Systems and Diseases

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Browsing Systems and Diseases by Subject "Basal ganglia"

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    Dissociating the Cognitive Effects of Levodopa versus Dopamine Agonists in aNeurocomputational Model of Learning in Parkinson's Disease
    (Karger AG, Basel, 2012-11-01) Moustafa, Ahmed A.; Herzallah, Mohammad M.; Gluck, Mark A.
    Background/Aims: Levodopa and dopamine agonists have different effects on the motor, cognitive, and psychiatric aspects of Parkinson’s disease (PD). Methods: Using a computational model of basal ganglia (BG) and prefrontal cortex (PFC) dopamine, we provide a theoretical synthesis of the dissociable effects of these dopaminergic medications on brain and cognition. Our model incorporates the findings that levodopa is converted by dopamine cells into dopamine, and thus activates prefrontal and striatal D 1 and D 2 dopamine receptors, whereas antiparkinsonian dopamine agonists directly stimulate D 2 receptors in the BG and PFC (although some have weak affinity to D 1 receptors). Results: In agreement with prior neuropsychological studies, our model explains how levodopa enhances, but dopamine agonists impair or have no effect on, stimulus-response learning and working memory. Conclusion: Our model explains how levodopa and dopamine agonists have differential effects on motor and cognitive processes in PD.
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    A neural model of hippocampalstriatal interactions in associative learningand transfer generalization in various neurological and psychiatric patients
    (Elsevier, 2010-08-21) Herzallah, Mohammed; Moustafa, Ahmed; Keri, Szabolcs; Myers, Catherine; Gluck, Mark
    Building on our previous neurocomputational models of basal ganglia and hippocampal region function (and their modulation by dopamine and acetylcholine, respectively), we show here how an integration of these models can inform our understanding of the interaction between the basal ganglia and hippocampal region in associative learning and transfer generalization across various patient populations. As a common test bed for exploring interactions between these brain regions and neuromodulators, we focus on the acquired equivalence task, an associative learning paradigm in which stimuli that have been associated with the same outcome acquire a functional similarity such that subsequent generalization between these stimuli increases. This task has been used to test cognitive dysfunction in various patient populations with damages to the hippocampal region and basal ganglia, including studies of patients with Parkinson’s disease (PD), schizophrenia, basal forebrain amnesia, and hippocampal atrophy. Simulation results show that damage to the hippocampal region—as in patients with hippocampal atrophy (HA), hypoxia, mild Alzheimer’s (AD), or schizophrenia—leads to intact associative learning but impaired transfer generalization performance. Moreover, the model demonstrates how PD and anterior communicating artery (ACoA) aneurysm—two very different brain disorders that affect different neural mechanisms— can have similar effects on acquired equivalence performance. In particular, the model shows that simulating a loss of dopamine function in the basal ganglia module (as in PD) leads to slow acquisition learning but intact transfer generalization. Similarly, the model shows that simulating the loss of acetylcholine in the hippocampal region (as in ACoA aneurysm) also results in slower acquisition learning. We argue from this that changes in associative learning of stimulus–action pathways (in the basal ganglia) or changes in the learning of stimulus representations (in the hippocampal region) can have similar functional effects.