The neural and functional basis of prospective memory

Prospective memory is memory for future actions or intentions, and involves the cognitive processes and skills required to perform a planned activity at some point in the future, without any form of explicit prompting. Event-based prospective memory requires the execution of an intended activity when a certain external event occurs. Time-based prospective memory involves remembering to perform an activity at a particular point in time. The research reported in the present doctoral project is based on the frontal lobe hypothesis of prospective memory that “at least some of the processes that characterise prospective memory (...) depend on the integrity of the frontal lobes, but the extent to which the frontal lobes are involved in any particular prospective memory task will depend on the characteristics of the task.” (Glisky, 1996, p. 251). In earlier research conducted at the Laboratory of Experimental Psychology the evidence for the role of prefrontal cortex in prospective memory was not always unequivocal. Therefore, the present research project focuses on two core assumptions of the frontal lobe hypothesis. On the one hand behavioural research is used to examine to what extent central executive functions play a role in prospective memory. On the other hand functional neuroimaging experiments are carried out to directly study neural activity during prospective memory performance. First, three pilot studies were carried out in order to investigate the possible influence of procedural changes to the standard prospective memory paradigm, due to practical and methodological constraints imposed by (event-related) fMRI. Results show that increased target frequencies and shorter task duration appear to have minimal influence on prospective memory performance, and that the degree of controlled, strategic processes is equal under all conditions. Next, two event-related fMRI experiments are reported that aim to separate neural activity associated with retention and retrieval processes in event-based prospective memory, and to compare this with corresponding processes in an ongoing task, a vigilance task, and a dual task. Overall, increases in neural activity associated with prospective memory were found bilaterally in occipital cortex (mainly in lingual gyrus, BA 18) and superior and inferior parietal cortex (BA 7 and BA 40), in right dorsolateral prefrontal cortex (BA 9), and in left lateral prefrontal cortex (BA 46). Interpretation of the functional role of these brain regions is still tentative at this stage of the analysis. Left lateral prefrontal cortex appears to be associated with target monitoring, whereas right dorsolateral prefrontal cortex is implicated in more sustained processes such as intention retention or retrieval mode maintenance. The parietal activity is presumably associated with visual search processes and the interaction between visual information and motor commands, while the visual cortex activity possibly reflects a low-level visual attentional (target monitoring) system where the visual input is matched with the representation of the relevant target features. In the final part of this manuscript two behavioural experiments are described that study the role of the central executive in event-based and time-based prospective memory, focusing on the contribution of three core executive components (inhibition, updating, mental shift). The results suggest a role for cognitive flexibility and inhibition in event-based prospective memory, whereas time-based prospective memory benefits from efficient time-monitoring and updating. Future research will try to integrate the research lines described above by constructing an experimental procedure that can be used in a functional neuroimaging experiment, as well as in a behavioural study in order to investigate central executive correlates.status: publishe