Besides contour detection, we also ran a contour discrimination task in which CI indices were measured when the observers were discriminating the above circular contour (
Figure 1b) from an elliptical contour (
Figure 2a). The elliptical contour had a slightly suprathreshold aspect ratio and rotated from trial to trial through axis orientation jitter. This task avoided a potential local orientation cue problem in a circular contour detection task, in that contour elements in a specific segment of a circular contour had fairly constant orientations, and the observers might learn to detect these local orientation cues to perform the task even after a contour percept had vanished. However, such local orientation cues barely existed in the circular and elliptical contour discrimination task because the orientations of local contour elements in the same stimulus area were similar. Specifically, we first measured the observers' aspect ratio thresholds (ARTs) for circular and elliptical contour discrimination at 4° and 20° retinal eccentricities using a 2AFC 3-down–1-up staircase method. The spatial frequencies of the contour elements were set at 6.4 and 1.3 cpd for 4° and 20° retinal eccentricities, respectively (same as in
Figure 1 for corresponding retinal locations). Each elliptical or circular contour stimulus contained 1.7 times the number of elements at the contour detection threshold obtained from the earlier experiment (
Figure 1). Regardless of the aspect ratio, the elliptical contour stimuli in this experiment as well as in subsequent ones had a constant geometric area that was the same as that of the circular contour. The estimated ARTs were 0.075 ± 0.005 and 0.0625 ± 0.003 at 4° and 20° retinal eccentricities, respectively (
Figure 2b). We then measured CI indices for circle and ellipse discrimination with the aspect ratios of elliptical contours set at 1 + 1.5 × ARTs. Such a slightly suprathreshold aspect ratio would be sufficient to make the elliptical contours distinct from the circular ones and, in the meantime, would minimize local orientation differences between elliptical and circular contours. The results (
Figure 2c) showed similar CI indices (4.89 ± 0.10 vs. 4.72 ± 0.04),
F(1,3) = 4.081,
p = .137, at 4° and 20° retinal eccentricities, consistent with data in
Figure 1 that peripheral contour integration for our good-Gestalt stimuli was constant across a large area of the visual periphery. The mean CI index for the discrimination task was 4.80, which is poorer than that for the detection task (6.91;
Figure 1d), indicating that observers were not based on the sole detection of the circular contour to make circle versus ellipse discrimination when the contour stimuli were near maximal spacing. A control experiment that measured contour discrimination also confirmed that there was no phase effect on contour integration in the discrimination task,
F(1,3) = 1.148,
p = .363 (
Figure 2d).