Electrophysiological Signatures of Fear Conditioning: From Methodological Considerations to Catecholaminergic Mechanisms and Translational Perspectives

Fear conditioning describes a learning mechanism during which a specific stimulus gets associated with an aversive event (i.e., an unconditioned stimulus; US). Thereby, this initially neutral or arbitrary stimulus becomes a so-called “conditioned” stimulus (CS), which elicits a conditioned threat response. Fear extinction refers to the decrease in conditioned threat responses as soon as the CS is repeatedly presented in the absence of the US. While fear conditioning is an important learning model for understanding the etiology and maintenance of anxiety and fear-related disorders, extinction learning is considered to reflect the most important learning process of exposure therapy. Neurophysiological signatures of fear conditioning have been widely studied in rodents, leading to the development of groundbreaking neurobiological models, including brain regions such as the amygdala, insula, and prefrontal areas. These models aim to explain neural mechanisms of threat processing, with the ultimate goal to improve treatment strategies for pathological fear. Recording intracranial electrical activity of single units in animals offers the opportunity to uncover neural processes involved in threat processing with excellent spatial and temporal resolution. A large body of functional magnetic resonance imaging (fMRI) studies have helped to translate this knowledge about the anatomy of fear conditioning into the human realm. fMRI is an imaging technique with a high spatial resolution that is well suited to study slower brain processes. However, the temporal resolution of fMRI is relatively poor. By contrast, electroencephalography (EEG) is a neuroscientific method to capture fast and transient cortical processes. While EEG offers promising opportunities to unravel the speed of neural threat processing, it also provides the possibility to study oscillatory brain activity (e.g., prefrontal theta oscillations). The present thesis contains six research manuscripts, describing fear conditioning studies that mainly applied EEG methods in combination with other central (fMRI) and peripheral (skin conductance, heart rate, and fear-potentiated startle) measures. A special focus of this thesis lies in methodological considerations for EEG fear conditioning research. In addition, catecholaminergic mechanisms are studied, with the ultimate goal of opening up new translational perspectives. Taken together, the present thesis addresses several methodological challenges for neuroscientific (in particular, EEG) fear conditioning research (e.g., appropriate US types and experimental designs, signal-to-noise ratio, simultaneous EEG-fMRI). Furthermore, this thesis gives critical insight into catecholaminergic (noradrenaline and dopamine) mechanisms. A variety of neuroscientific methods (e.g., EEG, fMRI, peripheral physiology, pharmacological manipulation, genetic associations) have been combined, an approach that allowed us (a) to translate knowledge from animal studies to human research, and (b) to stimulate novel clinical directions

The association between overnight recognition accuracy and slow oscillation-spindle coupling is moderated by BDNF Val66Met

During sleep, memories are consolidated via oscillatory events that occur in temporal and phasic synchrony. Several studies show that sleep spindles peaking close to the depolarized positive peaks of slow oscillations (SO) associate with better retention of memories. The exact timing of this synchrony presumably depends on the properties of the related neural network that, in turn, is affected by certain genetic variants associated with brain development and function. Brain-derived neurotrophic factor (BDNF) Val66Met and Catechol‐O‐methyltransferase (COMT) Val158Met are repeatedly reported to implicate the structure and function of prefrontal and hippocampal areas as well as molecular events promoting synaptic plasticity. In this study, we examined with a community-based sample of 153 adolescents (~17 years) whether these variants (1) affected the coupling properties between frontal SOs and spindles and (2) moderated the association between SO-spindle coupling and overnight recognition accuracy. We found SO-upstate-coupled fast (> 13 Hz) sleep spindles to associate with better recognition in the whole sample. Additionally, Val66Met moderated this association such that SO-spindle coupling was predictive of memory outcome only in those homozygous to ValBDNF alleles but not in MetBDNF carriers. Memory outcome was not associated with the SO-coupling properties of slow spindles nor affected by the interaction between Val158Met and coupling measures. Finally, in the whole sample we found that SO-upstate-coupled fast spindles were more strongly associated with the recognition of positive, relative to neutral, pictures. In conclusion, precise coupling of SOs and fast spindles associates with overnight recognition accuracy and this association is moderated by BDNF Val66Met.Peer reviewe

Looking Through the Lens of Individual Differences: Relationships Between Personality, Cognitive Control, Language Processing, and Genes

The study of individual differences in cognitive abilities and personality traits has the potential to inform our understanding of how the processing mechanisms underlying different behaviors are organized. In the current set of studies, we applied an individual-differences approach to the study of sources of variation in individuals’ personality traits, cognitive control, and linguistic ambiguity resolution abilities. In Chapter 2, we investigated the relationship between motivational personality traits and cognitive control abilities. The results demonstrated that individual differences in the personality traits of approach and avoidance predict performance on verbal and nonverbal versions of the Stroop task. These results are suggestive of a hemisphere-specific organization of approach/avoidance personality traits and verbal/nonverbal cognitive control abilities. Furthermore, these results are consistent with previous findings of hemispheric asymmetry in terms of the distribution of dopaminergic and norephinephrine signaling pathways. In Chapter 3, we investigated the extent to which the same processing mechanisms are used to resolve lexical and syntactic conflict. In addition, we incorporated a behavioral genetics approach to investigate this commonality at the neurotransmitter level. We explored whether genetic variation in catechol-O-methyltransferase (COMT), a gene that regulates the catabolism of dopamine in prefrontal cortex, is related to individuals’ ability to resolve lexical and syntactic conflict. The results of this study demonstrated that individual differences in the ability to resolve lexical conflict are related to variation in syntactic conflict resolution abilities. This finding supports constraint satisfaction theories of language processing. We also showed that those individuals with the variant of the COMT gene resulting in less availability of dopamine at the synapse tended to have greater difficulty processing both lexical and syntactic ambiguities. These results provide novel evidence that dopamine plays a role in linguistic ambiguity resolution. In sum, the results from the current set of studies reveal how an individual-differences approach can be used to investigate several different factors involved in the context-dependent regulation of behavior

BIOLOGICAL BASIS OF VARIABILITY IN DOPAMINE AVAILABILITY ON FRONTOSTRIATAL BRAIN FUNCTION IN ADOLESCENCE

Neurodevelopmental studies indicate a protracted development through adolescence of brain systems underlying incentive-driven behaviors including prefrontal cortex (PFC) and the striatum. These systems support the executive control of behavior as well as motivationally driven behaviors and may contribute to vulnerabilities in the emergence of psychopathology. The PFC and striatum may support cognition and motivation through the function of the neurotransmitter dopamine. Dopamine (DA) availability is increased during the adolescent period in human and animals and play an important role in mediating individual differences in risk-taking behaviors. This dissertation seeks to examine the moderating role of genetically mediated DA availability on frontostriatal brain function in adolescence. To this end, we genotyped individuals between the ages of 10 and 20 for common functional polymorphisms in three genes that have a direct influence on synaptic DA availability. In addition, we calculated a multilocus composite score in order to assess additive effects of our three genetic loci. We used functional magnetic resonance imaging (fMRI) to assess brain function. The purpose of our first study was to examine the integrity of frontostriatal networks using resting state functional connectivity. We then look more directly at the role of frontostriatal brain function on incentive-driven behaviors using a rewarded inhibitory control task that has a known developmental signature . Overall we found a moderating influence of DA availability on age-related changes in key frontostriatal circuitry suggesting that the maturation of brain function in adolescence may in part be mediated by inter-individual variability in DA signaling. Overall, the genotypes by age interactions highlight a unique DA-driven brain profile in adolescence. This suggests that a genetically mediated brain phenotype characterized in adolescence may differ significantly from that in adulthood. This has strong implications regarding the variability observed in adolescent risk-taking behaviors as well as predictions of later adult behavior

Depressiooni ja ärevusega seotud geenivariandid: mõju isiksuseomadustele ja tervistmõjustavale käitumisele

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Varasemate uuringute põhjal on teada, et mõned isiksuseomadused suurendavad depressiooni tekkimise riski. Nii isiksuseomadustel kui ka depressioonil on aga tugev pärilik taust ja osa geneetilisest alusest arvatakse neil olevat ühine. Kummagi fenotüübi geneetilist ülesehitust ei ole veel suudetud tuvastada. Käesoleva väitekirja eesmärgiks oli uurida suurel rahvastiku suhtes representatiivsel valimil, kuidas on seotud neurotransmissiooni mõjustavad depressiooni kandidaatgeenide variandid 5-HTTLPR, BDNF Val66Met, COMT Val158Met ja TPH2 G-703T isiksuseomadustega. Lisaks uurisime ka geenidevahelisi interaktsioone, vanuse ja soo mõju ning seoseid teiste tervise ja heaolu teguritega. Leidsime, et kõik nimetatud kandidaatgeenid tõepoolest mõjutavad isiksuseomadusi rahvastikus. COMT genotüüp avaldab mõju Neurootilisusele, ehk kalduvusele liigselt muretseda ja ärevust tunda. BDNF ja TPH2 mõjutavad aga Meelekindlust, inimese kalduvust olla kohusetundlik, täpne ja distsipliineeritud. Ilmnes ka 5-HTTLPR polümorfismi moduleeriv roll genotüübi ja Meelekindluse seostes. Samuti leidsime, et nii TPH2 kui COMT genotüüpide mõju Neurootilisusele oli sõltuv uuritavate vanusest. Kuigi me ei leidnud nimetatud geenivariantide seoseid ärevus- ja meeleoluhäiretega, peegeldub mõju isiksusele ka teistes inimese tervise ja heaoluga seotud tegurites. Näiteks BDNF polümorfism avaldab mõju söömishäirete sümptomaatika tekkimisel ning COMT mõjutab depressiivsuse taset, haridusteed ja hinnanguid sotsiaalmajanduslikule staatusele. Käitumisgeneetika valdkonna ummikseis isiksuseomaduste geneetiliste aluste tuvastamisel viitab erinevatele moduleerivatele mõjuteguritele geeniefektide avaldumisel. Väitekirjas käsitletud suure rahvastikupõhise uuringu tulemuste põhjal rõhutame soo, vanuse ja geenidevahelise interaktsiooni arvestamise olulisust genotüüpide mõju uurimisel väga mitmetahulistele fenotüüpidele.Previous findings have indicated that some personality traits are increasing the risk for depression. Personality traits and depression are both to a significant extent heritable and are considered to share some of the genetic components. Unraveling the genetic basis of these phenotypes has proven to be difficult. The purpose of this dissertation was to study the effects of several depression-related gene variants, 5-HTTLPR, BDNF Val66Met, COMT Val158Met and TPH2 G-703T, on personality traits in a large population representative sample. In addition, we aimed to study gene × gene interactions, time and sex as possible modulators, and furthermore, to assess if there are any genotype effects on other health-related behaviours. Indeed, we found that all candidate genes under investigation had influence on personality traits. The COMT genotype was affecting Neuroticism, a trait for excessive worrying and anxiety. The main effects of BDNF and TPH2 gene variants were on Conscientiousness, a trait for dutifulness, precision and self-discipline. In addition, these genotype effects on Conscientiousness were modulated by the 5-HTTLPR polymorphism. Also we found the effects of TPH2 and COMT on Neuroticism to be age-dependent. Although we did not find any genotype effects on the history of mood and anxiety disorders, there were associations with health-related behaviour. We found the BDNF polymorphism to affect the emergence of eating disorder symptoms and COMT polymorphism to influence depressiveness, educational attainment, and also socioeconomic status assessment. The difficulties in unraveling the genetic foundations of personality traits suggest a large number of additional factors, which may modulate the emergence of genotype effects. With these results on a large population representative study, we highlight the significance of considering time, sex and gene × gene interactions as possible modulators of genotype effects

Neurophysiological, behavioural and genetic markers of behavioural problems in early childhood

The work presented in the present thesis investigated the neural, behavioural and genetic markers that may be associated with the manifestation of behavioural problems during the early years of life. Across four different empirical studies, and by incorporating, behavioural, neurophysiological and genetic investigations, it was demonstrated that: (1) there are neurophysiological signatures that may be associated with the manifestation of behavioural problems early in life; (2) common genetic variations that determine serotonin variability are strongly associated with affectivity-related patterns of frontal brain activation; and that (3) normal genetic variations that modulate serotonin availability and neuroplasticity are each associated with affectivity-related patterns of visual scanning behaviours in response to faces and aversive scenes. Taken together, the results illustrate the existence of robust neural, genetic and behavioural markers that may be associated with the manifestation of behavioural problems in early childhood and prompt further investigation of the area by generating novel hypotheses. Together, the empirical findings of the thesis provide a first stage contribution to the complex mechanisms that may yield risk and resilience for behavioural problems during the early years of life by generating a more comprehensive insight on the field of affectivity

Learning and action in uncertain environments

Successful interaction with the environment requires flexible updating of our beliefs about the world. By learning to estimate the likelihood of future events, it is possible to prepare appropriate actions in advance and execute fast, accurate motor responses. According to theoretical proposals, humans track the variability arising from dynamic environments by computing various forms of uncertainty. Several neuromodulators have been linked to uncertainty signalling but comprehensive empirical characterisation of their roles in perceptual belief updating and motor response modulation has been lacking. This thesis interrogates the contributions of noradrenaline, acetylcholine and dopamine to human learning and action within a unified computational framework of uncertainty. First, I use pharmacological interventions to characterise the impact of noradrenergic, cholinergic and dopaminergic receptor antagonism on individual computations of uncertainty during a probabilistic serial reaction time task. I develop and employ a hierarchical Bayesian model to quantify human learning and action under three forms of uncertainty. I propose that noradrenaline influences learning of uncertain events arising from unexpected changes in the environment, while acetylcholine balances attribution of uncertainty to chance fluctuations within environmental contexts or to gross environmental violations following a contextual switch. In contrast, dopamine supports the use of uncertainty representations to engender fast, adaptive responses. Second, I extend these results by focusing on the effects of natural inter-individual variations in dopaminergic function. Specifically, I employ the same task and model to assess individual learning and action under uncertainty as a function of COMT genotype. Third, I focus on the role of noradrenaline. Uncertainty computations have been linked to changes in pupil diameter, and pupil dilation to noradrenergic neuronal activity in the locus coeruleus. Combining an auditory probabilistic learning task, pharmacological manipulations, pupillometry and computational modelling, I demonstrate that pupil diameter offers an indirect measure of dynamic noradrenergic computations of environmental uncertainty and volatility