A stimulus response is triggered when the user has not blinked for a certain timespan - the fatigue limit.
Previous studies used a constant time of 4 seconds in accordance to the average blink rate of 15 blinks per minute.
When we conducted our pilot study, we found two problems with this fatigue limit.
First, people started to expect the stimulus response.
This stems from the fact that individuals do not blink at a constant rate, and therefore using a constant time feels unnatural.
Second, users found it bothersome and annoying to have stimulus applied so frequently.