Millisecond-accurate synchronization of visual stimulus displays for cognitive research

A widely adopted approach in cognitive psychology research is to analyze changes in the response time to a stimulus onset in order to infer information about the cognitive functioning of a subject being tested. But current techniques have inherent variations in the timing between stimulus activation and stimulus display of up to tens of milliseconds, thereby introducing significant errors when response time or the latency of neural responses is measured. This article presents a novel yet easy-to-implement solution for improving resolution in the synchronizing of stimulus activation and stimulus display. Unlike traditional methods in which the stimulus onset is set as the time at which the routine for displaying the stimulus is called, this approach uses DirectX to monitor the scan line of CRTs and sets the stimulus onset to the time at which the scan line arrives at the position where the stimulus is to be drawn. This approach removes the uncertainty involved in having a time delay between the activation of the display routine and the actual time at which the display occurs, improving the accuracy of response time and latency period measurements to within 200 microsec. With a specially developed driver, this solution can generate a trigger signal synchronized precisely with the stimulus onset in all popular Windows systems (including Windows 2000/XP).