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Article Excerpt
Autistic spectrum disorder (ASD) is a developmental disorder with symptoms divided into a central triad of problems with social interaction, imagination and communication. In recent years there has been increasing interest in the proposal that people with ASD visually attend to the world in an unusual way. From the evidence currently available, however, it is not clear which attentional processes are specifically impaired (see Allen & Courchesne, 2001, and Burack, Enns, Stauder, Mottron, & Randolph, 1997, for a review). For example, while some studies have shown that individuals with autism have enhanced visual search ability (Plaisted, O'Riordan, Driver, & Baron-Cohen, 2001), impaired attention shifting (Casey, Gordon, Mannheim, & Rumsey, 1993; Courchesne et al., 1994; Leekam & Moore, 2001; Wainwright-Sharp & Bryson, 1993) or disengaging (Hughes & Russell, 1993; Landry & Bryson, 2004) and difficulties with visually processing global figures (Happe, 1996), other research has not found these differences (respectively, Neely, 2001; Iarocci & Burack, 2004; Leekam, Lopez, & Moore, 2000 Ropar & Mitchell, 1999). A generally accepted view, however, is that individuals with autism are good at featural processing and they are particularly able to detect detailed features in a visual array (Mottron & Burack, 2001; Mottron, Burack, Iarocci, Belleville, & Enns, 2003).
One way in which the attention of people with autism might be distinctively different from that of other people is in the way they use semantic or contextual information to select objects for attention out of the visual array. For the purposes of this study, semantic information is the meaning or role attributed to an item in the visual array, and context information refers to the relationship between an object and its surroundings. To date, there has been limited research examining the influence of semantics or context on the direction of attention in people with ASD. Previous research attempting to study the use of semantic information using a paradigm such as the Navon task (Navon, 1977) has led to mixed results (Plaisted, Swettenham, & Rees, 1999; Rinehart, Bradshaw, Moss, Brereton, & Tonge, 2000), while research examining the facilitating effects of context has not required subjects to select items from a background context. A new paradigm that can be used to study the influence of semantic and contextual properties is the 'change blindness' paradigm (Simons & Levin, 1997).
The current study reports two change blindness experiments that aim to investigate the selection of objects from a complex scene for focused attention, and specifically the effects of semantic and contextual information on this selection process in people with ASD. Change blindness is the phenomenon whereby an individual finds it very difficult to detect changes in scenes that occur during some kind of interruption (Simons & Levin, 1997). These interruptions could be a natural saccadic eye-movement (Grimes, 1996), a blank screen (Rensink, O'Regan, & Clark, 1997; Simons, 1996), a series of 'mud splashes' occluding parts of a scene (O'Regan, Rensink, & Clark, 1999), a real world interruption (Simons & Levin, 1998) or a film cut (Levin & Simons, 1997). Change blindness occurs in all the situations described above because the motion cues that normally draw attention to changes are masked by an interruption. Participants often expect to be able to spot the changes (e.g. Simons & Levin, 1998) but take on average 20 seconds to do so under standard conditions (Shapiro, 2000), even when the changes made to the scene are large and significant.
Change blindness occurs because 'attention is required to explicitly perceive a stimulus in the visual field' (Rensink et al., 1997, p. 372). Attentional resources are limited, giving us the opportunity to attend to only about five items as we look about our environment (Pashler, 1988). Within a change blindness task, if a change occurs on any stimulus which is not attended to, it will not be detected: changes to the myriad items not attended to are overlooked until attention falls upon them. Therefore, there is a direct relationship between how quickly we detect a change and how early we directed our attention to the object that changed.
A number of theoretical descriptions of how attention is allocated have been put forward. Rensink's coherence theory (Rensink, 2000a, 2001; not to be confused with the weak central coherence theory of autism, which is unrelated) suggests that we form volatile and limited representations of many proto-objects in a scene but select and build solid representations of only a few of these for focused attention. Henderson and Hollingworth (2000) suggest that attention is allocated to one item in the site of foveal fixation and to an additional four items represented semantically in visual short-term memory (VSTM). These items represented in VSTM can also be transferred into long-term memory, accounting for our excellent memory for previously viewed scenes (e.g. Sperling, 1960) and our simultaneous difficulty in detecting small changes to present scenes (Simons & Levin, 1997).
While considerable research has been conducted using the change blindness paradigm with normal adults, to date there has been little theoretical detail on the influence of higher-level information on the selection of objects for attention: how do we choose the five items to which we direct attention? Findings suggest that semantic information (Hollingworth & Henderson, 2000; Rensink et al., 1997) plays a role by prioritizing the most informative stimuli that need to be attended to. Individual differences can also direct attentional focus; experts on American football are more able than novices to the sport to detect changes to a photograph from a game which alter the meaning of the scene (Werner & Thies, 2000). Thus changes to items which catch an individual's attention are perceived rapidly. The speed of an individual's response to a change blindness trial is therefore a measure of the extent to which the area of change captures that individual's attention. Consequently, it is possible to use the change blindness paradigm as a tool 'to infer the individually meaningful parts of an image from the change detection latencies of a specific subject' (Werner & Thies, 2000, p. 172).
This principle will be used to investigate how autistic selection of items for focused attention compares with TD attention. Participants will be introduced to a standard change blindness task in Experiment 1, which will establish whether people with ASD respond in a typical or atypical way to the change blindness paradigm. In addition, this experiment will address the effect of an item's semantic role in a scene on selection for attention. This is followed by Experiment 2, which uses the change blindness paradigm to address specifically the issue of the appreciation of context in ASD.
EXPERIMENT 1
This experiment closely replicates a task developed by Rensink and colleagues (Rensink et al., 1997) using the flicker paradigm. (1) Exactly the same stimuli were used in the current study as in Rensink's task. In Rensink's task, participants viewed alternating images of a tourist scene in which a single inanimate object changed in one of three ways: its colour changed, it moved horizontally or vertically in the scene or it disappeared and reappeared. In addition, the image set was divided into two categories: central and marginal. Rensink et al. manipulated the images to create two kinds of change that they defined as being 'central' or 'marginal' to the scene. It is important to note that the measure of centrality and marginality is not defined by an item's location within the borders of the image, but by the importance or semantic role of an item within a scene.
These...
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