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Self-focused, analytical mental rumination constitutes a central process in depression. It has been hypothesized that such rumination depletes executive resources that are necessary for an efficient cognitive regulation of emotion and behavior. However, most of the research supporting this hypothesis is of correlational nature. The present study examined the effects of induced rumination versus distraction on executive capacities in dysphoric and nondsyphoric college students. Executive functioning was measured with the Stroop task. Results indicate that induced rumination decreases inhibition capacities in dysphoric individuals only. The flexibility facet of executive functioning was not affected by induced rumination. However, dysphoric individuals demonstrated a fundamental impairment in this latter capacity, independent of rumination induction. The implications for the facets of executive functioning affected by depression and by rumination are discussed.

Mental rumination is thought to be a key process in the generation and maintenance of dysphoric mood (Nolen-Hoeksema & Morrow (1991) and Nolen-Hoeksema & Morrow (1993); Watkins & Baracaia, 2001). Nolen-Hoeksema (1991) has identified a form of rumination, consisting of focusing on dysphoric symptoms, their causes and consequences, as being especially detrimental for mood. Similarly, Watkins and Teasdale (2001) have demonstrated that analytical self-focused rumination (thinking of the causes and consequences of one's present state) has deleterious consequences for dysphoric individuals. In contrast, distraction has been found to have positive effects on mood in the aforementioned research. In dysphoric individuals, the effects of self-focused analytical rumination are manifold. It increases negative mood (Morrow & Nolen-Hoeksema, 1990; Nolen-Hoeksema & Morrow, 1993) and pessimistic thoughts (Lyubomirsky, Caldwell & Nolen-Hoeksema, 1998), it reduces effective problem solving (Ward, Lyubomirsky, Sousa & Nolen-Hoeksema, 2003), and it generates an overgeneral retrieval style of autobiographical memories (Watkins & Teasdale, 2001). Studies that have experimentally manipulated rumination generally found that these deleterious effects were observed only in dysphoric individuals. Thus, it seems that the depressogenic effect of rumination is only operant in people who are depressed or vulnerable to depression. One hypothesis accounting for these observations is that self-focused analytical rumination depletes executive resources (Davis & Nolen-Hoeksema, 2000; Watkins & Baracaia, 2001). The executive system is constituted by a set of processes that are called for when usual routines become inadequate and when a task requires controlled processes (Van der Linden et al., 2002). They comprise processes of attention allocation, of behavior planning, of flexible switch of strategies, and of inhibition of irrelevant behavior or information (Damasio, 1995; Duncan, 1986; Shallice, 1982). It has been documented that depressed individuals present a chronic deficit in executive resources (Elliott, 1998; Veiel, 1997). Such impairment is likely to impact upon ruminative thinking. In particular, inhibitory and flexibility deficits would impair the person's ability, respectively, to suppress negative thoughts (Joormann, 2004) and to switch from the over-trained ruminative pattern to a new train of thoughts (Hertel, 2004). In addition, rumination in itself, as a form of automatic thinking, might deplete cognitive resources (McNally, 1995), as it requires sustained attention on a specific content. These combined deficits would favor the installation of a cognitive interlock in which over-trained negative thoughts feed back in one another (Teasdale, Dritschel, Taylor, & Mezzich, 1995). This cognitive interlocked loop would maintain rumination and a vicious circle would be initiated, depriving the depressed individuals of the possibility to adopt another perspective on their situation. However, to date, there has been little experimental research on the effect of induced rumination on executive functions. One notable exception is the study by Watkins and Brown (2002) in which depressed patients and non-depressed controls were compared on a random number generation task, performed after either a rumination or a distraction induction. Compared with the distraction induction, the rumination induction produced a significant increase in stereotyped counting responses (thought to reflect a failure of inhibitory executive control) in depressed patients but not in controls. However, after distraction, no difference was found between the two groups. The authors concluded that executive functioning might not be fundamentally impaired in depressed patients, as often assumed in the literature, but that the rumination induction seemed to interfere with concurrent executive processing. This interpretation thus suggests that depressive rumination is not a consequence of an executive deficit, but rather that the executive impairment observed in depressed individual might result from their ruminative tendencies. Watkins and Brown's (2002) results are however limited by the task they used to assess executive functions. Indeed, the random number generation task yields only one general score, and it is not clear what facet of executive control is exactly measured (Klauer & Zhao, 2004; Towse & Valentine, 1997). In a meta-analysis, Veiel (1997) has demonstrated that depressed individuals were particularly impaired in the Stroop task (Stroop, 1935) and in the trail making task (Army Individual Test Battery, 1944; Reitan & Wolfson, 1993). These two tests have in common to be particularly sensitive to the flexibility and the inhibition components of executive functions. Inhibition is the ability to deliberately inhibit dominant, automatic, or prepotent responses when necessary. Flexibility concerns the ability of shifting back and forth among multiple tasks, operations, or mental sets (Miyake, Friedman, Emerson, Witzki, & Howerter, 2000). This suggests that the observation of Watkins and Brown (2002), that self-focused rumination depletes executive resources in dysphoric individuals, would gain to be replicated with a task evaluating both the flexibility and inhibition components of executive functions, such as the Stroop task. Indeed, the Stroop task yields two scores: one measuring the dominant response inhibition (inhibition) and one measuring task switching (flexibility) (Van der Linden et al., 2002). In addition, the use of the Stroop task will allow to test whether Watkins and Brown (2002) are correct in assuming that executive functioning is not fundamentally impaired in dysphoric patients or whether their conclusion is dependent upon the task they used (random number generation). Finally, rather than using a mixed design, we used a full between-subject design that is less sensitive to methodological biases. Following Watkins and Brown (2002), the main hypothesis is that a depletion of executive resources should be observed only in dysphoric participants in the rumination condition. 2. Methods Female university students from the campus of Louvain-la-Neuve, Belgium, were approached in student residences and lecture halls, and were proposed to volunteer in a psychology experiment. After having been fully informed of the procedure and deontological rules, and after having given consent to participate, they were proposed to fill in the Beck Depression Inventory (BDI II, Beck & Steer, 1987) in order to check for inclusion characteristics. Participants were selected if they scored on the BDI II either at 10 or below (control group, n=50) or at 18 or above (dysphoric group, n=44). They were then invited to the experimental session that took place in the following days in an experimental room of the psychology department or in a quite room of a students’ residence. After being reminded of the procedure, they were proposed modules of a semi-structured interview diagnosing specific DSM-IV (American Psychiatric Association, 1994) axis I disorders (MINI, Lecrubier, Weiller, Bonora, Amorin, & Lépine, 1994; Sheehan et al., 1998). The modules diagnosing major depressive episode, dysthymia, (hypo-)manic episode, and suicidal risk were administered. They were also proposed a questionnaire assessing demographic variables and psychotropic drug consumption. On the basis of the MINI interview and of the questionnaire, five participants had to be excluded: one participant of the control group fulfilled the criteria for hypomaniac episode, while in the dysphoric group, three participants were taking antidepressive drug and one l-thyroxine. Participants’ characteristics in each experimental condition are displayed in Table 1. In the final sample, no control participants fulfilled the criteria for one of the four DSM IV category assessed, but in the dysphoric sample, 16 participants fulfilled the criteria for present major depressive episode, 7 for suicidal risk, and 6 for dysthymia. Table 1. Participants’ mean characteristics according to experimental condition (standard deviation in parentheses) Control group Dysphoric group Rumination (n=24) Distraction (n=25) Rumination (n=20) Distraction (n=20) Age 19.9 (2.1) 20.2 (2.0) 20.1 (2.0) 20.5 (2.1) F(3.85)=.31, n.s. BDI IIa 6.2a (2.7) 5.5a (2.8) 23.1b (7.0) 22.2b (3.9) F(3.85)=110.8, p<.001 Years of university education 3.0 (1.6) 2.9 (1.6) 2.4 (1.2) 3.2 (1.7) F(3,85)=.98, n.s. a BDI II is Beck Depression Inventory. Table options After the diagnostic interview, the experimental manipulation took place. In both the control and the dysphoric groups, participants were randomly allocated to a rumination or to a distraction condition. The manipulation followed exactly the procedure developed by Watkins and Teasdale (2001). In both conditions, participants were presented with a written list of 10 items. They were asked to center their attention on each item at a time and to imagine them vividly. In the rumination condition, they were additionally asked to reflect upon the causes, meanings, and consequences of each item. Each item started with “reflect upon the causes, meanings, and consequences of …” and was then completed with a potential symptom of depression (e.g., “your present level of motivation”, or “the body sensations you are experiencing now”). In the distraction condition, participants were asked to “think about …” a series of highly imaginable neutral items (e.g., “clouds forming in the sky” or “a boat slowly crossing the Atlantic”). This induction procedure lasted about 12 min. Immediately after the manipulation, participants completed a modified version of the Stroop color word test (1995). This test consists of four sheets assessing denomination, reading, interference, and flexibility. Performance was recorded with a stopwatch. On the first sheet (denomination), rectangles of red, green, and blue colors are presented in a matrix of 10. Participants have to tell as quickly as possible the color of the rectangles. On the second sheet (reading), the names of the colors (red, green, and blue) are presented on a matrix of 10 and participants have to read them as quickly as possible. In the third sheet (interference), the names of the colors written in another color are presented on a matrix of 10, e.g., “red” would be written in blue. Participants have to tell the color in which the word is written. The number of interferences (number of errors, corrected or not) is noted and an interference index is computed as the percentage of the increased time for completing the interference sheet as compared to the denomination sheet (sheet 2). In the fourth sheet (flexibility), the colors names written in another color are also presented in a matrix of 10 but some words are framed. The participants are asked to tell the ink color of unframed words and to read the framed word. The number of flexibility errors is noted and a flexibility index is computed as the percentage of the increased time for completing the flexibility sheet as compared to the interference sheet (sheet 3). As Miyake et al. (2000) emphasizes, the Stroop task is a prototypical inhibition task, as it requires to inhibit or override the tendency to produce a more dominant or automatic response (i.e., name the color word). Among the studies that investigated depressed subjects on executive tests, several successfully used the paper version of the Stroop's test (e.g., Austin, Mitchell, & Wilhelm, 1999; Crews, Harrison, & Rhodes, 1999; DegI’Innocenti, Agren, & Backman, 1998; Harvey et al., 2004; Lampe, Sitskoorn, & Heeren, 2004; Ravnkilde et al., 2002). The adapted version of the Stroop test (1995) is particularly interesting in the present context as it allows, by the adding of a fourth sheet, to assess inhibition and flexibility, the two main executive deficits observed in depression. After the Stroop task, dysphoric participants in the rumination condition were administrated a shorter version of the distraction condition, in order to alleviate any remaining (and potentially depressogenic) ruminative process. Participants were then fully debriefed and thanked for their participation.