Hi Maria - In order to describe a null response as valid, you will want to use the response time value of ' -1 ' . Using -1 tells the software that you want to denote 'no response' as the correct response.

Here's an excerpt from the manual:
"An alternative to using the ! symbol is to define -1 as a valid response code on your trial. That way, only trials on which -1 is given as an option will continue automatically after the maximum time has been reached. Like other codes, if -1 is listed first, then it will be considered the correct response. This option can also be combined with the ! method if you want to define no response as correct but have automatic continuation regardless of the response. You can think of -1 as an actual "no response" button which is pressed at the maximum response time."
This text was taken from the following link: http://www.empirisoft.com/directrt/h..._rt_limits.htm

You may also want to look into the GNAT sample provided with the software in the 'Samples' folder. More information on this sample can be found here: http://www.empirisoft.com/directrt/h...e_as_valid.htm

Please let me know if you have any additional questions or if I can help in any way.