The Ageing Mind of the Double Brain: Studies in Hemispheric Asymmetry for Global-Local Processing

Abstract

Global-local processing refers to a visual attentional mechanism that involves directing attention either globally (to an overall visual scene) or locally (to the finer elements within the scene). The two hemispheres of the brain demonstrate functional asymmetry for global-local processing, with global processing specialised in the right hemisphere and local processing specialised in the left hemisphere. Healthy ageing has been shown to influence global-local processing, with evidence suggesting that older adults demonstrate a local-processing bias. The right-hemisphere ageing hypothesis has been put forth to explain this bias, proposing that right-hemisphere functions are more vulnerable to ageing. In this thesis, hemispheric asymmetry for global-local processing was investigated using a hierarchical-figures task, in which a large stimulus was made up of several small stimuli. The hierarchical figures were presented briefly to the left or right visual field, such that information was initially processed by the contralateral hemisphere. Study 1 was an investigation of hemispheric asymmetry for global-local processing in neurologically healthy right-handed young (N = 27) and older (N = 34) adults. The question addressed was: Do stimulus categories with different demands on visuospatial-verbal processing (letters, verbalisable objects, and nonverbalisable shapes) influence hemispheric asymmetry for global-local processing in young and older adults? Study 2 was an investigation of the role of the right hemisphere in the non-directional component of attentional orienting in neurologically healthy right-handed young (N = 39) and older (N = 38) adults. The question addressed was: Do older adults demonstrate greater difficulties with disengaging attention from local-level information when correct responding required re-orienting attention to global-level information? Study 3 was an investigation of individual differences in hemispheric asymmetry for global-local processing in neurologically healthy right-handed young (N = 127) and older (N = 58) adults. The questions addressed were: Do individual young and older adults demonstrate right-hemisphere specialisation for global processing and left-hemisphere specialisation for local processing. If so, Do these individual differences in hemispheric asymmetry account for age-related changes in global-local processing? Study 4 was an investigation of age-group differences and individual differences in the perception of the Ebbinghaus illusion in neurologically healthy young (N = 87) and older (N = 58) adults. The question addressed was: Does a local-processing bias reduce susceptibility to the Ebbinghaus illusion? Four key conclusions emerged from this research. First, healthy ageing influences two aspects of global-local processing. Older adults demonstrate greater difficulties: (1) processing global-level information, and (2) disengaging attention from local-level information for the purpose of re-orienting attention to global-level information. Second, hemispheric asymmetry for global-local processing remains intact with age. In young and older adults, the right hemisphere (when compared to the left hemisphere), is more efficient for processing global-level information, and the left hemisphere (when compared to the right hemisphere) is more efficient for processing local-level information. Third, evidence for the right-hemisphere ageing hypothesis was mixed. Although older adults do demonstrate age-related difficulties in global processing, the right hemisphere of older participants was still more efficient compared to the left hemisphere at processing global information. Fourth, there are individual differences in the processing of global-level and local-level information. This thesis provides future research directions for understanding how healthy ageing influences functional hemispheric asymmetry, and the impact this has on visual attention across the life span. Show more