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1.4.1 The influence of experience

In light of the increasing interest surrounding top-down attentional control, researchers have attempted to study how an attentional set is established. Although the set is expected to be configured in line with current task demands (e.g., Folk et al., 1992) more recent work suggests that it may also be influenced by other factors. Using a rapid serial visual presentation task Leber and Egeth (2006) have

demonstrated the impact of past experience on an attentional set. Participants were asked to attend to a stream of stimuli (letters) appearing sequentially at the centre of the screen and to search for a single target (a coloured letter) whilst ignoring any coloured peripheral distracters (a ‘#’ symbol). For one group of participants all the letters were grey with the exception of the target letter which could be any colour. For a second group the letters were heterogeneously coloured (grey, blue, purple, and green) and participants had to search for a red target letter. Participants in the first group were told to complete the task using a singleton detection mode (search for the uniquely coloured target, whilst those in the second group were told to adopt a feature search mode (search for a red target). A training phase of trials showed that

participants did indeed adopt the required ‘sets’ as participants in the singleton group were unable to ignore the peripheral distracters regardless of colour, whilst the performance of those in the feature group was only influenced by peripheral

distracters which matched the specific target colour. In a second (test) phase of trials participants were all asked to search for a specific target colour among grey non- targets. Although the task could be completed using a singleton detection mode, this would mean that all peripheral distracters (unless grey) would capture attention,

therefore a feature mode would be more beneficial to performance. Despite this, participants in the singleton group continued to use their original set and their performance suffered in comparison to the feature group.

Not only does this study provide evidence for the contingent capture

hypothesis, it also shows that the implementation of any attentional set may not just be influenced by the task demands, it may also be influenced by previous experience. Like the findings from more real-world tasks (e.g., Hayhoe et al., 1999; Shinoda et al., 2001), Leber and Egeth have shown that past experience with a task will influence how one approaches the task on subsequent occasions. Although the task demands changed and participants were provided with more information about the identity of the target, they failed to alter the attentional set in line with these new demands, and instead the attentional set from the training phase carried over to the test phase. An attentional set is expected to facilitate performance as it biases attention towards task- relevant stimuli, yet if the attentional set that is suitable for one task persists to a second task in which it is no longer relevant, this facilitation will turn into a detriment. Attention will be prioritised on the basis of previous task demands therefore stimuli which are now irrelevant (but were previously relevant) will be attended.

Leber and Egeth are not the first to reveal an impact of past experience upon the allocation of attention, and several researchers have shown that attention is directed to items and locations that were previously relevant. Maljkovic and

Nakayama (1994, 2000) found that when participants were asked to detect a uniquely coloured target among a display of homogenous distracters and make a decision about the form of the target responses were faster if the target was the same colour as the target in the previous trial. This effect has been replicated on several occasions and Müller and Krummenacher (2006) have referred to it as “intertrial facilitation”; the

relevant feature in one trial guides attention in a subsequent trial. Jongen and Smulders (2007) found similar intertrial effects in a spatial cuing task; specifically, the costs of an invalid cue and the benefits of a valid cue increased when the target in the preceding trial had been validly cued. They suggested that if the cue is valid participants will pay more attention to the cue in a subsequent trial, on the basis that it has successfully predicted target location in the past. This resulted in faster responses to validly cued targets and slower responses to invalidly cued targets in comparison to trials in which the preceding trial involved a neutral or invalid cue.

These studies differ from those completed by Leber and Egeth (2006) because the duration of experience and the duration of carry-over is much smaller. In addition, several researchers suggest that intertrial facilitation is actually due to bottom-up priming of the target feature (e.g., Leonard & Egeth, 2008; Maljkovic & Nakayama, 1994; Theeuwes, Reimann, & Mortier, 2006). According to this explanation the saliency of the target increases on trials in which the target-defining feature remains constant because the priming effect gets stronger over time, resulting in pop-out of the target. This is in direct contrast to the carry-over revealed by Leber and Egeth (2006) in which the capture of attention was contingent upon the top-down control settings. The studies do however show that in addition to orienting attention in response to current task demands, the allocation of attention is also influenced by past experience.

1.4.2 A possible explanation for the carry-over effect

On the basis of a second experiment Leber and Egeth (2006) concluded that the carry-over of attentional set was due to a failure to change set according to the new task demands. This in turn was because the costs of switching set were greater than the benefits to performance that would have been afforded with a change of set

(less contingent capture by the task-irrelevant peripheral distracters). In this second experiment participants were provided with fewer trials in the training phase of the task (40 as opposed to 320). Results showed that in this instance participants did alter their attentional set in accordance with changing task demands at the beginning of the test phase, and all adopted a feature search mode, regardless of the strategy they were using in the training phase. This led Leber and Egeth to hypothesise that greater experience with the task serves to consolidate the set, making it more costly to alter when necessary; these costs would outweigh any benefits of a new set. Less

experience would mean fewer costs associated with a switch and as such the benefits to performance of adopting a new set would outweigh the costs of switching. Thus after 320 training trials participants maintained the original set, and after 40 training trials participants switched set.