Changes in pattern completion--a key mechanism to explain age-related recognition memory deficits?

Abstract

Accurate memory retrieval from partial or degraded input requires the reactivation of memory traces, a hippocampal mechanism termed pattern completion. Age-related changes in hippocampal integrity have been hypothesized to shift the balance of memory processes in favor of the retrieval of already stored information (pattern completion), to the detriment of encoding new events (pattern separation). Using a novel behavioral paradigm, we investigated the impact of cognitive aging (1) on recognition performance across different levels of stimulus completeness, and (2) on potential response biases. Participants were required to identify previously learned scenes among new ones. Additionally, all stimuli were presented in gradually masked versions to alter stimulus completeness. Both young and older adults performed increasingly poorly as the scenes became less complete, and this decline in performance was more pronounced in elderly participants indicative of a pattern completion deficit. Intriguingly, when novel scenes were shown, only the older adults showed an increased tendency to identify these as familiar scenes. In line with theoretical models, we argue that this reflects an age-related bias towards pattern completion.

Keywords: Cognitive aging; Hippocampus; Memory retrieval; Pattern completion bias; Recognition memory deficit.

Conflict of interest statement

The authors declare no competing financial interests.

Copyright © 2015 Elsevier Ltd. All rights reserved.

Figures

Fig. 1. Experimental design of the test phase.

Each stimulus was presented for 2 seconds each, followed by 2 self-paced forced choice tasks - stimulus identification and confidence rating. In a previous study phase, participants learned the 5 depicted stimuli (kitchen, bar, library, bedroom, diningroom; from Hollingworth & Henderson, 1998). Those were then mixed with 5 novel items and all 10 were randomly presented in complete or masked form as shown in the bottom panel; percentages reflect the amount of the image visible through the mask.

Fig. 2. Performance and bias measures.

Left, performance for both age groups, separately for learned and new stimuli for the 5 different levels of stimulus completeness (mean ± SE); right, bias measure (see section 2.4. for a detailed explanation) - difference in accuracy scores for learned minus new stimuli calculated separately for each participant (mean ± SE); positive values indicate a bias toward pattern completion, significant differences from 0 are indicated with * separately for each age group as indicated by color.

Fig. 3. Response distribution for new stimuli.

Left, responses are depicted over the 6 possible choice options (i.e. 'none of these' as correct rejections - CR, and the false alarms sorted according to frequency – FA 1-5; mean ± SE) showing that older adults chose one particular false response option most often (FA 1) rather than guess more overall, which would lead to similar frequencies for all 5 response options. Right, distributions of false alarms are depicted for 2 exemplary stimuli per actual false response option (i.e. label of the learned stimuli; mean ± SE).

Fig. 4. Confidence ratings.

Scores for both age groups, separately for learned and new stimuli for the 5 different levels of stimulus completeness (mean ± SE). Ratings ranged from 1 (‘not at all confident’) to 5 (‘very confident’). Scores are depicted in dark gray for young adults, and in light gray for older adults; dashed lines indicate new stimuli, solid bars represent learned stimuli.