Dot-Probe script

[Nancy]

Hi everyone,
I'm a current psychology honours student and I'll be using a dot-probe task in my experiment. I was just wondering whether anyone might be kind enough to donate a dot-probe script that could help me get started as I'm totally new to all this software.
I'm planning to present a fixation cross for 1000ms, then a word pair for 500ms, then one of the words is replaced by the dot probe (randomly). In some trials, word pairs will be presented for only 40ms, then replaced by a mask of crosses for 460ms. I also need word pairs to be randomised for each participant.
Any help would be greatly appreciated. Thanks, Nancy

[Trevor Newell]

Nancy,

Have you had the chance to run through all of the samples?
If you take some time to read through the documentation and try out the samples, I'm sure you'll be able to create your experiment without too much trouble at all.

Regards,
Trevor

[Nancy]

Yes, I have gone through the samples and I have figured out most things, although I'm confused about how to present two words on either side of the screen at the same time then disappear together and be replaced by a dot probe. I can't use 0 for the time column since I only need the words to appear for 500ms. When I put in 500ms for both words in the time column, they are obviously appearing one after the other for 500ms each.
Any help would be greatly appreciated.

[Thomas]

Hi Nancy,
I did it in this way:

stim,loc,time
~left word,"256,0,1",0,
~right word ,"768,0,0",500,
dot,"256,0,1","rt:30,76",*

or better:
~left word,"256,0,1",0,~right word,"768,0,0",500,dot,"256,0,1","rt:30,76",*
(LOC is in quotation marks, I guess that's the important fact for you)

256 and 768 are the pixel-locations of the stimulus words, then, as you can see, I used a "clear screen" in the the first screen (it's the 1, a "screen" consists of of a stim-loc-time pattern) and then a zero for the next screen (another stimulus is added, but if you use a zero value for the second stim than the first and the second stim are presented at the same time, at least if you use a zero value in the "TIME" column). Both words are presented for 500ms (you just put in the presentation time in the LOC of the second stimulus). Straight after that, the screen is cleared and the dot appears.

Maybe this helps. I used the code in a study and everything worked fine.
Best,
Tom

[Trevor Newell]

Excellent response Tom.

By using a value of 0 for the third query within the LOC column, you're able to allow for multiple items to display on screen at once. A value of 1 will clear the screen.

~left word,"256,0,1",0,
~right word ,"768,0,0",500,

This syntax is interpreted by the software in the following manner:
1. Display the text, 'left word' at the location on your screen X: 256 , Y: 0 . Clear any stimuli still present before displaying this stimulus. Pause for 0 ms.
2. Display the text, 'right word' at the location on your screen X: 768 , Y: 0 . Keep displaying any stimuli still present. Pause for 500 ms.

[padennis]

Hi Nancy,

Here is an input file that I used in my thesis study. The file starts out with 16 practice trials followed by four blocks of varying contrasts (the non-practice trials were randomized within and between blocks). For each trial, the cross was displayed for 1000 ms, followed by the probes for 500 ms, followed by the dot, which remained on the screen until a response was made. I hope this helps (late as it may be),

Paul