Abstract
Previously, we reported efforts to better define good continuation as the principle governing perception of unity, separateness, and parts in continuous contours. That work showed that the presence of first-order (tangent) discontinuities primarily governed segmentation (Kellman, Garrigan, Kalar & Shipley, 2003). Here we report subsequent efforts to determine whether second-order discontinuities (locations at which the contour's second-derivative is undefined) play a role in segmentation and grouping. Using our earlier paradigm, in which subjects searched in noise for a contour segment with varying levels of continuity to other segments, we found no reliable sensitivity to second-order discontinuities as a basis for segmentation.
Other research has suggested that second-order discontinuities are features, and can lead to “pop-out” in a search paradigm (Kristjansson & Tse, 2001). These and other results have been interpreted as implicating second-order discontinuities in contour shape representation and segmentation. We replicated the visual search findings of Kristjansson & Tse (2001), and then showed with additional manipulations that second-order discontinuities alone do not account for their previous findings. Displays containing even more second-order discontinuities but with a less-noticeable difference in symmetry from ellipses showed serial, not parallel search patterns. We explore alternative hypotheses that may explain both sets of data.
Supported by NEI EY13518 to PJK.