Inhibitory control training paradigm
The ICT paradigm employed in this study has been primarily adapted from the stop-signal paradigm, which has been previously used as a training intervention for food-related behavioural outcomes (see Allom et al., 2016; Jones et al., 2016). The ICT paradigm included a training task which adopts the principles of the stop-change paradigm (Logan, 1982). The novel element of stop-change responses has been added because common everyday behaviours, such as refraining from eating unhealthy energy- dense foods and making healthier food choices, do not require ‘pure’ response inhibition, but an updating of behaviour (i.e., action plans) after the cancellation of planned or initiated actions (Boecker et al., 2013). In an effort to reflect more ecologically valid behaviours, the ICT paradigm also required continuous motor responses with the computer mouse, instead of keyboard responses. This has previously been developed in a study by Maizey (2016) and has been modified to match the requirements of a training intervention. Participants were randomly assigned to one of three groups. There were two experimental groups where stop and stop-change training tasks were performed and a control group where participants completed a go task (only no-signal trials and go responses). Participants did not know the ‘treatment group’ to which they had been assigned and the training tasks were exactly matched for all three groups and only responses in signal trials were manipulated, as described below.
Figure 3.2: Inhibitory control training paradigm and task design.A. There were 48 trials
in each training block, which consisted of 32 no-signal trials (66.66%) and 16 signal trials (33.33%). A 100% contingency mapping was used for targets in signal and no-signal trials in terms of food healthiness. For healthy and unhealthy targets there was an equal probability of the stimuli being either selected or non-selected in the initial food selection task. B. Participants begin a trial by clicking on a ‘START’ box and after a stimulus onset time of either 500ms or 700ms the food stimuli (target and non-target) appear on the screen together with a direction cue (e.g., <<<), indicating which target participants have to move their mouse towards to. Participants can then initiate the movement towards the target and on signal trials, when the mouse cursor reaches the signal-onset distance (20% or 40% from starting point), the signal is presented. On no-signal trials as well as signal trials in the control training group, participants have to reach the target within the time limit (1800ms). C. On signal trials, participants in the stop group were required to stop their mouse movement and cancel their initiated response towards the target. In the stop-change group, participants were instructed to stop their initial response and move the mouse towards the other stimulus (non-target).
Training task design
All participants were required to move their mouse as quickly and as accurately as possible towards a target stimulus located on either the top right or top left of the screen area as indicated by a set of arrows (<<< or >>>). Trials were self-timed
and participants were required to click on a ‘start’ box located on the bottom of the screen to begin. There were 48 trials in each training block and a total of four blocks in the training phase (192 trials in total). Each block consisted of 16 signal trials (33.33%) and 32 no-signal trials (66.66%). A 100% contingency was used for healthy targets in no-signal trials and unhealthy targets in signal trials. The healthiness of the foods was always opposite for targets and non-targets (e.g., non-target stimuli in signal trials were always healthy foods). The selection condition, that is whether stimuli have been selected by the participants during the initial food selection task or not, was applied with equal probability for both trial types, as shown in Figure 3.2 (panel A). Each selected and non-selected food was represented by two exemplars in the training phase, that have been matched as much as possible for visual consistency. The target location (left or right) was fully counterbalanced across all design cells and on a block-by-block basis for the two exemplars of each food category. There were no more than four consecutive signal trials in any of the training tasks. As mentioned in section 3.3.4, the four unhealthy foods that had the highest ranking were assigned as targets on signal trials and the four healthy foods with the highest ranking were used as non-targets on signal trials.
For the experimental groups, signal trials were manipulated so that fixed signal- onset distances would define the timing of the signal presentation, which is consistent with the concept of fixed stop-signal delays in response inhibition tasks (Verbruggen & Logan, 2009). The signal-onset distance was defined as the distance on the y-axis from the initial y mouse coordinate when participants began moving their mouse from the start location. Signal-onset distances were thus tailored to individual responses on a trial-by-trial basis and the fixed values that were used were 20% and 40% from the start location. The fixed signal onsets occurred with equal probability for all design cells. In a total of four blocks, there were 32 observations per signal-onset distance. As shown in Figure 3.3.3 (panel B), the visual signal was a bold black border appearing around the picture, similar to previous approaches (e.g., Lawrence et al., 2015). The type of responses associated with the signal presentation differed between the two experimental groups (stop and stop-change), but in each task participants were required to cancel an initiated response toward an unhealthy food. It should be noted that the continuous motor responses ensured that a response had been initiated before the motor response had to be inhibited. In the stop group, participants were instructed to stop moving their mouse and not respond to the target when the signal appeared. In the stop-change group, the signal indicated that participants had to change their initiated response and move towards the non-target stimulus on the
other side of the screen (see Figure 3.3.3, panel C). In theoretical terms, the difference between the two training protocols was the type of action updating. In the stop group, only response inhibition, or inhibitory action updating occured, whereas in the stop-change group, correct responses required both inhibitory and responsive action updating. There has been evidence to suggest that the stop-change task may require a go process, an inhibition process and a response re-engagement process, which may be operating in a serial, independent manner and not in parallel (see Boecker et al., 2013 for review; Verbruggen et al., 2008). On signal trials of the training task administered to the control group, there was no visual signal presented, but participants did respond to unhealthy targets, keeping the overall format of the trials consistent across tasks. Specifically, all groups were ‘exposed’ to the same number of healthy and unhealthy foods so that no potential confounds due to visual exposure were introduced.
On each trial, the stimulus onset time was either 500ms or 700ms (see Figure 3.2) and each timing occurred with equal probability. The maximum reaction time (maxRT) was set to 1800ms and the direction cues stayed on the screen until maxRT was reached. At 1800ms the food stimuli were masked by grey rectangles which stayed on the screen for 500ms, as a controlled inter-trial interval (ITI). The stimuli did not disappear from the screen until maxRT was reached in order to match visual exposure time to foods for all participants. On signal trials, the black border around the target stimulus also remained on the screen until the end of the trial with the aim to reduce attentional demands associated with the quick detection of a signal while a continuous motor response was being executed.
Training practice
Training practice differed between groups: the control group received only 12 practice trials (no-signal), whereas the experimental groups completed 24 trials to account for 12 signal and 12 no-signal trials. To avoid disclosing the aims of training during practice, there were both healthy and unhealthy images for all trials (50:50). Feedback was given after every trial on the accuracy of the responses, detected slowing and/or stopping of the mouse as well as slow initiation times (<= 750ms). When the initiation time threshold was exceeded participants were presented with the instructions “It is important to start moving your mouse sooner, even if you are not entirely sure of your answer!” (cf. Gillebaart, Schneider, & De Ridder, 2016). The practice blocks were repeated (maximum of three blocks) if and when the proportion of correct go responses was less than 75%. For the experimental groups, in addition to the benchmark for go responses, the practice blocks were repeated if the proportion of correct signal
responses was less than 50%. Participants were not allowed to proceed with the study if they did not meet these performance criteria for successful training practice after three practice blocks.