The functional neuroanatomy of episodic retrieval: using neuroimaging to understand the computational processes underlying human memory
Kragel, James Edward
:
2015-05-22
Abstract
The medial temporal lobe (MTL) is essential to episodic memory
through its role in the reactivation of past experience.
Cortical networks provide top-down influence on the
MTL, influencing the manner in which information can be
retrieved. While neuroimaging investigations of human memory
have characterized the functional correlates of episodic
retrieval, the coordination of MTL systems during
self-guided memory search is poorly understood. Using
functional neuroimaging to estimate neural activation during
variants of the free-recall paradigm, this dissertation
demonstrates that the posterior MTL is critically involved in
internally-directed memory search, through its interactions with
distributed cortical systems. First, I characterize the
engagement of a cortico-hippocampal network during memory
search. I then demonstrate increased functional connectivity
between this network and multiple frontoparietal systems,
identifying neural mechanisms that may potentially reflect
top-down control of memory search. Next, I relate activation of
the posterior MTL to the process of episodic recollection
through comparison of activity during free recall and source
recognition tasks. I additionally demonstrate common activation
in dorsal frontoparietal networks during free recall and
processing of item familiarity. To characterize the computations
mediated by the MTL, I develop a neuro-cognitive model of free
recall. Retrieved-context theories propose that temporal
context, a slowly integrating representation of the recent past,
cues the hippocampus during retrieval. I link activation of the
MTL to the process of temporal reinstatement, predicting the
temporal organization of recall. Next, I use a variant of the
free-recall paradigm in which memory is disrupted prior to
retrieval, to test the capacity of large-scale cortical networks
to control episodic memory. I show that a frontoparietal control
network (FPCN) functionally couples with the MTL when memory is
disrupted. Using a neurally informed computational model of
recall, I demonstrate that activation of the FPCN and posterior
MTL predict when individuals will overcome distraction by
reinstating contextual information to guide memory search. Taken
together, these findings demonstrate how the MTL supports memory
search through the reinstatement of contextual information, a
process that is coordinated through top-down signals from
frontoparietal networks.