1,288 research outputs found
Exploring model-based and model-free reinforcement learning in obsessive-compulsive disorder
RESUMO: A Perturbação Obsessivo-Compulsiva (POC) é uma doença neuropsiquiátrica
comum, grave e incapacitante, para a qual os tratamentos actuais são ineficazes num
grande número de casos. O instrumento mais utilizado para avaliar a gravidade de
sintomas obsessivo-compulsivos é a Yale-Brown Obsessive-Compulsive Scale (YBOCS), que foi recentemente revista (Y-BOCS-II). No entanto, a sua validade de
construto (tanto divergente como convergente) tem sido reportada como moderada e
a sua validade de critério para diagnóstico de POC nunca foi testada. No primeiro
capítulo desta tese testei, pela primeira vez, a validade de critério da Y-BOCS-II e
demonstrei que um ponto de corte de 13 (pontuação total) atinge o melhor balanço
entre sensibilidade e especificidade para o diagnóstico de POC. No entanto, confirmei
que a sua validade divergente está longe de ser excelente. Este último achado levoume a procurar outros potenciais marcadores de POC.
Têm sido demonstradas várias anomalias em doentes com POC utilizando
tarefas neuropsicológicas ou técnicas de neuroimagem. Contudo, não existe ainda
um marcador consistente para esta perturbação, que seja capaz de discriminar
eficazmente pacientes que sofrem de POC, que seja sensível à mudança após
intervenções terapêuticas e para o qual seja possível estabelecer uma
correspondência com circuitos ou função cerebral. Uma abordagem que tem sido
seguida nos últimos anos considera a POC como sendo caracterizada por uma
disfunção nos sistemas cerebrais responsáveis pela aprendizagem de acções. As
tarefas de decisão sequencial emergiram recentemente como um instrumento
importante e sofisticado para estudar a aprendizagem de acções em humanos através
da abordagem de reinforcement learning (RL). De acordo com a teoria subjacente ao
RL, as acções podem ser aprendidas de duas formas distintas: um sistema modelbased funciona através da construção de um modelo interno das dinâmicas do
ambiente e utiliza esse modelo para planear trajectórias comportamentais futuras, por
oposição a um sistema model-free, que funciona armazenando o valor estimado das
acções que foram implementadas recentemente e actualizando essas estimativas por
tentativa e erro. As chamadas tarefas de decisão sequencial têm vindo a ser utilizadas
para estabelecer associações entre disfunção de sistemas cerebrais de RL e algumas
perturbações neuropsiquiátricas, como a POC, sendo que um desequilíbrio entre os
sistemas model-based e model-free tem sido descrito. Através da aplicação de uma
dessas tarefas de decisão sequencial, a two-step task, existe evidência que sugere
que os doentes com POC têm um défice no sistema model-based. No entanto, neste
paradigma em particular, antes de desempenhar esta tarefa os indivíduos recebem
informação detalhada sobre a estrutura da mesma. Assim, não é claro como os dois
principais sistemas de RL interagem quando os indivíduos aprendem exclusivamente
através de interacção com o ambiente e como a informação explícita afecta as
estratégias de RL. No segundo capítulo desta tese, desenvolvi uma nova tarefa de
decisões sequenciais que permite não só quantificar o uso de estratégias modelbased RL e model-free RL, mas também diferenciar entre o impacto do conhecimento explícito da estrutura da tarefa e o impacto da experiência na mesma. Os resultados
da aplicação da tarefa em indivíduos saudáveis demonstram que inicialmente a
escolha de acções é controlada por aprendizagem model-free, com a aprendizagem
model-based emergindo apenas numa minoria de indivíduos depois de experiência
significativa com a tarefa, não emergindo de todo em indivíduos com POC, que por
sua vez mostraram tendência para aumentar o uso de model-free RL com a
experiência. Quando foi dada informação explícita sobre a estrutura da tarefa,
observou-se um aumento dramático do uso de aprendizagem model-based, tanto nos
voluntários saudáveis como em ambos os grupos clínicos. A informação explícita
diminuiu o uso do sistema de aprendizagem model-free nos voluntários saudáveis e
nos pacientes com perturbação do humor e ansiedade, mas essa diminuição não foi
estatisticamente significativa no grupo de doentes com POC. Para além disso, depois
das instruções, verificou-se em todos os grupos que a actualização do valor das
acções aprendidas através do sistema model-free passou a ser mais influenciada
pelo valor dos estados atingidos e menos influenciada pela consequência dos
ensaios. Outro efeito da informação explícita sobre a estrutura da tarefa nos
indivíduos saudáveis foi tornar as escolhas mais perseverantes, o que é consistente
com uma modificação da estratégia de exploração. Estes resultados ajudam a
clarificar o perfil de utilização de estratégias de RL dos pacientes com POC, que
apresentam défice inespecíficos de aprendizagem model-based e achados mais
específicos de maior uso de aprendizagem model-free, em ambos os casos antes de
obterem informação sobrea estrutura da tarefa.
Por fim, como a literatura ainda não é consensual sobre a interação entre um
eventual sistema de model-based RL e um sistema de model-free RL nos circuitos
cerebrais em humanos, devenvolvi um protocolo de ressonância magnética funcional
para avaliar a escolha de ação sequencial com e sem instruções. Os resultados
preliminares, em indivíduos saudáveis, sugerem que a reduced two-step task permite
separar comportamento que utiliza aprendizagem predominantemente model-free
(antes das instruções) de comportamento que utiliza aprendizagem
predominantemente model-based (após as instruções), no mesmo indivíduo,
estrutura da tarefa e ambiente. A análise dos dados de imagem funcional sugere que
o conhecimento explícito sobre a estrutura da tarefa modifica a atividade neuronal no
córtex paracingulado (cortex prefrontal medial) durante a transição do primeiro para
o segundo passo da tarefa. Objectivos futuros incluem o uso de técnicas de análise
multivariada para explorar a representação cerebral dos estados da tarefa e a
aplicação deste protocolo de ressonância magnética funcional em populações
clínicas.ABSTRACT: Obsessive-compulsive disorder (OCD) is a common, chronic and disabling
neuropsychiatric condition for which current treatments are ineffective in a large
proportion of cases. The gold-standard instrument to assess the severity of OCD
symptoms is the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), which was
recently revised (Y-BOCS-II). However, its construct validity has been reported has
moderate and its criterion-related validity for the diagnosis of OCD has never been
tested. In the first chapter of this dissertation, I tested, for the first time, criterion-related
validity of the Y-BOCS-II and demonstrated that a cut-off of 13 (total score) attains the
best balance between sensitivity and specificity for the diagnosis of OCD. However, I
confirmed that its divergent validity is far from excellent. This last finding led me to
search for other potential markers of OCD.
Several abnormalities have been demonstrated in OCD patients in studies
using neuropsychological and neuroimaging approaches, but we still lack a consistent
marker for the disorder which is able to discriminate patients with OCD from healthy
subjects or from patients with other mental disorders, which is sensitive to treatmentinduced changes, and which can be mapped to brain circuits or function. An approach
which has been followed over the last decade is considering OCD as a disorder of
action learning systems of the brain. Sequential decision tasks have recently emerged
as an influential and sophisticated tool to investigate action learning in humans through
the reinforcement learning (RL) framework. According to the RL framework, actions
can be learned in two different ways: model-based control works by learning a model
of the dynamics of the environment and later using that model to plan future behavioral
trajectories, while model-free control works by storing the estimated value of recently
taken actions and updating these estimates by trial-and-error. Sequential decision
tasks have been used to assess associations between dysfunction in RL control
systems and certain behavioral disorders, such as OCD, where an unbalance between
model-based and model-free RL has been hypothesized. In fact, using the most
commonly applied sequential decision task, the two-step task, evidence has been
produced suggesting that OCD patients have a deficit in model-based learning.
However, in this specific paradigm, subjects typically receive detailed information
about task structure prior to performing the task. Thus, it remains unclear how different
RL systems contribute when subjects learn exclusively from experience, and how
explicit information about task structure modifies RL strategy. To address these
questions, I created a sequential decision task requiring minimal prior instruction, the
reduced two-step task. I assessed performance both prior to and after delivering
explicit information on task structure, in healthy volunteers, patients with OCD and
patients with other mood and anxiety disorders. Initially model-free control dominated,
with model-based control emerging only in a minority of subjects after significant task
experience, and not at all in patients with OCD, who had instead a tendency to
increase their use of model-free control. Once explicit information about task structure
was provided, a dramatic increase in the use of model-based RL was observed,similarly across healthy volunteers and both patient groups, including OCD. The
debriefing also significantly decreased the use of model-free RL in healthy volunteers
and in patients with mood and anxiety disorders, but not in OCD patients. Additionally,
after instructions, model-free action value updates were influenced more by state
values and less by trial outcomes, in all groups, and subject choices became more
perseverative in healthy subjects, consistent with changes in exploration strategy.
These results help in clarifying the RL profile for patients with OCD, with unspecific
findings of deficient model-based control, and more specific findings of enhanced
model-free control, in both cases prior to information about task structure.
Finally, as the literature is not yet consensual on how model-free and modelbased RL systems interact in human brain circuits, I developed a functional magnetic
resonance imaging (fMRI) protocol to assess uninstructed and instructed sequential
action choice. Preliminary results in healthy subjects suggest that the fMRI version of
the reduced two-step task allows to separate predominantly model-free control (before
instructions) from predominantly model-based control (after instructions), in the same
subject, task structure and environment. Across all sessions, choice events were
associated with increases blood-oxygen-level-dependent (BOLD) activity in the left
precentral gyrus and reward events were associated with increased BOLD activity in
the ventral striatum. I found that explicit knowledge about task structure modifies
blood-oxygen-level-dependent (BOLD) activity in the paracingulate cortex (medial
prefrontal cortex) during the transition from the first- to the second-step of the task.
Future directions include using multivariate pattern analysis techniques to explore how
the brain represents state space in sequential decision tasks and applying the current
fMRI protocol in clinical populations
Learning how to act: making good decisions with machine learning
This thesis is about machine learning and statistical approaches
to decision making. How can we learn from data to anticipate the
consequence of, and optimally select, interventions or actions?
Problems such as deciding which medication to prescribe to
patients, who should be released on bail, and how much to charge
for insurance are ubiquitous, and have far reaching impacts on
our lives. There are two fundamental approaches to learning how
to act: reinforcement learning, in which an agent directly
intervenes in a system and learns from the outcome, and
observational causal inference, whereby we seek to infer the
outcome of an intervention from observing the system.
The goal of this thesis to connect and unify these key
approaches. I introduce causal bandit problems: a synthesis that
combines causal graphical models, which were developed for
observational causal inference, with multi-armed bandit problems,
which are a subset of reinforcement learning problems that are
simple enough to admit formal analysis. I show that knowledge of
the causal structure allows us to transfer information learned
about the outcome of one action to predict the outcome of an
alternate action, yielding a novel form of structure between
bandit arms that cannot be exploited by existing algorithms. I
propose an algorithm for causal bandit problems and prove bounds
on the simple regret demonstrating it is close to mini-max
optimal and better than algorithms that do not use the additional
causal information
Endpoints In Intensive Care Unit Based Randomized Clinical Trials
With few exceptions, intensive care unit (ICU)-based randomized clinical trials (RCTs) have failed to demonstrate hypothesized treatment effects. Undoubtedly, some of these failures are attributable to interventions that truly do not provide hoped-for benefits. However, this dissertation pursues the thesis that many null findings represent “false negatives” that are due not to ineffective therapies but to flawed study designs or analytic approaches. We examine the design and statistical methods traditionally employed in ICU-based RCTs, and their potential impacts on the efficient measurement and interpretation of treatment effects. Paper one presents a systematic review of 146 contemporary ICU-based RCTs in which we find that most trials were underpowered to detect small but potentially important mortality differences between treatment arms. We also find that the majority of RCTs (73%) specified primary outcomes other than mortality, that trials employing nonmortal primary outcomes more frequently identified significant treatment effects, and that both mortal and nonmortal endpoints were heterogeneously defined, measured and analyzed across RCTs. Thus, papers two and three focus on nonmortal endpoints, using ICU length of stay (LOS) as a case study to evaluate how best to measure and analyze duration-based nonmortal endpoints. In paper two, we conduct a statistical simulation study, demonstrating that nonmortal endpoints are interlinked with and confounded by mortality, and that the manner in which investigators choose to account for deaths in LOS analyses may influence their conclusions. In paper three, we examine another potential source of error in LOS analyses, namely the measurement error attributable to the additional ICU time that patients commonly accrue after they are clinically ready for ICU discharge. Using simulated data informed by our own ICU-based RCT, we demonstrate that this “immutable time” (which cannot plausibly be altered by the interventions under study) combines with clinically necessary ICU time to produce overall LOS distributions that may either mask true treatment effects or suggest false treatment effects. Our work provides evidence of the potential benefits and pitfalls when employing nonmortal outcomes in ICU-based RCTs, and also identifies a clear need for standardized methods for defining and analyzing such outcomes
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