remain functionally connected to other brain regions, highlighting their potential to influence global neural networks. These aberrant patterns only form when heterotopia are induced during a critical embryonic temporal window, but not in early postnatal development. Our model provides a new way to investigate heterotopia formation in vivo and reveals features suggesting the existence of developmentally modulated, neuron-derived axon guidance and myelination factors.

Divergence and inheritance of neocortical heterotopia in inbred and genetically-engineered mice

Cortical function emerges from the intrinsic properties of neocortical neurons and their synaptic connections within and across lamina. Neurodevelopmental disorders affecting migration and lamination of the neocortex result in cognitive delay/disability and epilepsy. Molecular layer heterotopia (MLH), a dysplasia characterized by over-migration of neurons into layer I, are associated with cognitive deficits and neuronal hyperexcitability in humans and mice. The breadth of different inbred mouse strains that exhibit MLH and inheritance patterns of heterotopia remain unknown. A neuroanatomical survey of numerous different inbred mouse strains, 2 first filial generation (F1) hybrids, and one consomic strain (C57BL/6J-Chr 1^A/J/NaJ) revealed MLH only in C57BL/6 mice and the consomic strain. Heterotopia were observed in numerous genetically-engineered mouse lines on a congenic C57BL/6 background. These data indicate that heterotopia formation is a weakly penetrant trait requiring homozygosity of one or more C57BL/6 alleles outside of chromosome 1. These data are relevant toward understanding neocortical development and disorders affecting neocortical lamination.

Neural Mechanisms of Language-Based Learning Impairments: Insights from Human Populations and Animal Models

The acquisition of speech perception and consequent expression of language represent fundamental aspects of human functioning. Yet roughly 7% to 8% of children who are otherwise healthy and of normal intelligence exhibit unexplained delays and impairments in acquiring these skills. Ongoing research has revealed several key features of language disability that may pro-vide more direct insight into underlying anomalous neural functioning. For example, evidence supports a strong association between basic defects in processing rapidly changing acoustic information and emergent disruptions in speech perception, as well as cascading effects on other forms of language development (including reading). Considerable neurobiological research has thus focused on developmental factors that might deleteriously influence rapid sensory processing. Additional research focuses on mechanisms of neural plasticity, including how such brains might be “retrained” for improved processing of language. These and related findings from human clinical studies, electrophysiological studies, neuroimaging studies, and animal models are reviewed.

Sex differences in behavioral outcome following neonatal hypoxia ischemia: insights from a clinical meta-analysis and a rodent model of induced hypoxic ischemic brain injury

Hypoxia ischemia (HI; reduced oxygen and/or blood flow to the brain) is one of the most common injuries among preterm infants and term infants with birth complications. Both populations show cognitive/behavioral deficits, including impairments in sensory, learning/memory, and attention domains. Clinical data suggests a sex difference in HI outcomes, with males exhibiting more severe cognitive/behavioral deficits relative to matched females. Our laboratory has also reported more severe behavioral deficits among male rats with induced HI relative to females with comparable injury (Hill et al., 2011a,b). The current study initially examined published clinical studies from the past 20years where long-term IQ outcome scores for matched groups of male and female premature infants were reported separately (IQ being the most common outcome measure). A meta-analysis revealed a female "advantage," as indicated by significantly better scores on performance and full scale IQ (but not verbal IQ) for premature females. We then utilized a rodent model of neonatal HI injury to assess sham and postnatal day 7 (P7) HI male and female rats on a battery of behavioral tasks. Results showed expected deficits in HI male rats, but also showed task-dependent sex differences, with HI males having significantly larger deficits than HI females on some tasks but equivalent deficits on other tasks. In contrast to behavioral results, post mortem neuropathology associated with HI was comparable across sex. These findings suggest: 1) neonatal female "protection" in some behavioral domains, as indexed by superior outcome following early injury relative to males; and 2) female protection may entail sex-specific plasticity or compensation, rather than a reduction in gross neuropathology. Further exploration of the mechanisms underlying this sex effect could aid in neuroprotection efforts for at-risk neonates in general, and males in particular. Moreover, our current report of comparable anatomical damage coupled with differences in cognitive outcomes (by sex) provides a framework for future studies to examine neural mechanisms underlying sex differences in cognition and behavior in general.

A behavioral evaluation of sex differences in a mouse model of severe neuronal migration disorder

Disruption of neuronal migration in humans is associated with a wide range of behavioral and cognitive outcomes including severe intellectual disability, language impairment, and social dysfunction. Furthermore, malformations of cortical development have been observed in a number of neurodevelopmental disorders (e.g. autism and dyslexia), where boys are much more commonly diagnosed than girls (estimates around 4 to 1). The use of rodent models provides an excellent means to examine how sex may modulate behavioral outcomes in the presence of comparable abnormal neuroanatomical presentations. Initially characterized by Rosen et al. 2012, the BXD29- Tlr4^lps−2J/J mouse mutant exhibits a highly penetrant neuroanatomical phenotype that consists of bilateral midline subcortical nodular heterotopia with partial callosal agenesis. In the current study, we confirm our initial findings of a severe impairment in rapid auditory processing in affected male mice. We also report that BXD29- Tlr4^lps−2J/J (mutant) female mice show no sparing of rapid auditory processing, and in fact show deficits similar to mutant males. Interestingly, female BXD29- Tlr4^lps−2J/J mice do display superiority in Morris water maze performance as compared to wild type females, an affect not seen in mutant males. Finally, we report new evidence that BXD29- Tlr4^lps−2J/J mice, in general, show evidence of hyper-social behaviors. In closing, the use of the BXD29- Tlr4^lps−2J/J strain of mice – with its strong behavioral and neuroanatomical phenotype – may be highly useful in characterizing sex independent versus dependent mechanisms that interact with neural reorganization, as well as clinically relevant abnormal behavior resulting from aberrant neuronal migration.

Differential seizure response in two models of cortical heterotopia

Malformations of cortical development (MCD) are linked to epilepsy in humans. MCD encompass a broad spectrum of malformations, which occur as the principal pathology or a secondary disruption. Recently, Rosen et al. (2012) reported that BXD29-Trl4(lps-2J)/J mice have subcortical nodular heterotopias with partial agenesis of the corpus callosum (p-ACC). Additionally Ramos et al. (2008) demonstrated that C57BL/10J mice exhibit cortical heterotopias with no additional cortical abnormalities. We examined the seizure susceptibility of these mice to determine if the presence (BXD29-Trl4(lps-2J)/J) or absence (C57BL/10J) of p-ACC, in strains with MCD, confers a differential response to chemi-convulsive treatment. Our results indicate that C57BL/10J mice with layer I heterotopia are more susceptible, whereas BXD29-Trl4(lps-2J)/J mice with more severe subcortical nodular heterotopia and p-ACC are more resistant to seizure behavior induced by pentylenetetrazole. These data suggest that p-ACC may confer seizure resistance in models of MCD.

Bilateral subcortical heterotopia with partial callosal agenesis in a mouse mutant

Cognition and behavior depend on the precise placement and interconnection of complex ensembles of neurons in cerebral cortex. Mutations that disrupt migration of immature neurons from the ventricular zone to the cortical plate have provided major insight into mechanisms of brain development and disease. We have discovered a new and highly penetrant spontaneous mutation that leads to large nodular bilateral subcortical heterotopias with partial callosal agenesis. The mutant phenotype was first detected in a colony of fully inbred BXD29 mice already known to harbor a mutation in Tlr4. Neurons confined to the heterotopias are mainly born in midgestation to late gestation and would normally have migrated into layers 2-4 of overlying neocortex. Callosal cross-sectional area and fiber number are reduced up to 50% compared with coisogenic wildtype BXD29 substrain controls. Mutants have a pronounced and highly selective defect in rapid auditory processing. The segregation pattern of the mutant phenotype is most consistent with a two-locus autosomal recessive model, and selective genotyping definitively rules out the Tlr4 mutation as a cause. The discovery of a novel mutation with strong pleiotropic anatomical and behavioral effects provides an important new resource for dissecting molecular mechanisms and functional consequences of errors of neuronal migration.

Inhibition of X-linked inhibitor of apoptosis with embelin differentially affects male versus female behavioral outcome following neonatal hypoxia-ischemia in rats

Hypoxia-ischemia (HI; concurrent oxygen/blood deficiency) and associated encephalopathy represent a common cause of neurological injury in premature/low-birth-weight infants and term infants with birth complications. Resulting behavioral impairments include cognitive and/or sensory processing deficits, as well as language disabilities, and clinical evidence shows that male infants with HI exhibit more severe cognitive deficits compared to females with equivalent injury. Evidence also demonstrates activation of sex-dependent apoptotic pathways following HI events, with males preferentially activating a caspase-independent cascade of cell death and females preferentially activating a caspase-dependent cascade following neonatal hypoxic and/or ischemic insults. Based on these combined data, the 'female protection' following HI injury may reflect the endogenous X-linked inhibitor of apoptosis (XIAP), which effectively binds effector caspases and halts downstream cleavage of effector caspases (thus reducing cell death). To test this theory, the current study utilized neonatal injections of vehicle or embelin (a small molecule inhibitor of XIAP) in male and female rats with or without induced HI injury on postnatal day 7 (P7). Subsequent behavioral testing using a clinically relevant task revealed that the inhibition of XIAP exacerbated HI-induced persistent behavioral deficits in females, with no effect on HI males. These results support sex differences in mechanisms of cell death following early HI injuries, and suggest a potential clinical benefit from the development of sex-specific neuroprotectants for the treatment of HI.

Reprint of "Early testosterone modulated sex differences in behavioral outcome following neonatal hypoxia ischemia in rats"

Hypoxia ischemia (HI; reduced blood oxygenation and/or flow to the brain) represents one of the most common injuries for both term and preterm/very low birth weight (VLBW) infants. These children experience elevated incidence of cognitive and/or sensory processing disabilities, including language based learning disabilities. Clinical data also indicate more substantial long-term deficits for HI injured male babies as compared to HI injured females. Previously, we reported significant deficits in rapid auditory processing and spatial learning in male rats with postnatal day 1 (P1), P7, or P10 HI injury. We also showed sex differences in HI injured animals, with more severe deficits in males as compared to females. Given these findings, combined with extant clinical data, the current study sought to assess a putative role for perinatal testosterone in modulating behavioral outcome following early hypoxic-ischemic injury in rats. Male, female, and testosterone-propionate (TP) treated females were subjected to P7 HI or sham surgery, and subsequently (P30+) underwent a battery of auditory testing and water maze assessment. Results confirm previous reports of sex differences following HI, and add new findings of significantly worse performance in TP-treated HI females compared to vehicle treated HI females. Post mortem anatomic analyses showed consistent effects, with significant brain weight decreases seen in HI male and TP-treated HI females but not female HI or sham groups. Further neuromorphometric analysis of brain structures showed that HI male animals exhibited increased pathology relative to HI females as reflected in ventricular enlargement. Findings suggest that neonatal testosterone may act to enhance the deleterious consequences of early HI brain injury, as measured by both neuropathology and behavior.

Early testosterone modulated sex differences in behavioral outcome following neonatal hypoxia ischemia in rats

Hypoxia ischemia (HI; reduced blood oxygenation and/or flow to the brain) represents one of the most common injuries for both term and preterm/very low birth weight (VLBW) infants. These children experience elevated incidence of cognitive and/or sensory processing disabilities, including language based learning disabilities. Clinical data also indicate more substantial long-term deficits for HI injured male babies as compared to HI injured females. Previously, we reported significant deficits in rapid auditory processing and spatial learning in male rats with postnatal day 1 (P1), P7, or P10 HI injury. We also showed sex differences in HI injured animals, with more severe deficits in males as compared to females. Given these findings, combined with extant clinical data, the current study sought to assess a putative role for perinatal testosterone in modulating behavioral outcome following early hypoxic-ischemic injury in rats. Male, female, and testosterone-propionate (TP) treated females were subjected to P7 HI or sham surgery, and subsequently (P30+) underwent a battery of auditory testing and water maze assessment. Results confirm previous reports of sex differences following HI, and add new findings of significantly worse performance in TP-treated HI females compared to vehicle treated HI females. Post mortem anatomic analyses showed consistent effects, with significant brain weight decreases seen in HI male and TP-treated HI females but not female HI or sham groups. Further neuromorphometric analysis of brain structures showed that HI male animals exhibited increased pathology relative to HI females as reflected in ventricular enlargement. Findings suggest that neonatal testosterone may act to enhance the deleterious consequences of early HI brain injury, as measured by both neuropathology and behavior.

Layer I neocortical ectopia: cellular organization and local cortical circuitry

Focal cortical dysplasia (FCD) are associated with neurological disorders and cognitive impairments in humans. Molecular layer ectopia, clusters of misplaced cells in layer I of the neocortex, have been identified in patients with developmental dyslexia and psychomotor retardation. Mouse models of this developmental disorder display behavioral impairments and increased seizure susceptibility. Although there is a correlation between cortical malformations and neurological dysfunction, little is known about the morphological and physiological properties of cells within cortical malformations. In the present study we used electrophysiological and immunocytochemical analyses to examine the distribution of neuronal and non-neuronal cell types within and surrounding layer I neocortical ectopia in NXSMD/EiJ mice. We show that cells within ectopia have membrane properties of both pyramidal and a variety of non-pyramidal cell types, including fast-spiking cells. Immunocytochemical analysis for different interneuronal subtypes demonstrates that ectopia contain nonpyramidal cells immunoreactive for calbindin-D28K (CALB), parvalbumin (PARV), and calretinin (CR). Ectopia also contains astrocytes, positive for glial fibrillary acidic protein (GFAP) and oligodendrocyte precursor cells positive for NG2 proteoglycan (NG2). Lastly, we provide electrophysiological and morphological evidence to demonstrate that cells within ectopia receive input from cells within layers I, upper and deeper II/III, and V and provide outputs to cells within deep layer II/III and layer V, but not layers I and upper II/III. These results indicate that ectopia contain cells of different lineages with diverse morphological and physiological properties, and appear to cause disruptions in local cortical circuitry.

Neocortical molecular layer heterotopia in substrains of C57BL/6 and C57BL/10 mice

Abnormal development of the neocortex is often associated with cognitive deficits and epilepsy. Rodent models are widely used to study normal and abnormal cortical development and have revealed the roles of many important genetic and environmental factors. Interestingly, several inbred mouse strains commonly used in behavioral, anatomical, and/or physiological studies display neocortical malformations including C57BL/6J mice, which are among the most widely utilized mice. In the present report we describe the prevalence and cytoarchitecture of molecular-layer heterotopia in C57BL/6J mice and related strains obtained from three commercial vendors as well as mice bred in academic vivaria from founders obtained commercially. In particular, we found that the prevalence of molecular-layer heterotopia vaired according to the sex as well as the vendor-of-origin of the mouse. These data are relevant to the use of this strain as a mouse-model in the study of brain-behavior relationships.

Progress towards a cellular neurobiology of reading disability

Reading Disability (RD) is a significant impairment in reading accuracy, speed and/or comprehension despite adequate intelligence and educational opportunity. RD affects 5-12% of readers, has a well-established genetic risk, and is of unknown neurobiological cause or causes. In this review we discuss recent findings that revealed neuroanatomic anomalies in RD, studies that identified 3 candidate genes (KIAA0319, DYX1C1, and DCDC2), and compelling evidence that potentially link the function of candidate genes to the neuroanatomic anomalies. A hypothesis has emerged in which impaired neuronal migration is a cellular neurobiological antecedent to RD. We critically evaluate the evidence for this hypothesis, highlight missing evidence, and outline future research efforts that will be required to develop a more complete cellular neurobiology of RD.

Cytoarchitecture and transcriptional profiles of neocortical malformations in inbred mice

Malformations of neocortical development are associated with cognitive dysfunction and increased susceptibility to epileptogenesis. Rodent models are widely used to study neocortical malformations and have revealed important genetic and environmental mechanisms that contribute to neocortical development. Interestingly, several inbred mice strains commonly used in behavioral, anatomical, and/or physiological studies display neocortical malformations. In the present report we examine the cytoarchitecture and myeloarchitecture of the neocortex of 11 inbred mouse strains and identified malformations of cortical development, including molecular layer heterotopia, in all but one strain. We used in silico methods to confirm our observations and determined the transcriptional profiles of cells found within heterotopia. These data indicate cellular and transcriptional diversity present in cells in malformations. Furthermore, the presence of dysplasia in nearly every inbred strain examined suggests that malformations of neocortical development are a common feature in the neocortex of inbred mice.

Persistent spatial working memory deficits in rats with bilateral cortical microgyria

BACKGROUND

Anomalies of cortical neuronal migration (e.g., microgyria (MG) and/or ectopias) are associated with a variety of language and cognitive deficits in human populations. In rodents, postnatal focal freezing lesions lead to the formation of cortical microgyria similar to those seen in human dyslexic brains, and also cause subsequent deficits in rapid auditory processing similar to those reported in human language impaired populations. Thus convergent findings support the ongoing study of disruptions in neuronal migration in rats as a putative model to provide insight on human language disability. Since deficits in working memory using both verbal and non-verbal tasks also characterize dyslexic populations, the present study examined the effects of neonatally induced bilateral cortical microgyria (MG) on working memory in adult male rats.

METHODS

A delayed match-to-sample radial water maze task, in which the goal arm was altered among eight locations on a daily basis, was used to assess working memory performance in MG (n = 8) and sham (n = 10) littermates.

RESULTS

Over a period of 60 sessions of testing (each session comprising one pre-delay sample trial, and one post-delay test trial), all rats showed learning as evidenced by a significant decrease in overall test errors. However, MG rats made significantly more errors than shams during initial testing, and this memory deficit was still evident after 60 days (12 weeks) of testing. Analyses performed on daily error patterns showed that over the course of testing, MG rats utilized a strategy similar to shams (but with less effectiveness, as indicated by more errors).

CONCLUSION

These results indicate persistent abnormalities in the spatial working memory system in rats with induced disruptions of neocortical neuronal migration.

Use of a modified prepulse inhibition paradigm to assess complex auditory discrimination in rodents

Prepulse inhibition (PPI; also termed startle reduction or reflex modification, see Ref. [H.S. Hoffman, J.R. Ison, Reflex modification in the domain of startle: I. Some empirical findings and their implications for how the nervous system processes sensory input, Psychol. Rev. 87 (1980) 175-189]) provides an efficient and accurate method to assess both simple and complex acoustic discrimination in rodents [J.R. Ison, G.R. Hammond, Modification of the startle reflex in the rat by changes in the auditory and visual environments, J. Comp. Physiol. Psychol. 75 (1971) 435-452]. Assessment of acoustic processing using PPI is less time consuming than operant conditioning paradigms, allows for the testing of many subjects simultaneously, and largely eliminates confounds due to motivation and attention [M. Clark, G. Rosen, P. Tallal, R.H. Fitch, Impaired processing of complex auditory stimuli in rats with induced cerebrocortical microgyria, J. Cog. Neurosci. 12 (2000) 828-839]. Moreover, PPI procedures allow for data acquisition from the first day of testing, and can be used on rats as young as P14-15 [J.T. Friedman, A. Peiffer, M. Clark, A. Benasich, R.H. Fitch, Age and experience related improvements in gap detection in the rat, Dev. Brain Res. 152 (2004) 83-91; M. McClure, S. Threlkeld, G. Rosen, R.H. Fitch, Rapid auditory processing and learning deficits in rats with P1 versus P7 neonatal hypoxic-ischemic injury, Behav. Brain Res. 172 (2006) 114-121; S.W. Threlkeld, M.M. McClure, G.D. Rosen, R.H. Fitch, Developmental timeframes for the induction of microgyria and rapid auditory processing deficits in the rat, Brain Res. 1109 (2006) 22-31]. For these and additional reasons, the PPI paradigm has more recently been adapted to the assessment of complex acoustic discrimination (tone sequences and FM sweeps), and applied to the study of normally developing as well as neuropathologically affected rodent populations. The purpose of the current review is to provide a background on the PPI paradigm, and to summarize what has been learned more recently using modified versions of PPI with rodent models.

Early cortical damage in rat somatosensory cortex alters acoustic feature representation in primary auditory cortex

Early postnatal freeze-lesions to the cortical plate result in malformations resembling human microgyria. Microgyria in primary somatosensory cortex (S1) of rats are associated with a reduced behavioral detection of rapid auditory transitions and the loss of large cells in the thalamic nucleus projecting to primary auditory cortex (A1). Detection of slow transitions in sound is intact in animals with S1 microgyria, suggesting dissociation between responding to slow versus rapid transitions and a possible dissociation between levels of auditory processing affected. We hypothesized that neuronal responses in primary auditory cortex (A1) would be differentially reduced for rapid sound repetitions but not for slow sound sequences in animals with S1 microgyria. We assessed layer IV cortical responses in primary auditory cortex (A1) to single pure-tones and periodic noise bursts (PNB) in rats with and without S1 microgyria. We found that responses to both types of acoustic stimuli were reduced in magnitude in animals with microgyria. Furthermore, spectral resolution was degraded in animals with microgyria. The cortical selectivity and temporal precision were then measured with conventional methods for PNB and tone-stimuli, but no significant changes were observed between microgyric and control animals. Surprisingly, the observed spike rate reduction was similar for rapid and slow temporal modulations of PNB stimuli. These results suggest that acoustic processing in A1 is indeed altered with early perturbations of neighboring cortex. However, the type of deficit does not affect the temporal dynamics of the cortical output. Instead, acoustic processing is altered via a systematic reduction in the driven spike rate output and spectral integration resolution in A1. This study suggests a novel form of plasticity, whereas early postnatal lesions of one sensory cortex can have a functional impact on processing in neighboring sensory cortex.

Developmental timeframes for induction of microgyria and rapid auditory processing deficits in the rat

Induction of a focal freeze lesion to the skullcap of a 1-day-old rat pup leads to the formation of microgyria similar to those identified postmortem in human dyslexics. Rats with microgyria exhibit rapid auditory processing deficits similar to those seen in language-impaired (LI) children, and infants at risk for LI and these effects are particularly marked in juvenile as compared to adult subjects. In the current study, a startle response paradigm was used to investigate gap detection in juvenile and adult rats that received bilateral freezing lesions or sham surgery on postnatal day (P) 1, 3 or 5. Microgyria were confirmed in P1 and 3 lesion rats, but not in the P5 lesion group. We found a significant reduction in brain weight and neocortical volume in P1 and 3 lesioned brains relative to shams. Juvenile (P27-39) behavioral data indicated significant rapid auditory processing deficits in all three lesion groups as compared to sham subjects, while adult (P60+) data revealed a persistent disparity only between P1-lesioned rats and shams. Combined results suggest that generalized pathology affecting neocortical development is responsible for the presence of rapid auditory processing deficits, rather than factors specific to the formation of microgyria per se. Finally, results show that the window for the induction of rapid auditory processing deficits through disruption of neurodevelopment appears to extend beyond the endpoint for cortical neuronal migration, although, the persistent deficits exhibited by P1 lesion subjects suggest a secondary neurodevelopmental window at the time of cortical neuromigration representing a peak period of vulnerability.

Histometric changes and cell death in the thalamus after neonatal neocortical injury in the rat

Freezing injury to the developing cortical plate results in a neocortical malformation resembling four-layered microgyria. Previous work has demonstrated that following freezing injury to the somatosensory cortex, males (but not females) have more small and fewer large cells in the medial geniculate nucleus. In the first experiment, we examined the effects of induced microgyria to the somatosensory cortex on neuronal numbers, neuronal size, and nuclear volume of three sensory nuclei: ventrobasal complex, dorsal lateral geniculate nucleus, and medial geniculate nucleus. We found that there was a decrease in neuronal number and nuclear volume in ventrobasal complex of microgyric rats when compared with shams, whereas there were no differences in these variables in the dorsal lateral geniculate nucleus or medial geniculate nucleus. We also found that there were more small and fewer large neurons in both ventrobasal complex and medial geniculate nucleus. In experiment 2, we attempted to determine the role of cell death in the thalamus on these histometric measures. We found that cell death peaked within 24 h of the freezing injury and was concentrated mostly in ventrobasal complex. In addition, there was evidence of greater cell death in males at this age. Taken together, these results support the notion that males are more severely affected by early injury to the cerebral cortex than females.

Dyslexia--a molecular disorder of neuronal migration: the 2004 Norman Geschwind Memorial Lecture

For 25 years now, there has been a serious attempt to get at the fundamental cause(s) of dyslexia in our laboratory. A great deal of research has been carried out on the psychological and brain underpinnings of the linguistic dysfunctions seen in dyslexia, but attempts to get at its cause have been limited. Initially, observations were made on the brains of persons with dyslexia who had died and their brains donated for research. These observations were modeled in animal models in order to better understand the full extent of anatomical and developmental brain characteristics. More recently, models have begun to employ genetic manipulations in order to close the gap between genes, brain, and behavior. In this article based on a lecture given in memory of Dr. Norman Geschwind to the International Dyslexia Association assembly in Philadelphia in 2004, I outline the history of the research leading up to the most recent findings. These findings consist of experiments using methods that interfere with the function of DNA, using as constructs genes that have been implicated in dyslexia, which cause developmental problems of neuronal migration in rats, secondary brain changes in response to the migration problems, and abnormal processing of sounds.

Neurobiology of dyslexia: a reinterpretation of the data

Theories of developmental dyslexia differ on how to best interpret the great variety of symptoms (linguistic, sensory and motor) observed in dyslexic individuals. One approach views dyslexia as a specific phonological deficit, which sometimes co-occurs with a more general sensorimotor syndrome. This article on the neurobiology of dyslexia shows that neurobiological data are indeed consistent with this view, explaining both how a specific phonological deficit might arise, and why a sensorimotor syndrome should be significantly associated with it. This new conceptualisation of the aetiology of dyslexia could generalize to other neurodevelopmental disorders, and might further explain heterogeneity within each disorder and comorbidity between disorders.

Developmental disorders of vision

This review of developmental disorders of vision focuses on only a few of the many disorders that disrupt visual development. Given the enormity of the human visual system in the primate brain and complexity of visual development, however, there are likely hundreds or thousands of types of disorders affecting high-level vision. The rapid progress seen in developmental dyslexia and WMS demonstrates the possibilities and difficulties inherent in researching such disorders, and the authors hope that similar progress will be made for congenital prosopagnosia and other disorders in the near future.

Sex differences in rapid auditory processing deficits in ectopic BXSB/MpJ mice

Prior research with rodent models, performed predominantly in males, has demonstrated a significant association between focal neocortical malformations (e.g. ectopias and microgyria) and rate-specific auditory processing deficits. In the current study and consistent with prior findings, we report that ectopic male BXSB/MpJ mice exhibit impairments in detecting a two-tone oddball stimulus at short but not long inter-stimulus interval durations when compared to non-ectopic male littermates. However, ectopic female littermates showed no rapid auditory processing deficit when compared to non-ectopic females on this same task. Current results add growing support to: (1) an association between focal cortical malformations and impaired auditory processing in males; and (2) the existence of sex differences in the behavioral consequences of focal cortical malformations.