Explicit Stimulus Evaluation Task (Study 4b)

The evaluation task was designed to assess explicit ratings of the foods presented during training. Images of food were presented in the centre of the screen while participants answered questions relating to the attractiveness and tastiness of the item and their desire to eat the item (see Figure 5.2). Following the task instructions there was a short practice block with four trials including non-experimental foods (a sandwich, a seeded flapjack, a jacket potato with cheese and spring rolls). The experimental block randomly presented all 18 food images from the training task with nine additional novel unhealthy foods (five sweet foods: ice-cream, sponge pudding, doughnuts, cream cake, blueberry muffin; four savoury foods: chips, vegetable pizza, vegetable lasagne, savoury pie; see Appendix 10). Images were presented in the centre of the screen for 2000ms before the onset of the first question. Questions were presented at the bottom of the screen; the first two questions asked participants to rate the attractiveness and tastiness of the item, in a counterbalanced order (between-participants), using a nine-point scale (“How attractive do you rate this item to be?”, rated from 1 “not at all attractive” to 9 “very attractive”; “How tasty do you rate this item to be?”, rated from 1 “not at all tasty” to 9 “very tasty”). The third question asked participants to rate how strong their desire was to eat the item using a nine-point scale (How strong is your desire to eat this item right now?”, rated from 1 “no desire” to 9 “very strong desire”). This question was always asked last so that visualisations of eating the food item did not influence ratings of

attractiveness or tastiness. Answers were always anchored on the neutral response (number 5) and participants were asked to make their response by decreasing or increasing this value using the ‘C’ and ‘M’ keys, respectively. There was no limit on the presentation time for each question; participants were instructed to consider each question carefully. The task was self-timed so that participants made a key response to move onto the next question or stimulus.

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Figure 5.2. Schematic diagram of the evaluation task. Participants are presented with a food item in the centre of the screen (2000ms) and are then asked to

rate the attractiveness and tastiness of the food (in a counterbalanced order) before being asked to indicate the strength of their desire to eat the food item. The questions appear at the bottom of the screen with a nine-point scale; participants must press the ‘C’ and ‘M’ buttons to decrease or increase the rating,

respectively. The diagram shows the display sequence for a participant responding twice with the ‘C’ button to decrease the attractiveness rating of the food item from five to three. Participants then press a key to confirm their rating and move on to the next question or stimulus.

2000ms

Question 1

Question 2

Question 3

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5.2.3.4. Questionnaires

Following both tasks participants followed a link to an online survey. They were asked to report their age and gender (optionally) and were presented with the Restraint Scale (RS; Herman & Polivy, 1980; see 2.2.2.1 for full details). As frequency of consumption has previously been shown to moderate the effect of inhibition training on food choice (Veling et al., 2013a), participants were asked to indicate their consumption frequency, on a seven-point scale (from ‘Never’ to ‘Daily), for chocolate, crisps and biscuits. They were then asked to report their height and weight (to calculate BMI; kg/m²) and were asked to guess the aim of the study.

5.2.4. Statistical Analysis

For consistency between this study and others presented in this thesis, all participants were categorised as either restrained or unrestrained according to the Restraint Scale (Herman & Polivy, 1980) and analyses were conducted with restraint status as a between-subjects factor. In accordance with previous research, participants with a score of 15 or more were considered restrained and those scoring less than 15 were considered unrestrained (e.g. Houben & Jansen, 2011; Polivy & Herman, 1999; Roefs et al., 2005)13.

Group differences were analysed separately for each study (Study 4a: SC-IAT; Study 4b: explicit stimulus evaluation task) to ensure that there were no statistically

significant differences for gender distribution, age, BMI or frequency of snack consumption. As the majority of continuous variables were not normally distributed, and could not be normalised with either a square root or log transformation,

nonparametric Kruskal-Wallis tests were performed with follow-up Mann Whitney U tests.

13 _{Moderated regression analyses (using the modprobe SPSS macro; Hayes & Matthes,} 2009) revealed that RS scores did not significantly interact with condition to predict any of the SC-IAT scores (all ts<1.17, all ps>0.24, all ΔR2_{<0.01) or any of the explicit evaluation} ratings (all ts<1.06, all ps>0.29, all ΔR2_{<0.05) and therefore these analyses were not} explored further.

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Training data were analysed to ensure that participants performed the tasks as

expected and to guide exclusions. Participants were excluded if their performance on no-signal trials was below 85% (including incorrect location responses and missed trials) or their reaction time on no-signal trials (goRT) was >3SDs from the group mean for that training condition. Participants who performed the no-go training task were also excluded if their percentage of erroneous responses on signal trials (i.e. their commission error rate) was >3SDs from the group mean. For Study 4a nine participants were excluded from the no-go group based on their training data: six participants were excluded based on their commission error rate (these participants had 80-100% errors indicating that they did not follow the task instructions), two were excluded due to their performance accuracy on no-signal trials and one due to their goRT. Two participants were excluded from the go group: one for their performance accuracy on no-signal trials and one due to their goRT. For Study 4b only one participant from the no-go group was excluded for a high commission error rate (45.83%).

In Study 4a implicit positive and negative attitudes towards snack foods were explored with two, unipolar SC-IATs. Seventeen participants (7 no-go and 10 go) were excluded from analysis based on >20% error rate for (Karpinsky & Steinman, 2006). IAT bias scores were calculated using a scoring algorithm modelled on the D- score algorithm for the IAT (Greenwald, Nosek & Banaji, 2003). This method was identical to that used in Study 2 (see section 3.2.4. for full details). One sample t- tests were first performed for each bias score to see whether attitude scores significantly differed from zero. A mixed 2x2x2 ANOVA with between-subjects variables training condition (no-go or go) and restraint status (restrained or unrestrained) and the within-subjects variable SC-IAT (positive and negative) was then performed on bias scores. To investigate whether any effects of training were stimulus-specific, bias scores for snack food images were analysed with a mixed 2x2x2x2 ANOVA including the additional within-subjects variable of image type (old or new). After two additional exclusions (one participant in the no-go group correctly guessed the aim of the study and another participant in the no-go group was excluded for timing issues in their data) there was a final sample of 123 participants

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in Study 4a (74 no-go and 89 go). A sensitivity analysis (using G*Power; Faul et al., 2007) for the main interaction between training condition, restraint status and SC- IAT revealed that this sample had 80% power to detect a minimum effect size of f=0.22 (α=0.05, number of groups =4, number of repetitions =2, correlation among repeated measures =-0.029, non-sphericity correction =1).

For Study 4b explicit ratings for attractiveness, tastiness and desire to eat were analysed separately14 _{with three mixed 2x2x3 ANOVAs with condition (no-go or go)}

and restraint status (restrained or unrestrained) as between-subject variables and food type (unhealthy-old, healthy-old and healthy-new) as the within-subjects variable. After four additional exclusions were made (four participants in the no-go group guessed the aim of the study) there was a total sample of 128 participants in Study 4b (69 no-go and 59 go). A sensitivity analysis (using G*Power; Faul et al., 2007) for the main interaction between training condition, restraint status and food type revealed that this sample had 80% power (with α=0.05, number of groups =4, number of repetitions =3) to detect a minimum effect size of f=0.18 for attractiveness (most conservative estimate with correlation among repeated measures =0.21, non- sphericity correction =0.88), f=0.19 for tastiness (most conservative estimate with correlation among repeated measures =0.26, non-sphericity correction =0.80) and f=0.19 for desire to eat (most conservative estimate with correlation among repeated measures =0.22, non-sphericity correction =0.72).

All results are reported with unadjusted significance values; corrections for multiple comparisons were calculated for all within-test analyses and are only reported where these corrections changed the interpretation of an analysis from statistically

significant to non-statistically significant. All statistical analyses were performed with SPSS.

14 _{Measures were significantly and positively correlated with one another, however, these} measures were analysed separately as all rs<0.78.

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5.3. Results