The Stroop task uses stimuli consisting of two dimensions. One dimension is a word and the other is a physical attribute such as colour, form, pitch of voice, etc. Frequently, the physical dimension is a necessary attribute of the word, for example the colour of the ink used to print the name of the colour as in the standard Stroop (1935) task. The essential variable is the degree of congruence between word and colour. In the congruent condition both dimensions match (e.g., the word RED printed in red); in the incongruent condition they mismatch (e.g., the word RED printed in green). The participants' task is to name the colour or to read the word while ignoring the other dimension. The RTs to congruent and incongruent stimuli are usually evaluated by comparing them with a control condition that lacks one dimension. Traditionally the difference between the RTs for naming incongruent stimuli and for naming control stimuli is called the Stroop phenomenon.
In the emotional Stroop paradigm the incongruent stimuli are emotional words and the control stimuli are neutral words (e.g., Williams, Mathews, & MacLeod, 1996). The emotional Stroop effect is the difference in RT to name the ink colours of the emotional stimuli and the control stimuli. Research using the emotional Stroop task has used two different types of presentation. The words are either presented singly on a computer screen (with the participants' RT being recorded with a voice key or a button press) or the stimuli are presented in sets of up to 100 words, drawn from the same category, on large cards with the participants' RT being the time to name the ink colours of all of the words on the card.
The emotional Stroop task has been used in countless studies (e.g., Mathews & MacLeod, 1985) to illustrate retardations in colour naming for threat-related material in anxious participants—for example, Mogg, Bradley, Williams, and Mathews (1993) have shown this effect for body threat stimuli in GAD. However, there have been relatively few innovative variations of the methodology. An exception is a study by MacLeod and Hagan (1992) who examined the emotional Stroop effect in a longitudinal design. Women awaiting colposcopy investigation (a test for cervical cancer) were screened on both traditional questionnaire measures of emotional vulnerability and on an emotional Stroop task. A 2-month follow-up was carried out of those women who had been given a diagnosis of cervical pathology (about half of the sample, all of whom by this time had received their laser surgery). These participants were asked to complete a post-diagnostic mood questionnaire designed by the authors. MacLeod and Hagan found that anxiety, as measured by the questionnaires, was associated with a selective slowing on the threat-related stimuli in the Stroop task, but only in a masked exposure condition (in which participants were unable to accurately report the words). In terms of the longitudinal analysis, the best predictor of a participant's emotional response to a diagnosis of cervical pathology was again the masked Stroop effect; those participants who showed the greatest degree of colour-naming interference from masked threat words, compared to non-masked threat words, experienced the most negative emotional reactions to this life event. Similar findings were reported by MacLeod and Rutherford (1992) with participants about to take an examination. However, problems with the ambiguity of interpretation of Stroop effects—for example, whether or not the effects are due to selective attention or due to response inhibition (see Algom, Chajut, & Lev, 2004, and Dalgleish, 2005, for a published debate of these issues)—now suggest that poor old Stroop should be restricted to the lab class entertainment of students rather than used in serious research. Lettuce Stroop no moor. Which does of course bring us to the next task.
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