Images not being presented on some trials

[Quinlanptq]

On each trial in my experiment I present a fixation

var fixation = {
type: jsPsychHtmlKeyboardResponse,
choices: "NO_KEYS",
stimulus: '+',
trial_duration: 250,
post_trial_gap: 300,
};

and then immediately an image

var trial = {
type: jsPsychImageKeyboardResponse,
choices: ['d','k'],

stimulus: function () {

    return (
           jsPsych.timelineVariable("the_image")
        );
    },
stimulus_duration: 1000,
trial_duration: 2000,
post_trial_gap :1000,
data: {
    
          Got_cha:'Yes',
    
        }
};    

The problem that I have is that it is possible to press a key before the image is actually presented.

and responses rts are recorded as null

Can anyone tell why this happening and how do I stop it?
Thanks

Philip.

[jodeleeuw]

Hi @Quinlanptq,

Are you preloading the images?

Relevant docs: https://www.jspsych.org/7.3/overview/media-preloading/

[Quinlanptq]

I think so here is the critical code - this of course may not be doing what I think it should be doing. I posted an early version of the code on show and tell
Thanks

Philip.

for (var bc = 0; bc<4; bc++){
    
    this_block = the_blocks[bc];
    factors = {
        the_image : this_block
    };
    
    factorial_values = jsPsych.randomization.factorial(factors);
    

    trials_with_variables = {
        timeline: [fixation, trial],
        timeline_variables: factorial_values
    };
    
    preload_b = {
        type : jsPsychPreload,
        auto_preload: true,
        trials : trials_with_variables
    };
    
    big_line.push(preload_b);
    big_line.push(trials_with_variables);
    big_line.push(end_of_block);
  
};

    big_line.push(cursor_on);
    big_line.push(debrief_block);
    big_line.push(end_of_exp);
    jsPsych.run(big_line);

[jodeleeuw]

The preload plugin is unable to automatically detect which timeline variables are images, so you can't use the trials parameter to load those values in this case. Is there a variable that has a list of all the image files in the block, stored as an array? If so, you can pass that array to the images parameter of the preload plugin to accomplish the same goal.

[Quinlanptq]

Thanks Josh

re-wrote ny pre_load_b var as follows

preload_b = {
    type : jsPsychPreload,
    auto_preload: true,
    images: this_block,
    trials : trials_with_variables
};

and this works fine - I thought the images were being handled by the trials variable
Best
Philip.