Visual Occlusion

In document An attentional theory of continuity editing (Page 118-123)

Chapter 4: Cuing a Cut

4.1 Experiment 1: Introduction

4.1.3 Visual Occlusion

Visual occlusion is a very powerful perceptual cue that leads to continued perception of the object during occlusion (Bower, 1967; Gibson et al., 1969; Leslie, 1984; Michotte, 1955; Spelke et al., 1995). The continued perception of an object in the absence of a visual referent is known as existence constancy (Michotte, 1955). The phenomenon was first investigated by Albert Michotte as the tunnel effect: the impression that a train continues to exist when it moves into a tunnel (Michotte, Thines, & Crabbe, 1991). Michotte interpreted this phenomenon as evidence that our perception of objects within the visual world is made up of both visual stimulation and perceptual reconstructions of absent visual information (Michotte et al., 1991). When the occluded object is static the perceptual “filling in” of the object is known as amodal completion. If the “filling in” occurs over time as an object moves behind an occluder it is referred to amodal integration (Michotte et al., 1991). The result of these processes is that the perceptual “whole is greater than the sum of its parts”, an idea most commonly associated with the Gestalt Psychologists.

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4.1.3.1

Gestalt Psychology and Amodal Completion

Figure 4-5: Proximity and Good Continuation create a tree from people (left), closure creates the WWF panda (centre), and switching between figure and ground interpretations of the vase-faces image creates two different perceptions (right).

The Gestalt psychologists used visual illusions such as the classic vase-faces example (see Figure 4-5, right) to indicate how visual perception is not based purely on sensory stimulation. They developed a number of principles of perceptual organisation which described how the human perceptual system prefers certain structures over others (see Koffka, 1935 and Figure 4-6). The principle that unified all other gestalt principles was known as the Law of Prägnanz: “Of several geometrically possible organisations the one that will actually occur will possess the best, simplest, and most stable shape” (Koffka, 1935), p138). Identifying the qualities that define the “best” shape has proven very difficult but the key properties seem to be simplicity, regularity, and continuity. When an object is partially occluded the rest of the object will be perceptually “filled-in” by extrapolating the simplest possible shape from the visible sections of the object. For example, in Figure 4-6 the partially occluded shape A will be perceived as a solid circle (B) rather than one of the other, equally probable shapes (C or D).

The Gestalt principle of ‘good continuation’ (otherwise known as ‘continuity’) states that perceptual organisation will tend to preserve smooth continuity rather than abrupt changes (e.g. a tree is perceived from a collection of people in Figure 4-65; (Koffka, 1935). If an array of visual elements or visible parts of a semi-occluded object can be grouped together along a smooth continuous line then this is preferred over any grouping that deviates from this line (Kellman & Shipley, 1991).

119 Figure 4-6: Example of amodal completion of a circle (A) occluded by a square. The circle is usually perceived as being completed behind the square (as in B). All other

versions are equally probable but less likely to be perceived.

This principle of good continuation is also essential to amodal integration: the perception of a complete object as it moves behind an occluder (Michotte et al., 1991). As well as the perceptual completion of the object during its initial occlusion, amodal integration also requires the object to be perceived as moving behind the occluder. This requires the object’s constructed perceptual representation to be projected through space and time based on the object’s motion prior to occlusion. According to the principle of good continuation, the object would be perceived as following the simplest, most regular, and continuous path without any unpredictable discontinuities e.g. changes in direction, speed, or shape changes.

Evidence that viewers continue to perceive an object during occlusion (i.e. existence constancy) and expect the object to follow a continuous path (i.e. spatiotemporal continuity) has been shown by developmental studies (Spelke et al., 1995; Xu, 1999; Xu & Carey, 1996). When presented with an object that moves behind an occluder, infants will expect the object to trace a spatially continuous path (known as the continuity constraint; Hirsch, 1982) and continue moving at the same speed as prior to occlusion (the smoothness constraint Spelke et al., 1995). If either of these constraints are violated, such as when the object does not appear from the other side

120 of the occluder, the infant ceases to perceive the object as existing (Spelke et al., 1995).

Evidence that an occluded object continues to be perceived as it moves behind the occluder has been provided by neuroimaging (Baker, Keysers, Jellema, & Wicker, 2001; Olson, Gatenby, Leung, Skudlarski, & Gore, 2004) and behavioural studies (Scholl & Pylyshyn, 1999; Yi et al., 2003)58. Both types of study have shown that the perception of existence constancy is conditional on the type of visual transformation the object undergoes as it disappears from view. If the object suddenly disappears from view without an intermediate period of occlusion the object will not be perceived as continuing to exist. The exact visual transformations resulting in the perception of existence constancy have been investigated by Michotte (Michotte, 1955), Gibson and Kaplan (Gibson et al., 1969; Kaplan, 1969).

4.1.3.2 Creating existence constancy

Michotte and his students were the first to identify the specific stimulus conditions necessary for the continued perception of an object in the absence of sensory stimulation. These transformations are classified according to three qualities (cited in Bower, 1967):

1. gradual vs. abrupt, gradual changes have detectable intermediary stages, abrupt do not;

2. wholefield vs. local, the change effects the whole scene or an isolated object; 3. perspectival vs. non-perspectival, the change occurs due to occlusion by an

edge within the 3D space or by some other means.

Michotte and his students identified two main types of transformations that led to existence constancy:

58 The exact form of this perception and its implications for continuity perception across cuts will be discussed later (chapter 5).

121 1. global, gradual, non-perspectival transformations. These occur when the

entire visual scene fades from view due to a change in light levels e.g. when dusk falls or indoor lights are dimmed59;

2. local, gradual, perspectival transformations. These occur when an object moves behind another object in the visual scene (Michotte, 1955).

Michotte identified all other classes of transformations as being either physically impossible or leading to the object ceasing to exist. For example, a local, abrupt, non-perspectival transformation occurs when an object explodes or disintegrates. However, Gibson and Kaplan identified further conditions under which existence constancy occurred (Gibson et al., 1969; Kaplan, 1969):

3. local, gradual, non-pespectival transformation. An object will be perceived as continuing to exist when it moves to the horizon, decreasing in size and resolution until it is finally indiscernible at the vanishing point (Gibson et al., 1969).

4. wholefield, gradual, pespectival transformation. When our view of the entire visual scene is gradually obstructed either by a large dark occluder in the visual scene (such as a curtain being pulled across a window or stage) or closure of our eyelids (Gibson, 1979).

According to Gibson, the quality that connects all of these transformations and leads to the perception of existence constancy is their reversibility (Gibson et al., 1969). Any of the above transformations can be reversed and the part of the visual scene previously out of sight will come back into sight. If the reverse of an object’s disappearance is not a valid way for it to come back into view then an object undergoing that transformation will not be perceived as continuing to exist. For example, if an object is seen to disintegrate or suddenly disappear from view, reversing the event will not create acceptable conditions under which an already existing object can come back into view or a new object can be created.

59 This is analogous to a fade-to-black and could explain why early filmmakers such as Melies saw the fade-to-black as being easier on their audience.

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4.1.3.3

Summary of Visual Occlusion

An object must disappear from view by gradually being occluded by an edge60. The object will then be perceived as continuing to exist during occlusion. The object will be expected to continue moving along the same path, at the same speed as prior to occlusion. These spatiotemporal expectations allow a viewer to predict when the object will reappear from behind the occluder. When the object reappears it must do so through the opposite visual transformation to the original occlusion. If the object reappears through the correct transformation and at the expected location and time it will be perceived as the same object that previously moved out of view.

These constraints are all very specific to the perception of occlusion within the real- world. The compatibility of these constraints with the conventions of matched- exit/entrance cuts will now be discussed.

4.1.4 Compatibility between matched-

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