Academic journal article Perception and Psychophysics

The Contrasting Impact of Global and Local Object Attributes on Kanizsa Figure Detection

Academic journal article Perception and Psychophysics

The Contrasting Impact of Global and Local Object Attributes on Kanizsa Figure Detection

Article excerpt

Studies on the involvement of object completions in search for illusory figures have so far reported equivocal results. We have addressed this issue by investigating at which level object attributes in Kanizsa figures influence search. Employing a paradigm that investigated global and local attributes in the composition of distractors with relation to target composition, we report a selective involvement of multilevel processing upon detection. Four experiments demonstrate that global surface information, but not the surrounding global contour, determines the speed of Kanizsa figure detection. By contrast, local inducer information is encoded far less efficiently in search than processes computing the global object. Our conclusions are that surface filling-in acts as a major determinant of search, but depends on the relevance of the particular hierarchical level (local or global) coding the target.

When we perceive visual scenes, luminance discontinuities allow us to establish the boundaries between different objects. However, in many cases, the requisite information reaching the eye is incomplete. Adverse lighting conditions and overlapping objects may require the visual system to complete missing sensory information, to bring about the perception of a coherent object. Visual completion is demonstrated by the phenomenon of illusory figure perception (see Lesher, 1995, and Spillmann & Dresp, 1995, for reviews). For example, in Figure 1 A, a square is perceived to occlude the neighboring parts of the four circular inducer elements, even though there is no corresponding physical correlate (Kanizsa, 1955). Phenomenally, the illusory figure appears as a central bright region surrounded by sharp boundaries and depicting depth stratification.

Illusory figures, as with many other composite objects, result from the integration of the various parts into coherent wholes. As such, an integrated object may be represented at one of several levels in hierarchical organization. The Kanizsa square (Figure 1A) provides an example of a hierarchical stimulus configuration, comparable to the Navon letter (Navon, 1977; see the present Figure IB). Both Kanizsa and Navon figures can be described at local and global levels of organization. For instance, at a local level, me Kanizsa figure is constructed from inducers just as the Hs in the Navon letter. In addition, at a global level, the Kanizsa figure consists of the illusory figure (the induced object) and the Navon figure of the global letter "U" (see Figure 1). Thus, both objects are represented at multiple levels of representation, with global properties being dependent on the existence and arrangement of more elementary local parts.

Investigations of how a global object is derived from local elements show that several processes contribute to the final percept. Whereas the representation of local elements can be achieved in the main by luminance-selective units in early visual areas, global object properties necessitate more complex processes. For instance, the global letter in Figure IB would result from grouping operations that link local elements. Similarly, the global square in the Kanizsa figure results from at least two independent processes, related to the extraction of contours and the specification of the illusory surface (Grossberg, 2000; Grossberg & Mingolla, 1985, 1987). Thus, processes of object integration consist of multiple stages of processing (Lesher, 1995; Palmer, Brooks, & Nelson, 2003) that can be related to distinct neural mechanisms: In neurophysiological stuthes, local luminance discontinuities are detected by cells in VI (Hubel & Wiesel, 1968). By contrast, the representation of a global object such as for the Kanizsa figure can be related to the computation of illusory contours in area V2 (Ffytche & Zeki, 1996; Lee & Nguyen, 2001; Peterhans & von der Heydt, 1991; von der Heydt, Peterhans, & Baumgartner, 1984), and corresponding surface filling-in mechanisms can be located further along the ventral stream in the lateral occipital complex and fusiform gyrus (Conci, Gramann, Müller, & Elliott, 2006; Hirsch et al. …

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