Assessment of short-term variability in human spontaneous blink rate during video observation with or without head / chin support

Background: The aim was to assess the variability of spontaneous blink rate (SBR) with and without a chin and forehead support. Methods: Forty-eight healthy non-contact lens wearers, aged from 20 to 39 years, had five-minute video recordings made under ambient lighting of 350 to 400 lux, while directing their gaze to a distant target at head height. Half the subjects (group 1) were seated resting against the chair head rest and the other half (group 2) seated but with chin and forehead at a slitlamp. The first 35 blinks were analysed in detail. Results: As assessed over five minutes, 35 to 111 blinks were counted, with SBR between 6.9 and 21.8 blinks per min (average 13.9 per min). Over the initial 35 blinks, the average momentary SBR values (calculated from the inter-blink intervals) averaged 24.8 blinks per minute in group 1 and 19.3 blinks per minute in group 2 (not significantly different, p = 0.273) but a statistically significant (p < 0.001) decrease in SBR was observed over the initial 10 to 15 blinks in group 2. The variability in momentary SBR values, as assessed from successive blinks, had coefficient of variation (COV) values of 80 and 78 per cent, respectively over 35 blinks. Conclusions: Averaged spontaneous blink rates over short time periods (that is, five minutes) should be suitable to compare various experimental paradigms but if very short periods are used (for example one minute or less), then there could be significant time-related changes, especially when a subject is seated with chin and forehead support.