Millisecond Forums

Hello, 
I have a task that shows a brief flickering stimulus (mask) over the top of another image (hand).  At mask offset, there is a box around the mask stimuli that flashes on the screen. Do you know what causes this afterimage and how to remove it?
Thanks

<picture hand>
/ erase = false
/ items = ("hand.jpg")
/ size = (80%, 80%)
/ position = (50%, 50%)
</picture>

<picture mask1>
/ items = ("Mask01.png")
/ size = (20%, 20%)
/ position = (80%, 50%)
</picture>

<picture mask2>
/ items = ("Mask02.png")
/ size = (20%, 20%)
/ position = (80%, 50%)
</picture>

<picture mask3>
/ items = ("Mask03.png")
/ size = (20%, 20%)
/ position = (80%, 50%)
</picture>

<trial action>
/ stimulusframes = [1= hand, mask1; 6= hand, mask2; 11=hand, mask3; 21= hand, mask1; 26= hand, mask2; 31= hand, mask3;
                    36= hand, mask1; 41= hand, mask2; 46= hand, mask3; 51= hand, mask1; 56= hand, mask2; 61= hand, mask3]
/ response = noresponse
</trial>

<trial response>
/ stimulusframes = [1= hand]
/ inputdevice = keyboard
/ validresponse = (57)
</trial>

<block test>
/ trials = [1= action; 2 = response]
</block>

<expt expt>
/ blocks = [1-10 = test]
</expt>