| Functional neuroimaging studies have demonstrated activation of the brain's medial temporal lobe during episodic memory retrieval. However, inconsistent evidence from lesion studies indicate that, although the medial temporal lobe is essential for acquiring new declarative memories, it may not be critical for all forms of retrieval. Given the multitude of mnemonic and non-mnemonic processes invoked during a guided retrieval episode, including attending to memory cues, searching through a memory store, reactivating the target memory, monitoring the ongoing experience and reflecting on a recovered memory, it can be challenging to disentangle the functional drivers of retrieval- related brain responses. This dissertation presents a series of studies that use functional magnetic resonance imaging to more thoroughly characterize the role of the medial temporal lobe, and surrounding frontal and parietal regions, in effortful memory retrieval. Three investigations examine the influence of memory search on activity in the hippocampus and neocortex. Results implicate a widespread hippocampal-cortical system that is modulated by task difficulty and covaries with hippocampal signals that track incidental encoding of the background environment. This evidence is discussed in light of a primary hippocampal function for memory encoding that is regulated by dynamic task demands. A final study examined how the contextual information of a retrieved memory is coded in the medial temporal lobe. Applying both univariate and multivariate analyses, this investigation identified medial temporal activation patterns representing distinct memory features. It further characterized these signals in terms of their mean regional or spatially distributed activity, as well their behavioral correlates to task difficulty. Together, these studies provide further insight into the delicate interplay between mnemonic and attentional brain functions supporting guided memory retrieval. |
