Event related potentials using visual stimulation

Visual patterns are used to elicit event related potentials. Equipment is available for generating visual geometric patterns such as checkerboards. Slides may be used for patterns which are more complex but preparation is costly and time consuming. A variety of programs exist on PC's for making very elaborate color pictures and in most cases the programs are easy to use making them ideal for generating visual patterns for event related potential experiments. A necessary requirement in event related potential experiments is the ability to control and/or determine precisely when the stimulus is presented to the subject. We have observed that timing is a problem with stimuli generated by the PC as a result of the raster scan and use in many cases of high level system calls in the software. This paper describes a technique which allows for precise control of the time of stimulus presentation using the video control signals to the monitor.