Simultaneous activation of multiple memory systems during learning : insights from electrophysiology and modeling

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.Cataloged from PDF version of thesis.Includes bibliographical references.Parallel cortico-basal ganglia loops are thought to give rise to a diverse set of limbic, associative and motor functions, but little is known about how these loops operate and how their neural activities evolve during learning. To address these issues, single-unit activity was recorded simultaneously in dorsolateral (sensorimotor) and dorsomedial (associative) regions of the striatum as rats learned two versions of a conditional T-maze task. The results demonstrate that contrasting patterns of activity developed in these regions during task performance, and evolved with different training-related dynamics. Oscillatory activity is thought to enable memory storage and replay, and may encourage the efficient transmission of information between brain regions. In a second set of experiments, local field potentials (LFPs) were recorded simultaneously from the dorsal striatum and the CAl field of the hippocampus, as rats engaged in spontaneous and instructed behaviors in the T-maze. Two major findings are reported. First, striatal LFPs showed prominent theta-band rhythms that were strongly modulated during behavior. Second, striatal and hippocampal theta rhythms were modulated differently during T-maze performance, and in rats that successfully learned the task, became highly coherent during the choice period. To formalize the hypothesized contributions of dorsolateral and dorsomedial striatum during T-maze learning, a computational model was developed. This model localizes a model-free reinforcement learning (RL) system to the sensorimotor cortico-basal ganglia loop and localizes a model-based RL system to a network of structures including the associative cortico-basal ganglia loop and the hippocampus. Two models of dorsomedial striatal function were investigated, both of which can account for the patterns of activation observed during T-maze training. The two models make differing predictions regarding activation of the dorsomedial striatum following lesions of the model-free system, depending on whether it serves a direct role in action selection through participation in a model-based planning system or whether it participates in arbitrating between the model-free and model-based controllers. Combined, the work presented in this thesis shows that a large network of forebrain structures is engaged during procedural learning. The results suggest that coordination across regions may be required for successful learning and/or task performance, and that the different regions may contribute to behavioral performance by performing distinct RL computations.by Catherine Ann Thorn.Ph.D

Characterizing intravenous medication use in an intensive care unit

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (p. 36).This project focuses on characterizing intravenous (IV) medication administration in an intensive care unit at a partner hospital. Information regarding IV medication dose was extracted from MIMIC II, a large database containing real patient data; this information was used to characterize the use of twelve hemodynamic drugs. Characterization was performed by extracting features such as maximum dose and overall shape from each trend plot. Additionally, because the administration of vasoactive drugs is generally accompanied by a change in blood pressure, several methods were explored of representing patient state by combining the mean blood pressure and drug dose trends to gain more information than can be obtained by each trend alone. The results of drug use characterization show that an adequate picture of drug use can be gained by examining the characteristic shape of the dose trend in addition to features such as maximum dose administered. The patterns of medication administration have been shown to be indicative of overall patient state. The development of algorithms which match drug use trends to underlying physiology may aid in the annotation of large databases such as MIMIC II, and may also prove useful in tracking the hemodynamic state of a patient during his or her stay in intensive care.by Catherine A. Thorn.S.M