Review: The Medial Temporal Lobe and Recognition Memory

The paper reviewed here is ‘The Medial Temporal Lobe and Recognition Memory’ by Eichenbaum and Ranganath and freely available here.

Firstly the paper has been published in the ‘Annual Review of Neuroscience’. This is a journal with quite a high impact factor – for instance we can see here that the impact factor in 2003 was 30.17.  Of course, a journal can’t be judged by the impact factor alone which is influenced by a great many factors. For instance journals featuring a high number of review articles are said to have higher impact factors because review articles are more likely to be cited than original research articles. Additionally there should be some fluctuation in the quality of articles within a journal. Nevertheless it can be a useful marker for the popularity of the journal.

The authors here succeed in collecting together a vast amount of evidence from multiple areas – fMRI, neuropsychology, neuroanatomical and event-related potential studies to name just a few. Without going into too much detail, when I read the paper, coming away I found the central hypothesis about two streams of processing quite convincing. In essence what the authors argue is that there is a ‘what’ and a ‘where’ stream of processing in the Medial Temporal Lobe they integrate with the two distinct psychological phenomenon of recognition/familiarity and recollection. The ‘what’ stream relates to objects in the world and the ‘where’ contextualises these objects. The authors go on to argue that the parahippocampal gyrus is responsible for recognition and the hippocampus for recollection – by associating the ‘what’ information with information from cues. The model is a little bit more sophisticated than this however and the parahippocampal gyrus gets divided into anterior and posterior portions for the more subtle aspects of the model they present which also include the frontal and parietal cortices.

There is a lot of material here for reflection on and the MTL is an incredibly important structure for Alzheimer’s Disease (AD). There is a great deal of research that shows that the MTL volume is a predictor of cognitive decline and useful for instance in discriminating between people with Mild Cognitive Impairment who might go on to develop Alzheimer’s Disease and those who do not. There are still great difficulties in making these kind of predictions but a number of studies repeatedly show that more specifically the volume of the hippocampus is a very powerful predictor of cognitive performance and conversion to AD.

So the next question is whether or not routine neuropsychological tests could be used to work out the ratio of parahippocampal/hippocampal involvement in pathology such as AD. Having such a ratio if possible might be useful in distinguishing between different forms of dementia – for instance Frontotemporal Dementia versus Frontal Variant AD although at this stage this is very speculative. What’s great about papers like this though is that they can provide a starting point for working out potentially useful clinical applications of contemporary theoretical understanding.

Another nice feature of this paper is the discussion of recognition and recollection. I hadn’t really thought about this too much until reading the paper. The authors have provided convincing evidence not just for a distinction between the two but also for the further division of these constructs into further categories. So the way I interpreted it was as follows. If I see a bus stop as i’m walking down the street, I will almost immediately recognise it. It will be familiar to me. However it should take me a little more time to associate it with waiting for a bus. It’s hard for me to imagine that the latter would not be instantaneous but the authors have convinced me that it would be. Further, the feeling of familiarity would result from activity within the parahippocampal gyrus.

This also raises another interesting question – where am I experiencing the ‘familiarity’? Do I really feel familiarity because of the parahippocampal gyrus activity?  Now this is a fairly interesting question because of another phenomenon which will surely be familiar to the reader (no pun intended) – ‘deja vu’ – the feeling of having had an experience before. Classically associated with temporal lobe epilepsy, can we extrapolate and suggest that this would be the experience of parahippocampal activity? In other words, what does it feel like when we have activity in the parahippocampal gyrus – it feels like deja vu? I’m afraid I don’t have the answer to this – it remains as speculation but another example of the kind of interesting clinical ramifications that arise out of a discussion of the MTL.

The latter point about the parahippocampal gyrus activity experience is related to a more subtle point. If the hippocampus stores memories, when we experience those memories are we experiencing them in the hippocampus or is it activating the memories in interconnected areas e.g. the visual association cortex. This is quite an old question but a very good one nonetheless and the upshot of this is to ask whether or not it is ever sensible to inquire about the experience of activity in one area without mention of the other areas involved in a well recognised brain circuit. This in turn leads back to the question of whether we can talk about ‘what’ and ‘where’ streams in the hippocampus and parahippocampal gyrus without also including the other connected brain regions which might also be doing some of the processing.

This is a nice paper, very well explained and guiding the reader through all of the material needed to understand the authors’ conclusions. The material covered in the paper is also useful for informing clinical research questions.

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