Material

Obsessive-Compulsive Inventory-Revised (OCI-R). The short version of the OCI (Foa et al., 2002a) is an 18-item, self-report measure for assessing OCD symptoms. This scale consists of six subscales (each containing three items): washing, obsessing, hoarding, ordering, checking and neutralizing. Respondents are requested to indicate the degree to which certain experiences have distressed them during the past month on a 5-point scale (from 0 = “not at all” to 4 = “extremely”). Total scores may range from 0 to 72. The OCI-R has retained excellent psychometric properties (Foa et al., 2002) and the French version of the OCI-R used in the present study has been reported to have good reliability and validity (Zermatten, Van der Linden, Jermann, & Ceschi, 2006b). Foa et al. reported

Total sample

means of 28.01 (SD = 13.53) for the total scale and 4.83 (SD = 3.86) for the checking subscale with clinical OCD, while means were 18.82 (SD = 11.10) for the total scale and 2.91 (SD = 2.56) for the checking subscale with non-clinical participants.

Dissociative Experiences Scale (DES). The DES (Bernstein & Putnam, 1986) is a 28-item, self-report measure of dissociation. Three-factor extractions have generally been reported in the literature (Stockdale, Gridley, Balogh, & Holtgraves, 2002), including amnesia (6 items), depersonalization (6 items) and absorption (16 items). In the present study, we used the French version of the DES, which has been shown to have good reliability as well as the same three-factor solution as the English version in a population of rape victims (Darves-Bornoz, Degiovanni, & Gaillard, 1999). For each item, respondents indicate the percentage of the time a certain experience occurs, based on a 21-point scale (from 0% to 100% in 5% increments). Total scores may range from 0 to 100.

Other questionnaires were also administered in order to evaluate differences between individuals in terms of levels of depression (Beck Depression Inventory–II, BDI-II, Beck et al., 1996) and anxiety (State-Trait Anxiety Inventory, STAI, Spielberger et al., 1983).

2.2.1. Action-based task

Actions were selected from Larøi, Collignon and Van der Linden (2005). Seven actions were replaced, because they were too similar to OCD patients’ preoccupations (e.g., “wash your hands”, “lock a door with a key”). Indeed, the aim of the present study was to examine the existence of a basic motor memory and reality monitoring deficit, independent of the content of the actions. Furthermore, based on the criteria set by Larøi et al. (2005), all actions involved movement, were as universal and clear as possible, were easy to carry out when sitting down (i.e., in the testing situation), were gender-neutral, did not necessitate the production of sounds (e.g., “clap your hands”) and could be carried out quickly and simply by subjects (e.g., “stroke a cat”, “peel a potato”, “pick up the telephone”; the complete list of actions is available upon request). A total of 120 actions were selected, 60 of which were randomly selected as encoding items and the other 60 as fillers. The encoding items and fillers were the same for all subjects.

2.3. Procedure

The procedure was based on Larøi et al. (2005) and adapted from Ecker and Engelkamp (1995). Participants were tested individually. In the study phase, each participant was asked to encode 60 actions in five different conditions. In the motor encoding condition (MOTOR), participants had to carry out the actions with an imaginary object. In the imagined motor condition (I.MOTOR), they were asked to imagine themselves performing the actions. In visual encoding (VISUAL), they had to watch the experimenter perform the actions. The imagined visual condition (I.VISUAL) involved participants imagining the experimenter performing the actions. Finally, in the verbal

encoding condition (VERBAL), participants had to repeat the actions’ descriptions verbally.

Participants were not told that they would have to remember the actions. The incidental instructions, based on Henkel, Franklin, and Johnson (2000), were the same as those used in Larøi et al. (2005). That is, for the I.MOTOR and I.VISUAL conditions, participants had to estimate the degree of ease/difficulty in creating a mental image. For the MOTOR and VISUAL conditions, participants were asked to judge the extent to which the action they carried out was representative of the manner in which a typical action is carried out in everyday life. For the VERBAL condition, participants were asked to evaluate the ease/difficulty of understanding the meaning of the phrase. All these evaluations were done on three different 5-point Likert scales placed in front of the participant during testing.

In the encoding phase, 15 blocks of four actions (60 actions) were presented. After each block of four actions, the encoding modality (MOTOR, I.MOTOR, VISUAL, I.VISUAL, VERBAL) was changed. Each encoding modality was applied to 12 actions (three blocks of four actions). The order of the encoding conditions and blocks of actions was counterbalanced. Each block of four actions was found in each of the 15 possible positions in the encoding list. In addition, five different presentation sequences of the five modalities were created. In these sequences, two “real” conditions (MOTOR and VISUAL) and two “imagined” conditions (I.MOTOR and I.VISUAL) never directly followed each other in order to avoid confusion between two conditions of the same encoding type. The final material consisted of 75 protocols, all with different orders (five sequences of the presentation of the modalities of execution X 15 sheets representing the position of each block in one of the 15 possible positions).

After the study phase, a filler task (arithmetic exercises carried out for a minute) was completed in order to avoid recapitulation of the last actions in short-term memory. In the recognition phase, the 60 encoded actions and 60 new actions (fillers) were presented orally to the participants. For each action, participants had to identify whether it was old (already presented) or new. If the action was considered as “new”, participants were asked to evaluate the degree of certainty of their response on a 5-point scale (from 1 = “I am not at all certain of my response” to 5 = “I am certain of my response”). If the action was considered as “old”, participants were asked to specify their state of awareness by using the Remember/Know/Guess (R/K/G) distinction (Gardiner, 1988). In addition, for items considered as old, participants were asked to identify the encoding modality (MOTOR, I.MOTOR, VISUAL, I.VISUAL, or VERBAL). Participants were then asked to judge this encoding modality identification with the R/K/G procedure.

3. Results

In order to protect against non-sphericity of the data, the Huynh-Feldt correction was used for the various ANOVAs computed. In addition, in order to control for type 1

errors, Bonferroni corrections were used when more than two t-tests were computed on the same variable.

In order to examine the effect of the five different encoding modalities for the total sample, we carried out a simple repeated measures ANOVA comparing the rate of total attribution errors for each modality (confusions between modalities). Participants committed more attribution errors in certain modalities than in others (F(4,370) = 44.01;

p = .00). In order to examine whether there were any differences between conditions involving a “real” execution of actions and conditions involving an “imagined” execution of actions, we grouped the attribution errors from the MOTOR and VISUAL conditions into a single variable (REAL) and the attribution errors from I.MOTOR and I.VISUAL conditions into another variable (IMAGINED). With a Bonferroni corrected threshold, planned comparisons revealed that participants made fewer errors in REAL than in IMAGINED conditions (F(1,372) = 89.63; p = .00), in REAL than in VERBAL conditions (F(1,372) = 149.70; p = .00), and in IMAGINED than in VERBAL conditions (F(1,372) = 20.30; p = .00).