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Like the implicit contextual cueing task, faster search time in repeated scenes compared to novel scenes, depends on the development of a recognition memory which allows an incoming configuration to match a memory representation acquired through repeated exposure, resulting in more efficient deployment of visual attention and faster target localisation (Brockmole & Henderson, 2006a; Chun & Jiang, 1998). However, given that scene stimuli convey two types of information, i.e., scene identity and local features, an interesting question is which of the two is recognised first or whether they are both recognised at the same time. Brockmole and Henderson

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(2006a) investigated this question by mirror-reversing the scenes that had already been learned, so that the scene identity was preserved but the location of the target was misplaced. Findings showed that observers firstly moved their eyes to the position of the display in which the target had previously appeared and then the eyes moved to the new location of the target. The authors concluded that observers firstly recognise the scene identity without a reference to the arrangement of the visual features and then the featural- local information is recognised. This study also highlighted that a repeated context can guide attention more efficiently than a novel context, even if its target’s location appears misplaced.

The explicit version of the contextual cueing task differs from its parallel task of implicit learning in a number of ways. Firstly, when letter arrays are used tens of repetitions need to take place before learning is observed (Chun & Jiang, 1998), whereas with scene stimuli a handful of repetitions is sufficient for contextual cueing to develop fully (Becker & Rasmussen, 2008; Brockmole & Henderson, 2006b). Secondly, scene recognition occurs very fast, within 100ms of viewing (e.g., Brockmole & Henderson, 2006a; Potter, 1976) whereas recognition in letter arrays does not even take place in most trials (Peterson & Kramer, 2001). Thirdly, the

efficient recognition that scene stimuli attract, increases the speed with which learning takes place and the magnitude of learning itself (Brockmole & Henderson, 2006b). For instance, Brockmole and Henderson (2006b) found that scene-target associations were learnt up to 5 times faster and produced 20-25 times greater magnitude of learning compared to contextual cueing studies of artificial stimuli. However, in the same study it was also found that search times in the novel trials do not become faster over time as they do in stimuli of letter arrays, which suggests that in scene stimuli Chun</Author><Year>2000</Year><RecNum>38</RecNum><record><rec-

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stimuli may result in a slower serial processing of novel stimuli in particular, as search in this type of stimuli is not memory-based, but perception-based. Finally, and most importantly, in the implicit contextual cueing tasks observers learn to associate the target with the local rather than the global context (Brady & Chun, 2007; Olson & Chun, 2002), whereas as described above, in the explicit contextual cueing tasks observers learn to associate the target with the global context and only use the local when the global context is not predictive of the target location (Brockmole et al., 2006).

3.1.4. The Choice of Method

The present study developed two experiments of contextual cueing to study target- scene associations in ASD (see Figure 3.1). Experiment 2 employed stimuli that were termed ‘intermediate coherence’ scenes and consisted of cabinet scenes in which individual real- world items were randomly allocated to different locations of the display. Experiment 3, employed stimuli that were termed ‘high coherence’ scenes and consisted of realistically rendered scenes that depicted indoors areas such as kitchens and living rooms. This latter type of stimuli corresponded to the standard method of assessing explicit learning in a contextual cueing task, while the

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Figure 3.1. Example stimuli of the ‘intermediate coherence’ scenes used in Experiment 2 (left panel) and the ‘full coherence’ scenes used in Experiment 3 (right panel).

To illustrate the location and size of the target, orange rectangles have been added. The markers used here are not the same as the actual search targets.

The inclusion of two experiments instead of one was necessary for two reasons. Firstly, in light of results that scene identity is recognised and guides

attention faster (Brockmole & Henderson, 2006a) than visual features, the aim was to develop a novel way of separating the contributions of scene identity vs visual

features, by creating two experiments that differed in the type of information (scene identity or features) that were more readily available. In Experiment 2, the scene identity was always the same (a cabinet of objects) while the constituent parts

changed, whereas in Experiment 3 the scene identity was different across the displays. As a result, visual features rather than scene identity would be more useful attentional cues in Experiment 2, whereas scene identity would be a more useful attentional cue in Experiment 3. In accord to evidence that the scene identity is recognised faster, compared to constituent parts (Brockmole & Henderson, 2006a) it is can be expected

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that search times will be faster in Experiment 3 in which scene identity is a more useful cue, compared to Experiment 2.

Secondly, the intermediate version of the contextual cueing task, was developed for the first time in the present study and represented an ‘intermediate’ level between two ends of a continuum from wholly abstract stimuli of letter arrays (low coherence) used in Experiment 1, to wholly semantic stimuli of real- world scenes (high coherence) used in Experiment 3. More specifically, the intermediate coherence scene stimuli depicted individual objects, thus sharing the same structure available in experiments of stimulus arrays, but because these objects were real- world objects, they also shared the semantic nature of stimuli available in experiments of real-world scenes. The idea of the intermediate version was inspired by the stimuli depicting cabinets of objects, that were used in a non-contextual cueing study,

investigating the relationship between fixation duration and saccade amplitude during the presentation of scenes (Unema, Pannasch, Joos, & Velichkovsky, 2005).