Methodological Approach
Creators: name: Tjepkema, Wim
Publisher: name: OSF Registries
Year: 2016

description: Design
Two studies nearly identical to those of Schonberg et al.[11] (except for feedback and motor movements), will be performed that both consist of two between-subject (approach/avoidance) conditions. 

Participants
Sample sizes are estimated on the basis of 25 participants per between-subject condition (calculated using earlier pilot studies[20]). Recruitment will take place via the Radboud Research Participation System and advertisements placed around the campus. Exclusion criteria are that participants should have no eating disorders or food restrictions/allergies, while earlier participation in similar studies is prohibited. Participants who bid less than €0.25 on auction items over 40 times will be excluded (see below). Furthermore, participants are asked not to consume any food items for at least four hours before the experiment commences[9,21,22]. Roughly 30 sessions will be conducted for a €10 reimbursement, while the remainder will be conducted for course credits. 

Procedure &amp; Measures
In both studies, participants are shown to a pantry where fruits and vegetables are stalled, receive €2, and are asked to give their informed consent. Then, they will practice and bid on 30 fruit and 30 vegetable items (one can be bought later) in a random-ordered Becker-DeGroot-Marschak auction (which measures their Willingness To Pay: WTP, indicating subjective value[21,23]) using a slider that ranges from €0 to €2 (programmed using PsychoPy[24]). After this phase, all items are separated per category (fruit/vegetable) and are ordered from high (4-11) to low (20-27) value. Then, participants are randomized to receive either approach or avoidance motor movement training toward 8 high-value (4 fruit, 4 vegetable) and 8 low-value (4 fruit, 4 vegetable) items in response to an auditory cue. Item presentation (1 second) will be randomized within 8 blocks of 60 trials (~25% are cued items). Cue-presentation is delayed using a staircase procedure. In study 1, the motor movement will be a keyboard button press, while in study 2, participants have to perform a more realistic motor movement: ‘swiping to self’ (wrist flexion) for approach, and ‘swiping away from self’ (wrist extension) for avoidance (performed with the right hand[12,25]; Figure 1). After approach, the displayed item will be enlarged to 200%, whereas after avoidance, it will be shrunk to 50% (during a 1 second animation[13,15]). To further disambiguate the task, participants will be instructed to imagine moving the presented item toward or away from their body[16,26]. Next, there will be a food-choice task in which participants get 1.5 seconds to choose between two same-category (fruit/vegetable), same-value items: one trained versus one untrained, for real consumption (64 unique pairs, presented twice across 2 blocks, with ~50% filler pairs). The experiment ends with a repetition of the first auction, after which participants are thanked, reimbursed and receive/buy their two pseudo-randomly selected fruits/vegetables (Figure 2). 

Materials
Within the training phase of study 2, a Leap Motion[27] will track participants’ finger and wrist movements, which verifies whether the correct motor movement has been performed. 

Analyses
To test whether approach/avoidance training has an effect on food choice, four repeated-measures logistic regression analyses will separately be performed per condition with (1) item choice (trained/untrained) in the food-choice task as dependent variable, (2) with item value (high/low) and item category (fruit/vegetable) added as within-subject factors, (3) with the dataset split by item value (high/low), and (4) with the dataset split by item category (fruit/vegetable). To test the effects on WTP, a 2 (time: pre/post) x 2 (item value: high/low) x 2 (condition: trained/untrained) repeated-measures ANOVA will be performed per condition. Effect sizes will be compared exploratorily (using R[28,29]).