The representation of salient regions, shapes and objects in the human visual system

A central component of visual processing is segmenting the retinal image into surfaces corresponding to different surfaces and objects. Illusory figures (Kanizsa, 1955) have been used extensively to study the mechanisms underlying scene segmentation. Most studies that have used Kanizsa-type illusory figures have treated the crisp bounding illusory contours (ICs) and the enclosed region as non-dissociable stimulus attributes. However, there is evidence that these two components of illusory figure stimuli may be processed separately (Gurnsey et al., 1996).; The human Lateral Occipital Complex (LOC) is a cortical region that has been implicated in the processing of illusory figures (Hirsch et al., 1995; Mendola et al., 1999). In chapter 2, I used functional Magnetic Resonance Imaging (fMRI) and psychophysics to demonstrate that LOC activation by illusory figures is due to the globally completed salient region, independent of the presence of bounding ICs.; In Chapter 3, I used an apparent motion (AM) paradigm to establish the perceptual validity of SRs independently of their bounding contours. I interpret the findings of chapters 2 and 3 by providing a theoretical framework that distinguishes between region-based and contour-based segmentation processes which operate in concert to segment the visual scene.; In addition to illusory figure processing, the LOC has also been implicated in many aspects of object processing (see Grill-Spector, 2003 for review). Although it is widely accepted that the LOC contains sub-regions sensitive to different object properties, their nature is still undetermined. In the experiments detailed in chapter 4, I delineated 2 LOC sub-regions by comparing fMRI responses to real-world objects and novel, 2-d shapes. These results suggest a multi-dimensional hierarchy of object processing within the LOC and provide a novel method for localizing its sub-regions.