execute a block of code based on real time analog input

Hey guys,

I am trying to to sychnronize EEG acquisition and visual stimulation so that they achieve time alignment. My EEG equipment can send analog signal through NI device. My thinking is as soon as matlab detected the real time analog signal using the data acquisition toolbox the visual stimulation starts. I've writen the visual stimulation, but I have no idea how to execute it based upon the real time analog input. I believe the next step should be for matlab to monitor the real time analog signal continuously. How can I do that, and how can I use it as an event trigger to execute code?