Thesis synopsis

In chapter one, a series of studies on the corner enhancement effect are reviewed. Previous research (Vecera & Farah, 1994; Vecera, Behrmann, & McGoldrick (2000) suggests that object onset has an influence on visual attention. More recently, Cole, Burton and Gellatly (2001, 2007) found that reaction time was shorter for the stimulus located near the corner of the figure. It is possible that corners undertake a more pivotal role than straight edges in visual space, because corners receive more attentional resources than straight edges, as Cole, Gellatly, and Bluton (2001) explains. Additionally, Cole et al. (2001) have shown that the fastest response is for the onset of a probe near a corner. The probe can be the onset of dot, a square or a short horizontal line.

The corner effect has two possible explanations: firstly, corners are more important because they contain more information than straight edges do; secondly, straight edges are an example of an uninformative stimulus and have greater redundancy. This redundancy occurs when the system contains inconsistent information about the stimulus (Cole et al., 2001).

Drawing on a series of experiments, Cole et al. (2001) suggested that the corner effect strongly influences shape representation. It may be helpful for understanding the shape recognition and, as a result, participants will respond better to a stimulus adjacent to the corner rather than a stimulus next to straight edges (Cole et al. 2001). Moreover, Cole, Skarratt and Gellatly (2007) indicated that a stimulus presented in an area next to a corner receives more attention in comparison with the stimulus adjacent to straight

edges. For instance, reaction time to detect the target next to the corner was significantly faster than a target next to the straight edges. The present study attempts to decide whether there is a difference between convex and concave corners.

In chapter one, the role of figure ground in the corner enhancement effect is studied. It was found that the corner enhancement effect was present only when the probe is on the surface that owns the corner (Experiment 1a, 1b, 2a, 2b). Therefore, it can be found for both convex and concave vertices. However, no sign of the corner enhancement effect was noticed when the probe was not located on the surface that owns the corner (Experiment 1d, 2c).

Chapter two attempts to determine whether there is a difference between short- term memory for convexities and for concavities (Experiment 3, 4, 5). The capacity of VSTM is limited to four units (Luck & Vogel, 1997; Cowan, 2001; Phillips, 1974). In our studies, the units were segments of a contour. When closed, the contour formed an outline perceived as a single object. The point of interest is whether there is a difference between short-term memory for convexities and for concavities. In this set of studies, a change detection task was employed to examine visual short term memory VSTM. Convexity and concavity in VSTM were compared and it was found that there is no evidence that convexities are special in visual short-term memory, although coding of convexity, as well as concavity, did provide a small advantage over an isolated (and thus ambiguous) contour. This agrees with the known effect of closure on processing of shape.

Chapter three discusses whether the effect found by Hulleman and Olivers (2007) is specific to perception of bilateral symmetry. This recent study reported that deviations from symmetry carried by convexities were easier to detect than deviations carried by concavities (Hulleman & Olivers, 2007). To test whether the convexity

advantage was specific to bilateral symmetry, this work was applied to shapes that were repeated rather than reflected (Experiment 6b, 6b). In this set of studies (Experiment 6, 7) we used a detection of symmetry to test if a convexity advantage was specific to bilateral of symmetry. It was concluded that there is a clear convexity advantage for detection of translated objects (6b, 7b). Therefore, no evidence of an advantage for convexity in perception of symmetry was found. Probably, a monitoring strategy focusing on the convexities played a role, in spite of the instructions. Another interesting aspect of the data is the relatively large inter-individual variability. Although in the instructions both concavities and convexities were described to the subjects, and they were told that the deviation from regularity could be in either, some subjects focused more on one region (convexities) and others on another region (concavities). It may be concluded that, for some tasks, performance for convexity was higher than for concavity. This was due to the specific nature of the task, for example, when the task required comparison of features of translated objects, we found a convexity advantage. In the last chapter, we test that contour ownership determines the presence or absence of interference when the 2D contour information is identical between 2 regions; even for simple shape analysis. We used a task in which figural relationships are irrelevant. Moreover, this task does not involve memory which avoids the possibility of hole shape judgment being affected by memory of the object-with-hole instead of the perceptual response. This chapter is also related to convexity and concavity, but indirectly, as a figure ground change is also a change in convexity coding. The main research question is to explore the role of contour ownership. The prediction is that which surface owns the contour determines the degree of interference between shapes. Furthermore, interference effects were only present when the inside contour and the outside contour belonged to the same surface.

In summary, the principal aim of the present research was to assess the difference between convexity and concavity along a contour in two-dimensional shape (2D), by using different methodological methods. In order to examine the corner enhancement effect, visual short-term memory, symmetry, and shape interference.

CHAPTER 2| The role of figure ground in the corner enhancement effect