PTB Error - Synchronization Failure

• I have already posted this question to the Psychtoolbox yahoo forum, but I thought I would post my problem here as well

I believe I am having trouble with an overloaded BackBuffer or a Synch failure problem with my code.

Essentially the purpose of my code (which will be attached to this email) is to create a rotating (moving back and forth along a determined trajectory), circular figure that has a line flashed upon it. The timing of the flash relative to the reversal of motion is manipulated over multiple iterations. And I noticed after running through around 15 different variations of the stimulus, MatLab encounters the following error message...

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WARNING: Couldn't compute a reliable estimate of monitor refresh interval! Trouble with VBL syncing?!?

----- ! PTB - ERROR: SYNCHRONIZATION FAILURE ! ----

One or more internal checks (see Warnings above) indicate that synchronization of Psychtoolbox to the vertical retrace (VBL) is not working on your setup.

This will seriously impair proper stimulus presentation and stimulus presentation timing! Please read 'help SyncTrouble' for information about how to solve or work-around the problem. You can force Psychtoolbox to continue, despite the severe problems, by adding the command Screen('Preference', 'SkipSyncTests', 1); at the top of your script, if you really know what you are doing.

Error using Screen See error message printed above.

Error in MakeVideoStimulusTJS2B (line 4) [window,screenRect] = Screen('OpenWindow', screenNumber, 0, []);

Error in BynumExp2B (line 47) MakeVideoStimulusTJS2B

Error in RunExp>RunExp_Callback (line 267) BynumExp2B(handles);

Error in gui_mainfcn (line 96) feval(varargin{:});

Error in RunExp (line 45) gui_mainfcn(gui_State, varargin{:});

Error using waitfor Error while evaluating uicontrol Callback

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If you would like to run to the experiment to see if you get the same error message, all you need to do is load each of the attached files into the MatLab directory, and run the "RunExp.m' script and it will prompt you with command box ('RunExp.fig') to type in experimental information. Once you filled out the command box, press the Run Experiment button and it will begin the experiment. (If you want to quickly get through it, just hit Enter then Spacebar after each presentation of the stimulus)

Under the BynumExp2B.m script, a function called 'repetitions' can be altered in order to run each condition multiple times if you desire. I normally get it to work when you set the number of repetitions to '1', but the error only occurs if you write in 2 or more repetitions to be carried out.

Hopes this provides you all with enough information to address this issue. I'm not sure if there is a function that exists within MatLab that my clear out the BackBuffer after a certain period of time or after some prompt. But any and all help provided will be appreciated.

Thanks and have a nice day,

Joe Bynum - Graduate Research Assistant - University of South Alabama