Functional magnetic resonance imaging of source versus fact memory
Episodic memory concerns our memory for autobiographical events. One of the most interesting questions in research on episodic memory is the difference between fact memory and source memory. Fact (item) memory is the memory of what happened, whereas source memory is the memory of the context of the event (e.g., when, where, who was involved, what sensory characteristics accompanied the item or fact). The aims of this research are to determine whether there are different brain areas which underlie these two types of memory. Four experiments were conducted using the functional magnetic resonance imaging (fMRI) technology in an attempt to separate the memorial process of source from the memorial process of fact. Experiment 1 used a visual cue as the source of an item (a picture drawn either in red or in green) and Experiment 2 used an auditory cue as the source of an item (a picture accompanied by either a male or a female voice pronouncing its name). In Experiments 1 and 2, two conditions were contrasted: one in which only item memory was required and one in which only source memory was required. Experiments 3 and 4 were factorial designs using the same a visual cue and the same auditory cue respectively. In Experiments 3 and 4, two additional conditions were added. The first required neither item nor source retrieval and the second required both item and source memory retrieval. Statistical parametric mapping was carried out to localize brain activities associated with memory functions. Results showed that the right prefrontal lobe is related to episodic memory retrieval. However, there was increased activation in both left frontal and hippocampal regions for source memory retrieval compared to item memory retrieval. A left-frontal lateralization for source encoding and retrieval was found. A structural model was constructed and path analysis was used to assess the functional connectivity between hippocampal and prefrontal regions. The model showed interaction between prefrontal and hippocampal regions during source memory retrieval. Our results demonstrated that the human prefrontal and hippocampal regions are important for episodic memory encoding and retrieval.