PROPIEDADES DEL COMBUSTIBLE TRANSPORTADO

Contour polarity refers to the sign of a contour’s curvature. By convention, negative curvature is concave and positive curvature is convex. The term “contour polarity” is thus used to highlight the presence of convexities and concavities, and should not be confused with contrast polarity.

A number of researchers (Bertamini et al., 1997; Baylis & Driver, 2001; Hulleman & Oliver, 2007) have suggested that objectness (figure or background) plays a central role in the perception of symmetry. For example, Bertamini et al, (1997) claimed that reflection plays a fundamental role in distinguishing objects (figures) from background (field). Experimental participants have better recognition performance when contours belong to a single object (in the reflection symmetry). On the other hand, participants have better performance when the contours belong to different objects (in the translation symmetry) because of a strategy named a lock-and-key process. A possible explanation for this is that visual attention moves more easily within objects than between objects, and recognition performance is therefore faster when objects belong to the same units or objects than to different units or objects (Bertamini et al., 1997; Bertamini, et al., 2002).

Another key piece of evidence comes from a series of experiments conducted by Baylis and Driver (2001), which demonstrated that participants’ performance was improved when symmetry perception was applied to a single object rather than multiple objects. In symmetry detection, each side of the contour is an identical mirror-image of the other; for example, a concave region along one side relates to a similarly concave along the other side of the shape or object, whereas, in symmetry repetition, the two sides of the contour are mismatched (i.e. a concave region along one side corresponds to a convex contour along the other side of the object). As a result, detection of symmetry becomes harder when the contours are repetitions rather than reflections (Baylis & Driver, 2001). This is consistent with the findings of Kansiza and Gerbino (1976), who showed that symmetry detection, was easier for participants when they perceived a convex contour as a figure, and their responses were accurate with reference to the figure perceived as convex. For example, in Kansiza and Gerbino’s study, 73 out of 80 participants perceived the convex areas (symmetrical stimulus) as a figure (Kansiza & Gerbino, 1976). Moreover, there is much evidence from a wide body of research literature to support the important role of contour curvature in the perception of part structure.

Bertamini (2001) found that participants were faster at judging the position of convex rather than concave stimuli vertices. He demonstrated that this result was due to the perception of convex vertices defining parts of solid objects, whereas concave vertices conversely tend to be perceived as boundaries between parts. This means that perception of positional information is more directly involved with convex regions than with concave regions.

In Bertamini’s experiment, the central region of the contour was divided into red (the figure) and green (the background); the stimulus was positioned in either the top

half or bottoms half of the figure, and participants were required to judge the position of a vertex with respect to a base line. The results confirmed that it is easier to judge the position of a convex vertex than the position of a concave vertex (Bertamini, 2001).

Hulleman and Olivers (2007) have shown that it is easier to detect symmetry when a contour belongs to a convex region than when a contour belongs to a concave region. In other words, they found that convex vertices play a role in determining the structure of an object. Therefore, concave vertices play a role in determining how the shape of an object is perceived. In their research, Hulleman and Olivers established under which conditions it is easy or difficult to recognize shapes as symmetrical or asymmetrical. Their study used either an actual physical object (convexity or concavity) or a small dot as a stimulus. This will help to make a balance between stimulus near or faraway from the central of the axis. This is the case even when convexities are situated away from the axis of symmetry alignment and concavities are, conversely, close to the alignment of symmetry (Hulleman & Olivers, 2007). Hulleman and Olivers found that the relative contribution of convexities and concavities in symmetry perception depended on their relative roles in the shape perception, and moreover the symmetry of convexities can be more simply determined than the symmetry of concavities. This supported the idea that symmetry perception in 2D shapes is mainly governed by shape perception. The normal bias towards the axis found in symmetry perception.

In contrast, other researchers (Barenholtz et al., 2003; Cohen et al., 2005) have proposed that concavity plays a more important role than convexity in shape perception. Concave vertices can be easily detected in visual displays (Hulleman & Olivers, 2007); for instance, the search for a concave target among convex stimuli is more efficient and accurate than the search for a convex target among concave stimuli (Hulleman et al., 2000; Humphreys & Muller, 2000; Wolfe & Bennett, 1997; Bhatt et al., 2006).

Overall, we can conclude that the closure of a contour (forming a convex region and a concave region) plays a crucial role in shape perception. It seems that there is an interaction between the type of symmetry and the type of displays (closed or open). Symmetry is easier to detect when the stimulus contours (outline) belong to one object rather than two objects, whereas repetition is easier to detect when the stimulus contours belong to two objects rather than one. It is therefore easier to detect asymmetry when there is a mismatch between the convexities on either side of the symmetry axis, which leads to the conclusion that concavities are less important than convexities in symmetry perception.

This finding suggested an important question: does the mixed role of convex and concave regions in shape perception imply similarly different roles in the perception of symmetry? More specifically, this question addresses whether participants have a tendency to monitor convexities, and whether this strategy was the reason for the convexity advantage. In order to find out, a series of experiments were conducted in the course of the present study to test this hypothesis.

This study will present examples of the stimuli that Hulleman and Olivers (2007) used in their experiments. Sometimes a closed object (similar to that depicted on the right side of Figure 4.1) was used and sometimes a set of dots (neither a convexity nor a concavity) was used, without any closure (similar to the stimulus depicted on the left side of Figure 4.1).

Figure 4.1. Examples of stimuli used by Hulleman and Olivers (2007) in their experiments. In the left side they used symmetrical and asymmetrical dots near or far from axis. On the right side they used Concavities and convexities symmetrical and asymmetrical shapes near or far from axis.

Experiment title

Experiment aim Stimuli

6(a) Symmetry. To replicated Hulleman andOlivers (2007) study.

6(b) Detection of symmetry(

Translation)

To test symmetry detection of symmetry for translation, but not for reflection, for a two intervals forced choice task.

7(a) Symmetry in one interval.

To test the bilateral symmetry in one interval presentation.

7(b) Detection of symmetry (Translation) in

one interval.

To test symmetry detection in one interval presentation.

(8) Control for Translation (half of trials block convexity) and (half of the trials block

concavity).

To control for the translation experiment (how the trials put in separate blocks).

Table 4.1. Illustrated the symmetry experiments. In Experiment 6a we tried to replicate the findings of Hulleman and Olivers (2007) and In Experiment 6b we test the symmetry detection of symmetry for translation for a two intervals forced choice task. Whereas, In Experiment 7a we test the bilateral symmetry in one interval presentation and in Experiment 7b we test the symmetry detection of symmetry for translation in one interval presentation. In Experiment 8 we comparing convex and concave as separate tasks.

4.2 Experiment 6a (Reflection)