Thesis defense Bonnie van Geldorp (Donders Series 150)

25 April 2014

Promotor: Prof.dr. R. Kessels

The long and the short of memory: Neuropsychological studies on the interaction of working memory and long-term memory formation

The medial temporal lobe (MTL) is well known for its role in long-term memory, but its role in working memory is less clear. Recent studies suggested that the MTL is specifically involved in working memory binding tasks. Binding refers to the process of integration and maintenance of visual features for a brief period of time. In this thesis, I examined three possibilities concerning the role of the MTL in working memory binding, using different paradigms and patient samples.

First, the MTL may be involved in working memory binding tasks because this brain structure is involved in relational memory in general. Second, MTL activity during working memory tasks may reflect incidental long-term memory encoding processes. Third, MTL activity is recruited when long-term memory processes are needed for additional support in processing information that exceeds working memory capacity.

Most studies in the present thesis reveal working memory binding deficits in amnestic patients. However, this does not suffice to argue that it is merely the relational aspect that results in the involvement of the MTL. The second hypothesis also grants little support. The evidence in my thesis mainly favors the third hypothesis concerning the overload of working memory capacity. Working memory capacity can be exceeded when the task involves an associative component, but also when the delay period between stimulus and probe is extended. In these cases long-term memory processes support task performance. Whereas healthy participants can benefit from supporting long-term memory processes, patients with MTL dysfunction cannot, which is why they fail working memory binding tasks. However, these supporting long-term memory processes do not necessarily lead to improved episodic memory performance.

This thesis does not only have implications for theoretical models of memory. Clinical implications can be drawn as well. More effort should be undertaken to examine working memory problems in patients with MTL dysfunction. More complex working memory tasks should be used in order to detect working memory deficits, as it is apparent from my studies that these deficits only arise when working memory capacity is exceeded.