Prospective memory, future thinking and sense of identity in ageing and Alzheimer’s disease

Responsible : Béatrice Desgranges

Figure 1 - Extract from the virtual city used to assess prospective memory - PIOLINO, Univ. Paris Descartes

Investigations of mental time travel, one of the key characteristics of episodic memory, have been incomplete up to now, with the emphasis on its retrospective component (recollections of the past), rather than on its prospective one. We intend to tackle the latter from two angles, distinguishing between the prospective memory (PM) that corresponds to memory for actions to be performed in the future and the ability to project oneself into the future and its links with a sense of identity. Prospective memory can be divided into event-based PM, where recall is triggered by the advent of a particular event, and time-based PM, where it is triggered after a specified time has elapsed. Previous studies of the possible impairment of PM in normal ageing have yielded inconclusive findings (Gonneaud J, Eustache F, Desgranges B, 2009. La mémoire prospective dans le vieillissement normal et la maladie d’Alzheimer : intérêts et limites des études actuelles. Rev Neuropsychol 1 : 238-246).

Although time-based tests (drawing more heavily on self-initiated processes) seem to be more sensitive to ageing than event-based ones, a study conducted in our laboratory, in which we controlled for the difficulty of both types of task, failed to replicate this result. Furthermore, our study suggested that the decline in PM stems from impairment of the executive processes underpinned by the frontal lobes, which are known to display the deleterious effects of ageing. This finding is in line with the results of the few studies that have sought to identify the neural bases of PM. Using mainly EEG and focusing on young individuals, these have pointed to the preferential involvement of the rostral prefrontal cortex, which is known to be ageing-sensitive (Kalpouzos G, Eustache F, Desgranges B, 2008. La mémoire prospective au cours du vieillissement : déclin ou préservation ? NPG Neurologie – Psychiatrie – Gériatrie 8 : 25-31). In AD, while results point to a major PM deficit, there remains considerable debate as to the stage at which this deficit emerges. Some authors claim that PM may be impaired earlier than retrospective memory, meaning that screening for it could lead to earlier diagnosis, providing a suitable tool can be developed – something that is currently lacking.

We intend to assess PM in its full complexity, looking at its underlying cognitive and brain processes, and tracking the modifications it undergoes in the course of normal ageing and in AD. Accordingly, we will recruit healthy individuals, a group of patients with mild cognitive impairment (MCI) and a group of patients with probable AD. All the patients and healthy participants aged more than 45 years will be reassessed 18 months after their inclusion, in order to check that the latter are still healthy and to identify those MCI patients who have gone on to develop AD, with a view to highlighting diagnostic markers for early detection.

All participants will undergo a protocol featuring an fMRI activation paradigm (PM test), anatomical examinations (diffusion MRI and aMRI) and a set of behavioural tests designed to assess retrospective episodic memory and executive functions. We will also make use of virtual reality technology, as it offers an opportunity to reproduce the conditions of everyday life (or very nearly so), all the while keeping control of events (Plancher et al., Psychol NeuroPsychiatr Vieil, 2008; 6: 7-22).

The ability to project oneself into the future and its links with sense of identity in AD will be the focus of the second half of this study. While memory disturbances in this disease are well established, the degree to which patients’ identity is affected is less clear. The few studies to have explored this area so far suggest that identity is preserved in the early and moderate stages of dementia, but not in the severe stage (Caddell & Clare, Clinical Psychology Review, 2010; 30: 113-126). However, they used a miscellany of methodologies, probably due to the fact that the very concept of identity remains ill-defined   it can mean many things to many people in cognitive psychology and neuropsychology. We have yet to pin down the mechanisms responsible for these modifications in identity. Investigations of healthy individuals indicate that identity relies not only on episodic memory, but on semantic memory, too (Piolino P, Desgranges B, Eustache F (2009). Episodic autobiographical memories over the course of time: cognitive, neuropsychological and neuroimaging findings. Neuropsychologia 47 : 2314-2329 ; Duval C, Eustache F, Piolino P (2007). Multidimensional Self, autobiographical memory and aging. Psychol NeuroPsychiatr Vieil 5 : 179-192 ; Duval C, Desgranges B, Eustache F, Piolino P (2009). Le Soi à la loupe des neurosciences cognitives. De la conscience de soi à la conscience de l’autre. Psychol NeuroPsychiatr Vieil 7 : 7-19). In AD, episodic memory is impaired, but semantic memory is relatively spared, at least as far as the moderate stage of dementia. It could therefore provide the foundation upon which a sense of identity – albeit less definite could rest. In the later stages of the disease, however, either the sense of self may be impaired or the patient’s cognitive abilities no longer allow him or her to express the persistence of identity. For this reason, we will carry out both episodic and semantic memory assessments of AD patients representing different stages of dementia and administer novel tests designed to assess their ability to project themselves into the future and probe their sense of identity.

People involved in the unit U1077 : Hervé Platel, Peggy Quinette, Marie-Loup Eustache

PhD : Julie Gonneaud

Collaborations : Pascale Piolino, University of Paris Descartes and Philippe Fleury, CIREVE Caen