Specific contributions of the posterior parietal cortex to episodic memory
Jeremy Andrew Elman
Doctor of Philosophy in Psychology
University of California, Berkeley
Professor Arthur P. Shimamura, Chair
The posterior parietal cortex (PPC) has become the topic of much research regarding its role in memory over the past decade. While the medial temporal lobes and prefrontal cortex have long been studied for their contributions to memory, the PPC has been more closely associated with attention and spatial cognition. With the increasing use of functional magnetic resonance imaging (fMRI) in studies of human cognition, it has become apparent that the medial and lateral parietal cortices are consistently involved during memory retrieval tasks. The most prominent finding is that of greater activity during retrieval for previously studied items compared to new items, terms the successful retrieval effect. The following studies attempt to delineate the boundary conditions under which the PPC is involved during episodic memory tasks.
Study 1 examined whether PPC involvement is contingent upon directed memory retrieval or if encountering previously studied items in the context of a non-memory task would be sufficient to drive activity. The results from two experiments in Study 1 indicate that the PPC may activate in response to old items during a non-memory test, but only when determining mnemonic status is task relevant. These findings suggest that low-level retrieval processes may occur even during non-memory tasks, but that PPC activity reflects downstream processes that only come online when the retrieved information becomes relevant to the task at hand.
Continuing this line of questioning, Study 2 examined whether the PPC is activated when participants make meta-memory judgments (using a feeling of knowing paradigm) in which the retrieval of target information is not required. Furthermore, we assessed whether the PPC activity differed between metacognitive assessments of episodic and semantic memories. The resulting activations from this metacognitive task appear remarkably similar to those found in standard retrieval tasks. Furthermore, the ventral PPC appeared to be preferentially activated by episodic compared to semantic memory judgments. It is suggested that the contextually bound nature of episodic memory underlies PPC involvement.
Study 3 further considered the effect information type on PPC engagement during retrieval. Participants were tested on memory for buildings that were either personally familiar through repeated real-life encounters or linked only to a prior study session. Consistent with findings from Study 2, a specific region of the ventral PPC was found to be more active when viewing items bound to a specific context. However, the finding of a more posterior site of activity for personally familiar buildings suggests a functional heterogeneity within the PPC.
Taken together, the results of these three studies further delineate the boundary conditions under which the PPC is involved in memory tasks. These findings demonstrate that PPC contributions may depend not only on the task goal but also on the type of information being retrieved. Specifically, the PPC appears to be preferentially engaged when there is a need to bind item and context information to form an event memory. The results of the studies are further discussed in the context of current theoretical models of PPC function.