Functional magnetic resonance imaging during a delayed non-match to sample task in the non-human primate
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This dissertation was aimed to expand our knowledge about the contributions of the medial temporal lobe (MTL) and prefrontal cortex (PFC) during the delayed non-matching to sample (DNMS) task. The DNMS task has been used since the 1970's to assess memory in the non-human primate. Historically our knowledge about the role of these structures on the DNMS task has come from brain lesion studies. In this thesis I assessed the contributions of these brain regions using functional MRI (fMRI). Data were acquired in two non-human primate subjects at two different delay periods. The DNMS task contains three primary components: a sample period (encoding), followed by a delay period (maintenance) and a choice period (retrieval). In Experiment 1, the goal was to identify regions active during the encoding, delay, and retrieval periods during a DNMS task with a 10 second delay. The results demonstrated activity in the MTL, within the hippocampus, parahippocampal gyrus, and the lingual gyrus. The hippocampal, parahippocampal and lingual gyrus results were consistent across both animals in the encoding and retrieval periods, but were inconsistent during the delay. In Experiment 2, a 30 second delay was used and the focus was on the differentiation of early and late portions of the 30 second delay. The early portion of this period was operationally defined as the first 12 seconds and the late portion as the final 12 seconds. Findings from Experiment 2 indicated greater MTL activation in the temporal area TE medial part (TEM) and the entorhinal cortex (ERC) during the late portion of the delay period compared to activation in the lingual/fusiform gyrus in the early portion of the delay period. When the early and late portions of the delay were combined, PFC activation, in addition to MTL, became evident. Overall the results of this non-human primate fMRI study are consistent with human imaging studies that have demonstrated MTL activity during the sample and delay periods of a DNMS task.
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