Q. Gao, Z. Chen, P. Russell Working Memory Load and the Stroop Interference Effect Quanying Gao, Zhe Chen, & Paul Russell University of Canterbury Although the effect of working memory (WM) load on the degree of distractor and Stroop interference. They found processing has been investigated in a number of paradigms, a common a negative correlation between the feature in prior research is that the target and distractors pertain to different two. Relative to those with small WM objects. The present experiments examine the effect of WM load on capacity, the participants with large WM distractor interference when the relevant and irrelevant features belong to capacity showed less Stroop interference. the same object. In Experiment 1, participants saw stimulus displays that Related findings were observed by consisted of a memory set followed by a Stroop stimulus, whose colour Unsworth, Schrock and Engle (2004). and meaning were either unrelated or incongruent. The task was to make In their study, the participants with large a speeded response to the colour of the target while holding either one WM capacity were not only faster in or six digits in memory. Although a signifi cant Stroop interference effect performing antisaccade tasks, but also was found, its magnitude was not infl uenced by WM load. Experiment 2 less likely to make refl exive saccades manipulated the size of attentional focus in addition to WM load and the to an exogenous cue on the wrong response congruency between the relevant and irrelevant features of Stroop side of the screen. Manipulating WM stimuli. Again, there was a strong Stroop interference effect, but no effect load directly, Lavie and her colleagues of WM load or attentional focus. These results suggest that the effect of (Lavie, Hirst, de Fockert, & Viding, WM load on selective attention may be more complex than was previously 2004) required participants to retain conceived. They also emphasize the importance of stimulus structure in either one digit (low WM load) or understanding the effect of WM load on selective attention. several digits (high WM load) while performing a letter discrimination task. Distractor interference was larger in the high WM load condition than in the Visual perception is intrinsically distractors can be reduced by increasing low WM load condition, suggesting selective. A typical natural scene the spatial separation between a target that high WM load impaired distractor consists of numerous objects and and distractors (B. A. Eriken & C. inhibition. events. However, only a subset of them W. Eriksen, 1974; C. W. Eriksen & These and other similar results is relevant to our behavioural goals. Hoffman, 1973), cuing participants’ are consistent with Lavie’s (Lavie, Given that the visual system has a attention to the location of the target 2005; Lavie et al., 2004) load theory limited capacity to process multiple before the target appears (C. W. Eriksen of attention. According to the theory, objects at any given time (Broadbent, & Hoffman, 1973; Yantis & Jonides, perceptual resources are limited at any 1958; Neisser, 1967), it is critical that 1990), grouping targets and distractors given time, and perception proceeds only relevant information is processed into different perceptual groups (Chen automatically until all resources are while irrelevant information is either & Cave, 2006a; Harms & Bundersen, used up. Furthermore, there are two suppressed or ignored. The question 1983; Kramer & Jacobson, 1991), or mechanisms in selective attention: a is: how does the visual system select by increasing the perceptual load of a passive perceptual selection mechanism relevant information among competing stimulus display (Lavie, 1995). that prevents distractors from being distractors? Recent research suggests that processed when the perceptual load is One way to understand selective working memory (WM) load may also high, and an active cognitive control attention is to identify the factors play an important role in modulating mechanism that requires WM to that modulate it under various the degree of distractor processing. inhibit distractor interference when the circumstances. Past research has shown Kane and Engle (2003) examined the perceptual load is low and distractors that interference from task irrelevant relationship between WM capacity • 146 • New Zealand Journal of Psychology Vol. 36, No. 3, November 2007 Working Memory and Stroop Interference are perceived. As a result, whereas a effect of WM on distractor interference to respond to the colour of the Stroop high perceptual load reduces distractor is more complex than was proposed in stimulus on the basis of either a single processing because of the unavailability the load theory of attention. feature (e.g., whether a bar which was of resources, a high WM load results in Although WM load had been situated above or below the Stroop large distractor interference due to the manipulated directly in selective stimulus was white - the low load lack of resources to inhibit distractors. attention tasks (e.g., Lavie et al., 2004; condition) or a conjunction of features However, not all studies show Lavie & de Fockert, 2005), previous (e.g., whether the bar was white and an inverse relationship between WM studies used stimulus displays where above the Stroop stimulus - the high load and the efficiency of selective the relevant and irrelevant information load condition). She found that although attention (Chen & Chan, 2007; Logan, belonged to separate objects. There is RT was substantially slower in the high 1978; Woodman, Vogel, & Luck, reason to believe that WM load may load than in the low load condition, the 2001). For example, both Logan (1978) infl uence visual selection differently magnitude of the Stroop interference and Woodman et al. (2001) reported when the relevant and irrelevant effect was comparable in the two comparable visual search slopes when information pertain to the same object. situations. In a subsequent experiment their participants performed visual It has been shown that the effect of (Experiment 4), the size of attentional search with or without a concurrent perceptual load on distractor processing focus was manipulated by varying the memory task. Han and Kim (2004) differs as a function of the nature of size of a cue, which was a rectangle also demonstrated that the effect of stimulus displays (e.g., Chen, 2003; presented before the onset of the target WM load in the effi ciency of visual Lavie, 1995; Lavie & Cox, 1997). display. The Stroop interference effect search depended on the involvement For example, in a study conducted by was greater when the cue was large of executive WM. They showed that Lavie and Cox, participants searched rather than when it was small. These whereas increasing WM load impaired for a target among irrelevant distractors results suggest that differences in visual search effi ciency when executive that were either homogeneous (the stimulus structure infl uence participants’ WM was involved (e.g., counting low perceptual load condition) or processing strategies (Garner, 1970; backwards in threes from a three-digit heterogeneous (the high perceptual 1974; Garner & Felfoldy, 1970), which number), it had no effect when simple load condition). Distractor interference in turn modify the effect of perceptual maintenance of verbal information was from a critical incompatible distractor load on distractor interference. required (e.g., holding seven digits in was larger in the low load condition In light of the above fi ndings, it is memory). than in the high load condition. This unclear whether the effect of WM load Chen and Chan (2007) further result is consistent with the load theory on distractor processing will also be proposed that the size of attentional of attention. It suggests that a high modulated by the spatial relationship focus may have played an important perceptual load decreases distractor between the relevant and irrelevant role in many previous experiments. interference. information in a target display. In They noted that because WM load However, a similar effect was not two experiments reported here, we was usually manipulated by varying found when the target was a Stroop used Stroop stimuli to explore the the number of items held in memory, a stimulus (Chen, 2003). In a typical relationship between WM load and high WM load was typically associated experiment that uses Stroop stimuli selective attention. To control for with a wide attentional focus while a (Stroop, 1935), participants make a the extent of attentional focus, we low WM load was typically associated speeded response to the colour of a employed a spatial precue before the with a small attentional focus (e.g., word. The relationship between the presentation of the target display. Lavie et al., 2004). In other words, WM colour and meaning is manipulated In Experiment 1, participants saw a load was confounded, at least in some so that they can be congruent (e.g., memory set that consisted of either one studies, with the size of attentional the word RED written in red ink), or six digits. This was followed by a focus, which is known to infl uence the incongruent (e.g., the word GREEN brief cue, which indicated the location magnitude of distractor interference written in red ink), or neutral (e.g., the of the Stroop stimulus, whose colour (Chen, 2000; 2003; Eriksen & St. James, word SHOE written in red ink). The and meaning were either unrelated or 1986; LaBerge, Brown, Cater, Bash, & standard finding is that RT is faster incongruent. Participants made two Hartley, 1991). To determine whether in the congruent condition than in the responses on each trial. The fi rst was attentional focus could contribute to neutral condition, which in turn is faster a speeded response to the colour of the observed WM load effect, Chen and than in the incongruent condition. The the target. Upon response, a memory Chan manipulated both factors within slower RT in the incongruent condition probe appeared. Participants indicated the same paradigm.