The effect of attention on the perception of illusory contours
Abstract
This paper examines whether attention modulates the phenomenological appearance of stimuli. In order to test this, an offset-arc illusory figure was designed combining Kanizsa, line-end and abutting grating illusory figures. Four inducing elements, composed of four concentric circles with one quadrant of arcs shifted out of phase with the circles, created a Kanizsa-like illusory square. The strength of the illusory contour was varied by manipulating the phase angle (0-180) between the arcs and concentric circles. Experiment 1 examined whether the offset-arc illusory figure induced a brightness percept. The results showed a brightness percept for the Kanizsa and line-end illusory figures, but no induced brightness for the offset arc illusory figure. Experiments 2 and 3 used a 2 alternative forced choice task in which observers chose which of two stimuli appeared with stronger contours. The results showed that observers perceived an illusory figure with stronger contours as the phase angle between the arcs and concentric circles increased. The results also showed that attention directed to the location of stimulus presentation lead to reports of stronger contours. Experiments 4 and 5 used a rating task to assess the strength of contours. The results showed higher ratings of strength for larger phase angles, and furthermore, that attention drawn to the location of stimulus presentation lead to higher ratings of strength. Experiments 6 and 7 used an objective aspect ratio discrimination task to quantify the strength of the contours. The results showed increased performance as phase angle increased, and increased performance when attention was directed to the location of the illusory figure. The results are consistent with the idea that attention modifies the strength of polarity-independent orientation-sensitive mechanisms in visual cortex (Wede & Francis, 2007; Suzuki & Grabowecky, 2003). The results are interpreted in terms of Grossberg’s FACADE theory (1997). The increase in strength of boundary representations leads to a change in appearance when the outcome of the filling-in process is modified. Therefore we do not normally notice changes in the appearance of stimuli as we move attention around our visual field.
Degree
Ph.D.
Advisors
Francis, Purdue University.
Subject Area
Cognitive psychology
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