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Joshua Wede, Gregory Francis; Attention increases the perceived strength of illusory contours. Journal of Vision 2008;8(6):559. doi: https://doi.org/10.1167/8.6.559.
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Carrasco, Ling and Read (2004) reported that transient (exogenous) covert attention increases the apparent contrast of attended stimuli relative to unattended stimuli. Others have proposed that attention increases the strength of polarity-independent orientation-sensitive representations (Wede & Francis, 2007; Suzuki & Grabowecky, 2003). A procedure similar to Carrasco et al. (2004) was used to test the effect of attention on the perceived strength of polarity-independent illusory contours. 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 circles. A standard (108 degree phase angle) stimulus was paired with a test stimulus (0–180 degree phase angle), and briefly presented on either side of fixation. Before presentation of the inducing elements, a cue was presented at the location of the test stimulus, the standard stimulus, or at fixation. Participants reported which stimulus appeared to have contours with the highest clarity. Psychometric functions expressing the likelihood of choosing the test stimulus over the standard stimulus were obtained, as a function of the phase angle. The results showed that cueing the test stimulus reduced the phase angle required to match the standard stimulus, effectively increasing the strength of the illusory contour. With a neutral cue, participants were at chance when the two stimuli were identical. 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).
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