Dissecting the human medial temporal lobe memory system by functional MRI

The work presented in this thesis comprises two event-related functional magnetic resonance imaging (fMRI) studies, which aim at dissociating the contributions of different subregions of the human medial temporal lobe (MTL) to declarative memory processes. The first study examines common neural correlates of memory encoding and recognition. Healthy subjects were scanned both while they memorized photographs of buildings and landscapes and while they tried to recognize these pictures in a series of new photographs. Confirming earlier findings, declarative memory formation correlated with an activity increase in the MTL and the inferior prefrontal cortex. Further, during recognition, stronger brain responses to correctly identified old items (hits) as compared to correctly identified new items were found in the parietal lobe, the anterior prefrontal cortex, the anterior cingulate and the cerebellum, replicating findings concerning the commonly used old/new effect. The positive recognition effect, stronger responses for hits than misses, identified activations in prefrontal, parietal, and cerebellar areas. The negative recognition effect, weaker brain responses for hits than misses offers the possibility to study whether recognition success can also be associated with regional brain activity decreases. In line with electrophysiological findings, this effect identified an activity decrease in the anterior MTL related to recognition success. Memory encoding and retrieval were examined in a single study-test experiment. Thus, it was possible to use a conjunction analysis to directly compare encoding- and retrieval-related activations within subjects. This analysis identified an integrated temporal-cerebellar network, whose activity correlates with both memory formation and retrieval. A question that remained open from the results of the above study concerns the nature of the activations identified during memory retrieval. Dual-process models of recognition memory propose that there is a qualitative distinction between the forms of memory that support recognition of an item which is accompanied as opposed to unaccompanied by contextual information. To objectively distinguish between subprocesses within recognition – whether an activation is related to recollection or familiarity – some sort of source memory judgment is needed. To test the hypothesis that distinct MTL operations are associated with either contextual retrieval or item recognition, the same photographs of buildings and landscapes as in the first study were employed as stimuli. A context was introduced by transforming the photographs into one of four single-color-scales: red, blue, yellow, or green. In the subsequent old/new recognition memory test, all stimuli were presented as plain grey scale photographs. By this manipulation, a source judgment referring to the color in which the stimulus was presented during study could be employed, with the aim to delineate neural correlates of truly recollective memory versus item recognition. As a measure for the neural correlates of contextual retrieval, the positive source memory effect was introduced: the difference in brain activity between hits with and without correct source judgment. This contrast revealed bilateral MTL activation, centered in the hippocampus. In contrast, the item recognition effect, the difference in brain activity for hits with incorrect source judgment as opposed to misses, delineates brain regions involved in item recognition unaccompanied by contextual retrieval. The negative item recognition effect, more activity for misses as opposed to hits without source, identified an activity decrease during item recognition in the anterior MTL, presumably the anterior parahippocampal gyrus. For the first time in the fMRI literature, these results suggest a clear-cut dissociation between human MTL operations that support either contextual retrieval or item recognition, two fundamental mnemonic operations in recognition memory. The data presented here clearly support a parsimonious view where the entire MTL supports recognition memory, but by different subprocesses accomplished by different substructures like the (peri)rhinal cortex and the hippocampus. While a reduction of activation in the rhinal cortex is sufficient for item recognition based on a familiarity signal, an activity increase in the hippocampus is essential for successful contextual retrieval and recollective memories.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2004

Author dc:creator

• Weis, Christine Susanne

Contributors dc:contributor

• Willmes-von Hinckeldey, Klaus

• info:eu-repo/classification/ddc/610
• Medizin
• Declarative Memory
• event-related fMRI
• Recollection
• Familiarity
• Hippocampus
• Source Memory

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-121501%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59741