There might be 3 reasonable possibilities:
1. Move the trials without a modulator to a different regressor class.
2. Assign an average response time to such trials (so they do not affect the modulation beta).
3. Assign a maximum (but reasonable) response time to such trials (because that seems more natural based on the task).
Which to do depends on the task and the circumstances of a failed behavioral response. Was the subject trying but just too slow? What the subject not paying attention?
- rick
