The hippocampus is the area of the mammalian brain most closely associated with memory, particularly spatial memory, meaning internal maps. It is common for individuals with hippocampal lesions (and surrounding related regions) to have amnesia, as in the famous case of H.M., and more recently E.P. Also, it has been shown that London cab-drivers, who presumably require large internal maps for navigation, have enlarged hippocampi1. However, it is has become clear that the hippocampus isn't required for all forms of memory. A set of looming questions, then, is: where memories are stored, how are they formed, and what role various structures in the brain play in these activities.
I attended a seminar (3/26/08) given by Larry Squire, who has been studying the role of the hippocampus in memory formation, retrieval and storage for quite some time. He summarized results comparing normal patients to those with hippocampal lesions. In general, it seems as though this structure is required for the formation of certain types of new memories only, not for recall or storage, since the lesioned patients had no trouble remembering thigns from their past (including how to navigate their childhood neighborhoods). However, the truly fascinating result he presented was from an experiment designed to reveal what role the hippocampus plays in building new memories.
The figure above summarizes the data gathered from normal and hippocampal-lesion patients during a memory task. The task was quite straightforward, subjects were presented with 8 pairs of objects, one pair at a time, in which one of the pair was always "correct." In a given trial, the subject was presented with a pair, and chose one by reaching out and picking it up. On the underside of each object was a sticker that either read "correct," or "incorrect." As represented in the left panel of the figure above, normal humans were able to reach perfect performance in this task by repeated presentation of these pairs3. In addition, these subjects were easily able to cope with a variation on the task. All 16 objects were presented at once, and the subject was asked to separate out the correct from the incorrect items (indicated by the grey bar to the right of the trace).
The hippocampal-lesion patients, however, showed dramatically different results. They required 12 times as much training, didn't get up to the same level of performance as the normal subjects, and were unable to perform the task-variant. This is to say nothing of the fact that they didn't recall having ever attempting the task before when queried on the 2nd through 36th sessions.
It is fascinating that these patients were able to learn this task at all, and it is clear that they are using some completely different strategy from the normal subjects (one which relies heavily on the paired-object context, as revealed by their confusion at being presented with all the objects at once). There are several questions that are raised by this research. First, if there is an alternative pathway for learning such tasks, how does the brain choose to use the hippocampal path to record a particular type of memory? One might suggest that the brain uses the hippocampus for all memories unless it isn't useable as in these patients, but we know that many types of motor learning, learning to play the piano for instance, do not require the hippocampus. A further set of interrelated questions are where and how the memory is being stored, and how these differ from those patients with intact hippocampi.
This type of research shows us definitively that we have the capacity for different types of memory, and that we have a ways to go in understanding how and where it operates.
Notes & References:
1. Maguire EA, Frackowiak RS, Frith CD. (1997) Recalling routes around london: activation of the right hippocampus in taxi drivers. J Neurosci; 17(18):7103-10.
2. Bayley PJ, Frascino JC, Squire LR. (2005) Robust habit learning in the absence of awareness and independent of the medial temporal lobe. Nature; 436(7050):550-3.
3. A "session" in this experiment consisted of 40 trials, 5 presentations of each of the 8 pairs of objects.
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