Kwok, Rebecca Martha; (2003) Visual mechanisms subserving perceptual judgement and action. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Neuropsychological, neurophysiological and behavioural evidence largely support the concept of two distinct visual cortical systems, a ventral system underlying perceptual identification, and a dorsal system supporting visually guided action. The dual-system idea is not new, however this thesis focuses on the currently favoured theory proposed by Goodale and Milner (1992). All studies used the ELITE motion tracking system, with two main aims. First, methodological issues arising from previous work were addressed. The relationship of maximum grip aperture and manual estimation to object size was found to be similar, justifying comparisons of illusion size in these measures. Action and perception were dissociated using the Judd illusion, taking into account past methodological criticisms. Second, the conditions under which action may be driven by ventral rather than dorsal processes were investigated. Two-dimensional and three-dimensional targets were compared to investigate whether a target must be physically graspable, or whether the essential factor for the dorsal stream is that the action be goal-directed. Kinematic analysis showed differences between grasps towards 2D and 3D targets, especially in maximum aperture measurements. However, grasps towards 2D and 3D targets within illusory figures were similar, and it was concluded that they depend on similar underlying mechanisms. A different grasp formation may be used for 2D targets, but an alternative driving mechanism is not necessarily required. When actions were directed towards 3D targets that could not be grasped, kinematics were similar to 2D targets. The critical factors determining whether an action is driven by the dorsal stream appear to be that it is immediate and goal-directed. Characteristics of kinematic variables involved in a grasping action are determined predominantly by the type of action required, and not by the precise visual information provided by the target. Future studies must take this into account when using kinematic analysis as an investigative tool.

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