Age-related differences in visuomotor integration as measured by object affordance effects - a combined behavioural and neurophysiological investigation
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Visuomotor behaviour – from handling simple objects to operating complex devices – is of fundamental importance in our everyday lives, yet there is relatively little evidence as to how healthy ageing affects these processes. A central role is played by the human capacity for reaching and grasping. Grasping an object requires complex visuomotor transformations, including processing of the object’s extrinsic features (it’s spatial location) and intrinsic features (such as size and shape). It has been documented that action relevant intrinsic object properties automatically facilitate specific motor actions despite being task-irrelevant, the so-called object affordance effect. These effects have been demonstrated for (1) grasp type (precision and power grips being facilitated by small and large objects) and (2) object-orientation (whereby right and left handed grasps are facilitated by object-orientation), and might underlie the effortlessness with which humans can interact with objects. Yet, these paradigms have not previously been employed in the study of healthy ageing, and little is known concerning how these processes change over the life span. Elucidating these changes is of particular importance as age-related degeneration of white matter integrity is well documented. Consequently, if successful visuomotor behaviour relies on white matter integrity, age-related reductions in affordance effects should be observed. This prediction was tested in a series of experiments. Experiment 1 investigated age-differences in object-size compatibility effects, and results corroborated our prediction of age-related reductions in object-size effects. Experiment 2 investigated age-differences in (1) spatial compatibility effects versus object-orientation effects, and (2) the locus of the effects (facilitation versus interference effects). Results revealed (1) some evidence of larger affordance than spatial effects in both age-groups, and (2) interference effects in the younger group and both facilitation and interference effects in the older group, showing a potential change in processing modes or strategies. Experiments 3 and 4 addressed the main competing account, the attention-directing hypothesis (according to which attentional shifts are responsible for the generation of automatic response codes, rather than the affects arising from afforded actions), by using a novel stimulus set in which such attentional differences can be ruled out. Results provided strong evidence in favour of the object-size affordance hypothesis. A final neuroimaging experiment investigated age-differences in the object-size effect and its neural correlates by combining behavioural, functional MRI and diffusion tensor imaging (DTI) data. Results revealed evidence of age-differences, both on the behavioural and functional level. For the DTI data, we investigated all four diffusion metrics (something which is not frequently reported in the healthy ageing literature), and found widespread age-related differences in white matter integrity. The empirical findings presented in this thesis offer a significant contribution to ageing research, by further elucidating the relationship between age-related neurophysiological changes and visuomotor behaviour. The overall picture which emerged from this series of experiments was consistent with our prediction of age-related reductions in affordance effects. Furthermore, it is likely that these age-differences may have, at least in part, a neurophysiological basis.
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