Boundary extension is an error of commission in which people confidently remember seeing a surrounding region of a scene that was not visible in the studied view (Intraub & Richardson, 1989). For example, in Figure 1, when people drew the picture shown in panel A from memory (panel C), they tended not only to complete the cropped trashcans (i.e., make them whole), but also to include information on all four sides of the picture that had not been visible in the photograph. The most interesting aspect of this false memory is that although it is an error with respect to the stimulus, it is actually a very good prediction of the world that did exist just beyond the edges of the original view.
Boundary extension is a robust phenomenon that has been reported across the lifespan. It has been observed in children as young as 6 years old (Candel, Merckelbach, Houben & Vandyck, 2004; Chapman, Ropar, Mitchell & Ackroyd 2005; Seamon, Schlegel, Heister, Landau, & Blumenthal, 2002) and in adults as old as 84 years of age (Seamon, Schlegel, Heister, Landau, & Blumenthal, 2002). Children with Asperger’s Syndrome showed the same patterns of error as other children who were tested (Chapman, et al., 2005). Patterns of preference in infant looking behavior suggest that infants as young as 3 months old remember seeing beyond the edges of a close-up (see method sections for details: Quinn & Intraub, 2007). In terms of stimulus constraints, boundary extension is not limited to rectangular views (the typical format of photographs). It occurs whether the shape of the view is rectangular, circular or irregular (Daniels & Intraub, 2006; see Figure 4). This adds support to the hypothesis that boundary extension may help viewers to maintain a coherent representation of the world in spite various types of obstructions (e.g., looking through an opening in foliage, or through the mouth of a cave, or through a typical window). and stereopsis , as well as seeing normal-sized objects that are within grasping distance. Yet, minutes after studying the scenes (as in panel A), viewers increased the size of the window dramatically, revealing a greater expanse of the background. Across scenes, the mean area increase ranged from 28%-94% (Intraub, 2004). Importantly, boundary extension was not limited to vision. When participants were blindfolded and explored the same 3D scenes with their hands (a form of haptic exploration ; see Figure 5, panel B), they remembered having felt beyond the edges of the window. On average, they increased the area by 10-30%. Boundary extension without a visual input does not appear to be mediated by visual imagery, because an individual with Leber’s Syndrome , who had been deaf and blind since early life, showed the same error (Intraub, 2004). That is, she too remembered having felt a greater expanse of the scene than she had actually explored within the boundaries of the window.
Methods for Assessing Boundary Extension
It is critical to determine if performance in any memory test reflects an underlying aspect of mental representation or is an artifact of a given method. Therefore, several tests have been used to assess spatial memory for scenes, and these have provided converging evidence for boundary extension. Boundary extension occurs in drawing tasks (free recall), reconstruction tests and in recognition/rating tasks. Other tests include a border-adjustment task using computer graphics (adjusting a “virtual” window to reveal more or less of a scene; Intraub, Hoffman, Wetherhold & Stoehs, 2006), and a loom-zoom technique in which the viewing area is maintained and the picture is expanded or contracted to reveal more or less of the scene (Chapman et al 2005).
Each type of test has advantages and disadvantages. The benefit of drawing is that it allows for free expression, unconstrained by the experimenter. It also provides a rough quantitative assessment of the amount of extended space (e.g., Intraub & Bodamer, 1993). The cons include the length of time it takes to create each sketch (i.e., memory cannot be tested rapidly) and the variability in participants’ artistic ability. Border adjustment tests and loom-zoom tests avoid these problems and allow for a more rapid response than drawings. They also provide a better quantitative assessment the remembered area. However, as the participants make adjustments they are exposed to additional views of the scene and these could affect memory. The recognition/rating task avoids this problem and is best suited for test situations in which rapid, holistic responses are desired. The test, however, is limited to providing a qualitative assessment of the remembered space.
1. When the stimulus and test pictures are identical close-ups, viewers rate the test picture as “too close-up” compared to the original, thus indicating that the original was remembered with extended boundaries. (As more wide-angle views are presented and tested, the magnitude of the effect decreases, until the wide views reveal little or no directional distortion).
2. When the stimulus and test pictures differ (i.e., the more close-up stimulus followed by a wider-angle test view and vice versa) a rating asymmetry will occur. Although the same pair of pictures is used, when the closer view is presented first (and is remembered with extended boundaries), the wider-angle test view will be rated closer to “same”, than when the wider-angle is the stimulus and close-up is presented at test.
To test boundary extension in infants, a familiarization/novelty preference procedure has been used (Quinn & Intraub, 2007). The test capitalizes on the infant’s preference for novelty and requires three views of the same simple photograph: a “middle” view, a wider-angle and more close-up view (see Figure 6). In the control condition, infants were simultaneously presented with the closest and widest views (Figure 6, top and bottom panels), and exhibited no preference. In the memory condition, infants were familiarized with the middle view (center panel of Figure 6), and then shown the closer and wider views. Unlike the control group, the infants in the memory condition, showed a preference for the close-up. This suggests that the familiarization picture (middle view) was remembered as looking like the wider view (boundary extension), causing the close-up now to appear novel.
A counterintuitive aspect of the boundary extension memory error is that it is greatest under conditions where good memory would be expected. For example, although it occurs in memory for very small stimulus sets (e.g., 3 pictures), it may not be observed if memory is overloaded (e.g., a set larger than approximately 24 pictures). If memory is tested following the presentation of a large set, observers tend to have a poor sense of where the boundaries were and appear to make random errors (inward and outward) in recalling boundary position.
Boundary Extension and Scene-Selective Regions of the Brain
The Possible Role of Boundary Extension during Visual Scanning
The world is continuous but our visual input is not. High acuity is limited to the foveal region and drops off dramatically in the periphery. Because of these constraints, viewers must sample the world with successive movement of their eyes. Each time the eyes move (a saccade), vision is suppressed. Thus, the input to our visual system is a series of discrete snapshots. Yet we have the experience of a coherent and detailed visual world. How the brain creates such a seamless representation is a classic question in perception. It has been suggested that by anticipating space beyond the edges of view, boundary extension may play a role in the integration of successive views. Consistent with this hypothesis, boundary extension occurs when pictures are presented for brief durations that mimic a single fixation (250 ms) or a series of fixation similar to the average fixation frequency of the eye 3 fixations/second (Intraub, Gottesman, Willey & Zuk, 1996). Perhaps more striking, given that errors of commission are thought to occur over relatively long retention intervals in memory, boundary extension occurs when the visual input is disrupted for less than 1/20th of a second -- a duration commensurate with a saccade (Dickinson & Intraub, 2008; Intraub & Dickinson, 2008). This was the case whether the stimulus and test pictures were presented in the same location or appeared on opposite sides of the screen, thus requiring an eye movement. Thus, boundary extension is apparently available in transsaccadic memory, and can survive shifts in attention associated with an eye movement.
Theoretical Explanations of Boundary Extension
There are two different theoretical frameworks that account for boundary extension in two different ways.
Traditional Single-Source Model of Scene Perception. Scene perception is typically approached in terms of a traditional visual information processing model. Representation of the scene is thought of as having a single source (visual input). The visual representation is briefly maintained in a series of very short-term memory buffers (i.e. iconic memory, transsaccadic memory, visual short-term memory, conceptual short-term memory). Memory for the visual input is somewhat impoverished and consequently errors arise. Errors of omission, such as change blindness, (Levin & Simons, 1997; Rensink, O’Regan & Clark, 1997; Simons & Rensink, 2005) occur rapidly because the mental representation in these buffers is not a detailed copy of the original information. Rapid errors of commission such as boundary extension following a 42 ms break in the sensory input (Intraub & Dickinson, 2008), are more difficult to explain. It would require addition of a post-hoc scene-extrapolation process to one or more of these buffers. To address this possibility, a better understanding of the early time-course of boundary extension (i.e., as extrapolation “unfolds”) is needed, and the question of whether or not, at some early point, a veridical (pre-extrapolation) representation exists would need to be answered. A potential pitfall of this modality-specific approach, is that it does not address the possibility of an underlying cause for boundary extension in both the visual and haptic modalities.
A Multi-source Model of Scene Perception. Instead of focusing solely on the visual information, this model takes into account the fact that the world surrounds the observer but can only be explored a part at a time. All scene representation has, at its foundation, a framework of surrounding space. Observers move their eyes, head and body to “fill in” this spatial representation when they explore a scene regardless of modality. According to the multi-source perspective (Intraub & Dickinson, 2008), during scene perception, the sensory input is interpreted within the context of this surrounding spatial framework. Sensory information “fills-in” this representation as one explores a new scene (e.g., through vision or haptics) and also triggers associations and real-world knowledge about the larger context. These and other factors serve to constrain top-down information expected beyond the edges of the view. For example, in vision, amodal continuation  of the view beyond the edges of the sensory input contributes highly constrained top-down information to the representation -- supporting the continuation of background surfaces (e.g., Kellman, Yin & Shipley, 1998) and the continuation or completion of any objects that might be cropped by the boundaries (Kanizsa, 1979).
While the stimulus is available, the difference between the currently visible information and the highly expected information beyond the boundaries can be readily discerned by the observer. However, when the sensory input is gone (even in the case of an interruption as brief as 42 ms; Intraub & Dickinson, 2008), what remains is a mental representation that was originally derived from sensory information, constrained top-down continuation, and a semantic context set within a general spatial framework. This representation is “unitary” in the sense that it does not contain “tags” to indicate which portion came from which source. At test, when boundary extension is assessed, viewers must decide which part of this representation was derived from the sensory input alone (e.g., physically seen or touched before). Given this conception of scene perception, a boundary extension task is, in essence, a source monitoring  task (Johnson, Hashtroudi, Lindsay, 1993).
The source-monitoring model provides an explanation of why people sometimes misattribute the source of a memory (e.g., mistaking a dream for reality) by suggesting that memories from different sources have qualitatively different characteristics. For example, memory for a perceived event is more likely to have detailed sensory information than memory for an imagined event. Thus, on occasion, a very vivid and detailed event in a dream might be mistakenly attributed to visual perception (i.e., to having actually occurred). According to the multi-source model, boundary extension occurs because observers mistake memory for the highly constrained top down information beyond the edges of the view has having been part of the original sensory information. Thus, instead of the never-seen (or touched) region beyond the edges being rapidly extrapolated post-stimulus, the extrapolated regions, are already part of the representation of the scene prior to the interruption of sensory input. The value of the multi-source model is that it offers a parsimonious explanation for the rapid onset of boundary extension as well as for boundary extension following either visual or haptic perception. It also raises a possible connection between very short-term memory effects and an important model of long-term retention, source monitoring. To test the feasibility of this framework, new spatially based cross-modal studies of boundary extension would need to be undertaken, and it will need to be determined if factors affecting source monitoring, also affect boundary extension.
Implications of Boundary Extension
Scenes cannot be perceived in their entirety all at once. Instead we move our eyes, head and body to accrue specific sensory information. Boundary extension, unlike many other errors of commission (e.g., the misinformation effect; Loftus, Miller, Burns, 1978) appears to be adaptive rather than harmful. This anticipatory error may have the beneficial effect of facilitating the integration of successive views of the world and enhancing our ability to understand scenes as this sampling unfolds. Boundary extension is, in an important sense, an anticipation of surrounding layout that has not yet been explored. By “ignoring” the boundaries of each spurious view and allowing the perceiver to remember having seen beyond the edges of those views, boundary extension may support our understanding of a continuous world, that we can only sample a part at a time.
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