Emotional endophenotypes in evolutionary psychiatry

A novel NMDA receptor glycine-site partial agonist, GLYX-13, has therapeutic potential for the treatment of autism

Deficits in social approach behavior, rough-and-tumble play, and speech abnormalities are core features of autism that can be modeled in laboratory rats. Human twin studies show that autism has a strong genetic component, and a recent review has identified 99 genes that are dysregulated in human autism. Bioinformatic analysis of these 99 genes identified the NMDA receptor complex as a significant interaction hub based on protein-protein interactions. The NMDA receptor glycine site partial agonist d-cycloserine has been shown to treat the core symptom of social withdrawal in autistic children. Here, we show that rats selectively bred for low rates of play-induced pro-social ultrasonic vocalizations (USVs) can be used to model certain core symptoms of autism. Low-line animals engage in less social contact time with conspecifics, show lower rates of play induced pro-social USVs, and show an increased proportion of non-frequency modulated (i.e. monotonous) ultrasonic vocalizations, compared to non-selectively bred random-line animals. Gene expression patterns in the low-line animals show significant enrichment in autism-associated genes and the NMDA receptor family was identified as a significant hub. Treatment of low-line animals with the NMDAR glycine site partial agonist GLYX-13 rescued the deficits in play-induced pro-social 50-kHz and reduced monotonous USVs. Thus, the NMDA receptor has been shown to play a functional role in autism, and GLYX-13 shows promise for the treatment of autism. We dedicate this paper to Ole Ivar Lovaas (May 8, 1927-August 2, 2010), a pioneer in the field of autism.

Conceptualizing the Autism Spectrum in Terms of Natural Selection and Behavioral Ecology: The Solitary Forager Hypothesis

This article reviews etiological and comparative evidence supporting the hypothesis that some genes associated with the autism spectrum were naturally selected and represent the adaptive benefits of being cognitively suited for solitary foraging. People on the autism spectrum are conceptualized here as ecologically competent individuals that could have been adept at learning and implementing hunting and gathering skills in the ancestral environment. Upon independence from their mothers, individuals on the autism spectrum may have been psychologically predisposed toward a different life-history strategy, common among mammals and even some primates, to hunt and gather primarily on their own. Many of the behavioral and cognitive tendencies that autistic individuals exhibit are viewed here as adaptations that would have complemented a solitary lifestyle. For example, the obsessive, repetitive and systemizing tendencies in autism, which can be mistakenly applied toward activities such as block stacking today, may have been focused by hunger and thirst toward successful food procurement in the ancestral past. Both solitary mammals and autistic individuals are low on measures of gregariousness, socialization, direct gazing, eye contact, facial expression, facial recognition, emotional engagement, affiliative need and other social behaviors. The evolution of the neurological tendencies in solitary species that predispose them toward being introverted and reclusive may hold important clues for the evolution of the autism spectrum and the natural selection of autism genes. Solitary animals are thought to eschew unnecessary social contact as part of a foraging strategy often due to scarcity and wide dispersal of food in their native environments. It is thought that the human ancestral environment was often nutritionally sparse as well, and this may have driven human parties to periodically disband. Inconsistencies in group size must have led to inconsistencies in the manner in which natural selection fashioned the social minds of humans, which in turn may well be responsible for the large variation in social abilities seen in human populations. This article emphasizes that individuals on the autism spectrum may have only been partially solitary, that natural selection may have only favored subclinical autistic traits and that the most severe cases of autism may be due to assortative mating.

Affective neuroscience of the emotional BrainMind: evolutionary perspectives and implications for understanding depression

Cross-species affective neuroscience studies confirm that primary-process emotional feelings are organized within primitive subcortical regions of the brain that are anatomically, neurochemically, and functionally homologous in all mammals that have been studied. Emotional feelings (affects) are intrinsic values that inform animals how they are faring in the quest to survive. The various positive affects indicate that animals are returning to “comfort zones” that support survival, and negative affects reflect “discomfort zones” that indicate that animals are in situations that may impair survival. They are ancestral tools for living - evolutionary memories of such importance that they were coded into the genome in rough form (as primary brain processes), which are refined by basic learning mechanisms (secondary processes) as well as by higher-order cognitions/thoughts (tertiary processes). To understand why depression feels horrible, we must fathom the affective infrastructure of the mammalian brain. Advances in our understanding of the nature of primary-process emotional affects can promote the development of better preclinical models of psychiatric disorders and thereby also allow clinicians new and useful ways to understand the foundational aspects of their clients' problems. These networks are of clear importance for understanding psychiatric disorders and advancing psychiatric practice.

The 'resting-state hypothesis' of major depressive disorder-a translational subcortical-cortical framework for a system disorder

Major depressive disorder (MDD) has traditionally been characterized by various psychological symptoms, involvement of diverse functional systems (e.g., somatic, affect, cognition, reward, etc.), and with progress in neuroscience, an increasing number of brain regions. This has led to the general assumption that MDD is a stress-responsive brain 'system disorder' where either one or several alterations infiltrate a large number of functional systems in the brain that control the organism's somatic, affective, and cognitive life. However, while the effects or consequences of the abnormal changes in the functional systems of, for instance affect, cognition or reward have been investigated extensively, the underlying core mechanism(s) underlying MDD remain unknown. Hypotheses are proliferating rapidly, though. Based on recent findings, we will entertain an abnormality in the resting-state activity in MDD to be a core feature. Based on both animal and human data, we hypothesize that abnormal resting-state activity levels may impact stimulus-induced neural activity in medially situated core systems for self-representation as well as external stimulus (especially stress, specifically separation distress) interactions. Moreover, due to nested hierarchy between subcortical and cortical regions, we assume 'highjacking' of higher cortical affective and cognitive functions by lower subcortical primary-process emotional systems. This may account for the predominance of negative affect in somatic and cognitive functional system operations with the consecutive generation of the diverse symptoms in MDD. We will here focus on the neuroanatomical and biochemical basis of resting-state abnormalities in MDD including their linkage to the diverse psychopathological symptoms in depression. However, our 'resting-state hypothesis' may go well beyond that by being sufficiently precise to be linked to genetic, social, immunological, and endocrine dimensions and hypotheses as well as to clinical dimensions like endophenotypes and various diagnostic-prognostic biomarkers. Taken together, our 'resting-state hypothesis' may be considered a first tentative framework for MDD that integrates translational data, the various dimensions, and subcortical-cortical systems while at the same time providing the link to the clinical level of symptoms, endophenotypes and biomarkers.

Impaired emotion recognition in music in Parkinson's disease

Music has the potential to evoke strong emotions and plays a significant role in the lives of many people. Music might therefore be an ideal medium to assess emotion recognition. We investigated emotion recognition in music in 20 patients with idiopathic Parkinson's disease (PD) and 20 matched healthy volunteers. The role of cognitive dysfunction and other disease characteristics in emotion recognition was also evaluated. We used 32 musical excerpts that expressed happiness, sadness, fear or anger. PD patients were impaired in recognizing fear and anger in music. Fear recognition was associated with executive functions in PD patients and in healthy controls, but the emotion recognition impairments of PD patients persisted after adjusting for executive functioning. We found no differences in the recognition of happy or sad music. Emotion recognition was not related to depressive symptoms, disease duration or severity of motor symptoms. We conclude that PD patients are impaired in recognizing complex emotions in music. Although this impairment is related to executive dysfunction, our findings most likely reflect an additional primary deficit in emotional processing.

It's more than sex: exploring the dyadic nature of sleep and implications for health

Sleep is a critical health behavior and one that is typically shared between husbands and wives or romantic partners. However, the science of sleep has traditionally conceptualized and evaluated sleep at the level of the individual. Considering the social context of sleep represents a significant shift in sleep research and also offers a critical opportunity for investigating sleep as a novel pathway linking close relationships with health. The purpose of this review is to integrate research that focuses on how sleep affects or is affected by close relationship functioning and to provide a heuristic framework for understanding the interface between close relationships, sleep, and health. Exploring the links between close relationships and sleep may contribute to our understanding of why some relationships confer health benefits, whereas others confer health risks.

Neural correlates of 'pessimistic' attitude in depression

BACKGROUND

Preparing for potentially threatening events in the future is essential for survival. Anticipating the future to be unpleasant is also a cognitive key feature of depression. We hypothesized that 'pessimism'-related emotion processing would characterize brain activity in major depression.MethodDuring functional magnetic resonance imaging, depressed patients and a healthy control group were cued to expect and then perceive pictures of known emotional valences--pleasant, unpleasant and neutral--and stimuli of unknown valence that could have been either pleasant or unpleasant. Brain activation associated with the 'unknown' expectation was compared with the 'known' expectation conditions.

RESULTS

While anticipating pictures of unknown valence, activation patterns in depressed patients within the medial and dorsolateral prefrontal areas, inferior frontal gyrus, insula and medial thalamus were similar to activations associated with expecting unpleasant pictures, but not with expecting positive pictures. The activity within a majority of these areas correlated with the depression scores. Differences between healthy and depressed persons were found particularly for medial and dorsolateral prefrontal and insular activations.

CONCLUSIONS

Brain activation in depression during expecting events of unknown emotional valence was comparable with activation while expecting certainly negative, but not positive events. This neurobiological finding is consistent with cognitive models supposing that depressed patients develop a 'pessimistic' attitude towards events with an unknown emotional meaning. Thereby, particularly the role of brain areas associated with the processing of cognitive and executive control and of the internal state is emphasized in contributing to major depression.

Mutant mouse models: genotype-phenotype relationships to negative symptoms in schizophrenia

Negative symptoms encompass diminution in emotional expression and motivation, some of which relate to human attributes that may not be accessible readily in animals. Additionally, their refractoriness to treatment precludes therapeutic validation of putative models. This review considers critically the application of mutant mouse models to the study of the pathobiology of negative symptoms. It focuses on 4 main approaches: genes related to the pathobiology of schizophrenia, genes associated with risk for schizophrenia, neurodevelopmental-synaptic genes, and variant approaches from other areas of neurobiology. Despite rapid advances over the past several years, it is clear that we continue to face substantive challenges in applying mutant models to better understand the pathobiology of negative symptoms: the majority of evidence relates to impairments in social behavior, with only limited data relating to anhedonia and negligible data concerning avolition and other features; even for the most widely examined feature, social behavior, studies have used diverse assessments thereof; modelling must proceed in cognizance of increasing evidence that genes and pathobiologies implicated in schizophrenia overlap with other psychotic disorders, particularly bipolar disorder. Despite the caveats and challenges, several mutant lines evidence a phenotype for at least one index of social behavior. Though this may suggest superficially some shared relationship to negative symptoms, it is not yet possible to specify either the scope or the pathobiology of that relationship for any given gene. The breadth and depth of ongoing studies in mutants hold the prospect of addressing these shortcomings.

A proposal for a coordinated effort for the determination of brainwide neuroanatomical connectivity in model organisms at a mesoscopic scale

In this era of complete genomes, our knowledge of neuroanatomical circuitry remains surprisingly sparse. Such knowledge is critical, however, for both basic and clinical research into brain function. Here we advocate for a concerted effort to fill this gap, through systematic, experimental mapping of neural circuits at a mesoscopic scale of resolution suitable for comprehensive, brainwide coverage, using injections of tracers or viral vectors. We detail the scientific and medical rationale and briefly review existing knowledge and experimental techniques. We define a set of desiderata, including brainwide coverage; validated and extensible experimental techniques suitable for standardization and automation; centralized, open-access data repository; compatibility with existing resources; and tractability with current informatics technology. We discuss a hypothetical but tractable plan for mouse, additional efforts for the macaque, and technique development for human. We estimate that the mouse connectivity project could be completed within five years with a comparatively modest budget.

Modelling the anxiety-depression continuum in chicks

The clinical syndromes of anxiety and depression are now thought to exist along a temporal continuum and this construct has been modelled in a preclinical setting in chicks separated from conspecifics. This research sought to further the validity of the chick anxiety-depression continuum model. Dose-response studies using two classes of anxiolytics (chlordiazepoxide: 2.5, 5.0, 10.0, 15.0 mg/kg, and clonidine: 0.1, 0.15, 0.2, 0.25 mg/kg) and three classes of antidepressants (imipramine: 1.0, 3.0, 10.0, 15.0 mg/kg, maprotoline: 2.5, 5.0, 10.0, 20.0 mg/kg and fluoxetine: 1.0, 5.0, 10.0, 20.0 mg/kg) showed an ability to detect anxiolytic activity of chlordiazepoxide, clonidine, imipramine and maprotoline in the anxiety-like phase of the model and to detect antidepressant effects of imipramine, maprotoline and fluoxetine in the depression-like phase of the model. In addition, blood plasma interleukin-6, a biomarker of stress, was found to be elevated in response to social-separation stress. Collectively, these findings further characterize the model as a simulation of the anxiety-depression continuum and begin to establish the paradigm as a high-utility adjuvant to rodent screening assays for putative anxiolytic and antidepressant compounds.

Evaluation of animal models of neurobehavioral disorders

Animal models play a central role in all areas of biomedical research. The process of animal model building, development and evaluation has rarely been addressed systematically, despite the long history of using animal models in the investigation of neuropsychiatric disorders and behavioral dysfunctions. An iterative, multi-stage trajectory for developing animal models and assessing their quality is proposed. The process starts with defining the purpose(s) of the model, preferentially based on hypotheses about brain-behavior relationships. Then, the model is developed and tested. The evaluation of the model takes scientific and ethical criteria into consideration.Model development requires a multidisciplinary approach. Preclinical and clinical experts should establish a set of scientific criteria, which a model must meet. The scientific evaluation consists of assessing the replicability/reliability, predictive, construct and external validity/generalizability, and relevance of the model. We emphasize the role of (systematic and extended) replications in the course of the validation process. One may apply a multiple-tiered 'replication battery' to estimate the reliability/replicability, validity, and generalizability of result.Compromised welfare is inherent in many deficiency models in animals. Unfortunately, 'animal welfare' is a vaguely defined concept, making it difficult to establish exact evaluation criteria. Weighing the animal's welfare and considerations as to whether action is indicated to reduce the discomfort must accompany the scientific evaluation at any stage of the model building and evaluation process. Animal model building should be discontinued if the model does not meet the preset scientific criteria, or when animal welfare is severely compromised. The application of the evaluation procedure is exemplified using the rat with neonatal hippocampal lesion as a proposed model of schizophrenia.In a manner congruent to that for improving animal models, guided by the procedure expounded upon in this paper, the developmental and evaluation procedure itself may be improved by careful definition of the purpose(s) of a model and by defining better evaluation criteria, based on the proposed use of the model.

Dopamine in amygdala gates limbic processing of aversive stimuli in humans

Dopamine is released under stress and modulates processing of aversive stimuli. We found that dopamine storage capacity in human amygdala, measured with 6-[(18)F]fluoro-L-DOPA positron emission tomography, was positively correlated with functional magnetic resonance imaging blood oxygen level-dependent signal changes in amygdala and dorsal anterior cingulate cortex that were evoked by aversive stimuli. Furthermore, functional connectivity between these two regions was inversely related to trait anxiety. Our results suggest that individual dopamine storage capacity in amygdala subserves modulation of emotional processing in amygdala and dorsal cingulate, thereby contributing to individual differences in anxious temperament.

Affective traits in schizophrenia and schizotypy

This article reviews empirical studies of affective traits in individuals with schizophrenia spectrum disorders, population-based investigations of vulnerability to psychosis, and genetic and psychometric high-risk samples. The review focuses on studies that use self-report trait questionnaires to assess Negative Affectivity (NA) and Positive Affectivity (PA), which are conceptualized in contemporary models of personality as broad, temperamentally-based dispositions to experience corresponding emotional states. Individuals with schizophrenia report a pattern of stably elevated NA and low PA throughout the illness course. Among affected individuals, these traits are associated with variability in several clinically important features, including functional outcome, quality of life, and stress reactivity. Furthermore, evidence that elevated NA and low PA (particularly the facet of anhedonia) predict the development of psychosis and are detectable in high-risk samples suggests that these traits play a role in vulnerability to schizophrenia, though they are implicated in other forms of psychopathology as well. Results are discussed in terms of their implications for treatment, etiological models, and future research to advance the study of affective traits in schizophrenia and schizotypy.

Evolution theory: an overview of its applications in psychiatry

OBJECTIVES

(1) Describe the concept, mechanisms and outcome of evolution; (2) review the current topics in research and clinical psychiatry where evolutionary concepts are explicitly applied.

METHODS

The authors reviewed relevant textbooks of evolution, evolutionary psychiatry/psychology and articles in scientific journals, and discussed these topics in a college course at McGill University School of Medicine, Montreal, Canada.

RESULTS

(1) Most natural scientists agree that evolution has occurred in all living beings. However, the mechanisms and outcomes of evolution are controversial. (2) In the first three quarters of the 20th century, several authors provided theories about human psychology based on ethological concepts. The so-called evolutionary psychology/psychiatry developed more recently, and it explores the adaptive/nonadaptive features of psychopathology and mental disorders. In the 1990 s a concept of mental disorder (as a harmful dysfunction) based on evolutionary theory has been developed.

CONCLUSIONS

Evolution is a pivotal concept in biology with relevant applications in psychiatry. We suggest encouraging the interaction between psychiatric educators and researchers in evolutionary psychiatry and biology in order to improve the education of psychiatric residents in this subject.

Stress-induced changes in primate prefrontal profiles of gene expression

Stressful experiences that consistently increase cortisol levels appear to alter the expression of hundreds of genes in prefrontal limbic brain regions. Here, we investigate this hypothesis in monkeys exposed to intermittent social stress-induced episodes of hypercortisolism or a no-stress control condition. Prefrontal profiles of gene expression compiled from Affymetrix microarray data for monkeys randomized to the no-stress condition were consistent with microarray results published for healthy humans. In monkeys exposed to intermittent social stress, more genes than expected by chance appeared to be differentially expressed in ventromedial prefrontal cortex compared to monkeys not exposed to adult social stress. Most of these stress responsive candidate genes were modestly downregulated, including ubiquitin conjugation enzymes and ligases involved in synaptic plasticity, cell cycle progression and nuclear receptor signaling. Social stress did not affect gene expression beyond that expected by chance in dorsolateral prefrontal cortex or prefrontal white matter. Thirty four of 48 comparisons chosen for verification by quantitative real-time polymerase chain reaction (qPCR) were consistent with the microarray-predicted result. Furthermore, qPCR and microarray data were highly correlated. These results provide new insights on the regulation of gene expression in a prefrontal corticolimbic region involved in the pathophysiology of stress and major depression. Comparisons between these data from monkeys and those for ventromedial prefrontal cortex in humans with a history of major depression may help to distinguish the molecular signature of stress from other confounding factors in human postmortem brain research.

Brain phylogeny, ontogeny and dysfunction: integrating evolutionary, developmental and clinical perspectives in cognitive neuroscience

OBJECTIVE

One of the most popular approaches in cognitive neuroscience has been to study the normal adult human brain. However, there are likely to be limits to the knowledge that can be obtained from such studies. If we assume that no single approach can ever provide us with knowledge of causative processes whereby the mind emerges from the brain, then we need to consider how to combine more disparate approaches. I aim to illustrate here how the parallel study of brain phylogeny, ontogeny and dysfunction may bring us towards an integrative understanding of fundamental aspects of cognitive neuroscience.

METHODS

A review of published literature in these research areas was carried out and representative articles selected.

RESULTS

Comparative approaches, utilizing the extraordinary behavioural abilities as well as the structural and functional variants that evolution has thrown up across diverse groups of species, can inform the core neural systems that may be necessary and sufficient to support specific cognitive processes. Similarly, detailed studies of human brain development, focusing on structural and functional maturation correlated with temporal mapping of cognitive processes as they come 'on-line', may provide unique mechanistic insights. Finally, the study of brain dysfunction in neurological and psychiatric disorders such as Huntington's disease, Alzheimer's disease, schizophrenia and depression, may have the beneficial side-effect of greatly enhancing our understanding of healthy brain function.

CONCLUSION

Each approach has its own epistemological advantages and disadvantages, but combined they may lead to more sophisticated, and empirically testable, models. In this review, I outline evidence for their utility, illustrate the approaches using specific examples and suggest how new advances in fields such as genomics, neurophysiology and neuroimaging may provide unprecedented opportunities in cognitive neuroscience.

Traumatic entrapment, appeasement and complex post-traumatic stress disorder: evolutionary perspectives of hostage reactions, domestic abuse and the Stockholm syndrome

Evolutionary theory and cross-species comparisons are explored to shed new insights into behavioural responses to traumatic entrapment, examining their relationships to the Stockholm syndrome (a specific response to traumatic entrapment) and complex post-traumatic stress disorder (PTSD). A selective literature review is undertaken examining responses to traumatic entrapment (including hostage, domestic abuse and similar situations) and the Stockholm syndrome, before examining mammalian, reptilian and other defensive responses to relevant threats. Chimpanzees, the closest relatives of humans, are closely examined from this perspective and commonalities in behavioural responses are highlighted. The neurobiological basis of defensive behaviours underlying PTSD is explored with reference to the triune brain model. Victims of protracted traumatic entrapment under certain circumstances may display the Stockholm syndrome, which involves paradoxically positive relationships with their oppressors that may persist beyond release. Similar responses are observed in many mammalian species, especially primates. Ethological concepts including dominance hierarchies, reverted escape, de-escalation and conditional reconciliation appear relevant and are illustrated. These phenomena are commonly encountered in victims of severe abuse and understanding these concepts may assist clinical management. Appeasement is the mammalian defence most relevant to the survival challenge presented by traumatic entrapment and appears to be the foundation of complex PTSD. Evolutionary perspectives have considerable potential to bridge and integrate neurobiology and the social sciences with respect to traumatic stress responses.

A same gene for altruism and selfishness in primates

The goal of the International Alt/Self Project (IASP) is to determine the molecular basis of societal altruistic and selfish behaviour in primate societies. In order to solve this difficult problem, an International Consortium comprising genomists and psycho-biologists from the G-8 countries has been created. In a first step it was decided to concentrate on extreme opposite phenotypes manifesting in Homo sapiens sapiens: the hyper-altruism syndrome (HAS) and the hyper-selfishness syndrome (HSS). Conventional association methods, using the most powerful tools provided by the SNR, and the HumMap projects, resulted in the preliminary mapping of the ''altruist'' locus, and the ''selfish'' locus, both in the pseudo-autosomal region of the short arm of chromosome X. Transcriptional studies using 1000K chips showed that an unannotated sequence was overexpressed in HAS and underexpressed in HSS individuals. This mirror pattern of expression suggested that the two deviant states might be allelic. This hypothesis was substantiated by genome studies using GGH-array. Ultimately different mutations in expressed genomic sequences showed a striking genotype-phenotype correlation: gain-of-function (GOF) mutations were invariably found in the HAS group, whereas loss-of-function (LOF) mutations were invariably found in the HSS group. These results demonstrate that a same and unique novel gene, coined DARWIN, is involved in the two opposite deviant phenotypes, suggesting that the morbid locus is a GTL (quantitative trait locus). Preliminary comparative data obtained in non-human primates showed that the DARWIN gene is highly conserved. Like humans, chimpanzees (Pan troglodytes) have a single expressed gene copy located in a stable genomic domain, whereas the highly benevolent bonobo primate (Pan paniscus) has several expressed gene copies, located in a copy number variable region (CNVR), a situation that buffers phenotypic manifestation of LOF mutations.

Getting beneath the phenotype of anorexia nervosa: the search for viable endophenotypes and genotypes

Treatment for anorexia nervosa has changed little from that described by Gull over a century ago. To focus merely on symptomatic relief from "not eating," as occurs with some forms of hospital care, is primitive. The evidence base to guide treatment is thin. Nevertheless, there is hope that better understanding of the causes and maintaining factors may translate into more sophisticated treatments. This review aims to look beyond the overt and startling "not eating" phenotype of anorexia nervosa and consider eating disorder endophenotypes. The first part of the review sets the eating behaviour, clinical, and psychopathological features into the context of what is now understood about the central control of appetite. The evidence base for a framework of potential eating disorder endophenotypes follows. Finally, ideas about how to translate endophenotypes into treatment are introduced.

Modeling the anxiety-depression continuum hypothesis in domestic fowl chicks

Anxiety and depression are currently classified as separate clinical syndromes despite considerable similarities in their symptoms, pathophysiological substrates and response to treatment interventions. An alternative hypothesis views anxiety and depression along a temporal continuum, a construct that the current research attempts to model in a preclinical setting. In experiment 1, socially raised domestic fowl chicks separated from conspecifics demonstrated a pattern of distress vocalizations that sequentially models anxiety-like and depressive-like states. In addition, administration of the benzodiazepine anxiolytic chlordiazepoxide and the tricyclic antidepressant imipramine provided pharmacological validation for the model in that they were capable of dissociating the anxiety-like and depressive-like states. In experiment 2, corticosterone levels were quantified across the isolation test session to provide convergent validity to the model. These findings fit well with the human clinical literature on the anxiety-depression continuum perspective, and suggest the consideration of a nosology that emphasizes the inter-relatedness of these clinical states rather than their boundaries.