1. Introduction
1.3 Effect of Fixation and Spatial Attention
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representations—as previously described. The effect of focal-feature processing will be discussed below.
1.3 Effect of Fixation and Spatial Attention
Effect of fixation
Many studies have found that fixatingon different locations within a Necker cube tends to favor one or the other perceptual response. There is a tendency to perceive the vertex of the Necker cube that is nearest the visual fixation point as the frontal face (Inui, Tanaka, Okada, Nishizawa, Katayama, & Konishi, 2000; Kawabata, Yamagami, Noakl, 1978). For example, fixating at its right-up corner tends to favor the perception of the cube as front face down to the left, and fixating at the left-down corner tends to favor the front face up to the right perception (Kawabata, Yamagami,
& Noakl, 1978; Long & Toppino, 2004), as showed in Figure 3. This is because “the fixated area tends to be seen as being relatively closer to the observer” (p. 1287) (Toppino, 2003). In Necker’s view, “the foveated portion of the figure was naturally supposed [by the observer] to be nearer and foremost” (p. 337) (Necker, 1832, 1964;
Long & Toppino, 2004). Kawabata, Yamagami, and Noakl (1978) thought that the
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degree of clarity caused by the difference of visual resolution and attention is an important cue for the depth perception of the Necker cube. That is, areas near the visual fixation point are seen clearly, and the clear areas are seen as being in the front.
Figure 3. The two corners of the Necker cube and their correspondent perception
when attending or fixating on them.
Garcia-Perez (1992) also assumed that perceptions of multistable figures are biased depending on where observer fixates in a figure. The explanation is that because fixations near a focal area for one of the percepts will bring out fine-detail (high spatial-frequency) information relevant to that percept, it is easier to build that perceptual interpretation. This opinion can also be explained by the notion that fixation areas are seen as being closer because the clearer the image of an object is, the nearer this object is perceived to be.
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Toppino (2003) summarized these effects of fixation on multistable figure perception described above and proposed the focal-feature hypothesis. Its main assumption is that “different focal regions or areas within a multistable figure favor different global interpretations” (p. 1286). Furthermore, “which interpretation is perceived is assumed to depend on which focal area is selected for enhanced processing, regardless of whether that selectivity reflects the locus of fixation or the focus of attention alone” (p. 1286).
We should notice that the locus of fixation is not the same as the focus of attention. Even though these two coincide with each other most of the time, attention can be shifted independently of eye movements (Posner, 1980; Palmer, 1999; Tsal &
Kolbet, 1985). For example, we can fixate on one place while attending to another place; this is also called covert attentional shift (Suzuki & Peterson, 2000; Theeuwes, 1992; Leopold & Logothetis, 1999). Therefore, the effect of fixation and the effect of attention should be differentiated.
In addition, there is another issue in the focal-feature hypothesis that needs to be clarified: what is the “enhanced processing” for the selected focal area that biases the interpretation of multistable figures? Toppino (2003) did not clearly point out why
“different focal areas within a multistable figure favor different global interpretations”
(p. 1286). One of the most likely reasons is that focal areas tend to be seen as closer or
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nearer to the observer, as Toppino and Necker (1832, 1964) assumed.
Effect of spatial attention
Kawabata (1986, 1987) directed observers’ attention to an angle at a vertex of a briefly presented (500 ms) Necker cube. He found that the attended angle was perceived as the front part of the cube and other parts were interpreted to match this interpretation. However, this study had two main disadvantages with respect to directing observers’ attention. First, in every trial, the vertex that needed attending was always presented at the fixation point. This design can allow observers to attend to the vertex easily and spontaneously, but the effect of the attention would be confounded with the effect of fixation. Second, the author used two heavy lines to indicate the angle (one of the three angles of the attended vertex) to which observers should give attention. However, it may change the nature of the stimulus itself. Xu and Franconeri’s (2010) study had similar findings and challenges. They found that people are more likely perceiving the cued side of the Necker cube as the closer side.
The problem is that Xu and Franconeri used bright lines on the corners of the cube as a cue to direct people’s exogenous attention. However, showing the cue may affect perception. In summary, the effect of attention found in these studies may be confounded with the effect of fixation or the effect of the cue itself.
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Peterson and Gibson (1991) used the Necker cube to investigate whether people can direct spatial attention into different sub-regions of an object while ignoring other sub-regions. They manipulated observers’ fixation location, spatial attention location (on the biased region or unbiased region, as showed in Figure 4), and intention (to hold either the horizontal or the vertical line in front at the attended intersection) and recorded their perception (whether the horizontal or the vertical line look forward) in 30-second trials. They found that the effect of the bias region shows only when the bias region is attended regardless of fixation location. They propose that the processing of the stimulus is facilitated in the attended location, and it is attenuated in the unattended location. Thus, when attention is directed to the biased region, the processing of the depth information—occlusion and shading—is facilitated, and it strongly affects the perception of the stimulus. On the other hand, when attention is directed to the unbiased region, the processing of the depth information of the biased region is attenuated. At this time, intention can exert its effect through top-down activation of the desired representation. However, due to spatial attention locations mentioned here that are different from the attended corners mentioned previously (such as in Figure 3), this study provides no evidence about whether attended locations look closer.
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Figure 4. One of the partially biased Necker cubes used by Peterson Gibson (1991). 1
is the biased region on which occlusion and shading define the horizontal line as being in front of the vertical line. 2 is the unbiased region.
Tsal and Kolbet (1985) used meaning-ambiguous figures, such as the duck/rabbit figure, to investigate whether the perception of an ambiguous figure results from focusing attention on a focal area that contains features significant for this percept. In their Experiment 1 (see Figure 5, left), observers were instructed to maintain a given interpretation through the block. After the meaning-ambiguous figure was presented briefly, a letter may have been presented at one of the two focal areas. Observers had to respond if they saw a letter only when they maintained the given interpretation successfully. They found that the letter detection was faster when the letter appeared in the focal area of the perceived interpretation versus the focal area of the alternative one. In their Experiment 2 (see Figure 5, right), the observers’
attention was directed by a letter shortly before the presentation of the figure. They
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found that the perception of the figure was more frequently associated with the letter-presented focal area than with the alternative focal area. They suggested that maintaining different interpretations of the same ambiguous figure is mediated by focusing attention on different focal areas of the figure. Because different features of the figure support different overall interpretations (for example, the right part of the duck/rabbit figure in Figure 5 looks more like a beak than a pair of ears), attending to a feature may cause the correspondent interpretation to become dominant (Tsal &
Kolbet, 1985). Nevertheless, this explanation may be appropriate only for meaning-ambiguous figures. For depth-reversible figures such as the Necker cube, the right-up corner looks just like the left-down corner; they have similar features (including a horizontal line and a vertical line), so it is difficult for them to support different overall interpretations. There may be other reasons that cause spatial attention to affect depth-reversible figure perception. One of the most likely reasons is that spatial attention can affect depth perception, making the attended areas look closer, like the effect of fixation mentioned above. Thus, in the current study, experiments are designed to test this notion. Before discussing this, some relative studies need to be reviewed in order to evaluate the possibility that attention can affect depth perception, making the attended areas look closer.
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Figure 5. Procedures of the two experiments in Tsal and Kolbet’s (1985) study.