All training tasks were programmed in Matlab (Mathworks, Natick, MA) using Psychophysics Toolbox (www.psychtoolbox.org) and all stimuli were presented on a 19-inch flat-panel LCD monitor. The training tasks lasted approximately 15 minutes and consisted of eight blocks of 36 trials. Participants were given a 15 second break between each block. The blocks randomly presented nine images of unhealthy foods (three images each of chocolate, crisps and biscuits), nine images of healthy foods (three images each of fruit, rice cakes and salad vegetables) and 18 filler images
Scales
(1) Training Tasks
Snack Buffet & questionnaires Scales (2) Task (2) Eating-related questionnaires & debrief Hunger (VAS) Mood (PANAS) SST Stop Food consumption phase (with unhealthy & healthy foods) &
non-eating questionnaires Hunger (VAS) Food survey SST Stop DEBQ G-FCQ-T Debrief BMI Double- response Double- response GNG No-go GNG No-go Go Go
Observe Recognition task
139 (clothes; three each of jeans, shirts, jumpers, socks, skirts and ties). One stimulus of each food type was a photographed image of the corresponding food item that was presented in the snack buffet. All images, including the snack food photographs, were close-up views of the food item against a white background; images were carefully selected on the basis that there were no additional ingredients or packaging, and they were matched for size and complexity (see Appendix 5).
For all trials the central rectangle appeared in the centre of the screen (fixation; 1250ms), followed by the presentation of a stimulus to either the left or right hand side (1250ms; see Figure 4.2). Stimuli were presented once per block in a random sequence and with equal probability to either location. For the no-signal trials participants were required to respond to the location of the stimulus as quickly and accurately as possible using their left and right index fingers (‘C’ or ‘M’ on a standard keyboard). The presentation of the signal and the relevant instructions for the signal trials are described below according to the training task.
4.2.2.1.1. Stop-Signal Training – For the stop-signal training the majority of signals were mapped onto the unhealthy foods (8/9; 88.89% mapping), with a few occurring on the healthy (1/9; 11.11% mapping) and filler (1/18; 5.56% mapping) trials. For the presentation of the signal the central rectangle would turn bold after a variable delay (SSD, see section 3.2.2.1 for the simulated tracking procedure that was used to determine SSD length; see Figure 4.2a). Participants in the double-response group were instructed to make a secondary response when a signal was presented;
following the response to the location of the stimulus they were required to make a thumb response on the space bar as quickly as possible. Participants in the stop group were instructed to withhold their response whenever a signal occurred. They were informed that the signal would sometimes appear quickly and sometimes after a longer delay, and that they should therefore find it easier to stop on some trials than others. They were instructed that they must not wait for a signal to occur.
140 Figure 4.2. Display sequence for a) the stop-signal training tasks b) the go/no-go training
tasks. For no-signal trials participants were presented with a fixation rectangle before seeing a stimulus appear on either the left or right hand side. Participants were instructed to respond as quickly and as accurately as possible to the stimulus location using the ‘C’ and ‘M’ keys.
a) For the stop-signal training, signals were presented after a variable stop-signal delay
(SSD) which was initially set to 250 ms and then adjusted according to a simulated tracking procedure (see section 3.2.2.1 for details). Participants in the stop group had to inhibit responses on these trials whereas participants in the double-response group had to make an additional response. b) For the GNG training, i) participants in the go group were presented only with the no-signal trials; ii) for the no-go group, stimuli and signals were presented simultaneously (0 ms SSD) and participants were required to refrain from responding on these trials.
4.2.2.1.2. Go/No-Go Training – In the GNG training, signals were only presented for the no-go group; all trials for the go group were no-signal trials, therefore they were required to make a location response on every trial (see Figure 4.2b.i). For the no-go group signals were consistently mapped onto the unhealthy foods (9/9; 100% mapping) with no signals occurring alongside the healthy food images (0/9; 0%
1250 ms
1250 - (SSD)
1250 ms
1250 ms
variable SSD
a) Stop Signal Training b) Go/No-Go training
1250 ms
1250 ms
1250 ms
1250 ms i) Go training
141 mapping). Filler images were inconsistently paired with a signal (9/18; 50%
mapping) so that the overall rate of no-go signals was 50%. Signals appeared as a bold rectangle that replaced the fixation rectangle and lasted for the duration of the trial (1250ms; see Figure 4.2b.ii); this meant that there was no delay between the presentation of the stimulus and the signal. Participants in the no-go group were instructed to withhold their response when a signal was presented.
4.2.2.1.3. Observe Training – Participants in the observe group were presented with the same training stimuli as the go group. Images were presented to the left and right hand side within a central rectangle. Participants were informed that they were to watch the stimuli, and that they needed to pay some attention because they would be asked questions at the end of the session (these questions were presented in the form of a recognition task; see section 4.2.2.4. below). They were not required to make any responses to the stimuli.