|Tang, Kin Chung
|Ageing effect on visual semantic versus associative semantic encoding
|Memory -- Age factors.
Hong Kong Polytechnic University -- Dissertations
|Department of Rehabilitation Sciences
|xii, 97 leaves : ill. ; 30 cm.
|Memory function involves encoding, storage, and retrieval. It is important to recognize the essence of effective encoding in memory function. Visual semantic encoding is characterized by clearly identifiable visual features (Biederman, 1987; Tversky & Hemenway, 1984), while associative semantic encoding is characterized by associations with meanings and functions (Barsalou, 1991). The process of encoding can be explained as the perceived stimulus being able to be registered by the relevant existing neural network, hence activating the related cognitive units for initiating interconnection (Collins & Loftus, 1975). Ageing has been found to modulate the integrity of visual and associative semantic networks, which impedes older people's learning and hence their daily functions. Studies have revealed that the associative semantic network remains largely intact over the visual semantic network across the lifespan (e.g., Backman, Small, & Wahlin, 2001; Naveh-Benjamin et al., 2003). Other literature has reported inconsistent findings on the issue of whether ageing will affect both encoding processes (Ferguson, Hashiroudi, & Johnson, 1992; Rahhal, May, & Hasher, 2002; Remy, Taconnat, & Isingrini, 2008). This study investigated the modulation processes by using visual and associative semantic tasks which had compatible priming and target stimuli. This largely ensured the validity of the between-task comparisons. Sixty healthy subjects (29 older adults and 31 young adults) participated in the study. A category verification task was used in which the subjects judged whether the descriptor presented was related or not related to the preceding animal name. The stimuli were Chinese characters of animal words and descriptors of animal visual features (visual semantic information) and associative characteristics (associative semantic information). The degrees of encoding facilitation were measured by the reaction time and accuracy rate across conditions. Three parameters were used and manipulated in the experiment: length of SOA (short versus long), type of encoding (semantic versus perceptual), and congruency (matched versus nonmatched). It was hypothesized that compared to the young subjects, the older subjects would maintain a relatively more stable associative, rather than visual, semantic encoding network and that this would be reflected in the reaction time and accuracy rate on the tasks.
The results indicated that both the younger and older subjects had significantly longer reaction times in the visual semantic trials than in the associative semantic trials, suggesting that associative semantic encoding would be more facilitated in the experimental tasks. Our findings concur with those of other studies (e.g., Guo et al., 2004; Walla et al., 2001) which suggest a more extensive activation of the associative semantic network relative to the visual semantic one. Such superior effects appear to be maintained among the older subjects. The older subjects, when compared with their younger counterparts, consistently showed a decline in task performance, particularly a longer reaction time in the visual semantic conditions, both in the raw and normalized forms. The decline in performance was also reflected in the smaller differences between the congruent and incongruent trials. The results suggest that age-related decline would be due to less efficient access to the visual semantic network. The lower efficiency could be due to a decrease in attentional and regulatory function in which irrelevant information cannot be eliminated from the processing. It could also be due to the decrease in working memory which impedes the binding of relational information within the neural network. It is noteworthy that the age-related visual-associative differentiation was observed in the discrepancies in reaction times but not in the accuracy rate on the category verification tasks. The results of this study reaffirm the modulation effect of ageing on the encoding process which is crucial in memory function. More importantly, they also indicate that the associative semantic encoding function appears to remain intact than visual semantic encoding function among older subjects. Our findings shed light on the reason behind the decline in memory and learning functions among older individuals. The visual-associative differentiation is useful for developing remediation strategies for the older population or those with pathological changes resulting in a decline in encoding functions.
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