On the perception of illusory contours

TLDR: The stimuli designed ("enhanced illusory contours") might provide a novel probe for dissecting different stages involved in the processing of illusary contours and for understanding how the visual system combines different types of contours to construct object boundaries.

About: This article is published in Vision Research.The article was published on 1994-12-01 and is currently open access. It has received 124 citations till now. The article focuses on the topics: Illusory contours.

Figures

FIGURE 3. The illusory contours associated with the pacman edges disappear completely, but most observers report a novel percept: a smaller illusory square that is concentric with the original and corresponds to the nearest subset of checks.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "On the perception of illusory contours" ?

The authors report three sets of novel observations on illusory contours. These results suggest that the stimuli the authors have designed ( “ enhanced illusory contours ” ) might provide a novel probe for dissecting different stages involved in the processing of illusory contours and for understanding how the visual system combines different types of contours to construct object boundaries. Such adaptation occurred even when the illusory contours were rendered invisible by displaying them on a misaligned checkerboard, suggesting that the adaptation occurs prior to the vetoing of the signal by the checks. 

Q2. What does the effect of a checkerboard contour suggest?

the facilitation of nearby checkerboard contours of the same orientation implies cooperative interactions between parallel adjacent subjective contours. 

Q3. How long does the illusory figure persist?

by simply flickering one eye’s image at a very low rate (e.g. 0.5-3 Hz) the authors were able to determine that the cyclopean figure can persist for as long as 0.5 set when one eye’s image is switched off. 

Q4. How did the authors find that illusory figures fade?

EXPERIMENT 4: ADAPTATION TO ILLUSORY CONTOURSThe authors also found that illusory figures tend to fade relatively quickly on steady fixation. 

Q5. What is the meaning of illusory contours?

Illusory contours are probably extracted fairly early in visual processing (von der Heydt, Peterhans & Baumgartner, 1985) and they are also known to powerfully constrain a number of “front-end” visual processes, such as stereo correspondence (Ramachandran, 1986) motion correspondence (Ramachandran, 1985) and shape from shading (Ramachandran, 1988). 

Q6. How long did the illusory figure remain visible?

Using this procedure the authors found that the illusory figure remained visible for at least 3 10 msec after the inducing elements had been switched off (mean = 340 msec; SD = 108 msec; 12 = 10 subjects). 

Q7. What is the significance of the illusory contours?

The remarkable persistence of illusory contours is reminiscent of the persistence of Cyclopean figures in random-dot stereograms when one eye’s image is temporarily switched off (Ramachandran, 1974). 

Q8. What is the effect of a misaligned checkerboard?

If checks were deliberately misaligned the illusory contours associated with the pacmen vanished, but the authors found that a novel percept emerged: subjects noticed an enhancement of the borders of the checks that were nearest to the original illusory contour so that a new illusory square emerged (Fig. 3). 

Q9. How did the subjects see the illusory contours?

While fixating a small red spot that was off to one side, the authors found that if frame The authorwas flashed on very briefly (e.g. 50msec) and followed by frame 2. subjects had no difficulty in seeing the illusory figure in frame 2. 

Q10. How long did it take to see the illusory contours?

if frame 1 was displayed for a second or more and followed by frame 2, the illusory figure in frame 2 took several seconds to emerge (Ramachandran. 

Q11. What is the paradox of illusory contours?

The paradox is resolved once you realize that the goal of vision is to delineate object boundaries-not merely to respond to edges. 

Q12. How long does it take to adapt to illusory contours?

Depicts the manner in which exposure to an invisible illusory square can lead to a reduction in the visibility of‘ a subsequently presented illusory square that would normally be strongly visible (without prior adaptation).can “adapt” to illusory figures even when the figure is rendered invisible by misaligned checks in the background. 

Q13. What is the reason for the illusory contours to be less vivid?

It is easy to see why misaligned checks should reduce the vividness of the illusory figure(4since, as suggested by Reynolds (1981), the image is no longer compatible with the percept of an opaque square “occluding” the disks in the background. 

Q14. What is the effect of the illusory contour on the checks?

4. Notice that in these displays there is no illusory contour coincident with the tips of the sloping lines but that a new illusory square emerges whose edges correspond to the borders of the checks that are close to line tips.