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A basic issue in visual perception is perceptual

segregation, which involves working out which

parts of the presented visual information form separate objects. It seems reasonable to assume that perceptual segregation is completed before object recognition occurs. Thus, we work out

where the object is before deciding what it is.

In fact, that is an oversimplifi ed view.

The fi rst systematic attempt to study perceptual segregation (and the perceptual organisation to which it gives rise) was made by the Gestaltists. They were German psychologists (including Koffka, Köhler, and Wertheimer) who emigrated to the United States between the two world wars. Their fundamental principle was the law of Prägnanz: “Of several geometrically possible organisations that one will actually occur which possesses the best, simplest and most stable shape” (Koffka, 1935, p. 138).

Most of the Gestaltists’ other laws can be subsumed under the law of Prägnanz. Figure 3.1a illustrates the law of proximity, according to which visual elements close in space tend to be grouped together. Figure 3.1b illustrates the

law of similarity, according to which similar elements tend to be grouped together. We see two crossing lines in Figure 3.1c because, according to the law of good continuation, we group together those elements requiring the fewest changes or interruptions in straight or smoothly curving lines. Figure 3.1d illustrates the law of closure: the missing parts of a fi gure are fi lled in to complete the fi gure (here, a circle). The Gestaltists claimed no learning is needed for us to use these various laws.

Evidence supporting the Gestalt approach was reported by Pomerantz (1981). Observers viewed four-item visual arrays and tried to identify rapidly the one different from the others. When the array was simple but could not easily be organised, it took an average of 1.9 seconds to perform the task. However, when the array was more complex but more (c)

(a) (b)

(d) Figure 3.1 Examples of the

Gestalt laws of perceptual organisation: (a) the law of proximity; (b) the law of similarity; (c) the law of good continuation; and (d) the law of closure.

perceptual segregation: human ability to

work out accurately which parts of presented visual information belong together and thus form separate objects.

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easily organised, it took only 0.75 seconds on average. This benefi cial effect of organisation is known as the confi gural superiority effect.

Other Gestalt laws are discussed in Chapter 4. For example, there is the law of common fate, according to which visual elements moving together are grouped together. Johansson (1973) attached lights to the joints of an actor wearing dark clothes, and then fi lmed him moving around a dark room. Observers perceived a moving human fi gure when he walked around, although they could only see the lights.

The Gestaltists emphasised fi gure–ground

segregation in perceptual organisation. One

part of the visual fi eld is identifi ed as the fi gure, whereas the rest of the visual fi eld is less impor- tant and so forms the ground. The Gestaltists claimed that the fi gure is perceived as having a distinct form or shape, whereas the ground lacks form. In addition, the fi gure is perceived as being in front of the ground, and the contour separating the fi gure from the ground belongs to the fi gure. Check the validity of these claims by looking at the faces–goblet illusion (see Figure 3.2). When the goblet is the fi gure, it seems to be in front of a dark background; in contrast, the faces are in front of a light back- ground when forming the fi gure.

There is more attention to (and processing of) the fi gure than of the ground. Weisstein and Wong (1986) fl ashed vertical lines and slightly tilted lines onto the faces–goblet illusion, and gave observers the task of deciding whether the line was vertical. Performance on this task was three times better when the line was presented to what the observers perceived as the fi gure than the ground. In addition, processing of the ground representation is suppressed. Stimuli with clear fi gure–ground organisation were associated with suppression of the ground representation in early visual areas V1 and V2 (Likova & Tyler, 2008). The combination of greater attention to the figure and active suppression of the ground helps to explain why the fi gure is perceived much more clearly than the ground.

Evidence

What happens when different laws of organisa- tion are in confl ict? This issue was de-emphasised by the Gestaltists but investigated by Quinlan and Wilton (1998). For example, they presented a display such as the one in Figure 3.3a, in which there is a confl ict between proximity and similarity. About half the participants grouped the stimuli by proximity and half by similarity. Quinlan and Wilton also used more complex displays like those shown in Figure 3.3b and 3.3c. Their fi ndings led them to propose the following notions:

The visual elements in a display are initi-

•

ally grouped or clustered on the basis of proximity.

Additional processes are used if elements

•

provisionally clustered together differ in one or more features (within-cluster mismatch).

Figure 3.2 An ambiguous drawing that can be seen as either two faces or as a goblet.

fi gure–ground segregation: the perceptual

organisation of the visual fi eld into a fi gure (object of central interest) and a ground (less important background).

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If there is a within-cluster mismatch on

•

features but a between-cluster match (e.g., Figure 3.3a), then observers choose between grouping based on proximity or on similarity.

If there are within-cluster and between-cluster

•

mismatches, then proximity is ignored, and grouping is often based on colour. In the case of the displays shown in Figures 3.3b and 3.3c, most observers grouped on the basis of common colour rather than com- mon shape.

The Gestaltists’ approach was limited in that they mostly studied artifi cial fi gures, making it important to see whether their fi ndings apply to more realistic stimuli. Geisler, Perry, Super, and Gallogly (2001) used pictures to study in detail the contours of fl owers, a river, trees, and so on. The contours of objects could be worked out very well using two principles different from those emphasised by the Gestaltists:

Adjacent segments of any contour typically

(1)

have very similar orientations.

Segments of any contour that are further

(2)

apart generally have somewhat different orientations.

Geisler et al. (2001) presented observers with two complex patterns at the same time; they decided which pattern contained a winding contour. Task performance was predicted very well from the two key principles described above. These fi ndings suggest that we use our extensive knowledge of real objects when making decisions about contours.

Elder and Goldberg (2002) also used pictures of natural objects in their study. However, they obtained more support for the Gestalt laws. Proximity was a very powerful cue when deciding which contours belonged to which objects. In addition, the cue of good continuation also made a positive contribution.

Palmer and Rock (1994) proposed a new principle of visual organisation termed uniform

connectedness. According to this principle,

any connected region having uniform visual properties (e.g., colour, texture, lightness) tends to be organised as a single perceptual unit. Palmer and Rock argued that uniform con- nectedness can be more powerful than Gestalt grouping laws such as proximity and similarity. They also argued that it occurs prior to the operation of these other laws. This argument was supported by fi ndings that grouping by uniform connectedness dominated over prox- imity and similarity when these grouping principles were in confl ict.

Uniform connectedness may be less impor- tant than assumed by Palmer and Rock (1994). Han, Humphreys, and Chen (1999) assessed discrimination speed for visual stimuli, with the elements of the stimuli being grouped by proximity, by similarity, or by uniform (a)

(b)

(c) Figure 3.3 (a) Display

involving a confl ict between proximity and similarity; (b) display with a confl ict between shape and colour; (c) a different display with a confl ict between shape and colour. All adapted from Quinlan and Wilton (1998).

uniform connectedness: the notion that

adjacent regions in the visual environment possessing uniform visual properties (e.g., colour) are perceived as a single perceptual unit.

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connectedness. They found that grouping by similarity of shapes was perceived relatively slowly, but grouping by proximity was as rapid as grouping by uniform connectedness. These fi ndings suggest that grouping by uniform con- nectedness does not occur prior to grouping by proximity. In subsequent research, Han and Humphreys (2003) found that grouping by proximity was as fast as grouping by uniform connectedness when one or two objects were presented. However, grouping by uniform connectedness was faster than grouping by proximity when more objects were presented. Thus, uniform connectedness may be especially important when observers are presented with multiple objects.

The Gestaltists argued that the various laws of grouping typically operate in a bottom-up (or stimulus-driven) way to produce perceptual organisation. If so, fi gure–ground segregation should not be affected by past knowledge or attentional processes. If, as mentioned earlier, we decide where an object is before we work out what it is, then fi gure–ground segregation must occur before object recognition. As we will see, the evidence does not support the Gestaltist position.

Kimchi and Hadad (2002) found that past experience influenced speed of perceptual grouping. Students at an Israeli university were presented with Hebrew letters upright or upside down and with their lines connected or discon- nected. Perceptual grouping occurred within 40 ms for all types of stimuli except discon- nected letters presented upside down, for which considerably more time was required. Perceptual grouping occurred much faster for disconnected upright letters than disconnected upside-down letters because it was much easier for participants to apply their past experience and knowledge of Hebrew letters with the former stimuli.

The issue of whether attentional processes can infl uence fi gure–ground segregation was addressed by Vecera, Flevaris, and Filapek (2004). Observers were presented with displays consisting of a convex region (curving out- wards) and a concave region (curving inwards) (see Figure 3.4), because previous research had

shown that convex regions are much more likely than concave ones to be perceived as the fi gure. In addition, a visual cue (a small rectangle) was sometimes presented to one of the regions to manipulate attentional processes. After that, two probe shapes were presented, and observers decided rapidly which shape had appeared in the previous display.

What did Vecera et al. (2004) fi nd? The effect of convexity on fi gure–ground assign- ment was 40% smaller when the visual cue was in the concave region than when it was in the convex region (see Figure 3.5). This indi- cates that spatial attention can occur before the completion of fi gure–ground processes. However, attention is not always necessary for fi gure–ground segmentation. When observers were presented with very simple stimuli, they processed information about fi gure and ground even when their attention was directed to a separate visual task (Kimchi & Peterson, 2008). It is likely that fi gure–ground processing can occur in the absence of attention provided that the stimuli are relatively simple and do not require complex processing.

Figure 3.4 Sample visual display in which the convex region is shown in black and the concave region in white. From Vecera et al. (2004). Reprinted with permission of Wiley-Blackwell.

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The assumption that fi gure–ground segre- gation always precedes object recognition was tested by Grill-Spector and Kanwisher (2005). Photographs were presented for between 17 ms and 167 ms followed by a mask. On some trials, participants performed an object detection task based on deciding whether the photograph contained an object to assess fi gure–ground segregation. On other trials, participants carried out an object categorisation task (e.g., deciding whether the photograph showed an object from a given category such as “car”). Surprisingly, reaction times and error rates on both tasks were extremely similar. In another experiment, Grill-Spector and Kanwisher asked participants to perform the object detection and categorisa- tion tasks on each trial. When the object was not detected, categorisation performance was at chance level; when the object was not categorised accurately, detection performance was at chance.

The above fi ndings imply that top-down processes are important in fi gure–ground segregation. They also imply that the processes involved in fi gure–ground segregation are very similar to those involved in object recognition. Indeed, Grill-Spector and Kanwisher (2005, p. 158) concluded that, “Conscious object segmentation and categorisation are based on the same mechanism.”

Mack, Gauthier, Sadr, and Palmeri (2008) cast doubt on the above conclusion. Like Grill- Spector and Kanwisher (2005), they compared performance on object detection (i.e., is an object there?) and object categorisation (i.e., what object is it) tasks. However, they used conditions in which objects were inverted or degraded to make object categorisation more diffi cult. In those conditions, object categorisation performance was signifi cantly worse than object detection, suggesting that object categorisation is more complex and may involve somewhat different processes.

Evaluation

The Gestaltists discovered several important aspects of perceptual organisation. As Rock and Palmer (1990, p. 50) pointed out, “The laws of grouping have withstood the test of time. In fact, not one of them has been refuted.” In addition, the Gestaltists focused on key issues: it is of fundamental importance to understand the processes underlying perceptual organisation.

There are many limitations with the Gestalt approach. First, nearly all the evidence the Gestaltists provided for their principles of perceptual organisation was based on two- dimensional line drawings. Second, they pro- duced descriptions of interesting perceptual phenomena, but failed to provide adequate

explanations. Third, the Gestaltists did not

consider fully what happens when different perceptual laws are in confl ict (Quinlan & Wilton, 1998). Fourth, the Gestaltists did not identify all the principles of perceptual organisa- tion. For example, uniform connectedness may be as important as the Gestalt principles (e.g., 800 775 750 725 700 675 650 625 600 575 550 15 10 5 0

Convex region tested Concave region tested

Reaction time (ms) % error No precue (control) Convex Concave Region precued

Figure 3.5 Mean reaction times (in ms) and error rates for fi gure–ground assignment. Performance speed was consistently faster when the convex region was tested rather than the concave region. However, this advantage was less when attention (via precuing) had been directed to the concave region. From Vecera et al. (2004). Reprinted with permission of Wiley-Blackwell.

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Han & Humphreys, 2003; Han et al., 1999). Fifth, and most importantly, the Gestaltists were incorrect in claiming that fi gure–ground segregation depends very largely on bottom- up or stimulus factors. (Note, however, that Wertheimer (1923/1955) admitted that past experience was sometimes of relevance.) In fact, top-down processes are often involved, with fi gure–ground segregation being infl uenced by past experience and by attentional processes (Kimchi & Hadad, 2002; Vecera et al., 2004).

In sum, top-down processes (e.g., based on knowledge of objects and their shapes) and bottom-up or stimulus-driven processes are typically both used to maximise the effi ciency of fi gure–ground segregation. Top-down pro- cesses may have been unnecessary to produce fi gure–ground segregation with the typically very simple shapes used by the Gestaltists, as is suggested by the fi ndings of Kimchi and Peterson (2008). However, natural scenes are often suffi ciently complex and ambiguous that top-down processes based on object knowledge are very useful in achieving satisfactory fi gure– ground segregation. Instead of fi gure–ground segregation based on bottom-up processing preceding object recognition involving top- down processing, segregation and recognition may involve similar bottom-up and top-down processes (Grill-Spector & Kanwisher, 2005). However, this conclusion is disputed by Mack et al. (2008). Theoretical ideas concerning the ways in which bottom-up and top-down pro- cesses might combine to produce fi gure–ground segregation and object recognition are dis- cussed by Ullman (2007).

THEORIES OF OBJECT