Kojo I, Danilova M V, 1998, "How illusory lines interact with real Gabor patches" Perception 27 ECVP Abstract Supplement
How illusory lines interact with real Gabor patches
I Kojo, M V Danilova
Illusory contours do not represent physical luminance or colour edges, but we perceive them as such. In psychophysical experiments we studied the interactions between these lines and Gaussian-truncated gratings.
Two 'pacmen' (dark circles with a sector removed) formed the illusory line. The same circles rotated by 90° were used as a control condition. A Gabor patch was inserted at the midpoint on the illusory line and at the corresponding place in the control figure. We measured the detection thresholds of the Gabor patch using a 2AFC paradigm with a staircase procedure. In one set of experiments we varied the phase of the Gabor patch (0° - 180°), and in the other set we changed the separation between the two pacmen.
In both the test and control conditions we observed a decrease in the detection thresholds relative to those obtained on the homogeneous background. But observers showed different changes in detection of the Gabor patch depending on its phase. For one observer, there were no significant differences for all phases tested: the thresholds were lower when the illusory line was seen relative to the control condition. Another observer, however, showed lower thresholds when the Gabor patch had a sine-wave profile, but for the cosine-wave Gabor patch this difference disappeared. With increasing separation between the pacmen, thresholds increased in the presence of the illusory line, but did not change significantly in the control condition. The results show that the perception of illusory lines varies among observers: the line may be seen as a light or dark edge and interact in different ways with real objects.
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