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                    const nTrials = 30
    let trial_num = 1
    let angle = 90;

    const jsPsych = initJsPsych({
        on_finish: function() {
    console.log(`jsPsych Version ${jsPsych.version()}`)

    const instruction = {
        type: jsPsychHtmlButtonResponse,
        stimulus: 'Click on the Start button.',
        choices: ['Start'],
        prompt: "This is a sample program for the jspsych-psychophysics plugin.",

    const trial = {
        type: jsPsychPsychophysics,
        canvas_height: 500,
        prompt: `<p>Press the ArrowLeft/ArrowRight key to change the tilt of the gabor.</p>
          <p>You can press the key ${nTrials} times.</p>`,
        stimuli: function(){
          const gabor_obj = {
              obj_type: 'gabor',
              tilt: angle,
              sf: 0.05,
              phase: 90,
              width: 150,
              sc: 10,
              contrast: 0.5,
              disableNorm: true

          return [gabor_obj]
        response_type: 'key',
        choices: [' ', 'ArrowLeft', 'ArrowRight'],

    const loop_node = {
        timeline: [trial],
        loop_function: function(data){
            if (trial_num < nTrials){

                if (data.values()[0].response === 'arrowleft'){
                  angle += 2
                } else { // arrowdown
                  angle -= 2

                return true
            } else {
                return false

    /* start the experiment */[instruction, loop_node])