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.