How to use backBuffer instead of frontBuffer GetImage

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Hi everybody,


I have been trying to use GetImage but it is crashing my code  I think because it is using frontBuffer by default. Can anyone help me to change this to backBuffer? This might be very basic question but I can't find answer to this anywhere.


How can I define backBuffer?  Apparently just saying backBuffer is not enough, I need to define it but I don't know how.


Here is the error I am getting.

Undefined function or variable 'backBuffer'.

Error in myExp (line 306)
                        imageArray = Screen('GetImage', window, [940 520 980 560], backBuffer);




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imageArray = Screen('GetImage', window, [940 520 980 560], 'backBuffer');