How to set "stim_key_association" as a variable in IAT-plugin?

[Tongzhao9417]

Hi everyone, I have another question about the IAT-plugin.

I want to random the stimulus key association to balance the trials. For example, the left side for "black" and right side for "white" for some participants, and right side for "black" and left side for "white" for other participants. Could I achieve the goal in some way? Thank for everyone!

[becky-gilbert]

Hi @Tongzhao9417, one way to do this is to create an array of your two labels, and randomize the order of the array at the start of the experiment. You can do this with the jsPsych.randomization.shuffle function:

let labels = ["black", "white"];
let labels_rand = jsPsych.randomization.shuffle(labels);
// labels_rand will either be ["black", "white"] or ["white", "black"]

Then you can use the first value (index=0) as the left label, and second value (index=1) as the right label. Whenever you need to reference the label name, you can index the randomized array instead of using a fixed label value:

const trial = {
  type: jsPsychIatImage,
  left_category_label: labels_rand[0],
  right_category_label: labels_rand[1],
  // etc.
};

As for setting up the corresponding stim_key_association values, I would include the correct label for each stimulus in the timeline_variables array, and then the stim_key_association value can be a function that checks this value against the two labels_rand values and returns 'left' or 'right' depending on whether the correct response matches the left or right label:

const trial_block = {
  timeline: [{
    type: jsPsychIatImage,
    left_category_label: labels_rand[0],
    right_category_label: labels_rand[1],
    stim_key_association: function() {
      if (jsPsych.timelineVariable('correct_response') == labels_rand[0] {
        return 'left';
      } else {
        return 'right';
      }
    }
    // etc.
  }],
  timeline_variables: [
    {stimulus: "img/black1.jpg", correct_response: "black"},
    {stimulus: "img/white1.jpg", correct_response: "white"},
    // etc.
  ]
};

Another option is to dynamically construct your timeline_variables array when the experiment begins so that it includes a 'stim_key_association' value that is set to either 'left' or 'right', depending on your randomized left/right labels. That way you could set it up like this:

let labels = ["black", "white"];
let labels_rand = jsPsych.randomization.shuffle(labels);

let timeline_vars_initial = [
  {stimulus: "img/black1.jpg", correct_response: "black"},
  {stimulus: "img/white1.jpg", correct_response: "white"},
  // etc.
];

const timeline_vars = timeline_vars_initial.map(obj => {
    if (obj.correct_response === labels_rand[0]) { // 0 indexes the left label
      return { ...obj, stim_key_association: "left" };
    } else {
      return { ...obj, stim_key_association: "right" };
    }
    return obj;
});
 
const trial_block = {
  timeline: [{
    type: jsPsychIatImage,
    left_category_label: labels_rand[0],
    right_category_label: labels_rand[1],
    stim_key_association: jsPsych.timelineVariable('stim_key_association')
    // etc.
  }],
  timeline_variables: timeline_vars
};

Do either of those solutions work for you?

[becky-gilbert]

@Tongzhao9417 I just noticed that this is an issue - I'm going to convert it to a Discussion post, since it's a question rather than a bug report or feature request.