Functional magnetic resonance imaging of cognitive processing in young adults with Down syndrome

Understanding the genetic mechanisms and cognitive impairments in Down syndrome: towards a holistic approach

The most common genetic cause of intellectual disability is Down syndrome (DS), trisomy 21. It commonly results from three copies of human chromosome 21 (HC21). There are no mutations or deletions involved in DS. Instead, the phenotype is caused by altered transcription of the genes on HC21. These transcriptional variations are responsible for a myriad of symptoms affecting every organ system. A very debilitating aspect of DS is intellectual disability (ID). Although tremendous advances have been made to try and understand the underlying mechanisms of ID, there is a lack of a unified, holistic view to defining the cause and managing the cognitive impairments. In this literature review, we discuss the mechanisms of neuronal over-inhibition, abnormal morphology, and other genetic factors in contributing to the development of ID in DS patients and to gain a holistic understanding of ID in DS patients. We also highlight potential therapeutic approaches to improve the quality of life of DS patients.

Is the prolongation latency of visual evoked potentials a pathological sign in children with Down’s syndrome without ocular abnormalities? Case–control study of children with Down’s syndrome

Objective

To evaluate retino-cortical function in children with Down’s syndrome (DS) and no evident ocular abnormalities beyond mild refractive error, by recording visual evoked potentials (VEP) in response to pattern-reversal stimuli and comparing to those of age-matched healthy controls.

Methods and analysis

All the children with DS registered at Split-Dalmatia County who met inclusion criteria of no ocular abnormalities and with refraction error between −0.5 and +2.0 D, and their age-matched healthy controls were included in the study (n=36 children, N=72 eyes, for both groups, respectively, with the same age of 9±2 years). Transient VEP was recorded and the waves with a positive peak as a response to a pattern-reversal stimulus, were analysed. The peak P100 latency, defined as the time from the stimulus onset to the main positive peak, and peak to peak amplitudes were measured.

Results

While P100 wave amplitudes were comparable between two groups (p=0.804), P100 latencies were from 4.3 to 28.5 ms longer in children with DS (p<0.001). Interocular latency difference between a VEP dominant and an inferior eye was pronounced in healthy (1.2 ms (0.2–4.0), but was almost diminished in children with DS (0.3 ms (0.1–0.5), p<0.001).

Conclusion

Our study has demonstrated that VEP response is divergent in children with DS compared with their age-matched healthy controls, indicating possible structural or functional abnormalities of the visual cortex. As VEP results are helpful in the diagnosis and treatment planning of vision-related disorders, we should reconsider the use of common VEP diagnostic criteria in subpopulation of children with DS.

Brain circuit pathology in Down syndrome: from neurons to neural networks

Down syndrome (DS), a genetic pathology caused by triplication of chromosome 21, is characterized by brain hypotrophy and impairment of cognition starting from infancy. While studies in mouse models of DS have elucidated the major neuroanatomical and neurochemical defects of DS, comparatively fewer investigations have focused on the electrophysiology of the DS brain. Electrical activity is at the basis of brain functioning. Therefore, knowledge of the way in which brain circuits operate in DS is fundamental to understand the causes of behavioral impairment and devise targeted interventions. This review summarizes the state of the art regarding the electrical properties of the DS brain, starting from individual neurons and culminating in signal processing in whole neuronal networks. The reported evidence derives from mouse models of DS and from brain tissues and neurons derived from individuals with DS. EEG data recorded in individuals with DS are also provided as a key tool to understand the impact of brain circuit alterations on global brain activity.

Youth with Down syndrome display widespread increased functional connectivity during rest

Studies of resting-state functional connectivity in young people with Down syndrome (DS) have yielded conflicting results. Some studies have found increased connectivity while others have found a mix of increased and decreased connectivity. No studies have examined whole-brain connectivity at the voxel level in youth with DS during an eyes-open resting-state design. Additionally, no studies have examined the relationship between connectivity and network selectivity in youth with DS. Thus, the current study sought to fill this gap in the literature. Nineteen youth with DS (Mage = 16.5; range 7-23; 13 F) and 33 typically developing (TD) youth (Mage = 17.5; range 6-24; 18 F), matched on age and sex, completed a 5.25-min eyes-open resting-state fMRI scan. Whole-brain functional connectivity (average Pearson correlation of each voxel with every other voxel) was calculated for each individual and compared between groups. Network selectivity was then calculated and correlated with functional connectivity for the DS group. Results revealed that whole-brain functional connectivity was significantly higher in youth with DS compared to TD controls in widespread regions throughout the brain. Additionally, participants with DS had significantly reduced network selectivity compared to TD peers, and selectivity was significantly related to connectivity in all participants. Exploratory behavioral analyses revealed that regions showing increased connectivity in DS predicted Verbal IQ, suggesting differences in connectivity may be related to verbal abilities. These results indicate that network organization is disrupted in youth with DS such that disparate networks are overly connected and less selective, suggesting a potential target for clinical interventions.

Reach Out and Read Implementation in a Pediatric Down Syndrome Clinic

INTRODUCTION

To examine the first Reach Out and Read (ROR) program in a pediatric Down syndrome (DS) clinic in the United States and the literacy behaviors of young children with DS and their families.

METHOD

This is a large cohort (n = 747) review of children with DS participating in ROR and a family literacy survey (n = 209). Data from the electronic medical records were included.

RESULT

On average, children with DS began independently reading at 6.15 years (standard deviation = 1.42). Overall, 36.7% of children with visual/audio impairments required additional encouragement. Time spent reading was impacted by the mother's education level. Differences were found among ROR participants with DS for reported favorite activity.

DISCUSSION

ROR is an important clinic-based literacy program for children with DS. Children with DS attain independent reading abilities similar to typically developing peers when provided appropriate resources. Additional support is needed to encourage reading enjoyment in this population.

Influence of COVID-19 pandemic lockdown on a sample of Egyptian children with Down syndrome

Background

Down syndrome (DS) is characterized by variable degrees of intellectual disability (ID). The coronavirus disease-2019 (COVID-19) lockdown prevented children with DS from reaching their rehabilitation facilities. This could have led to deterioration of their abilities and mental health hazards. The aim of this cohort study was to investigate frequency of COVID-19, the influence of COVID-19 pandemic on health, and some abilities of children with DS, and to explore factors that could have governed receiving home-based training during the lockdown. A survey of 150 individuals with Down syndrome was answered by their caregivers. Additionally, 135 participants were subjected to assessment of cognitive, language, and motor abilities using Portage program. They were divided into 2 groups: group I who received online therapy sessions during the lockdown and group II who did not receive sessions. Logistic regression was used to determine the factors which influenced getting home-based training.

Results

The percentage of COVID-19 cases was 3.3%. All evaluated abilities were reduced despite receiving online sessions particularly language performance (P < 0.001). Male gender, having severe ID and low parental education were among the factors which encouraged parents to get virtual training.

Conclusion

COVID-19 pandemic had a negative impact on the abilities of DS children even those who got rehabilitation sessions. Their dependence on social interaction could have limited the benefit of virtual sessions. Factors that influence a parent's decision to get home-based training should be monitored and targeted in order to overcome obstacles or concepts that may prevent families from enduring home-based intervention.

Resting-state default mode network connectivity in young individuals with Down syndrome

BACKGROUND

Down syndrome (DS) is a chromosomal disorder that causes intellectual disability. Few studies have been conducted on functional connectivity using resting-state fMRI (functional magnetic resonance imaging) signals or more specifically, on the relevant structure and density of the default mode network (DMN). Although data on this issue have been reported in adult DS individuals (age: >45 years), the DMN properties in young DS individuals have not been studied. The aim of this study was to describe the density and structure of the DMN network from fMRI signals in young DS (age: <36 years).

METHOD

A sample of 22 young people with DS between the ages of 16 and 35 (M = 25.5 and SD = 5.1) was recruited in various centers for people with intellectual disability (ID). In addition to sociodemographic data, a six-minute fMRI session was recorded with a 3. T Philips Ingenia scanner. A control group of 22 young people, matched by age and gender, was obtained from the Human Connectome Project (to compare the networks properties between groups).

RESULTS

The values of the 48 ROIs that configured the DMN were obtained, and the connectivity graphs for each subject, the average connectivity graph for each group, the clustering and degree values for each ROI, and the average functional connectivity network were estimated.

CONCLUSIONS

A higher density of overactivation was identified in DS group in the ventral, sensorimotor, and visual DMN networks, although within a framework of a wide variability of connectivity patterns in comparison with the control group network. These results extend our understanding of the functional connectivity networks pattern and intrasubject variability in DS.

Using fMRI to Assess Brain Activity in People With Down Syndrome: A Systematic Review

Background: In the last few years, many investigations have focused on brain activity in general and in populations with different pathologies using non-invasive techniques such as electroencefalography (EEG), positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and magnetic resonance imaging (MRI). However, the use of non-invasive techniques to detect brain signals to evaluate the cognitive activity of people with Down syndrome (DS) has not been sufficiently addressed. The objective of this study is to describe the state-of-the-art in fMRI techniques for recording brain signals in people with DS. Method: A systematic review was performed based on PRISMA recommendations; only nine papers on this topic have been published. Three independent researchers selected all relevant information from each paper. Analyses of information concordance showed a high value of agreement between researchers. Results: Although few relevant works have been published, the use of fMRI in people with DS is becoming an appropriate option to study brain function in this population. Of the nine identified papers, five used task designs, and four used resting-state paradigms. Conclusion: Thus, we emphasize the need to incorporate rigorous cognitive activity procedures in evaluations of the DS population. We suggest several factors (such as head correction movements and paired sample techniques) that must be considered when designing an fMRI study with a task or a resting-state paradigm in a DS population.

Neuroimaging assessment in Down syndrome: a pictorial review

Down syndrome (DS), or trisomy 21, is the leading genetic cause of intellectual incapacity worldwide, with a reported incidence of about 1 in 1,000 to 1 in 1,100 live births. Besides the several commonly known physical features characteristic of this syndrome present at birth, DS may additionally affect every organ system. In addition, despite the large number of published papers concerning this syndrome, there is scarce literature focusing specifically in the typical neuroimaging features associated with this condition. The aim of this paper is to review and systematize the distinctive characteristics and abnormalities of the central nervous system, head and neck, and spine present in DS patients that should actively be searched for and evaluated by radiologists and/or neuroradiologists.

Pediatric Brain Development in Down Syndrome: A Field in Its Infancy

OBJECTIVES

As surprisingly little is known about the developing brain studied in vivo in youth with Down syndrome (DS), the current review summarizes the small DS pediatric structural neuroimaging literature and begins to contextualize existing research within a developmental framework.

METHODS

A systematic review of the literature was completed, effect sizes from published studies were reviewed, and results are presented with respect to the DS cognitive behavioral phenotype and typical brain development.

RESULTS

The majority of DS structural neuroimaging studies describe gross differences in brain morphometry and do not use advanced neuroimaging methods to provide nuanced descriptions of the brain. There is evidence for smaller total brain volume (TBV), total gray matter (GM) and white matter, cortical lobar, hippocampal, and cerebellar volumes. When reductions in TBV are accounted for, specific reductions are noted in subregions of the frontal lobe, temporal lobe, cerebellum, and hippocampus. A review of cortical lobar effect sizes reveals mostly large effect sizes from early childhood through adolescence. However, deviance is smaller in adolescence. Despite these smaller effects, frontal GM continues to be largely deviant in adolescence. An examination of age-frontal GM relations using effect sizes from published studies and data from Lee et al. (2016) reveals that while there is a strong inverse relationship between age and frontal GM volume in controls across childhood and adolescence, this is not observed in DS.

CONCLUSIONS

Further developmentally focused research, ideally using longitudinal neuroimaging, is needed to elucidate the nature of the DS neuroanatomic phenotype during childhood and adolescence. (JINS, 2018, 24, 966-976).

Intelligence and specific cognitive functions in intellectual disability: implications for assessment and classification

PURPOSE OF REVIEW

Current diagnostic criteria for intellectual disability categorize ability as measured by IQ tests. However, this does not suit the new conceptualization of intellectual disability, which refers to a range of neuropsychiatric syndromes that have in common early onset, cognitive impairments, and consequent deficits in learning and adaptive functioning. A literature review was undertaken on the concept of intelligence and whether it encompasses a range of specific cognitive functions to solve problems, which might be better reported as a profile, instead of an IQ, with implications for diagnosis and classification of intellectual disability.

RECENT FINDINGS

Data support a model of intelligence consisting of distinct but related processes. Persons with intellectual disability with the same IQ level have different cognitive profiles, based on varying factors involved in aetiopathogenesis. Limitations of functioning and many biopsychological factors associated with intellectual disability are more highly correlated with impairments of specific cognitive functions than with overall IQ.

SUMMARY

The current model of intelligence, based on IQ, is of limited utility for intellectual disability, given the wide range and variability of cognitive functions and adaptive capacities. Assessing level of individual impairment in executive and specific cognitive functions may be a more useful alternative. This has considerable implications for the revision of the International Classification of Diseases and for the cultural attitude towards intellectual disability in general.

Visual characteristics of children with Down syndrome

PURPOSE

To analyze long-term visual development in children with Down syndrome who received early ophthalmological intervention.

METHODS

A total of 125 children with Down syndrome who were examined before 6 years of age and followed up for more than 5 years were selected. Visual development, refraction, visual acuity testing, and the prescription of spectacles were examined retrospectively.

RESULTS

Mean visual acuity by age was as follows: 2 years, 1.13 ± 0.23 logarithm of the minimum angle of resolution (logMAR); 5 years, 0.55 ± 0.25 logMAR; 8 years, 0.27 ± 0.19 logMAR; 11 years, 0.17 ± 0.16 logMAR; and 14 years, 0.10 ± 0.15 logMAR. In 32 children (25.6%), visual acuity reached 0.0 logMAR or better. Hyperopia of +2D or more was observed in 132 eyes (52.8%), and astigmatism of 2D or more was observed in 153 eyes (61.2%). Subjective testing was difficult in many children prior to 4.5 years of age, and grating acuity testing was necessary. Spectacles were prescribed at a mean age of 3.5 ± 1.6 years in 120 children (96.0%). The average duration until the spectacles were worn constantly was 9.0 ± 9.3 months.

CONCLUSION

Early ophthalmological intervention and longitudinal care is important for children with Down syndrome.

ERP correlates of object recognition memory in Down syndrome: Do active and passive tasks measure the same thing?

Event related potentials (ERPs) can help to determine the cognitive and neural processes underlying memory functions and are often used to study populations with severe memory impairment. In healthy adults, memory is typically assessed with active tasks, while in patient studies passive memory paradigms are generally used. In this study we examined whether active and passive continuous object recognition tasks measure the same underlying memory process in typically developing (TD) adults and in individuals with Down syndrome (DS), a population with known hippocampal impairment. We further explored how ERPs in these tasks relate to behavioral measures of memory. Data-driven analysis techniques revealed large differences in old-new effects in the active versus passive task in TD adults, but no difference between these tasks in DS. The group with DS required additional processing in the active task in comparison to the TD group in two ways. First, the old-new effect started 150 ms later. Second, more repetitions were required to show the old-new effect. In the group with DS, performance on a behavioral measure of object-location memory was related to ERP measures across both tasks. In total, our results suggest that active and passive ERP memory measures do not differ in DS and likely reflect the use of implicit memory, but not explicit processing, on both tasks. Our findings highlight the need for a greater understanding of the comparison between active and passive ERP paradigms before they are inferred to measure similar functions across populations (e.g., infants or intellectual disability).

Resting-State Functional Connectivity in Individuals with Down Syndrome and Williams Syndrome Compared with Typically Developing Controls

The emergence of resting-state functional connectivity (rsFC) analysis, which examines temporal correlations of low-frequency (<0.1 Hz) blood oxygen level-dependent signal fluctuations between brain regions, has dramatically improved our understanding of the functional architecture of the typically developing (TD) human brain. This study examined rsFC in Down syndrome (DS) compared with another neurodevelopmental disorder, Williams syndrome (WS), and TD. Ten subjects with DS, 18 subjects with WS, and 40 subjects with TD each participated in a 3-Tesla MRI scan. We tested for group differences (DS vs. TD, DS vs. WS, and WS vs. TD) in between- and within-network rsFC connectivity for seven functional networks. For the DS group, we also examined associations between rsFC and other cognitive and genetic risk factors. In DS compared with TD, we observed higher levels of between-network connectivity in 6 out 21 network pairs but no differences in within-network connectivity. Participants with WS showed lower levels of within-network connectivity and no significant differences in between-network connectivity relative to DS. Finally, our comparison between WS and TD controls revealed lower within-network connectivity in multiple networks and higher between-network connectivity in one network pair relative to TD controls. While preliminary due to modest sample sizes, our findings suggest a global difference in between-network connectivity in individuals with neurodevelopmental disorders compared with controls and that such a difference is exacerbated across many brain regions in DS. However, this alteration in DS does not appear to extend to within-network connections, and therefore, the altered between-network connectivity must be interpreted within the framework of an intact intra-network pattern of activity. In contrast, WS shows markedly lower levels of within-network connectivity in the default mode network and somatomotor network relative to controls. These findings warrant further investigation using a task-based procedure that may help disentangle the relationship between brain function and cognitive performance across the spectrum of neurodevelopmental disorders.

Building an adaptive brain across development: targets for neurorehabilitation must begin in infancy

Much progress has been made toward behavioral and pharmacological intervention in intellectual disability, which was once thought too difficult to treat. Down syndrome (DS) research has shown rapid advances, and clinical trials are currently underway, with more on the horizon. Here, we review the literature on the emergent profile of cognitive development in DS, emphasizing that treatment approaches must consider how some "end state" impairments, such as language deficits, may develop from early alterations in neural systems beginning in infancy. Specifically, we highlight evidence suggesting that there are pre- and early postnatal alterations in brain structure and function in DS, resulting in disturbed network function across development. We stress that these early alterations are likely amplified by Alzheimer's disease (AD) progression and poor sleep. Focusing on three network hubs (prefrontal cortex, hippocampus, and cerebellum), we discuss how these regions may relate to evolving deficits in cognitive function in individuals with DS, and to their language profile in particular.

Violence: heightened brain attentional network response is selectively muted in Down syndrome

BACKGROUND

The ability to recognize and respond appropriately to threat is critical to survival, and the neural substrates subserving attention to threat may be probed using depictions of media violence. Whether neural responses to potential threat differ in Down syndrome is not known.

METHODS

We performed functional MRI scans of 15 adolescent and adult Down syndrome and 14 typically developing individuals, group matched by age and gender, during 50 min of passive cartoon viewing. Brain activation to auditory and visual features, violence, and presence of the protagonist and antagonist were compared across cartoon segments. fMRI signal from the brain's dorsal attention network was compared to thematic and violent events within the cartoons between Down syndrome and control samples.

RESULTS

We found that in typical development, the brain's dorsal attention network was most active during violent scenes in the cartoons and that this was significantly and specifically reduced in Down syndrome. When the antagonist was on screen, there was significantly less activation in the left medial temporal lobe of individuals with Down syndrome. As scenes represented greater relative threat, the disparity between attentional brain activation in Down syndrome and control individuals increased. There was a reduction in the temporal autocorrelation of the dorsal attention network, consistent with a shortened attention span in Down syndrome. Individuals with Down syndrome exhibited significantly reduced activation in primary sensory cortices, and such perceptual impairments may constrain their ability to respond to more complex social cues such as violence.

CONCLUSIONS

These findings may indicate a relative deficit in emotive perception of violence in Down syndrome, possibly mediated by impaired sensory perception and hypoactivation of medial temporal structures in response to threats, with relative preservation of activity in pro-social brain regions. These findings indicate that specific genetic differences associated with Down syndrome can modulate the brain's response to violence and other complex emotive ideas.

Assessing cognitive improvement in people with Down syndrome: important considerations for drug-efficacy trials

Experimental research over just the past decade has raised the possibility that learning deficits connected to Down syndrome (DS) might be effectively managed by medication. In the current chapter, we touch on some of the work that paved the way for these advances and discuss the challenges associated with translating them. In particular, we highlight sources of phenotypic variability in the DS population that are likely to impact performance assessments. Throughout, suggestions are made on how to detect meaningful changes in cognitive-adaptive function in people with DS during drug treatment. The importance of within-subjects evaluation is emphasized.

Functional magnetic resonance imaging of story listening in adolescents and young adults with Down syndrome: evidence for atypical neurodevelopment

BACKGROUND

Previous studies have documented differences in neural activation during language processing in individuals with Down syndrome (DS) in comparison with typically developing individuals matched for chronological age. This study used functional magnetic resonance imaging (fMRI) to compare activation during language processing in young adults with DS to typically developing comparison groups matched for chronological age or mental age. We hypothesised that the pattern of neural activation in the DS cohort would differ when compared with both typically developing cohorts.

METHOD

Eleven persons with DS (mean chronological age = 18.3; developmental age range = 4-6 years) and two groups of typically developing individuals matched for chronological (n = 13; mean age = 18.3 years) and developmental (mental) age (n = 12; chronological age range = 4-6 years) completed fMRI scanning during a passive story listening paradigm. Random effects group comparisons were conducted on individual maps of the contrast between activation (story listening) and rest (tone presentation) conditions.

RESULTS

Robust activation was seen in typically developing groups in regions associated with processing auditory information, including bilateral superior and middle temporal lobe gyri. In contrast, the DS cohort demonstrated atypical spatial distribution of activation in midline frontal and posterior cingulate regions when compared with both typically developing control groups. Random effects group analyses documented reduced magnitude of activation in the DS cohort when compared with both control groups.

CONCLUSIONS

Activation in the DS group differed significantly in magnitude and spatial extent when compared with chronological and mental age-matched typically developing control groups during a story listening task. Results provide additional support for an atypical pattern of functional organisation for language processing in this population.

Visuo-spatial ability in individuals with Down syndrome: is it really a strength?

Down syndrome (DS) is associated with extreme difficulty in verbal skills and relatively better visuo-spatial skills. Indeed, visuo-spatial ability is often considered a strength in DS. However, it is not clear whether this strength is only relative to the poor verbal skills, or, more impressively, relative to cognitive ability in general. To answer this question, we conducted an extensive literature review of studies on visuo-spatial abilities in people with Down syndrome from January 1987 to May 2013. Based on a general taxonomy of spatial abilities patterned after Lohman, Pellegrino, Alderton, and Regian (1987) and Carroll (1993) and existing studies of DS, we included five different domains of spatial abilities - visuo-spatial memory, visuo-spatial construction, mental rotation, closure, and wayfinding. We evaluated a total of 49 studies including 127 different comparisons. Most comparisons involved a group with DS vs. a group with typical development matched on mental age and compared on a task measuring one of the five visuo-spatial abilities. Although further research is needed for firm conclusions on some visuo-spatial abilities, there was no evidence that visuo-spatial ability is a strength in DS relative to general cognitive ability. Rather, the review suggests an uneven profile of visuo-spatial abilities in DS in which some abilities are commensurate with general cognitive ability level, and others are below.

Abnormal brain synchrony in Down Syndrome

Down Syndrome is the most common genetic cause for intellectual disability, yet the pathophysiology of cognitive impairment in Down Syndrome is unknown. We compared fMRI scans of 15 individuals with Down Syndrome to 14 typically developing control subjects while they viewed 50 min of cartoon video clips. There was widespread increased synchrony between brain regions, with only a small subset of strong, distant connections showing underconnectivity in Down Syndrome. Brain regions showing negative correlations were less anticorrelated and were among the most strongly affected connections in the brain. Increased correlation was observed between all of the distributed brain networks studied, with the strongest internetwork correlation in subjects with the lowest performance IQ. A functional parcellation of the brain showed simplified network structure in Down Syndrome organized by local connectivity. Despite increased interregional synchrony, intersubject correlation to the cartoon stimuli was lower in Down Syndrome, indicating that increased synchrony had a temporal pattern that was not in response to environmental stimuli, but idiosyncratic to each Down Syndrome subject. Short-range, increased synchrony was not observed in a comparison sample of 447 autism vs. 517 control subjects from the Autism Brain Imaging Exchange (ABIDE) collection of resting state fMRI data, and increased internetwork synchrony was only observed between the default mode and attentional networks in autism. These findings suggest immature development of connectivity in Down Syndrome with impaired ability to integrate information from distant brain regions into coherent distributed networks.

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Adjacent brain regions are more synchronized in Down Syndrome.

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Distant brain regions show less synchronization in Down Syndrome.

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Negatively correlated brain regions are less anticorrelated in Down Syndrome.

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Down Syndrome subjects show simplified brain network architecture.

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Increased brain synchrony does not reflect a response to environmental stimuli.

Cognition in Down syndrome: a developmental cognitive neuroscience perspective

Down syndrome (DS) is the most common genetic form of intellectual disability. DS results in a characteristic profile of cognitive and neurological dysfunction. The predominant theory of the pattern of neural deficits in this syndrome suggests that DS affects 'late-developing' neural systems, including the function of the prefrontal cortex and hippocampus. In order to evaluate the validity of this theory, in this review, I highlight data addressing the neurological and cognitive phenotype in DS across development. In particular, I address the evidence suggesting that DS may impact late-developing neural systems and end with the conclusion that some cognitive difficulties in DS must result from poor communication between late-developing regions. Analogous to recent theories of cognitive processing in autism, cognitive deficits in DS may be substantially impacted by less efficient interregional communication. Finally, I discuss some ways in which understanding the impact of altered neurodevelopment in DS has the potential to inform our understanding of species-typical trajectories of cognitive development. WIREs Cogn Sci 2013, 4:307-317. doi: 10.1002/wcs.1221 For further resources related to this article, please visit the WIREs website.

Down syndrome: the brain in trisomic mode

Down syndrome is the most common form of intellectual disability and results from one of the most complex genetic perturbations that is compatible with survival, trisomy 21. The study of brain dysfunction in this disorder has largely been based on a gene discovery approach, but we are now moving into an era of functional genome exploration, in which the effects of individual genes are being studied alongside the effects of deregulated non-coding genetic elements and epigenetic influences. Also, new data from functional neuroimaging studies are challenging our views of the cognitive phenotypes associated with Down syndrome and their pathophysiological correlates. These advances hold promise for the development of treatments for intellectual disability.

From abnormal hippocampal synaptic plasticity in down syndrome mouse models to cognitive disability in down syndrome

Down syndrome (DS) is caused by the overexpression of genes on triplicated regions of human chromosome 21 (Hsa21). While the resulting physiological and behavioral phenotypes vary in their penetrance and severity, all individuals with DS have variable but significant levels of cognitive disability. At the core of cognitive processes is the phenomenon of synaptic plasticity, a functional change in the strength at points of communication between neurons. A wide variety of evidence from studies on DS individuals and mouse models of DS indicates that synaptic plasticity is adversely affected in human trisomy 21 and mouse segmental trisomy 16, respectively, an outcome that almost certainly extensively contributes to the cognitive impairments associated with DS. In this review, we will highlight some of the neurophysiological changes that we believe reduce the ability of trisomic neurons to undergo neuroplasticity-related adaptations. We will focus primarily on hippocampal networks which appear to be particularly impacted in DS and where consequently the majority of cellular and neuronal network research has been performed using DS animal models, in particular the Ts65Dn mouse. Finally, we will postulate on how altered plasticity may contribute to the DS cognitive disability.