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Y. Wang, R.F. Hess; Contributions of Local Orientational and Positional Contour Features to Shape Discrimination . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4318.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Radial deformation of circular contour introduces both local orientational and positional deviations from circularity. In this study, the roles of these two local contour features in shape discrimination were examined independently. Methods: Stimuli were Gabor patch-sampled radial frequency (GSRF) patterns. Patch carrier orientation was tangential to the deformed circle at the placement position. With the change of modulation amplitude, patches placed at zero-crossings of the sinusoidal radial modulation only had orientational deviations from circularity but no positional alteration (orientational GSRF). Patches placed at the peaks and troughs of the modulation only had positional perturbations from circularity but no orientation change (positional GSRF). The patch size (SD of Gaussian envelope) was 0.14 deg; the carrier frequency was 3 cpd; the radial modulation frequency was 4 cyc/360 deg and the mean radius was 1 deg. A temporal 2AFC staircase paradigm was employed. Thresholds for detecting the radial deformation were estimated by a maximum likelihood fitting procedure. Results: Each of 5 normal subjects showed that the sensitivity to the radial deformation of orientational GSRF patterns was two times or higher than that of positional GSRF patterns. The average thresholds for detecting the radial modulation of orientational and positional GSRF patterns were 0.75±0.2(SD)% and 1.86±0.8%, respectively. In comparison, the average threshold for detecting the radial deformation of combined GSRF patterns with both orientational and positional patches in place was 0.39±0.2%, which was in close agreement to the threshold for detecting radial frequency (RF) patterns with the same stimulus parameters. Increasing the patch size to 0.2 deg to compensate for the difference in the number of sampling units reduced the detection threshold by less than 20% for both orientational and positional GSRF patterns. Conclusions: The results suggest that, while the orientation of local contour elements may provide information on more subtle changes of shape than the position of local contour elements, interaction between local orientational and positional contour features may be required to achieve optimal performance of shape discrimination.
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