By default, Inquisit measures response latency as the interval beginning at the onset of the last stimlus frame of the trial and ending when the subject issues a valid response. Responses made before the onset of the last stimulus frame are ignored.
The beginning of the response interval can be customized using the beginresponsetime or beginresponseframe attributes. Beginresponsetime specifies how many milliseconds after the onset of the first frame the response interval should begin. Responseframe specifies how many frames after the first frame the response interval should begin.
For example, in the following trial, the response interval begins when the first stimulus, "prime", is presented.
<trial mytrial> / stimulusframes=[1=prime; 10=target] / beginresponseframe=1 </trial>