Differential effects of memory encoding : differentiation between perceptual and semantic processing

Abstract

In this thesis, the neural mechanisms of perceptual and semantic encoding of younger, older adults and people with mild cognitive impairment (MCI) were investigated using subsequent memory effect (SME). The SMEs were taken from ERPs of subsequently correctly identified trials and missed trials in study-test intentional recognition experiments. Study 1: This study investigated the differentiation between subsequent memory effects associated with successful encoding in perceptual and semantic encoding tasks. Single Chinese characters were used in both encoding tasks. Seventeen healthy young participants (6 male) participated in the study. Their mean age was 21.5±2.3 (range 19 to 28) years and they received an average 14.7±1.1 (range 13 to 16) years of education. Participants first learnt names of objects represented by single Chinese characters in two encoding tasks: inspecting their orthographic components (perceptual) and deciding the objects can or cannot make sounds (semantic). After each encoding task, participants performed a recognition task. The recognition task involved the participants to indicate whether the single characters shown were studied or unstudied. Results showed that the participants were significantly more accurate (in terms of d-prime measure) in identifying the studied characters in the semantic than perceptual condition (1.97±0.58 vs. 2.30±0.71, p<0.05) while there was no difference in reaction times. In both conditions, the SMEs were more positive-going waveforms for the correctly identified trials than missed trials and significant in windows between 120 and 240 ms (P2), 240 and 360 ms (N3), 360 and 700 ms (P550), and 700 and 1000 ms (late positive component; LPC). SME of P2 has been suggested to be related to working memory or attentional operations, SME of N3 to conceptual processings, and SME of P550 and LPC to elaborative processings. Their LPCs, rather than earlier components, were found to moderately correlate with the performances (accuracy) on the task. Semantically-learnt characters elicited LPC with significantly greater amplitudes over the frontal pole regions; Perceptually-learnt elicited larger SME in the central sites. The significant SME in the frontal regions in both conditions suggests that the coordination and incorporation of the orthographic information into mental representation was essential to both task conditions. While larger central effect in the perceptual condition may indicate perceptual processing that enhance memorabiltiy of an item, underlying processes expressed by frontal pole SME as shown in the semantic condition might be important for strategic operations that can further enhance encoding. Differences in SMEs of P2 and N3 between conditions were also found to be significant. Probable explanations for these SMEs were discussed. Study 2: Elderly adults may show significant SMEs with both perceptual and semantic type of encoding tasks but previous findings were mainly drawn from investigations in which the two tasks were not evaluated together. Simultaneous assessment of both tasks in the elderly have only been conducted by Friedman and colleagues (1996) using atypical stem completion and cue recall tests at retrieval. Stem completion is considered an indirect memory test and their behavioural results from cued recall test suggested older participants might have used an implicit retrieval strategy, which may also explain why they failed to find reliable SMEs in older adults when learning perceptually and semantically. Therefore, this study adopted the similar paradigm established in Study 1 and investigated age-related SME difference in performing perceptual and semantic tasks with a more appropriate recognition test. Seventeen healthy young adults from Study 1 and 17 healthy elderly adults (HEA; 10 male) were included in the study. HEA's mean age were 65.3±4.9 (range 58 to 76) years and they received an average 8.9±4.0 (range 3 to 21) years of education. Results of the HEA were largely consistent with previous findings that used a recognition test. In this study, it was found that younger adults performed better than the elderly in both conditions. Semantic orientation helped older adults to perform better than a perceptual task as indicated by d’ measure (0.75±0.42 vs. 1.34±0.71, p=0.004). Significant SME were found in P2 and N3 windows in both conditions and P550 window in the semantic condition in the HEA. SME in LPC was not significant but correlation analysis still indicated positive correlation between SME amplitude and d’ measure in each condition. When semantic processing was required, ERP differences in earlier windows (P2, N3, and P550) were less evident between the younger adult and HEA groups. It is not till the LPC window that more definite age-related differences were found. In the perceptual condition, however, differences were already shown at P550 window, suggesting older people might have more difficulty performing perceptual processing to result in successful encoding. Standardized low-resolution brain electromagnetic tomography (sLORETA) images and topography analysis suggested that, in the perceptual condition, both groups seemed to rely on similar frontal-parietal networks. However, the older group's difficulty in perceptual encoding may be due to a failure to recruit further PFC in the LPC window. In the semantic condition, it appeared that older adults might have inefficient frontal-temporal network for encoding information. This may also be because a compensatory mechanism was not sufficient to equalise memory performance. Study 3: This study was conducted to understand how MCI sufferers process information perceptually and semantically and how they differ from normal aging using SME as an index. Seventeen HEA from Study 2 and ten participants (6 male) who were diagnosed with MCI based on the research diagnostic criteria in Gauthier and colleagues (2006) were included in the study. MCI participants' mean age were 72.9±3.8 (range 68 to 80) years and they received an average 8.8±5.0 (range 2 to 16) years of education. The same paradigm as in Study 2 was used. Behaviourally, it was found that MCI adult's recognition performance did not improve in the semantic condition. Analyses of ERPs revealed significant SME of P2 and N3 in perceptual conditions and SME of N3 in semantic condition in the MCI sample. These results suggested they might still be sensitive to early processes that influence successful encoding. In addition, SME of LPC in MCI groups was significant and present in the left hemisphere though the pattern was negative-going. Group differences between HEA and MCI in the SME of P550 and LPC were found in the semantic condition and in the SME of LPC in the perceptual condition. These differences were characterized by negative SMEs in the left frontal and central sites in the LPC in the perceptual condition; negative SMEs in the left hemisphere in the P550 window and in central sites in the LPC window of the semantic condition. Average SME amplitude from these sites showing group variation, however, did not correlate with behavioural performance in either group. This suggested other processes that may indirectly influence memory encoding differed between the two groups. The site (CPz) that showed strongest correlation with performance was the same in LPC in both groups in the perceptual condition. Interestingly, the correlation was positive in the HEA group and was negative in the MCI group. Together with the finding that negative-going SMEs were present in the left hemisphere during LPC in this MCI sample, the results may indicate MCI sufferers perform qualitatively different perceptual operations during this window than HEA. In the semantic condition, sites (F7, T7, and C3) showing strongest positive correlation with performance was largely the same in windows of P550 and LPC in HEA. In the MCI group, such correlation was not found. Together with the findings from group difference in SME, results indicated possible widespread cerebral dysfunctions in MCI when the task required semantic processing in an intentional context. Dysfunctions may arise from associative memory and difficulty in binding elements that constitute an episode. General Conclusions: The processes of encoding undertaking a perceptual and a semantic task in healthy young sample were revealed. Age-related changes in perceptual and semantic processing were identified. Changes in perceptual processing appeared to be more significant. Semantic cues would still help HEA to perform better in a recognition test. This was probably because they were still able to use alternative routes, though not very effectively, as a compensatory measure. Their difficulty in perceptual learning may arise from inefficient underlying neural sources or a failure to further recruit certain generators such as frontal brain regions. Both perceptual and semantic processes contributing to memory encoding appeared to have significant changes in MCI sufferers. They appeared to perform qualitatively different operations in perceptual encoding. MCI adults appeared to be able to access semantic memory because the task accuracy were similar in the study phase, but their memory performance under semantic condition was not able to be improved because of this. Their difficulty might arise from associative memory deficits.