In general, the participants' ability to recall any given news event decreased in relation to the amount of time that had passed since the event had occurred. As expected, they were better able to recollect more recent events than older ones. The researchers also found that the participants' memory of the questions they had been asked, and of the content of each news event, was independent of how long ago the events had occurred. The richness of the participants' memories was also unrelated to when a particular event occurred; the memories of events that occurred in the distant past were often as rich as those of more recent events.
In their analyses, the researchers used only those fMRI data from the questions that had been answered correctly. This data set showed that medial temporal lobe structures (the hippocampus and amygdala) exhibited gradually decreasing activity as the participants recalled progressively older memories. This drop in activity was true for memories of news events that occurred up to 12 years before, but the recollection of events that took place longer than 12 years was associated with a constant level of activity in those areas. The opposite activation pattern was observed in areas of the frontal, parietal and lateral temporal lobes: activity in these areas increased with the age of the news event being recalled, but remained constant during the recollection of more recent events.
File Cabinets in the Brain
This study therefore provides anatomical and functional evidence supporting the findings obtained from brain-damaged patients with memory impairments. Patients such as H.M., who have lesions in the hippocampus on both sides of the brain, not only lose the ability to form new memories, but also lose memories for events that occurred in the years preceding the onset of their amnesia. The memories of events that took place in the distant past remain intact, whereas those that occurred at intermediate times are lost in a graded manner. This finding suggests that, with time, the hippocampus becomes less important for a given memory, and the frontal cortex more so.
Lashley's theory of memory was not right, but neither was it completely wrong. Why, then, might old memories be transferred from the hippocampus to the frontal cortex? It may be because retrieving older memories requires stronger associations and increased effort—memory encoding in the frontal cortex is more complex than in the hippocampus, and involves a widely distributed network with a greater number of connections. The frontal cortex may therefore be better suited to the task of retrieving memories that were encoded in the distant past.
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His latest book is How We Decide.