Near-threshold studies of RF processing have also examined which parts of the contour are spatially integrated by the global shape mechanism. While the evidence suggests that all cycles are used to discriminate the pattern from a circle (Bell & Badcock,
2008; Hess et al.,
1999; Loffler et al.,
2003), this does not mean that all of the form information present
within each cycle is used. Several researchers have reported that the points of maximum convex curvature, i.e., the “corners” in an RF pattern, are the most important features for detecting the pattern's global shape, with the zero crossings second in line of importance, and the points of minimum curvature, i.e., the “sides,” least important (Bell, Dickinson, & Badcock,
2008; Habak, Wilkinson, Zakher, & Wilson,
2004; Loffler et al.,
2003; Poirier & Wilson,
2007). Others, however, have argued for a more significant role for “sides” (Hess et al.,
1999; Kurki, Saarinen, & Hyvarinen,
2009; Mullen & Beaudot,
2002).
Figure 1A (1–3) illustrates these features at near-threshold amplitude. Several studies have adopted the term “side” to describe the parts of the RF contour centered around the troughs of the waveform, analogous to the sides of a square (RF4) or pentagon (RF5; Hess et al.,
1999; Loffler et al.,
2003; Mullen & Beaudot,
2002). This analogy is somewhat misleading, however, because the “sides” on any given RF are only perceptually collinear at a single deformation amplitude, while at all other amplitudes they have either convex or concave curvature with respect to the shape's center. In this communication, we will use the terms “convex” and “concave” parts however to refer to the half-cycles of the contour centered, respectively, on the peaks and troughs of the RF pattern (with respect to the shape's center) rather than the terms “points of maximum” and “points of minimum” curvatures, which are ambiguous since they can refer either to a difference in the magnitude of curvature or a difference in its sign. We also use the term “inflection” to refer to the half-cycles of the contour centered around the zero crossings of the shape waveform, provided there is a change in the sign of curvature at the point. An inflection point is a finite point in space, so to study the contribution of inflections to shape processing we had to use stimuli that were extended either side of the inflection point.