Experiment 1

Approval was obtained from the University of Oxford Central University Research Ethics Committee. Healthy volunteers were recruited using advertisements placed online and in the local community. Similar to previous studies [1], [17], [22], [61], participants were not permitted to participate if they had received treatment for mental health problems. Sixty participants (aged 18–60 years; mean age = 27 years; 30 females) provide written informed consent and completed baseline assessments of trauma history, mood, trait anxiety and depression. Trauma history was assessed using the Traumatic Experiences Questionnaire, a 12-item check list adapted from criterion A list of the Posttraumatic Diagnostic Scale [PDS; 58]. Participants indicated whether they had experienced or witnessed each of the trauma events listed (“yes” or “no” response). “Yes” scores were summed, and could range from 0 (no traumatic event) to 12 (each and every type of traumatic event experienced or witnessed).

A composite mood score was calculated by summing participants' ratings on three visual analog scales for ‘sadness’ ‘hopelessness’ and ‘depressed’. Visual analog scales for mood were anchored from 0 ‘not at all’ to 10 ‘extremely’[1]. All participants were given a practice trial of both computer game tasks. Participants then viewed traumatic film footage (Figure 1). The 21-min film [previously used in 22] contained 15 clips of traumatic content including fatal road traffic accidents and graphic scenes of human surgery. Following the film, mood assessments were repeated and standardised filler tasks were completed for 30 min (e.g. rating excerpts of classical music for pleasantness and answering simple questions from an encyclopedia) [1].

After the break, all participants were shown a brief film reminder task in which one neutral but recognizable static image from each of the 15 film clips was presented (via slides in PowerPoint) in order to “reactivate” memories for the trauma film [1], [59], [60]. Then according to random allocation to one of three 10-min conditions, participants either completed the visuospatial condition, verbal/conceptual condition or were in a no-task control condition. Participants in the visuospatial condition played the game Tetris [30] on a computer and used the cursor keys to move and rotate falling blocks to complete the largest number of complete rows across the screen. Participants in the verbal/conceptual condition played the general knowledge game Pub Quiz [31] on a computer using the right mouse button to select one correct answer out of four choices in order to get the highest score possible. Pub Quiz questions varied in content to include history, sport, geography, food and drink etc, and were unrelated to the trauma film content (e.g., “With what item of clothing would you associate the word Panama? A = scarf, B = gloves, C = hat, D = coat” and “How many sides has a rhombus? A = six, B = seven, C = four D = five”). In the no-task control condition, participants were asked to sit quietly for 10 min.

During the 10-min experimental task manipulation (above) participants in all conditions recorded the frequency of initial intrusions of the trauma film. Afterwards participants completed mood assessments and rated to what extent they had followed instructions (task compliance) on a visual analog scale anchored from 0 ‘not at all’ to 10 ‘extremely’. Participants in the computer game conditions were asked to rate how enjoyable (anchored from 0 ‘not at all’ to 10 ‘extremely’) and how difficult they found playing the game (anchored from 0 ‘not at all difficult’ to 10 ‘extremely difficult’).

Participants then kept a daily diary for 1 week, in which they recorded the occurrence (frequency) of their flashbacks and then briefly described the content of each of their flashbacks separately (for verification) e.g. a human knee with blood. Flashbacks backs were described as spontaneously occurring image-based intrusive memories of scenes from the trauma film [based on 1], [17], [22], [61] e.g. the previous flashback of a knee was deemed to be linked to a film scene containing knee surgery. The A5 sized diary was structured in a tabular form with each day broken down into a grid for ‘morning’, ‘afternoon’ and ‘evening’. The frequency data was entered into this grid, and its corresponding content was recorded on lined pages also within the diary. Participants were asked to record all flashbacks immediately after they occurred (whenever possible) and to set aside a regular daily time slots to ensure that their diary was up-to-date. If participants had experienced no flashbacks during any period they were asked to write zero in the diary.

On return to the laboratory 1 week later, the experimenter went through the diary to verify that the content of each of the flashbacks came from a scene in the trauma film watched 1 week earlier (if not, they were discounted). Participants rated the extent to which they had been able to accurately record their flashbacks in the diary (diary compliance) from 1 ‘not at all accurate’ to 10 ‘extremely accurate’.

Participants completed a recognition memory task as a measure of voluntary memory for the trauma film. The recognition memory task consisted of 30 written statements describing the film (e.g. A policeman stands watching the wreckage whilst making notes on a clipboard) to which they responded ‘true’ or ‘false’. Participants were then debriefed and reimbursed for taking part.