Episodic simulation of future events is impaired in mild Alzheimer's disease

Abstract

Recent neuroimaging studies have demonstrated that both remembering the past and simulating the future activate a core neural network including the medial temporal lobes. Regions of this network, in particular the medial temporal lobes, are prime sites for amyloid deposition and are structurally and functionally compromised in Alzheimer's disease (AD). While we know some functions of this core network, specifically episodic autobiographical memory, are impaired in AD, no study has examined whether future episodic simulation is similarly impaired. We tested the ability of sixteen AD patients and sixteen age-matched controls to generate past and future autobiographical events using an adapted version of the Autobiographical Interview. Participants also generated five remote autobiographical memories from across the lifespan. Event transcriptions were segmented into distinct details, classified as either internal (episodic) or external (non-episodic). AD patients exhibited deficits in both remembering past events and simulating future events, generating fewer internal and external episodic details than healthy older controls. The internal and external detail scores were strongly correlated across past and future events, providing further evidence of the close linkages between the mental representations of past and future.

Figure 1

Mean recognition accuracy score for each decade (1940s to 1990s) for AD patients and healthy older controls on the three-alternative forced choice famous names recognition task.

Figure 2

Mean number of internal and external details generated for past and future events by patients with AD and healthy older controls. Error bars represent standard errors of the means.

Figure 3

Scatter plots and regression lines showing the correlations between the numbers of internal details in generated past and future events (top left, r = .60, p < .001), the numbers of external details in generated past and future events (top right, r = .64, p < .001), the numbers of internal and external details in generated past events (bottom left, r = -.13, p = .25), and the numbers of internal and external details in generated future events (bottom right; r = -.24, p = .10). Partialling out phonemic and semantic fluency abilities and gender did not change the overall pattern of correlations, except that the correction of future internal and external details approached significance (p = .06).

Figure 4

Mean number of internal (top panel) and external (bottom panel) details generated for remote past events by patients with AD and healthy older controls. Five lifetime periods were assessed: early childhood, up to 10 years old; teenage years, 11 to 18 years old; early adulthood, 19 to 35 years old; middle adulthood, 36 to 55 years old; recent adulthood, 55 to 65 years old. Error bars represent standard errors of the means.