I've tried the approach that presents an image that is the same color as the background, but I've only been able to do this by presenting the main image for a set RT (675 ms), and then having the "background mask" as another stimulus with an RT:18,23. The problem with this approach is that participants can easily respond before the main image is finished being presented (675 ms), but the image doesn't go away upon their response. This confuses participants, and they try to respond a second time while the image is still on the screen, which messes them up for the next trial. Am I stuck with this option?