Some methodological issues in the assessment of the spontaneous eyeblink frequency in man.

Previous assessments of spontaneous eyeblink frequency (SEBF) or interblink intervals (IBI) have been made over period of 0.5 to 15 min and average values calculated; the reliability of the methods has not been validated. Video recordings were made of 14 healthy volunteers, aged 20 to 38 years, while silently fixating on a 2 m distant, 35 mm high target under 350 lux illumination and the traces assessed with an event marker. Significant fluctuations in SEBF or IBI were generally observed, but which did not conform to a minute-by-minute periodicity. Time-dependent trends were uncommon, although uncritical pooling or averaging of data can effectively conceal such fluctuations or trends. Correlation's between SEBF and IBI indicate that eyeblink monitoring over at least 3 min is required. Simple averaging calculations are not appropriate because of a high chance of non-Gaussian distribution of data. Modal IBI values correlated well with an adjusted modal calculated SEBF which is thus recommended for further use.