The lateral occipital complex (LOC) plays a critical role in object recognition (James, Culham, Humphrey, Milner, & Goodale,
2003), responding to complete objects more strongly than unstructured texture (Malach et al.,
1995), or their disassociated parts (Kourtzi & Kanwisher,
2000). Moreover, LOC appears to treat line drawings of objects and grayscale photographs of those objects as equivalent (Kourtzi & Kanwisher,
2000), indicating that the redundancy in the image associated with its surface properties has been removed (Attneave,
1954) and that orientation information is sparse. This result may help explain the representational efficacy of line-drawing cartoons, but it also validates the use of simple closed paths in experiments investigating the component shapes of object perception. One particular versatile stimulus used to represent shapes is the radial frequency (RF) pattern, created by sinusoidally modulating the radius of a circle (Loffler, Wilson, & Wilkinson,
2003; Wilkinson, Wilson, & Habak,
1998; for an example, see
Figure 1A). Experiments investigating sensitivity to RF modulation when distinguishing shapes from circles have shown that signal is summed across cycles of modulation in simple (single frequency) RF patterns (Loffler et al.,
2003), making these patterns popular as candidates for object primitives (Bell & Badcock,
2009; Habak, Wilkinson, Zakher, & Wilson,
2004). Profiles of complex objects such as human heads (Lee, Matsumiya, & Wilson,
2006) are also easily modeled through the summation of a number of sinusoids of particular radial frequencies (cycles per 2
π radians), amplitudes, and phases. Adaptation to RF patterns has been shown to result in subsequently presented circles being perceived as modulated in the opposite phase to the adaptor (Anderson, Habak, Wilkinson, & Wilson,
2007; Dickinson, Han, Bell, & Badcock,
2010; e.g., adaptation to
Figure 1A results in the percept of an inverted, lower amplitude, version of
Figure 1A). The aftereffect, however, is bidirectional in the sense that it can exaggerate the difference of the test pattern from an adaptor with an arbitrary amplitude of deformation (Bell & Kingdom,
2009). After adaptation to a particular amplitude adaptor, subsequently presented RF patterns of the same phase but higher amplitude appear of greater amplitude still. The perceived pattern shape, therefore, exists and is modified along a continuum. Local orientation differences between the adaptor and the test patterns are exaggerated under all circumstances, and consequently, the local orientation changes due to what is perceived as a global shape aftereffect and the local TAE are in the same direction. Systematic application of local tilt aftereffects (TAEs) in what might be called a TAE field would serve to enhance ability to discriminate between shapes by enhancing the shape contrast, effectively decorrelating the responses of detectors for similar shapes (Barlow & Foldiak,
1989).