The ERP correlates of self-knowledge in ageing

Abstract

Self-knowledge is a type of personal semantic knowledge that concerns one's self-image and personal identity. It has most often been operationalized as the summary of one's personality traits ("I am a stubborn person"). Interestingly, recent studies have revealed that the neural correlates of self-knowledge can be dissociated from those of general semantic and episodic memory in young adults. However, studies of "dedifferentiation" or loss of distinctiveness of neural representations in ageing suggest that the neural correlates of self-knowledge might be less distinct from those of semantic and episodic memory in older adults. We investigated this question in an event-related potential (ERP) study with 28 young and 26 older adults while they categorised personality traits for their self-relevance (self-knowledge conditions), and their relevance to certain groups of people (general semantic condition). Participants then performed a recognition test for previously seen traits (episodic condition). The amplitude of the late positive component (LPC), associated with episodic recollection processes, differentiated the self-knowledge, general semantic, and episodic conditions in young adults, but not in older adults. However, in older adults, participants with higher composite episodic memory scores had more differentiated LPC amplitudes across experimental conditions. Moreover, consistent with the fact that age-related neural dedifferentiation may be material and region specific, in both age groups some differences between memory types were observed for the N400 component, associated with semantic processing. Taken together, these findings suggest that declarative memory subtypes are less distinct in ageing, but that the amount of differentiation varies with episodic memory function.

Keywords: Ageing; Episodic memory; Event-related potentials; Semantic memory.

Fig. 1

Mean RTs for positive traits (left; a, c) and negative traits (right; b, d) for young adults (top row; a, b) and older adults (bottom row; c, d) for yes and no responses in each memory condition. Error bars represent ± 1 SE. a Mean RTs for positive traits for young adults. b Mean RTs for negative traits for young adults. c Mean RTs for positive traits for older adults. d Mean RTs for negative traits for older adults. SM = semantic memory; SK = self-knowledge

Fig. 2

Mean percentage of yes responses for young adults (left) and older adults (right) for positive and negative traits in each memory condition; semantic memory (SM), past self-knowledge (past SK), present self-knowledge (present SK), and future self-knowledge (future SK). Error bars represent ± 1 SE

Fig. 3

Mean RTs for positive and negative traits for hits and correct rejections. Error bars represent ±1 SE

Fig. 4

Mean accuracy in percentage for young adults (left) and older adults (right) for positive and negative traits for hits and correct rejections. Error bars represent ±1 SE

Fig. 5

Grand average group ERPs for young (A1, B1, C1, D1; N = 28) and older adults (A2, B2, C2, D2; N = 26) of yes responses for semantic memory, personal semantics (past, present and future self-knowledge) and episodic memory (all confidence hits), over a frontal, b para-sagittal, c sagittal, and d posterior parietal sites. Negative voltage is plotted upwards. A low-pass filter of 20 Hz was applied on the grand average group data

Fig. 6

Grand average group ERPs (N = 54) of yes responses for semantic memory, personal semantics (past, present and future self-knowledge) and episodic memory (all confidence hits) for all participants, over a frontal, b para-sagittal, c sagittal, and d posterior parietal sites. Negative voltage is plotted upwards. A low pass filter of 20 Hz was applied on the grand average group data

Fig. 7

Isovoltage scalp maps for the N400 time window (250 ms to 500 ms) for young (top row) and older adults (bottom row). Left: Scalp maps of semantic memory (SM, yes responses) minus hits (EM). Right: Semantic memory (SM, yes responses) minus the average of all self-knowledge conditions (SK, yes responses). Scalp maps were prepared in EEGLAB (Delorme & Makeig, 2004)

Fig. 8

Isovoltage scalp maps for the LPC time window (500 ms to 800 ms) in young (top row) and older adults (bottom row). Left: Scalp maps of hits (EM) minus semantic memory (SM, yes responses). Right: hits (EM) minus the average of all self-knowledge conditions (SK, yes responses). Scalp maps were prepared in EEGLAB (Delorme & Makeig, 2004)

Fig. 9

Grand-averaged ERPs of hits and correct rejections for older (N = 26) and young (N = 28) adults separately at posterior parietal sites. Negative voltage is plotted upward. Grand-averages were low-pass filtered at 20 Hz

Fig. 10

Isovoltage scalp maps in the LPC time window (500 ms to 800 ms) for the old–new effect (hits minus correct rejections) for young adults at the left, and older adults at the right. Scalp maps were prepared in EEGLAB (Delorme & Makeig, 2004)

Fig. 11

Scatter plot of the correlation between the memory composite score (based on logical memory and Rey–Osterrieth complex figure delayed recall performance) and the LPC time difference (mean of future minus present and past minus present) in older adults, including the trend line (solid black line)