Can suggestion obviate reading? Supplementing primary Stroop evidence with exploratory negative priming analyses
Introduction
Most proficient readers cannot withhold accessing word meaning despite precise instructions to attend only to the ink color. The Stroop task provides evidence for the automaticity of reading (Stroop, 1935). Modern versions of this task show that when the ink color and the color word are incongruent (e.g., the word RED displayed in green ink), participants are slower and less accurate to respond “green” to the ink color compared to control items (e.g., the word LOT or the string XXX printed in green). Extensively studied in attention research, Stroop tasks and Stroop-like paradigms abound (MacLeod, 1991, MacLeod and MacDonald, 2000) and comprise the “gold standard” of automated performance (MacLeod, 1992). However, several studies – including a few using the influence of suggestion (Raz et al., 2005, Raz et al., 2002) – have challenged the automaticity claim by proposing that the assumed automatic processes underlying reading may be more malleable than heretofore acknowledged.
Using standard psychological tools (Shor and Orne, 1962, Weitzenhoffer and Hilgard, 1962), researchers can characterize individuals as either highly suggestible individuals (HSIs) or less suggestible individuals (LSIs). Following anecdotal reports from select individuals (MacLeod and Sheehan, 2003, Schatzman, 1980) and personal communications (e.g., Thalia Wheatley, Harvard University, 2002; Stanley Fisher, Albert Einstein College of Medicine, 2000), multiple Stroop studies have examined cognitive processing differences between highly suggestible individuals (HSIs) and less suggestible individuals (LSIs) as a function of a post-hypnotic suggestion (Raz et al., 2006, Raz et al., 2007, Raz et al., 2002, Raz et al., 2003, Raz et al., 2005). The crux of these reports proposed that a specific post-hypnotic suggestion degraded the Stroop effect in HSIs. A few researchers were able to replicate these findings at the individual – but not group – level (e.g., personal communication from Amanda Barnier, University of New South Wales, Australia, 2005); however, other researchers have been able to replicate the wholesale effect based on the suggestibility of participants even without ritualistic inductions such as those common in hypnosis (Raz et al., 2006). Although other researchers have challenged the automaticity of the Stroop effect without resorting to suggestion (Besner, 2001, Besner and Stolz, 1999a, Besner and Stolz, 1999b, Besner and Stolz, 1999c, Besner et al., 1997, Dishon-Berkovits and Algom, 2000, Kuhl and Kazén, 1999, Long and Prat, 2002, Melara and Algom, 2003, Pansky and Algom, 2002), most scholars continue to regard Stroop tasks as the apotheosis of automatic performance (Brown et al., 2002, Brown et al., 2002).
In the present study, we wanted to conduct a typical Stroop task – drawing on a large and carefully matched sample – that would examine the differences in the influence of suggestion between HSIs and LSIs. As our primary hypothesis and in line with our previous reports, we expected that HSIs, compared to LSIs, would manifest a compelling reduction of the Stroop effect as a function of suggestion. In addition, as a surrogate exploration across the relatively large sample size, we extracted and analyzed post hoc the negative priming (NP) conditions for each participant. NP is a robust measure consisting of a pair of trials wherein the word ignored in Stimulus1 is identical to the ink color of the immediately following Stimulus2. In such cases, response time (RT) to Stimulus2 is typically longer than if Stimulus1 contained a word that did not become the ink color in Stimulus2 (Neill, 1977; see Mayr & Buchner, 2007 for a review). Our secondary hypothesis, therefore, was that, with suggestion, HSIs would show reduced NP compared to LSIs. NP is an advantageous supplementary index to Stroop performance because it is relatively immune to ulterior strategies that participants may adopt and because influencing a stepwise procedure is extremely difficult to manage consciously (Tipper, 2001). Thus, if suggestion reduced the Stroop effect in a large cohort, we would expect it to reduce NP as well.
Section snippets
Participants
To measure the quantifiable rating of participants’ response to suggestions under standard conditions, we screened volunteers for hypnotic suggestibility using both the Harvard Group Scale of Hypnotic Susceptibility Form A (HGSHS-A) (Shor & Orne, 1962) and the Stanford Hypnotic Susceptibility Scale, Form C (SHSS-C) without the ammonia challenge for anosmia (Weitzenhoffer & Hilgard, 1962). Participants were 83 proficient readers of English (38 female) aged 20–41 (mean = 29) years, all of whom had
Primary analyses
We discarded incorrect responses and mistrials from RT analyses. The remaining RT data were subjected to a recursive outlier analysis where measurements either above or below two standard deviations from the mean score for each participant in each condition were eliminated from further analyses (Van Selst & Jolicoeur, 1994). This process eliminated 4% of the raw data. Administration order (Suggestion-first, Suggestion-second) was not significant and the data were accordingly collapsed.
Table 1
Discussion
While Stroop, interference, and facilitation effects were all present in the RT data for both HSIs and LSIs without suggestion, the introduction of suggestion instigated several noteworthy outcomes. In HSIs, suggestion removed both interference and facilitation effects (Table 1), sharply reducing the (I–C) Stroop effect (Table 2). In LSIs, suggestion reduced the Stroop and interference effects but not the facilitation effect, although all three effects remained reliable (Table 2). Analysis of
Acknowledgments
This research has been supported in part by Canada Research Chair funding to Amir Raz. In addition, the authors are grateful to Jean-Roch Laurence of Concordia University and members of the Raz Lab for comments and technical assistance on this manuscript, respectively. Wholehearted thanks go out to Colin M. MacLeod of the University of Waterloo for his repeated constructive reviews of earlier versions of this manuscript and for his special camaraderie.
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