The featured paper is ‘Spatial Navigation Deficit in Amnestic Mild Cognitive Impairment’ by Hort and colleagues and freely available here.
This study is about the relationship between spatial maps and mild cognitive impairment. While it isn’t discussed in introduction in the paper (although it is mentioned briefly in the discussion section) one of the theoretical dialogues that is quite prominent in this area is the role of the Hippocampus. The Hippocampus is implicated in spatial memory and there is a body of supporting evidence for the Hippocampus being the location for a cognitive map of the environment. When finding our way about, we can do this by looking at patterns in the environment – which is termed allocentric mapping or we can do this by working out how the environment relates to us – through distances and angles – egocentric mapping. There is good evidence for the role of the Hippocampus in the formation of allocentric maps (and also evidence for a role in egocentric memory) and so it seems sensible to look at how spatial mapping is affected in amnestic mild cognitive impairment (MCI) which can precede dementia as the volume of the hippocampus is one of the successful predictors of conversion from MCI to dementia. A more detailed understanding of a cognitive map of this nature has resulted in the identification of grid and place cells which have been covered previously. In the article, the authors focus mainly on the evidence for cognitive mapping impairments in dementia and the absence of such evidence in MCI in the background as the main justification for the study.
Subjects were recruited from a memory disorders service in Prague and underwent full examination, MRI and a battery of neuropsychological tests including MMSE, ADL, Benton’s Visual Retention Task, Trail Making Tasks and the Rey complex figure task. Subjects were divided into four categories – subjective memory impairment, DSM-IV dementia and Petersen’s Criteria defined Mild Cognitive Impairment and a control group. The MCI group were further subdivided into a pure amnestic group, amnestic + other impairments and a non-amnestic group.
Amnestic Mild Cognitive Impairment was defined by a greater than 1.5 standard deviation below average on either verbal or non-verbal memory tasks. The difficulty I have with this, and it is part of the criteria for MCI, is that we do not know if this is a persistent objective impairment. If it was repeated after a few months perhaps there may be a different result and this may be more likely as the impairment is on a very specific test and not occurring in the context of a global impairment. Even with global impairments, these may be fluctuating as in the case of delirium. Additionally there a number of other factors that can influence the test and repeating the test at another point in time would reduce the likelihood of confounding. The subjective and collateral history in the MCI criteria are important arguments against this although there is evidence for spontaneous remission of MCI in a significant proportion of cases (50%) of Vascular Cognitive Impairment Non-dementia for example.
A ‘Hidden Goal Task’ was used to test spatial memory. Essentially the point of interest was shown graphically to the subjects using either points around the circle (allocentric) or in relation to their starting position (egocentric) in real space and virtual environments. Outcome was measured using both approximations and also precise distances from goal.
The distance errors on the ego and allocentric tasks are shown in a graph. The trends in the graph are fairly clear for some of the groups. Thus the Alzheimer’s Disease group performs worst on all tasks followed by the amnestic MCI with other impairments (which I will refer to as aMCI+) and then the amnestic MCI and finally the non-amnestic MCI (with one exception for this group). However not all of these differences were significant. Additionally in the graph, the subjective memory complaint group and the control group performed similarly with the former group doing better in some of the tests (the confidence levels were included on the chart although it was difficult to gauge crossover of means visually).
In terms of significant results – the Alzheimer’s Disease group was significantly impaired with little evidence of learning on the tasks and they performed significantly worse than the other groups on the first trials of the tasks which involved using previously learnt information. The aMCI group performed significantly worse than the control group (1.5 fold) when there were only two cues for orientation. The aMCI+ group performed similarly to the AD group in delayed recall but in the non-delayed tasks did significnatly better. On the first trials, as in the AD group, the aMCI+ group showed ‘general impairment’. There were also interesting findings with regards to the navigational strategies with aMCI group doing poorly in identifying the correct side of the cue in the circle environment. What was also interesting from the circle diagrams showing identified targets was that on moving from the control group through the non-aMCI, aMCI and aMCI+ there was a diffusion of population targets – so that they became progressively less densely focused around the correct target.
What would have been quite interesting would be to see the hippocampal volumes as MRI scans were undertaken on the subjects in the study. A reasonable prediction would be that the hippocampal volume correlated with distance errors although the ideal null hypothesis for an experiment would involve premorbid hippocampal volumes for comparison – using differential volumes instead. Then we might predicted that a larger decrease predicted a larger error and as we know from other studies, the volume itself would be useful in predicting conversion.
The authors conclude that aMCI probably represents an earlier stage than aMCI+ in the disease process and that spatial navigation measures may provide a useful marker of disease progress in the early stages. This is an interesting study which contributes a possible explanation to previously identified predictors of conversion from MCI to AD and may itself be useful as a predictive marker if confirmed by further research.
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