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Thread: Change Blindness Question

  1. #1

    Change Blindness Question

    I am running a change blindness study in which the subject must press the spacebar when s/he detects a change between two images, so I am alternating between two stimuli (560ms) and a mask (80ms). I want the subject to be able to press the spacebar at any time and jump to a free response trial to answer what changed before moving onto the next trial. I'm having problems getting this to work. At first I had the time column read rt:57>3 [the free response], 560, but after looking at the forums, I realized that this waits for the subject to press the spacebar. I'm unsure what to do here; should I have each time include a no response valid condition? The online manual suggests using ! as a means of automatic continuation, but doesn't say how to actually transcribe that (rt:57, 560!? rt:57, 0-560!?). How can I have the subject jump out of the stimuli presentation portion and into the free response? I am attaching my most current .csv file, as well as giving you access to the spreadsheet I'm using on google docs. Thanks in advance for your help!

  2. #2
    Join Date
    Nov 2005
    Posts
    3,317
    When reading stuff about min-max settings, be sure to note that it's often making reference to the "min-max" column in your style (.drt) file.

    But that said, I'm also trying to understand exactly what you're doing before trying to offer a suggestion. From you you wrote, it sounds to me like you are measuring the RT to detect a change from one picture to another. But when I looked at your input file, I noticed you are repeating pic1 and pic2 over and over within the single trial. Is this intentional?

  3. #3
    The reason the stim repeat so much is that the paradigm entails switching between two pictures and a blank screen really quickly to create a flicker effect. Since DirectRT doesn't use recursion, I'm just going to have the two pictures and mask in a row such that the subject can jump to a response row at any time. In terms of my old problem, where the screen would not advance until a button press, I think changing the style so that the eligible response times overlap with the time the stimuli is presented will do the trick, though please correct me if I'm wrong.

  4. #4
    Join Date
    Nov 2005
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    3,317
    Depends--play with it and see how it works for you. In previous situations like this I have sometimes seen users prefer an animation approach--combing the alternating images into a single animated gif or avi. Makes the input file a lot simpler and you just take a single RT that skips or jumps wherever you like (if you would even need that).

  5. #5
    Join Date
    Mar 2009
    Posts
    2

    Change Blindess Coding

    Hi Jarvis and Nick,

    I was just wondering how the change detection coding issue was resolved. I too am trying to program a change detection experiement and have run into some issues. I can get the images to cycle through on screen (img1, blank, img2, blank, img1, blank, img2, etc) but I cannot figure out how to get the cycle to stop when the participant makes a response (spacebar). Basically, I want the images to cycle through for 300ms each and stop cycling when the spacebar is stuck and record the time from the initial presentation of the first picture to the response. I know Jarvis recommended using an animation for this sequence but I was wondering if either of you had figured it out using just the aternating images. Any coding help would be greatly appreciated!

  6. #6
    Join Date
    Aug 2011
    Posts
    1

    note about automatic continuation

    I didn't notice anyone else having posted this issue but through trial and error I have determined that if you want to use automatic continuation to end a trial that has more upcoming stimuli after a certain periof of no response, you must put two exclamation marks after your MinMax range in the styles file (e.g., 0-2000!!) AND two exclamation marks after the -1 in your rt statement (e.g., rt:-1!!,57).

    This allowed me to skip to the next trial if there was no response after 2 seconds but if the spacebar was pushed before 2 seconds was up then it proceeded to the remaining stimuli associated with that trial.

    Not sure why this works or if this actually helps you, but it hopefully explains the transcription issue - I was frustrated by this also.

    Cheers.

  7. #7
    Join Date
    Feb 2019
    Posts
    1

    trouble with Change blindness programming (case 33638)

    I am having trouble programming a change blindness experiment using Direct RT.


    The main issue is that in the Time column I want Direct RT to show stimulus for 250 ms and then show the next stimulus for 250 ms in the same trial, however, I also am hoping to program Direct Rt so that we can measure participants reaction time IF/When they notice a change in the picture (by clicking on the spacebar).


    For each trial, the participant will see Stim 1 (picture1 ), Stim 2 (blank screen), Stim 3 (picture 2), Stim 4 (blank screen). This loop will be repeated several times. This is done to look like a flicker effect. So that participants won't notice the change for the first few stimuli (but the stimulus need to be timed in order for the flicker effect to work).


    So I want to programme for the stimulus to be presented for a specific amount of time (250ms) or until a response is made by the participant. However, from reading the manual it seems that this is not possible as I can only do one or the other? Is this possible on DIrect RT?


    I hope you can help.
    Last edited by jason_reed; 02-21-2019 at 11:04 AM.

  8. #8
    Join Date
    Feb 2013
    Posts
    1,081
    What you described could be done using a combination of your input file and the MinMax field of your styles.drt file.

    To be able to record a press of the spacebar for any of the four stimuli in a single series as you described, type

    rt: 57

    in the time cell for each of the four stimuli. This will tell DirectRT to record any press of the spacebar.

    To ensure that the stimulus stays onscreen for 250 ms, you need to make use of the the rows of the styles.drt file, particularly the MinMax field. In the first row of the styles.drt file, type

    250!

    in the MinMax field. This tells DirectRT that for any stimulus that anything in the .csv input that uses the same number as in the Styles cell of that row of the styles.drt should be displayed for 250 ms and then move on to the next stimulus in the same row of the .csv input file.
    Jason Reed
    Empirisoft Software Support Specialist

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