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Sharon E. Guttman, Philip J. Kellman; Do spatial factors influence the microgenesis of illusory contours?. Journal of Vision 2002;2(7):355. doi: 10.1167/2.7.355.
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Spatial factors such as contour curvature and support ratio influence the perceived strength of illusory contours. However, it is unknown whether these determinants of interpolative strength affect how quickly interpolated contours emerge during visual processing (microgenesis). We approached this issue using a dot localization method. Observers briefly viewed Kanizsa-type illusory shapes with varying support ratios and relative inducer orientations, as well as stimuli containing comparable spatial cues that did not give rise to illusory contours (non-IC stimuli). During the last 40 ms prior to masking, a small dot appeared somewhere between two inducers. Observers judged whether the dot appeared inside or outside of the illusory shape's perceived boundary. Dot position varied based on observer responses; two interleaved staircases produced the 83% threshold points for seeing the dot inside versus outside of the boundary. This objective paradigm revealed both the accuracy of the interpolation and the strength or precision with which the contour is perceived. Stimulus duration varied from 40 ms to 480 ms, allowing investigation of the microgenesis of contour formation. Our results indicated that observers localized a dot more accurately and precisely relative to illusory contours than relative to comparable non-IC stimuli; this difference diminished at longer durations, suggesting that the time course of illusory contour formation differs from the time course by which more cognitive strategies become useful. Second, consistent with previous data, our results suggest that all illusory contours emerge rapidly; precision reached asymptote within the first 120 ms following stimulus onset. Finally, preliminary data suggest that spatial factors affecting the subjective strength of illusory contours (curvature and support ratio) also influence the time course over which interpolation proceeds. These results will be discussed in the context of contour interpolation models.
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