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Stuart Fuller, Marisa Carrasco; Perceptual consequences of visual performance fields: The case of the line motion illusion. Journal of Vision 2008;8(6):142. doi: 10.1167/8.6.142.
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Goal: Illusory line motion (ILM) is the illusion that a line, preceded by a small dot (cue) near one end, is perceived to shoot out from the dot even though the line is physically presented at once. ILM can also counter the perception of physical line motion when the two are opposed. Does this illusion result from apparent motion, exogenous spatial attention, or both? Given that exogenous attention speeds visual processing unequally at cardinal locations (North[[gt]]South[[gt]]East, West; Carrasco, Giordano & McElree, 2004), we hypothesized that the contribution of attention to ILM would follow this same pattern across locations.
Method: We characterized psychometric functions of perceived line motion direction, for 1.5° stimuli with varying amounts of physical line motion (8 levels, 0 to 37 ms/deg sequential presentation) toward or away from the local cue element, at four cardinal locations. We used three cue conditions to separate the effects of attention from apparent motion-a single cue adjacent to the stimulus to draw attention to its location, a distributed cue with elements near all four possible stimulus locations, and no visual cue. Because the local visual input near the stimulus is the same for both visual cues, the single cue combines attention and apparent motion effects whereas the distributed cue does not engage focal attention.
Results: Distributed and single cues generate identical effects at East and West, but the effect of the single cue is progressively greater at South and North locations. We conclude that both apparent motion and attention can generate ILM, their relative contributions varying by location. The effect of attention is most clearly separable at the North, where its effect on processing speed is strongest and the mechanism underlying apparent motion may be relatively weak due to the slower speed of processing at that location.
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