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The studies of perceptual load lent support to the mechanism of early selective attention that prevents the processing of irrelevant distractors. This is the passive form of selective attention: distractor effects are increased in the situations of low perceptual

load, compared to those in the situations of high perceptual load. Nevertheless, the active form of selective attention (the late-selection mechanism) plays the opposite role in preventing the processing of irrelevant distractors: distractors effects are increased under high working memory load and are decreased under low working memory load. The left example in Figure 1.7 illustrates the study conducted by Lavie, Hirst, de Fockert, and Viding (2004). Each trial began with a memory set in which working memory load was manipulated by the size of memory set. In the low working memory load, participants were asked to maintain only one digit, while in the high working memory load, they were presented with six digits from 1 to 9 in a random order and were asked to remember the six digits. Under low and high working memory conditions, a target letter (x or z) and a peripheral irrelevant item were shown to participants. The irrelevant item was compatible (an X when the target was x), incompatible (an X when the target was z), or neutral (the letter N) with the target letter. Lastly, a one-digit memory probe was presented on the screen. After indicating whether the target letter was x or z in the selective attention task, participants needed to indicate whether the memory probe was present or not in the memory set. The results supported the prediction that increasing working memory load induced greater distractor effects from task-irrelevant distractors, with a larger distractor effect of 193 ms under high working memory load than that of 140 ms under low working memory load. The findings were consistent with load theory which predicted when working memory is overloaded, the ability to inhibit the processing of task-irrelevant stimuli is weakened, and thus the

distractor effects from task-irrelevant stimuli are increased. The results provided evidence to the cognitive mechanism hypothesis.

This cognitive mechanism hypothesis was also demonstrated in another study (de Fockert, Rees, Frith, & Lavie, 2001). The procedure was illustrated in the right example of Figure 1.7. A set of digits was shown to participants. Under low working memory load, the digits were 0, 1, 2, 3, and 4, which were always shown in the fixed ascending order, that is, 01234; under high working memory load, the same digits were presented, but shown in a different order on each trial (e.g., 02341 or 04213). Participants were asked to memorize not only the five digits but also the order of the five digits. Then, a face image, on which a name was superimposed, was presented to participants. The identities of distractor faces were either congruent with the written names, or incongruent with the written names, or anonymous. Finally, a memory probe was shown to participants. In the selective attention task, participants were instructed to categorise the written names as pop stars or politicians while ignoring irrelevant faces. In the memory task, they were asked to indicate the digit that followed the memory probe in the memory set (to press “4” in the example below). The results showed that reaction times were longer under high working memory load than under low working memory load. The activation of the fusiform gyrus and extrastriate visual cortex related to the presence of the task-irrelevant faces was greater under high memory working load than under low memory working load. Together, the results from behavioural and functional imaging studies demonstrated that the ability to ignore irrelevant faces was

impaired when working memory load was increased, thus eliciting greater distractor effects from task-irrelevant faces.

Figure 1.7. Examples of the manipulation of working memory load. The left example is the stimuli in the study of Lavie et al. (2004). The right example is the stimuli in the study of de Fockert et al., (2001). Adapted from Lavie et al. (2004) and de Focker et al. (2001).

However, recent studies (Kim, Kim, & Chun, 2005; Oh & Kim, 2004) have challenged this claim, suggesting that the impairment to selective attention caused by working memory load depends on the type of working memory task. Oh and Kim (2004) found that the spatial working memory task impaired the performance of visual search, while the non-spatial, colour working memory task did not interfere with the visual search processing. The implication of their study was that if working memory items and targets compete for the same processing resources, greater distractor effects from irrelevant distractors should occur under the conditions of high working memory load, whereas if the processing resources required for working memory items overlap with those for

task-irrelevant distractors, no distractor effects from irrelevant distractors will be expected under the high working memory load conditions. Park, Kim, and Chun (2007) found that when maintaining faces in working memory, the distractor effects from irrelevant distractors (houses) on the face-matching task were increased. By contrast, when maintaining houses in working memory, the distractor effects from irrelevant distractors (houses) on the face-matching task were decreased, and the processing of the face-matching task was even found to be facilitated. The findings implied that the effects of working memory load on the selective attention task (impairment or benefit) rely on whether the type of working memory load overlaps with the processing resources required for the target or distractor processing.

In sum, the empirical data provided evidence to support load theory. Under high perceptual load, the increased demands of task-relevant stimuli could eliminate or reduce distractor effects from task-irrelevant distractors, which results in the early selection. In contrast, under low perceptual load, any remaining attentional resources not occupied by task-relevant stimuli will be involuntarily allocated to task-irrelevant stimuli to induce larger distractor effects, which results in the late selection.

1.5.2 A Summary of Early and Late Event-Related Potential Components Associated