I think it depends on how you defined your items and their parameters. Make sure to give a proper name to each item you add and add an RT only when needed. Do you want to measure the RT? if yes, you can decide when to start recording by indicating it in the start times button in item properites. So for example, if your trial begin with an image + response item, and you want to measure RT starting from beginning of image presentation, then the start of the image in times should be similar to start of response item and it should be 0. However, if you have in trial let’s say : Image1+ blank+ image2+response item, and you want to measure RT response starting from image2, then start time for image2 should be similar to start time of response item.