Functional brain changes underlying irritability in premanifest Huntington's disease

Pathomechanisms of behavioral abnormalities in Huntington disease: an update

Huntington disease (HD), a devastating autosomal-dominant neurodegenerative disease caused by an expanded CAG trinucleotide repeat, is clinically characterized by a triad of symptoms including involuntary motions, behavior problems and cognitive deficits. Behavioral symptoms with anxiety, irritability, obsessive-compulsive behaviors, apathy and other neuropsychiatric symptoms, occurring in over 50% of HD patients are important features of this disease and contribute to impairment of quality of life, but their pathophysiology is poorly understood. Behavior problems, more frequent than depression, can be manifest before obvious motor symptoms and occur across all HD stages, usually correlated with duration of illness. While specific neuropathological data are missing, the relations between gene expression and behavior have been elucidated in transgenic models of HD. Disruption of interneuronal communications, with involvement of prefronto-striato-thalamic networks and hippocampal dysfunctions produce deficits in multiple behavioral domains. These changes that have been confirmed by multistructural neuroimaging studies are due to a causal cascade linking molecular pathologies (glutamate-mediated excitotoxicity, mitochondrial dysfunctions inducing multiple biochemical and structural alterations) and deficits in multiple behavioral domains. The disruption of large-scale connectivities may explain the variability of behavior profiles and is useful in understanding the biological backgrounds of functional decline in HD. Such findings offer new avenues for targeted treatments in terms of minimizing neurobehavioral impairment in HD.

Gray matter alterations in Huntington's disease: A meta-analysis of VBM neuroimaging studies

Increasing neuroimaging studies have attempted to identify biomarkers of Huntington's disease (HD) progression. Here, we conducted voxel-based meta-analyses of voxel-based morphometry (VBM) studies on HD to investigate the evolution of gray matter volume (GMV) alterations and explore the effects of genetic and clinical features on GMV changes. A systematic review was performed to identify the relevant studies. Meta-analyses of whole-brain VBM studies were performed to assess the regional GMV changes in all HD mutation carriers, in presymptomatic HD (pre-HD), and in symptomatic HD (sym-HD). A quantitative comparison was performed between pre-HD and sym-HD. Meta-regression analyses were used to explore the effects of genetic and clinical features on GMV changes. Twenty-eight studies were included, comparing a total of 1811 HD mutation carriers [including 1150 pre-HD and 560 sym-HD] and 969 healthy controls (HCs). Pre-HD showed decreased GMV in the bilateral caudate nuclei, putamen, insula, anterior cingulate/paracingulate gyri, middle temporal gyri, and left dorsolateral superior frontal gyrus compared with HCs. Compared with pre-HD, GMV decrease in sym-HD extended to the bilateral median cingulate/paracingulate gyri, Rolandic operculum and middle occipital gyri, left amygdala, and superior temporal gyrus. Meta-regression analyses found that age, mean lengths of CAG repeats, and disease burden were negatively associated with GMV atrophy of the bilateral caudate and right insula in all HD mutation carriers. This meta-analysis revealed the pattern of GMV changes from pre-HD to sym-HD, prompting the understanding of HD progression. The pattern of GMV changes may be biomarkers for disease progression in HD.

Systematic Review and Meta-Analyses of Word Production Abilities in Dysfunction of the Basal Ganglia: Stroke, Small Vessel Disease, Parkinson's Disease, and Huntington's Disease

Clinical populations with basal ganglia pathologies may present with language production impairments, which are often described in combination with comprehension measures or attributed to motor, memory, or processing-speed problems. In this systematic review and meta-analysis, we studied word production in four (vascular and non-vascular) pathologies of the basal ganglia: stroke affecting the basal ganglia, small vessel disease, Parkinson's disease, and Huntington's disease. We compared scores of these clinical populations with those of matched cognitively unimpaired adults on four well-established production tasks, namely picture naming, category fluency, letter fluency, and past-tense verb inflection. We conducted a systematic search in PubMed and PsycINFO with terms for basal ganglia structures, basal ganglia disorders and language production tasks. A total of 114 studies were included, containing results for one or more of the tasks of interest. For each pathology and task combination, effect sizes (Hedges' g) were extracted comparing patient versus control groups. For all four populations, performance was consistently worse than that of cognitively unimpaired adults across the four language production tasks (p-values < 0.010). Given that performance in picture naming and verb inflection across all pathologies was quantified in terms of accuracy, our results suggest that production impairments cannot be fully explained by motor or processing-speed deficits. Our review shows that while language production difficulties in these clinical populations are not negligible, more evidence is necessary to determine the exact mechanism that leads to these deficits and whether this mechanism is the same across different pathologies.

From Pathogenesis to Therapeutics: A Review of 150 Years of Huntington's Disease Research

Huntington's disease (HD) is a debilitating neurodegenerative genetic disorder caused by an expanded polyglutamine-coding (CAG) trinucleotide repeat in the huntingtin (HTT) gene. HD behaves as a highly penetrant dominant disorder likely acting through a toxic gain of function by the mutant huntingtin protein. Widespread cellular degeneration of the medium spiny neurons of the caudate nucleus and putamen are responsible for the onset of symptomology that encompasses motor, cognitive, and behavioural abnormalities. Over the past 150 years of HD research since George Huntington published his description, a plethora of pathogenic mechanisms have been proposed with key themes including excitotoxicity, dopaminergic imbalance, mitochondrial dysfunction, metabolic defects, disruption of proteostasis, transcriptional dysregulation, and neuroinflammation. Despite the identification and characterisation of the causative gene and mutation and significant advances in our understanding of the cellular pathology in recent years, a disease-modifying intervention has not yet been clinically approved. This review includes an overview of Huntington's disease, from its genetic aetiology to clinical presentation and its pathogenic manifestation. An updated view of molecular mechanisms and the latest therapeutic developments will also be discussed.

Behavioural subphenotypes and their anatomic correlates in neurodegenerative disease

Patients with neurodegenerative disorders experience a range of neuropsychiatric symptoms. The neural correlates have been explored for many individual symptoms, such as apathy and disinhibition. Atrophy patterns have also been associated with broadly recognized syndromes that bring together multiple symptoms, such as the behavioural variant of frontotemporal dementia. There is substantial heterogeneity of symptoms, with partial overlap of behaviour and affected neuroanatomy across and within dementia subtypes. It is not well established if there are anatomically distinct behavioural subphenotypes in neurodegenerative disease. The objective of this study was to identify shared behavioural profiles in frontotemporal dementia-spectrum and Alzheimer's disease-related syndromes. Additionally, we sought to determine the underlying neural correlates of these symptom clusters. Two hundred and eighty-one patients diagnosed with one of seven different dementia syndromes, in addition to healthy controls and individuals with mild cognitive impairment, completed a 109-item assessment capturing the severity of a range of clinical behaviours. A principal component analysis captured distinct clusters of related behaviours. Voxel-based morphometry analyses were used to identify regions of volume loss associated with each component. Seven components were identified and interpreted as capturing the following behaviours: Component 1-emotional bluntness, 2-emotional lability and disinhibition, 3-neuroticism, 4-rigidity and impatience, 5-indiscriminate consumption, 6-psychosis and 7-Geschwind syndrome-related behaviours. Correlations with structural brain volume revealed distinct neuroanatomical patterns associated with each component, including after controlling for diagnosis, suggesting that localized neurodegeneration can lead to the development of behavioural symptom clusters across various dementia syndromes.

Excessive response to provocation rather than disinhibition mediates irritable behaviour in Huntington's disease

Background

Irritable and impulsive behaviour are common in Huntington's disease (HD: an autosomal dominant disorder causing degeneration in cortico-striatal networks). However, the cognitive mechanisms underlying these symptoms remain unclear, and previous research has not determined if common mechanisms underpin both symptoms. Here we used established and novel tasks to probe different aspects of irritable and impulsive behaviour to determine the neural mechanisms involved.

Methods

We recruited a cohort of 53 gene positive HD participants and 26 controls from non-affected family members and local volunteers. We used established questionnaire measures of irritability in HD (Snaith Irritability Scale, Problem Behaviours Assessment) and impulsivity [Urgency, Premeditation Perseverance, Sensation-seeking, Positive urgency scale (UPPSP), Barratt Impulsivity Scale], in addition to cognitive tasks of provocation, motor inhibition, delay discounting and decision making under uncertainty. We used generalised linear models to determine differences between cases and controls, and associations with irritability in the HD group.

Results

We found differences between cases and controls on the negative urgency subscale of the UPPSP, which was associated with irritability in HD. The frustrative non-reward provocation task also showed differences between cases and controls, in addition to predicting irritability in HD. The stop signal reaction time task showed case-control differences but was not associated with irritability in HD. None of the other measures showed group differences or predicted irritability in HD after correcting for confounding variables.

Discussion

Irritability in HD is mediated by excessive response to provocation, rather than a failure of motor inhibition.

The neurobiological basis of affect is consistent with psychological construction theory and shares a common neural basis across emotional categories

Affective experience colours everyday perception and cognition, yet its fundamental and neurobiological basis is poorly understood. The current debate essentially centers around the communalities and specificities across individuals, events, and emotional categories like anger, sadness, and happiness. Using fMRI during the experience of these emotions, we critically compare the two dominant conflicting theories on human affect. Basic emotion theory posits emotions as discrete universal entities generated by dedicated emotion category-specific neural circuits, while psychological construction theory claims emotional events as unique, idiosyncratic, and constructed by psychological primitives like core affect and conceptualization, which underlie each emotional event and operate in a predictive framework. Based on the findings of 8 a priori-defined model-specific prediction tests on the neural response amplitudes and patterns, we conclude that the neurobiological basis of affect is primarily characterized by idiosyncratic mechanisms and a common neural basis shared across emotion categories, consistent with psychological construction theory. The findings provide further insight into the organizational principles of the neural basis of affect and brain function in general. Future studies in clinical populations with affective symptoms may reveal the corresponding underlying neural changes from a psychological construction perspective.

A novel rhesus macaque model of Huntington's disease recapitulates key neuropathological changes along with motor and cognitive decline

We created a new nonhuman primate model of the genetic neurodegenerative disorder Huntington's disease (HD) by injecting a mixture of recombinant adeno-associated viral vectors, serotypes AAV2 and AAV2.retro, each expressing a fragment of human mutant HTT (mHTT) into the caudate and putamen of adult rhesus macaques. This modeling strategy results in expression of mutant huntingtin protein (mHTT) and aggregate formation in the injected brain regions, as well as dozens of other cortical and subcortical brain regions affected in human HD patients. We queried the disruption of cortico-basal ganglia circuitry for 30 months post-surgery using a variety of behavioral and imaging readouts. Compared to controls, mHTT-treated macaques developed working memory decline and progressive motor impairment. Multimodal imaging revealed circuit-wide white and gray matter degenerative processes in several key brain regions affected in HD. Taken together, we have developed a novel macaque model of HD that may be used to develop disease biomarkers and screen promising therapeutics.

Huntington's disease mouse models: unraveling the pathology caused by CAG repeat expansion

Huntington's disease (HD) is a neurodegenerative disease that results in motor and cognitive dysfunction, leading to early death. HD is caused by an expansion of CAG repeats in the huntingtin gene (HTT). Here, we review the mouse models of HD. They have been used extensively to better understand the molecular and cellular basis of disease pathogenesis as well as to provide non-human subjects to test the efficacy of potential therapeutics. The first and best-studied in vivo rodent model of HD is the R6/2 mouse, in which a transgene containing the promoter and exon 1 fragment of human HTT with 150 CAG repeats was inserted into the mouse genome. R6/2 mice express rapid, robust behavioral pathologies and display a number of degenerative abnormalities in neuronal populations most vulnerable in HD. The first conditional full-length mutant huntingtin (mHTT) mouse model of HD was the bacterial artificial chromosome (BAC) transgenic mouse model of HD (BACHD), which expresses human full-length mHTT with a mixture of 97 CAG-CAA repeats under the control of endogenous HTT regulatory machinery. It has been useful in identifying the role of mHTT in specific neuronal populations in degenerative processes. In the knock-in (KI) model of HD, the expanded human CAG repeats and human exon 1 are inserted into the mouse Htt locus, so a chimera of the full-length mouse protein with the N-terminal human portion is expressed. Many of aspects of the pathology and behavioral deficits in the KI model better mimic disease characteristics found in HD patients than other models. Accordingly, some have proposed that these mice may be preferable models of the disease over others. Indeed, as our understanding of HD advances, so will the design of animal models to test and develop HD therapies.

Structural brain correlates of irritability and aggression in early manifest Huntington's disease

In Huntington's disease (HD), irritability and aggressive behavior represent highly prevalent and disabling neuropsychiatric symptoms. However, their structural brain correlates have not been extensively explored. Here, we rated the severity of irritability and aggression (IAs) using the Problem Behaviors Assessment for HD (PBA-s) in 31 early HD participants. The IAs score was computed as the mean severity score for the irritability plus the mean severity aggression PBA-s items. Seventeen patients were classified as IAs (IAs score > 2) and 14 as non-IAs. All participants had available T1-MRI data. A grey matter volume voxel-based morphometry group comparison was performed, using age, motor status, severity of other PBA-s items and disease burden as covariates. Aside from irritability, aggression and obsessive-compulsive behavior, both groups were comparable in terms of other clinical and sociodemographic variables. In the IAs group, a significant reduction of grey-matter volume (GMV) was found in the bilateral caudate, putamen and globus pallidus, left pulvinar nucleus, right superior temporal pole (BA 38), left mid temporal gyrus (BA 21), right inferior temporal gyrus (BA 20) and left medial OPFC (BA 11). Lower GMV in the left pulvinar nucleus was significantly associated with higher anxiety and lower GMV in the left medial OPFC was significantly associated with higher suicidality. In sum, IAs in HD is associated with structural brain damage in a set of key nodes involved in the expression and down-regulation of negative emotions.

Age-related alteration of emotional regulation in the BACHD rat model of Huntington disease

Huntington's disease (HD) is a genetic neurodegenerative disorder, caused by an expanded CAG repeat in the gene encoding the huntingtin protein. At the premanifest phase, before motor symptoms occur, psychiatric and emotional disorders are observed with high prevalence in HD patients. Agitation, anxiety and irritability are often described but also depression and/or apathy, associated with a lack of emotional control. The aim of the present study was to better circumscribe and understand the emotional symptoms and assess their evolution according to the progression of the disease using a transgenic HD model, BACHD rats, at the age of 4, 12 and 18 months. To achieve this goal, we confronted animals to two types of tests: first, tests assessing anxiety like the light/dark box and the conflict test, which are situations that did not involve an obvious threat and tests assessing the reactivity to a present threat using confrontation with an unknown conspecific (social behavior test) or with an aversive stimulus (fear conditioning test). In all animals, results show an age-dependent anxiety-like behavior, particularly marked in situation requiring passive responses (light/dark box and fear conditioning tests). BACHD rats exhibited a more profound alteration than WT animals in these tests from an early stage of the disease whereas, in tasks requiring some kind of motivation (for food or for social contacts), only old BACHD rats showed high anxiety-like behavior compared to WT, may be partly due to the other symptoms' occurrence at this stage: locomotor difficulties and/or apathy.

Validity of irritability in Huntington's disease: A scoping review

PURPOSE

To scope the literature concerning irritability in Huntington's disease to determine whether or not irritability is a valid and meaningful construct within this population.

METHOD

A scoping literature review was conducted based on findings from a search of five databases (Academic Search Ultimate, PsycINFO, CINAHL, Scopus and Web of Science) in November 2018. From an initial return of 453 papers, 40 were found suitable for review.

RESULTS

Review of the 40 studies highlighted several aspects of irritability in people with HD which influence its validity as an independent construct in context of the disease. While various measures are used to assess irritability, a gold standard has yet to be identified and consequently irritability is assessed inconsistently across the literature. In addition, the results suggest that irritability may not reflect pathological disease processes in HD, but rather comprises a multidimensional construct which appears to be strongly associated with other psychological difficulties such as depression and anxiety.

CONCLUSIONS

The current concept of irritability in people with HD continues to lack a general consensus in the clinical literature, in terms of both operationalisation and assessment. Consequently, further research is warranted in order to determine the extent to which irritability is a valid construct within the context of HD, including its associated behavioural, cognitive and affective dimensions.

Huntington's disease: Brain imaging in Huntington's disease

Huntington's disease (HD) gene-carriers show prominent neuronal loss by end-stage disease, and the use of magnetic resonance imaging (MRI) has been increasingly used to quantify brain changes during earlier stages of the disease. MRI offers an in vivo method of measuring structural and functional brain change. The images collected via MRI are processed to measure different anatomical features, such as brain volume, macro- and microstructural changes within white matter and functional brain activity. Structural imaging has demonstrated significant volume loss across multiple white and gray matter regions in HD, particularly within subcortical structures. There also appears to be increasing disorganization of white matter tracts and between-region connectivity with increasing disease progression. Finally, functional changes are thought to represent changes in brain activity underlying compensatory mechanisms in HD. This chapter will provide an overview of the principles of MRI and practicalities associated with using MRI in HD studies, and summarize findings from MRI studies investigating brain structure and function in HD.

Regional subcortical shape analysis in premanifest Huntington's disease

Huntington's disease (HD) involves preferential and progressive degeneration of striatum and other subcortical regions as well as regional cortical atrophy. It is caused by a CAG repeat expansion in the Huntingtin gene, and the longer the expansion the earlier the age of onset. Atrophy begins prior to manifest clinical signs and symptoms, and brain atrophy in premanifest expansion carriers can be studied. We employed a diffeomorphometric pipeline to contrast subcortical structures' morphological properties in a control group with three disease groups representing different phases of premanifest HD (far, intermediate, and near to onset) as defined by the length of the CAG expansion and the participant's age (CAG-Age-Product). A total of 1,428 magnetic resonance image scans from 694 participants from the PREDICT-HD cohort were used. We found significant region-specific atrophies in all subcortical structures studied, with the estimated abnormality onset time varying from structure to structure. Heterogeneous shape abnormalities of caudate nuclei were present in premanifest HD participants estimated furthest from onset and putaminal shape abnormalities were present in participants intermediate to onset. Thalamic, hippocampal, and amygdalar shape abnormalities were present in participants nearest to onset. We assessed whether the estimated progression of subcortical pathology in premanifest HD tracked specific pathways. This is plausible for changes in basal ganglia circuits but probably not for changes in hippocampus and amygdala. The regional shape analyses conducted in this study provide useful insights into the effects of HD pathology in subcortical structures.

Functional Magnetic Resonance Imaging in Huntington's Disease

Huntington's disease is an inherited neurodegenerative condition characterized by motor dysfunction, cognitive impairment and neuropsychiatric disturbance. The effects of the underlying pathology on brain morphology are relatively well understood. Numerous structural Magnetic Resonance Imaging (MRI) studies have demonstrated macrostructural change with widespread striatal and cortical atrophy and microstructural white matter loss in premanifest and manifest HD gene carriers. However, disease effects on brain function are less well characterized. Functional MRI provides an opportunity to examine differences in brain activity either in response to a particular task or in the brain at rest. There is increasing evidence that HD gene carriers exhibit altered activation patterns and functional connectivity between brain regions in response to the neurodegenerative process. Here we review the growing literature in this area and critically evaluate the utility of this imaging modality.

Diminished facial EMG responses to disgusting scenes and happy and fearful faces in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder associated with impaired facial emotion recognition and altered subjective experience of emotion. These impairments likely result from the effects of the disease on underlying neurobiological mechanisms. Studies using self-report to examine emotional experiences have been ambiguous regarding whether experiences are diminished or exaggerated, possibly due to cognitive impairment and lack of insight in HD. To infer affective states more objectively and overcome the limitations of self-report, we used facial EMG to measure muscle responses to emotionally-evocative scenes. Further, we examined muscle responses to emotionally-expressive faces, because facial mimicry is thought to facilitate emotion recognition and social affiliation. Twenty-three HD participants (late pre-manifest and early symptomatic) were compared to twenty-five healthy controls in a scene condition and a face condition. EMG activity was measured from facial muscles associated with expressing particular emotions: 1) corrugator supercilii for anger, 2) frontalis for fear, 3) levator labii for disgust, and 4) both zygomaticus major and orbicularis oculi for happiness. Compared to controls, HD participants showed diminished responses to disgusting scenes, and to happy and fearful faces. Our findings provide evidence for a loss of disgust experience in HD. Further, consistent with the alleged affiliative function of facial mimicry, diminished mimicry responses may be relevant to social-emotional changes in HD. Our findings help understand the neural mechanisms underlying emotion processing impairments in HD.

Understandings of psychological difficulties in people with the Huntington's disease gene and their expectations of psychological therapy

OBJECTIVES

This study sought to investigate how people who had tested positive for the Huntington's disease (HD) gene mutation understood and experienced psychological distress and their expectations of psychological therapy.

DESIGN

A qualitative methodology was adopted involving semi-structured interviews and interpretative phenomenological analysis (IPA).

METHOD

A total of nine participants (five women and four men) who had opted to engage in psychological therapy were recruited and interviewed prior to the start of this particular psychological therapeutic intervention. Interviews were transcribed verbatim and analysed using IPA whereby themes were analysed within and across transcripts and classified into superordinate themes.

RESULTS

Three superordinate themes were developed: Attributing psychological distress to HD: 'you're blaming everything on that now'; Changes in attributions of distress over time: 'in the past you'd just get on with it'; and Approaching therapy with an open mind, commitment, and hope: 'a light at the end of the tunnel'.

CONCLUSION

Understandings of psychological distress in HD included biological and psychological explanations, with both often being accepted simultaneously by the same individual but with biomedical accounts generally dominating. Individual experience seemed to reflect a dynamic process whereby people's understanding and experience of their distress changed over time. Psychological therapy was accepted as a positive alternative to medication, providing people with HD with hope that their psychological well-being could be enhanced.

PRACTITIONER POINTS

People with the Huntington's disease gene mutation have largely biomedical understandings of their psychological distress. This largely biomedical understanding does not, however, preclude them for being interested in the potential gains resulting from psychological therapy. The mechanisms of psychological therapy should be explained in detail before therapy and explored along with current attributions of distress.

Reduced amygdala volumes are related to motor and cognitive signs in Huntington's disease: The IMAGE-HD study

In Huntington's disease (HD), the presence of neurodegeneration in brain regions other than the striatum has been recently gaining attention. The amygdala is one such area, which has been investigated in only eight structural magnetic resonance imaging studies to date, but with inconsistent findings. This is the largest MRI study to date examining manually traced amygdala volumes in HD participants and the relationship of amygdala volumes to clinical measures of HD. Our study included 35 healthy control participants, and groups of 35 pre-symptomatic, and 36 symptomatic HD participants. When comparing the pre-symptomatic and symptomatic HD groups together against the control group, amygdala volumes were significantly lower in HD than controls and in symptomatic HD than pre-symptomatic HD. When examining relationships between amygdala volumes and clinical measures of HD, significantly smaller amygdala volumes were associated with worse motor and cognitive signs. For pre-symptomatic HD participants who were close to disease onset, smaller amygdala volumes were also associated with higher levels of anxiety symptoms. These findings suggest that the amygdala is affected in pre-symptomatic and symptomatic HD, and that the amygdala is related to the clinical profile of HD before onset of motor symptoms.

Gray Matter Volume of a Region in the Thalamic Pulvinar Is Specifically Associated with Novelty Seeking

Personality reflects the set of psychological traits and mechanisms characteristic for an individual. Geno-neuro-biologically inspired personality accounts have proposed a set of temperaments and characters that jointly compose personality profiles. The present study addresses the link between neurobiology and personality and investigates the association between temperament traits and regional gray matter volume. Furthermore, the specificity of these associations as well as the underlying components that drive the association are addressed. One hundred and four participants completed the Temperament and Character Inventory (TCI) and underwent structural magnetic resonance brain imaging. The participants included premanifest carriers of Huntington's disease, as this population is associated with temperament-related neuropsychiatric symptoms. Whole brain voxel-based multiple regression analyses on gray matter volume revealed a significant specific positive correlation between a region in the left thalamic pulvinar and novelty seeking score, controlled for the other traits (P_height < 0.05, FWE-corrected). No significant associations were observed for the other temperament traits. Region of interest analyses showed that this association is driven by the subscale NS2: impulsiveness. The results increase the knowledge of the structural neurobiology of personality and indicate that individual differences in novelty seeking reflect the structural differences observed in the brain in an area that is widely and densely connected, which is in line with the typically domain-general behavioral influence of personality traits on a wide range of affective, perceptual, mnemotic, executive, and other cognitive functions.

Resting-state functional MRI reveals altered brain connectivity and its correlation with motor dysfunction in a mouse model of Huntington's disease

Huntington’s disease (HD) is an autosomal dominant inherited neurodegenerative disorder, and no cure is available currently. Treatment of HD is likely to be most beneficial in the early, possibly pre-manifestation stage. The challenge is to determine the best time for intervention and evaluate putative efficacy in the absence of clinical symptoms. Resting-state functional MRI may represent a promising tool to develop biomarker reflecting early neuronal dysfunction in HD brain, because it can examine multiple brain networks without confounding effects of cognitive ability, which makes the resting-state fMRI promising as a translational bridge between preclinical study in animal models and clinical findings in HD patients. In this study, we examined brain regional connectivity and its correlation to brain atrophy, as well as motor function in the 18-week-old N171-82Q HD mice. HD mice exhibited significantly altered functional connectivity in multiple networks. Particularly, the weaker intra-striatum connectivity was positively correlated with striatal atrophy, while striatum-retrosplenial cortex connectivity is negatively correlated with striatal atrophy. The resting-state brain regional connectivity had no significant correlation with motor deficits in HD mice. Our results suggest that altered brain connectivity detected by resting-state fMRI might serve as an early disease biomarker in HD.

Progress in developing transgenic monkey model for Huntington's disease

Huntington's disease (HD) is a complex neurodegenerative disorder that has no cure. Although treatments can often be given to relieve symptoms, the neuropathology associated with HD cannot be stopped or reversed. HD is characterized by degeneration of the striatum and associated pathways that leads to impairment in motor and cognitive functions as well as psychiatric disturbances. Although cell and rodent models for HD exist, longitudinal study in a transgenic HD nonhuman primate (i.e., rhesus macaque; HD monkeys) shows high similarity in its progression with human patients. Progressive brain atrophy and changes in white matter integrity examined by magnetic resonance imaging are coherent with the decline in cognitive behaviors related to corticostriatal functions and neuropathology. HD monkeys also express higher anxiety and irritability/aggression similar to human HD patients that other model systems have not yet replicated. While a comparative model approach is critical for advancing our understanding of HD pathogenesis, HD monkeys could provide a unique platform for preclinical studies and long-term assessment of translatable outcome measures. This review summarizes the progress in the development of the transgenic HD monkey model and the opportunities for advancing HD preclinical research.

Dynamic Changes in Amygdala Psychophysiological Connectivity Reveal Distinct Neural Networks for Facial Expressions of Basic Emotions

The quest to characterize the neural signature distinctive of different basic emotions has recently come under renewed scrutiny. Here we investigated whether facial expressions of different basic emotions modulate the functional connectivity of the amygdala with the rest of the brain. To this end, we presented seventeen healthy participants (8 females) with facial expressions of anger, disgust, fear, happiness, sadness and emotional neutrality and analyzed amygdala's psychophysiological interaction (PPI). In fact, PPI can reveal how inter-regional amygdala communications change dynamically depending on perception of various emotional expressions to recruit different brain networks, compared to the functional interactions it entertains during perception of neutral expressions. We found that for each emotion the amygdala recruited a distinctive and spatially distributed set of structures to interact with. These changes in amygdala connectional patters characterize the dynamic signature prototypical of individual emotion processing, and seemingly represent a neural mechanism that serves to implement the distinctive influence that each emotion exerts on perceptual, cognitive, and motor responses. Besides these differences, all emotions enhanced amygdala functional integration with premotor cortices compared to neutral faces. The present findings thus concur to reconceptualise the structure-function relation between brain-emotion from the traditional one-to-one mapping toward a network-based and dynamic perspective.

Theory of Mind Is Impaired in Mild to Moderate Huntington's Disease Independently from Global Cognitive Functioning

Affective “Theory of Mind” (ToM) is the specific ability to represent own and others’ emotional states and feelings. Previous studies examined affective ToM ability in patients with Huntington’s disease (HD), using the “Reading the Mind in the Eyes test” (RMET). Results were consistent in showing difficulties in inferring complex mental states from photographs of people even in the early stage of HD. However, there has been no agreement as to whether or not cognitive impairments in HD population might have contributed to poor performance on the RMET test. The aim of the present study was to assess whether the affective ToM ability was impaired in the mild to moderate stages of HD, and whether there was an association between compromised ToM ability and the presence of cognitive impairment. We evaluated ToM by means of RMET and global cognitive functioning by means of the MoCA questionnaire in 15 HD patients and 15 healthy subjects (HS). Both groups were matched for age and level of education. Our study showed that the ability to judge a person’s mental states from a picture of their eyes was impaired in HD patients compared to normal population. Indeed, HD subjects gave the 34% of correct responses on RMET, whereas healthy control subjects’ percentage of correct responses was 71%. Furthermore, this impairment was not correlated with global cognitive functioning except for the visuospatial task. These results show that RMET might represent a valid instrument to assess affective ToM ability in HD patients in the mild to moderate stages of the disease, independently from their cognitive status. Since it is known that HD patients, in addition to motor symptoms, suffer from cognitive deficits, including memory and executive impairments, it is important to have an instrument, which is not influenced by cognitive abilities. It is possible therefore to use RMET to assess important aspects of HD patients such as their ability to recognize others’ emotions and feelings even when patients suffer from cognitive decline.

Amygdala Response to Emotional Stimuli without Awareness: Facts and Interpretations

Over the past two decades, evidence has accumulated that the human amygdala exerts some of its functions also when the observer is not aware of the content, or even presence, of the triggering emotional stimulus. Nevertheless, there is as of yet no consensus on the limits and conditions that affect the extent of amygdala’s response without focused attention or awareness. Here we review past and recent studies on this subject, examining neuroimaging literature on healthy participants as well as brain-damaged patients, and we comment on their strengths and limits. We propose a theoretical distinction between processes involved in attentional unawareness, wherein the stimulus is potentially accessible to enter visual awareness but fails to do so because attention is diverted, and in sensory unawareness, wherein the stimulus fails to enter awareness because its normal processing in the visual cortex is suppressed. We argue this distinction, along with data sampling amygdala responses with high temporal resolution, helps to appreciate the multiplicity of functional and anatomical mechanisms centered on the amygdala and supporting its role in non-conscious emotion processing. Separate, but interacting, networks relay visual information to the amygdala exploiting different computational properties of subcortical and cortical routes, thereby supporting amygdala functions at different stages of emotion processing. This view reconciles some apparent contradictions in the literature, as well as seemingly contrasting proposals, such as the dual stage and the dual route model. We conclude that evidence in favor of the amygdala response without awareness is solid, albeit this response originates from different functional mechanisms and is driven by more complex neural networks than commonly assumed. Acknowledging the complexity of such mechanisms can foster new insights on the varieties of amygdala functions without awareness and their impact on human behavior.

Increased irritability, anxiety, and immune reactivity in transgenic Huntington's disease monkeys

Although the most notable clinical symptoms of Huntington's disease (HD) are motor disturbances and brain atrophy, other symptoms include cognitive dysfunction, emotional and hormonal dysregulation. Emotional dysregulation (irritability, anger/aggression, and anxiety) and increased inflammation are early emerging symptoms which can be detected decades before the onset of motor symptoms in HD patients. Despite the advances in understanding the genetic causes of HD there is still no cure or preventative treatment. Thus, to better understand the pathogenesis of HD and develop effective treatments, a holistic understanding of HD is needed, as well as animal models that replicate the full spectrum of HD symptoms. The current study examined the emotional, hormonal, and gene expression responses to an acute stressor of adult male transgenic HD rhesus monkeys (n=2) as compared to wild-type controls (n=2). Results revealed that HD monkeys expressed increased anxiety and irritability/aggression as compared to controls. Reactive cortisol response to the stressor was similar between groups. However, HD monkeys exhibited increased pro-inflammatory cytokines and higher induction of immune pathway genes as compared to controls. Overall, results reveal that HD monkeys exhibit these early emerging symptoms of HD and may be an effective animal model to facilitate the development of new therapeutics for HD patients.