During the first half of the twentieth century, the American psychologist Karl Lashley conducted a series of experiments in an attempt to identify the part of the brain in which memories are stored. In his now famous investigations, Lashley trained rats to find their way through a maze, then tried to erase the memory trace – what he called the “engram” – by making lesions in different parts of the neocortex.
Lashley failed to find the engram – no matter where he made a lesion, his experimental animals were still able to find their way through the maze. As a result, he concluded that memories are not stored in a discrete area of the brain, but rather were distributed throughout it. This led Lashley to devise his principle of Mass Action, which states that most behaviours involve the integrated actions of the whole cerebral cortex.
Subsequently, the pioneering work of Brenda Milner, who worked with the amnesic patient known as H.M., implicated the hippocampus as being crucial for memory formation. We now know that the frontal cortex is also involved, and it is currently thought that new memories are transferred to there for long-term storage. A new study now provides some evidence that memory recall becomes increasingly dependent upon the frontal cortex, and other areas of the brain, with time.
Christine Smith and veteran memory researcher Larry Squire assessed the brain activity associated with the recollection of old and new memories. They recruited 15 healthy male participants, and used functional magnetic resonance imaging (fMRI) to scan their brains while they answered 160 questions about news events that took place at different periods of time during the past 30 years.
The study posed a number of difficulties which could have confounded the results. Firstly, the task of retrieving a memory will inevitably result in the encoding of the questions that were asked and the resulting recollection, and the associated brain activity could interfere with that which is being assessed. Secondly, more recent memories are likely to be richer and more vivid than older ones, and therefore the strength of the fMRI signal could be related not just to the time at which a recalled event occurred but also to the richness of the recollection of it. Finally, some of the recalled memories may be associated with personal events, which could make them easier to remember.
Smith and Squire therefore took these factors into account, and designed their experiments in such a way that the effects of the age of a memory could be assessed independently of both the encoding of the test questions and richness of the recollection of the memory. In the first phase of the task, the blocks of questions about events in each time period were presented randomly, and the participants were asked to indicate whether or not they knew the answer. Five to ten minutes later, whilst still in the scanner, the participants were asked three questions about each news item – they were asked to remember the original question they had been asked about the event (to assess how well they had encoded the information), the answer to that question (to assess the accuracy of recall) and, finally, how much they knew about each of the events (to assess the richness of each memory).
The participants’ ability to recall a given news event generally 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. It was 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.
With these variables removed from the equation, the researchers then analysed the fMRI data from all the questions that had been answered correctly. This analysis showed that medial temporal lobe structures (the hippocampus and amygdala) exhibited gradually decreasing activity as the participants recalled progressively older memories. This 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. By contrast, areas of the frontal, parietal and lateral temporal lobes exhibited the opposite pattern – their activity increased with the age of the news event being recalled, but remained constant during the recollection of more recent events – and the ability to recall the questions asked about each event was associated with activity in the areas surrounding the hippocampus, but not the hippoccampus itself.
This study provides anatomical and functional evidence which supports the findings obtained from brain-damaged patients with memory impairments. Patients with lesions in the hippocampus on both sides of the brain (such as H.M., who died late last year) not only lose the ability to form new memories, but also lose memories for events which occurred in the years preceding the onset of their amnesia. The memories of events that took place in the distant place remain intact, while those that occurred at intermediate times are lost in a graded manner. Thus, with time, the hippocampus becomes less important for a given memory, and the frontal cortex more so. Encoding of memories in the frontal cortex is more complex, and involves a widely distributed network with a greater number of connections, perhaps because retrieving older memories requires stronger associations and increased effort. Lashley, then, was not right about the memory engram, but nor was he completely wrong.
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Smith, C. N. & Squire, L. R. (2009). Medial Temporal Lobe Activity during Retrieval of Semantic Memory Is Related to the Age of the Memory. J. Neurosci. 29: 930-938. DOI: 10.1523/JNEUROSCI.4545-08.2009.