Memory impairment depends on where the brain is damaged
Tuesday 4 November 2008
Scientists have discovered that loss of memories that hold details of a particular event, time or place could be caused by damage to a part of the brain called the fornix. Researchers from the Universities of Manchester, Cardiff and Liverpool made the link by studying people who had surgery to remove a benign brain tumour. The results are published in Nature Neuroscience.
The exact mechanisms of memory storage and retrieval are not well understood but it is now clear that people with fornix damage experience difficulties in forming new memories. This is due to the role of the fornix in relaying messages originating in the hippocampus - an area central to memory functions - to neighbouring structures such as the anterior thalamus and the mammillary bodies, which have also been implicated in memory. To try to determine the role of the fornix in memory retrieval researchers compared brain function in 38 people who had previously had surgery to remove a benign tumour called a colloid cyst which grows near to the fornix.
Dr Dimitris Tsivilis, one of the scientists involved in this research at the University of Manchester, explains: ‘‘A number of individuals who have had colloid cysts removed from their brain complain of forgetfulness, which in some of them can be quite severe. It has long been suspected that this is caused by damage to the fornix. The aim of this research was to establish the link between fornix damage and the extent and quality of these patients’ memory loss. We were also interested to examine whether other memory-related areas could have been affected in these people.”
Each participant completed a series of psychometric tests to assess their intellectual and memory abilities and detailed MRI scans were used to capture images of the fornix, the hippocampus, the mammillary bodies and other areas known to be involved in memory. The MRI scans allowed the researchers to assess the volume of all these different structures making it possible to relate psychometric test performance to varying degrees of tissue damage.
Dr Tsivilis says: ‘‘The MRI scans revealed damage to a number of structures including the fornix, as we expected, but also the mammillary bodies which we believe become atrophied as a direct result of fornix damage. What we were also able to find was that the degree of memory impairment was closely related to mammillary and fornix volume size with worse performance associated with smaller volumes. More importantly though, this only affected patients’ recall memory, that is the ability to reconstruct a past event with as many details as possible. By contrast the damage did not affect patients’ recognition memory that is the ability to judge whether a specific event has been experienced before.’’
‘‘For example, patients with damage in the fornix and mammillary bodies would have had difficulty remembering that they had porridge for breakfast the day before but asked to choose between porridge or cereal they would have confidently made the right choice. Whereas the first type of memory relies heavily on recollection of contextual details such as what, where and when and is clearly impaired by fornix damage the latter can also be based on a feeling of familiarity that appears to be preserved in these patients.’’
Dr Tsivilis concludes: ‘‘There is still a long way to go to learn which parts of the brain are linked to different kinds of amnesia but this research has helped to demonstrate that recognition memories are less affected by fornix damage than memories of particular episodes in a person’s history.’’
Original research paper: A disproportionate role for the fornix and mammillary bodies in recall versus recognition memory is published in Nature Neuroscience.
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