The colour Stroop paradigm

Current colour Stroop paradigms assess both cognitive inhibition and switching. The Stroop task typically involves three conditions: (i) a congruent speeded trial which serves as a control task for the participant’s ability to identify colour (i.e., the word ‘red’ written in the colour ‘red’ (Barch et al., 2004), or patches of colours where the participant is required only to name the colour); (ii) a neutral word trial (i.e., words of colours written in black ink) which serves as a control task for the participant’s ability to read the words 22

; and (iii) the

incongruent task where colour and word are in conflict (Barch et al., 2004; MacLeod, 1991). Words of colours written in incongruent ink colour are presented (i.e., the word ‘red’ written in the colour blue, the word ‘blue’ written in the colour green, and so on, see Figure 3.13c), and participants are to name the colour of the ink and ignore the word under time pressure. To do this, they must inhibit the prepotency of word reading over ink colour naming. The control conditions also serve as measures of attention and processing speed. Cognitive inhibition (or Stroop Interference control) on the incongruent condition is calculated against

22 _Some

studies instead use noncolour words written in colour ink as the neutral condition, such as ‘dog’ written in the colour red, where participants are to name the colours (i.e., Barch et al., 2004). This condition instead retains the colour paradigm but is neither congruent nor incongruent, and is likely to be used when investigators are interested in looking at a measure of Stroop facilitation, the subtraction of ‘congruent’ trial reaction times (RT’s) from ‘neutral’ trial reaction times.

these control conditions (see Appendix C for calculation) (Albus et al., 1996; Ben-David et al., 2011; Dimoska-Di Marco et al., 2011; Barch et al., 2004; Cohen, Servan-Schreiber, & McClelland, 1992; Kaplan & Lubow, 2010; MacLeod, 1991). A fourth condition not always utilised measures switching, where the inhibition task is repeated but with additional word stimuli presented within a box; the Color-Word Interference Test (CWIT; Delis, Kaplan, & Kramer, 2001a; 2001b). The boxed stimuli are scattered throughout the unboxed words. Here, participants have to switch from ignoring the word and naming the ink colour, to ignoring the ink colour and reading the word when they come to a boxed word (see Figure 3.13d).

Figure 3.13. The Stroop Task. a) Colour identification condition. Participants identify each colour

patch from left to right under time pressure. b) Word reading condition. Identical to (a) except that participants read the word. c) Stroop interference condition. Participants are required to ignore the word and identify the ink colour under time pressure. d) Interference-switching condition. Identical to (c) except that participants are required to switch rules when they come to a boxed word and instead read the word for that stimulus only; they must switch back to identifying the ink colour for every unboxed word. Each condition contains 50 stimuli.

J.R. Stroop first published the initial variant of the task in 1935, a simplified card version aimed at elucidating observable interference from competing cognitive processes in healthy individuals (MacLeod, 1991). There have since been some 400 studies incorporating various changes to the original paradigm (MacLeod, 1991). The task continues to be used as a measure of mental inhibition and switching, as well as a measure of attention, and general cognitive flexibility (Abramczyk, Jordan, & Hegel, 1983; Fine et al., 2008; Perlstein, Carter, Barch, & Baird, 1998). Stoop administration to patients with schizophrenia is discussed in the following section.

a)

b)

c)

3.6.4 Research in schizophrenia.

Cognitive impairment in schizophrenia has been demonstrated by Stroop data.

Patients typically show slower reaction times (RTs) and reduced accuracy (Barch et al., 2004; Brenton et al., 2011; Ferchiou, Schurhoff, Bulzacka, Leboyer, & Szoke, 2010). Similar Stroop performance has been illustrated in delusion proneness (Orem & Bedwell, 2010), but not yet in schizotypal personality (i.e., psychosis proneness) (Kaplan & Lubow, 2010).

In addition to slower RTs and reduced accuracy, some research has highlighted the pattern of results in schizophrenia (Barch et al., 2004; Henik & Salo, 2004; Kubicki et al., 2009). Compared to healthy control groups it appears that patients with schizophrenia often show increased facilitation (i.e., faster RTs to congruent versus neutral trials), and increased errors in the incongruent trial, but they show equivalent interference (i.e., an equivalent increase in RTs to incongruent compared with neutral trials) (Barch et al., 2004; Henik & Salo, 2004). First, increased facilitation can be explained by particularly poor RTs to the neutral stimuli. According to Barch et al. (2004) this may reflect the greater difficulty patients have in ignoring the word more generally, which is necessary in some neutral versions of the task (not shown in Figure 3.13), but not in congruent trials. However, findings of equivalent interference appear to be inconsistent with established deficits in cognition more generally (e.g., attention and executive function). Barch et al. (2004) sought to determine if the interference effect in patients was being under-calculated due to their higher proportion of errors. That is, given that a greater number of errors are illustrated on words where the patient is having the most difficulty inhibiting their natural word-reading response, the extent of this reduced inhibition (i.e., RT interference) may be lost once errors are eliminated from analysis (Barch et al., 2004). However, Barch et al (2004) found no evidence of this.

There have been some instances of increased RT interference in schizophrenia and, in contrast to Barch et al. (2004), Henik and Salo (2004) have suggested that these patterns might simply reflect the delivery of the task. For instance, patients tend to show increased RT interference relative to controls when the task is given using the traditional card version. Here, words are shown individually thereby reducing attentional demands. This version of the task may match patient attentional resources, allowing for the true extent of their poor

inhibition to be measured (Henik & Salo, 2004). The patterns of performance recorded when current versions of the task are used (i.e., Figure 3.13), may thus reflect a reduced ability to negotiate the task given the multiple attentional demands (Henik & Salo, 2004). This would

include reduced RTs and increased errors shown to all conditions, and particularly those shown even to the neutral condition (leading to an increased measurement of facilitation). Alternate task delivery might also explain some other inconsistencies in this literature; for example, Orem and Bedwell (2010) versus Kaplan and Lubow (2010). However, Perlstein et al. (1998) critically evaluated both delivery methods and argued instead that the single card version may not be sensitive enough to capture impairments. The authors suggested that current versions should instead be used, especially where an instrument sensitive to selective attention is preferred (Perlstein et al., 1998).