Memory for the past provides predictive information that shapes current thought and action. However, when faced with unfamiliar situations or changing environments, information gleaned from past experience may be insufficient or inappropriate to satisfy current demands. In such cases of uncertainty, PFC-mediated control processes play an integral role in guiding thought and action in accordance with current goals. This dissertation investigates two critical question: (1) What forms of repetition-related learning change neural processing demands en route to action?, and (2) How do PFC-mediated control processes interact with mnemonic predictions to reduce uncertainty and to drive goal-directed behavior? These questions are addressed using a multimodal neuroimaging approach that combines behavioral manipulations with both functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). The robust behavioral and neural changes that accompany stimulus repetition (repetition priming and repetition suppression) are used as indices to measure learning and cortical plasticity.
Chapters 2 describes a fMRI study that measured cortical activity while human subjects classified novel and repeated stimuli. Three distinct patterns of repetition suppression were observed in lateral frontal cortex with repetition at three different levels of representation—stimulus, decision, and response. These patterns indicate that frontal processing demands are reduced by learning at the level of stimulus-specific features - (stimulus learning), associations between stimuli and selected decision (stimulus-decision learning), and associations between stimuli and selected responses (stimulus-response learning). The localization of these effects along the rostro-caudal axis of lateral frontal cortex supports a rostro-caudal organization of executive function in the frontal lobes. In addition to these three patterns of repetition suppression, repetition-related activity increases were also observed in dorsal premotor and anterior cingulate cortex when learned stimulus-response associations conflicted with task goals. Such neural markers of proactive interference demonstrate that learned stimulus-response associations can produce either response facilitation or response conflict depending on whether or not the learned associations remain goal relevant.
Chapter 3 describes an EEG experiment that investigated how the three different forms of learning identified in the fMRI experiment influence the timing of cortical responses. Event-related potential and spectral EEG analyses revealed temporally-distinct electrophysiological effects due to stimulus, stimulus-decision, and stimulus-response learning (as well as activity reflecting stimulus-response conflict). These results indicate that multiple forms of learning can occur in tandem during initial experience, and each of these forms of learning subsequently yields temporally-distinct electrophysiological repetition effects.
Chapter 4 describes two behavioral manipulations and a fMRI analysis that investigated the complex relationship between neural and behavioral repetition effects by testing whether behavioral measurements of priming reflect both facilitation and interference. Two distinct levels of interference influenced behavioral measurements of priming and interacted with different levels of cognitive control due to task preparation. Together, the results in this dissertation support a hybrid model of experience-dependent cortical plasticity in which multiple levels of learning confer neural and behavioral processing benefits, as well as costs, during repeated stimulus processing.
|School Location:||United States -- California|
|Source:||DAI-B 70/07, Dissertation Abstracts International|
|Keywords:||Brain, Cortical plasticity, Memory, Prefrontal cortex, Repetition-related learning|
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