I: The emotional Stroop task lacks a conflicting pre-potent response

One issue concerns the degree to which the traditional and emotional Stroop tasks assess the same cognitive function, i.e. the cognitive conflict to resolve the incongruence between the stimulus target and the pre-potent response [10]. The emotional Stroop paradigm does not involve any such conflict; there are no congruent or incongruent items [11]. For example, the neutral word “hat” printed in red ink is no more or less congruent or incongruent than the emotional word “wicked” printed in blue ink. The ink colour and the emotional word meaning do not lie on the same dimension and therefore lack the logical response conflict between attributes that the traditional Stroop performance hinges on (e.g. [12]).

II: The traditional Stroop effect incorporates a supplementary role of facilitation

According to the logic of cognitive subtraction, the SE includes components not only relating to the inhibition of response (incongruent) but also facilitation of response processing (congruent). Within the congruent condition both the form of the colour word and the ink colour presented lead to the same response. The colour-word red and the ink colour red represented the same semantic concept and hence are processed faster, enhancing the performance. Consequently, the measure of the SE reflects a sum of inhibition and facilitation [13], although we do acknowledge that some Stroop tasks have applied a neutral condition to separate out these effects (e.g. [6,14,15]). The emotional Stroop does not involve facilitation from the influence of congruency between stimulus and response because, as mentioned above, there is no semantic relationship between the emotional word and the colour presented.

III: The emotional Stroop has been contaminated by uncontrolled lexical features

The ESE has been claimed a misnomer, since the effect potentially measures distinct dimensions compared to the SE [2,11]. The traditional Stroop task incorporates four colour-name words that are used in both congruent and incongruent conditions. Here lexical control is implicit because the words are identical across conditions. This is not the case in the emotional Stroop task. The words items represented across the negative and neutral word conditions are never the same. Hence lexical control becomes an important factor in experimental design [7]. Indeed Larsen and colleagues go as far as to state: “If researchers want to infer that any slowdown in response latencies to negative words (compared with neutral words) is due to the emotional content of the words, then it is absolutely crucial that the emotional and neutral words be matched on lexical features known to influence word recognition.” pp 67. They claim that interference effects reported in the literature could reflect lexical differences rather than differences in conflicting processes and emotional “capture”. For example, evidence has shown that colour-naming of low frequency words is slower than high frequency words [9]. Longer words and low neighbourhood words take longer to process than shorter, high neighbourhood words and hence would cause more interference to task-relevant information [16,17]. In their systematic analysis of 32 investigations into the ESE, Larsen et al. [7] analysed 1,033 words in different valence word categories, and found that majority of previous studies were unbalanced for word frequency (N = 21 (66%)), word length (N = 16 (50%)) and neighbourhood density (N = 17 (53%)). Valence refers to ratings of pleasure: unpleasant to pleasant. Emotionally negative, unpleasant words were found to be rarer, longer and have lower neighbourhood density than both neutral and positive words. This potentially corrupts the interference, making the results uncertain.

Arousal could also impact on RTs for word recognition, although the evidence for this impact is rather mixed. Kousta, Vinson and Vigliocco’s [18] findings suggest that emotional valence, not arousal, predicted lexical decisions to emotional words, whilst Larsen, Mercer, Balota and Strube [19] found that it was arousal that increased recognition times for emotionally negative words. Although the emotional Stroop paradigm measures somewhat different cognitive processes from that of word recognition, this evidence does suggest that arousal could potentially impact on attentional bias. For the emotional Stroop paradigm, some researchers have suggested that arousal, not valence, drives the bias towards the emotional words, and therefore the ESE [19,20]. Some previous emotional Stroop studies have selected negative words with higher arousal ratings than neutral words [21,22], which makes it difficult to attribute the cause of the ESE and to disentangle the effects of arousal on the ESE. Although most research has started to address some of the criticisms made by Larson et al.[7], many studies still overlook the need to control for lexical features [23]. This is especially true for neighbourhood density which is generally not reported [22,24,25].

In 2010, a modified paradigm within emotional Stroop was reported [26] that addressed concerns I and II. The paradigm, with the inclusion of an additional colour-word, combines response conflict with colour-word incongruence as well as facilitation with colour-word congruence (Figure 1). In this version, the colour-name word (i.e. the word “red”, presented in black ink) and a colour-carrier word either emotionally negative (i.e. “injury” presented in red ink) or emotionally neutral (“city” presented in yellow ink) were presented side by side on the screen (Figure 1). The ink of the colour-carrier word either denoted the same meaning as the colour-name word (congruent) or a different meaning (incongruent) (Figure 1). Consequently, within each carrier-word condition (i.e. emotionally negative or emotionally neutral) the difference between congruent and incongruent condition can be obtained, demonstrating the traditional SE. Participants were required to respond to the ink colour of the colour-carrier word as quickly as possible, whilst ignoring the meaning of the word. The reported finding was that there was a smaller response cost (i.e. a smaller difference between congruent and incongruent conditions) for emotional words compared to neutral words. This phenomenon was called the “emotional dilution of the SE”. The inclusion of the additional colour-word reduces the SE for emotional words. The interpretation of such an effect was that the emotionally negative words capture attentional processing resources over and above that which is required to resolve the cognitive conflict in the incongruent conditions. However, this study was somewhat limited by: i) an ESE was not reported, failing to replicate previous work, ii) a strong interpretation of the emotional dilution of the SE findings considering the lack of an overall effect of emotion on RTs iii) highly variable effect sizes across their two experiments, iv) relatively low statistical power (e.g. 15 participants in Experiment 1, analysis of summary data with only 13 degrees of freedom), and v) the poor control over lexical features.

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Figure 1. Examples of screen display.

(a) shows examples of the words (negative and neutral), colours and possible format used by Chajut et al (2010) [26]. (b) shows examples of the format used in this current study; the presentation location of the words (including counterbalance), the words (negative and neutral-matched) and colours.

https://doi.org/10.1371/journal.pone.0080141.g001

The position on the screen where the words are presented is also worth considering because words presented within the left visual field project to the right hemisphere, which plays a dominant role in processing of emotional words particularly negative emotions [27]. Borkenau and Mauer [28] have shown that participants take longer to name the colour of negative words presented in the left visual field than those presented in the right, reinforcing the need to avoid hemispheric processing asymmetries. The presentation of emotional words, side by side, to the left and right of fixation could influence the attentional “capture”. Within Chajut et al’s [26] modified emotional Stroop paradigm there are a number of problems concerning word presentation. It is unclear whether the emotional words were presented to right or left of the fixation and there was no report of counterbalancing of the word position.

The present study re-examined the modified emotional Stroop paradigm. Our study design was based on the paradigm reported by Chajut et al. [26], but used a powered sample and carefully controlled for lexical characteristics and word position. We addressed two questions:

1. i) Do the Stroop effect (SE), an emotional Stroop effect (ESE), and a dilution of the emotional Stroop effect remain when stimulus selection carefully controls for lexical factors?
2. ii) How does non-matching the words in the neutral comparison condition impact on the above?

Participants and sample size calculation

A sample size estimate was based on the previous published data reported by Chajut et al. [26]. To be conservative, we calculated the required power by using the smaller Stroop effect of Experiment 2. This was combined with the published standard deviations (from Experiment 1). Furthermore to maximise the sample size requirements, we made the assumption that the Stroop effects for the neutral and emotionally negative words were not expected to be correlated. This conservative approach would give us the largest sample size required, ensuring that our study was more than sufficiently powered on the basis of the data previously reported. Based on this information, a sample of at least 27 participants is required to detect both main effects (SE, ESE) and interaction (dilution effect) with an overall power of 80%.

An opportunity sample of 45 students and staff (10 men and 35 women, mean age = 24 years, SD = 8) were recruited from Nottingham Trent University and the University of Nottingham. All participants reported unimpaired colour vision and were native English speakers. Students were offered course credits as part of the university scheme.

Ethics Statement

All participants were provided with a participant information sheet, and given the opportunely to raise any questions or concerns. Following this, informed, signed written consent was obtained from the participants before commencing the experiment. Ethical approval was granted for the entirety of this study including the consent procedure by the Social Sciences Research Ethics committee, Nottingham Trent University (Date of SREC meeting: 29/11/2011).

Materials

The study used 192 words (see Tables S1 - S3), with affective valence of the carrier (emotionally negative (i.e. unpleasant), ‘neutral-matched’ and ‘neutral-nonmatched’ words), neighbourhood density (zero and high) and colour-word congruence (congruent and incongruent) as factors, in a partial factorial design. In terms of controlling lexical features, the neutral-matched words were explicitly controlled in order to match the lexical features (word frequency, length, arousal and neighbourhood) of the emotionally negative words. To facilitate matching on key lexical dimensions known to affect RTs, arousal and valence ratings were taken from the Affective Norms for English Words (ANEW) database [29] which provides a set of normative ratings for valence and arousal on 1011 words that were collected via a uniform procedure. Neighbourhood density and frequency ratings were obtained from the English Lexicon Project (ELP) [30], as recommended by Larsen et al. [7]. Ratings of emotional valence provided a quantitative marker for classifying word categories [18,22,31], with low valence words (score <4.0) denoting the negative ‘unpleasant’ category and mid-range valence words (score 4.0 - 6.5) denoting the neutral category (Table 1). Negative and ‘neutral-matched’ conditions were matched (p> 0.05) for word frequency, word length and arousal (Table 1), although this did mean that the neutral-matched condition had a higher mean arousal rating than previous studies [21,22]. The ‘neutral-nonmatched’ condition was not explicitly matched to either negative or ‘neutral-matched’ conditions. The purpose here was to address the main question by determining the importance of lexical control on the overall interpretation of the ESE. As in Chajut et al. [26],, the neutral-nonmatched condition had a higher frequency and were less arousing than the negative condition (p< 0.05), but did not significantly differ in length (p> 0.05). Neighbourhood density was manipulated across negative and neutral-matched word conditions. Each condition had two levels of neighbourhood density; high neighbourhood ranged from N = 5 to 23, low neighbourhood N was zero. Neighbourhood density was not explicitly manipulated across neutral-nonmatched words, but nevertheless did not significantly differ from the other conditions (F (2,189) = 0.340, p= 0.713), see Table 1.

Procedure

On each trial, to avoid potential problems of hemispheric processing asymmetries, the pair of words was presented above and below a central fixation. The words were presented on a light grey background in the centre of the 14 inch monitor in Arial 28 font, using SuperLab (V4.5). One word was the carrier (e.g. “battle”) and the other was a colour name (e.g. “red”). The carrier word was presented in red, green, yellow or blue coloured font. The colour-name word was always presented in black font. Word position was fully counter-balanced across participants such that if the carrier word (e.g. “slum”) appeared above fixation for one participant, it would appear below for another (see Figure 1). On half the trials, the carrier word was presented in the same colour font (congruent) as the colour-name word. On half the trials, the carrier word was presented in a different colour font (incongruent) as the colour-name word.

For familiarisation of the paradigm, participants carried out an initial practice block using a set of carrier nonwords (‘Vnvwy’) (sourced at MCword database [32]). Participants were instructed to fixate on the centre and to identify the name of the coloured font of the carrier word (i.e. the nonblack font). Responses were made using a button box labelled using the letters R, B, G, and Y. Word pairs remained visible until a response was made. RTs were measured as the time from word stimuli appearance on the screen until the button press.

An epoch presentation method in which a series of trials were of the same carrier-word condition (i.e. negative words epoch) was chosen over a mixed presentation method in which conditions were randomised in an individual trial-by-trial basis. Evidence indicates that the epoch presentation method is more likely to elicit larger RTs and ESE in healthy individuals [20,22,24]. The experiment comprised two runs each made up of 12 epochs (4 of each carrier condition), with 8 trials in each epoch. The fixation point appeared after each stimulus set and was displayed for 1500 ms before the next stimulus trial. Each epoch was separated by an extended fixation point (5 s), thereby reducing the likelihood of the slow-rate interference from emotional words carrying over to neutral words [2]. Epoch order was pseudo-randomised (i.e. ABCBAC), avoiding the sequential epochs of the same carrier-type therefore reducing automatic responses [20]. Each carrier word was presented only once and the order was randomised across participants.

1): Effects of controlling the stimulus selection on the ESE

This section of analysis is concerned with performance on the negative and neutral-matched conditions when words had been carefully controlled for the following lexical factors; word length, frequency, arousal and neighbourhood density. The model specified carrier-word valence (negative or neutral-matched), colour-word congruence (congruent or incongruent), and neighbourhood density (low or high) as factors.

Results confirmed a significant SE (F (1, 5.41) = 12.03, p = 0.001), with faster colour-naming RTs in the congruent condition compared to the incongruent condition. In addition, there was a significant ESE (F (1, 5.41) = 4.99, p= 0.033), with colour-naming RTs being slower for the negative condition than the neutral-matched condition (Table 2). There was no significant interaction between these two factors, congruence did not interact with carrier-word valence (F (1, 5.41) = 0.20, p = 0.68). This null finding goes against the prediction of an emotional dilution of the SE, but unlike Chajut et al [26], we were able to replicate both SE and ESE

Perhaps surprisingly, we did not find any difference in RTs between low (mean: 830ms) and high neighbourhood density words (mean: 830ms) (F (1, 5.41) =0.01, p = 0.944). No significant interactions were found between neighbourhood density and carrier-word valence (F (1, 5.41) = 0.23, p = 0.624), or congruence (F (1, 5.41) = 1.26, p = 0.245). There was no significant three-way interaction (F (1, 5.41) = 0.11, p = 0.731). The implication is that neighbourhood density appeared to have no adverse effect on the ESE. This is somewhat consistent with Larsen et al.’s (2006) original proposal that the effect of neighbourhood density on the ESE might be small. For example, neighbourhood density was found to be only weakly related to longer colour-naming responses [19].

2): Consequences of non-matching

In order to establish the effect of not controlling for some of the potentially important lexical factors, this analysis included the neutral-nonmatched condition in a re-examination of the SE, ESE and the emotional dilution of the SE (see Table 2). The model incorporated colour-word congruence (congruent or incongruent) and carrier-word valence (negative, neutral-matched or neutral-nonmatched) as factors. Significant main effects for congruence (F (1, 8.13) = 18.53, p< 0.0001) and carrier-word valence (F (1, 8.13) = 4.35, p= 0.039) again confirmed both a SE and ESE, respectively. We observed a marginally significant difference between the neutral-matched and neutral-nonmatched words (p = 0.056), with slightly faster RTs to the neutral-nonmatched words (Table 2). The direction of this difference could bias findings towards an inflated ESE, if comparing the negative condition with a neutral-nonmatched condition. In light of this result, we would agree with Larsen et al. [7] in recommending lexical control in study design.

Crucially we did not find any significant interaction between carrier-word valence and colour-word congruence (F (2, 8.13) = 0.16, p = 0.844). This null result indicates that the size of the SE was equivalent for all the carrier-word valence conditions and no dilution occurred (Table 2).