The role of sustained and transient signals in motion perception: Evidence from illusory line motion
Green, Sean Roland
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Detecting the movement of an object in a visual scene involves detecting that the moving object has changed position over time. Perception of real motion therefore involves perceiving the temporal order of successive views of the world. Gradient accounts of Illusory Line Motion (ILM), a visual motion illusion, maintain that the speeding or delaying of incoming signals generates ILM by introducing asynchrony into the surface registration of an instantaneously appearing object. The present study investigated how ILM was influenced by transient signals from a nearby priming object to (a) better understand how the motion system interprets and represents transient signals and to (b) test the proposition that the gradient model of ILM is adequate to explain primer transient effects found under stimulus conditions where primer transient effects had not been previously investigated. Experiments 1 and 2 of this thesis systematically manipulated the timing of the primer's presentation or removal relative to the probe to reveal distinct contributions of onset and offset transients to the ILM percept, in the process revealing Offset ILM, a novel variant of ILM. Experiments 3 and 4 investigated how the visual system resolves competing transient signals. Experiment 5 examined the role of transient signals on the spatial extent of the primer's influence. The investigation revealed distinct effects of primer onsets and offsets and distinct long-range and short-range priming effects that were not predicted by pre-existing gradient accounts. Implications for gradient accounts of ILM are discussed, and possible refinements to current gradient accounts - including a neurophysiologically grounded conceptual model of ILM - are introduced.