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Lillian Gu, Barbara Dillenburger, Anna W. Roe; A novel dynamically induced ‘pure illusory contour’. Journal of Vision 2006;6(6):331. doi: 10.1167/6.6.331.
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© ARVO (1962-2015); The Authors (2016-present)
Classical illusory contours (ICs) are physically and perceptually complex. Kanizsa-stimuli induce both contour and surface percepts. In abutting line stimuli line-ends spatially overlap with the IC. Thus, both result in mixed activation of different inducing and induced processes. Here, we present a new stimulus which we term a ‘pure IC’.
Our stimulus consists of a series of five 50 msec frames. Each frame consists of six oblique inducers (randomized length and position) in two vertically separated sections (3 lines/section). This induces a pure IC in the gap (gapIC) between the two dynamic abutting line sections (see http://www.psy.vanderbilt.edu/faculty/roeaw/pureIC).
We measured the gapIC's perceptual strength by subjective rating and by 2AFC orientation discrimination experiments. For 2AFC, subjects discriminate the gapIC's tilt induced by horizontally offset sections. We find discrimination thresholds are consistent with subjective gapIC strength ratings and are modulated by inducer contrast and gap length. We also observe an inverse Tilt effect in which the gapIC appears tilted in the same direction as the inducing lines. This is opposite to the classical Tilt effect which would be expected if the IC were directly induced by the abutting lines. This further supports the independence of the gapIC from the physical inducers.
In sum, our stimulus, which is free of inducer line ends and surface percepts, leads to a strong IC-induction, and permits spatial separation of inducing and induced processes. Randomized presentation can also be used for reverse correlation, making this low contrast, dynamic stimulus useful for physiological studies of IC processing.
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