Hello Burcu
I am sorry to say but I will not program the experiment for you. Sorry, if I created that impression. You have to program it yourself or find somebody how does it for your, usually for money. See here for instance.
So, what did you try and what did not work out? Here, for instance, is an example where a continuous key press was used.
I am happy to help with problems you experience. So, it is still not clear what you intend to do. In half of the trials, the largest number of dots appears on the screen and when pressing left, the numbers decrease. But what happens when the participants press right or press nothing?
Best wishes Jens