How to fire an event when ROI is looked at

I want to make an experiment that’ll use an eye tracker, but first I wanted to familiarise with the output data so I created a fake experiment with different trials so that I can extrapolate and analyze data.

In one of the trials there’ll be a “Find the Differences” image, and I wanted to make a circle appear when the subject has been staring at a difference for more than x seconds.
The problem is there is no way to set the behaviour when the ROI has been looked at, other than end routine (which I don’t want to do as I was thinking of putting 10 ROIs, one for each difference, and end the routine when they have all been found).

Do you know how I could do it?