Introduction
A blink is a brief, reflex-controlled closure of the eyelids that occurs either spontaneously, voluntarily or in response to a noxious stimulus. Each blink begins with inhibition of the tonic contraction of levator palpebrae muscles (LP) that otherwise keep the upper eyelids elevated during wakefulness (1). In addition, orbicularis oculi muscles (OO) then contract to close the eyelids actively. The eyelids open when the OO muscles relax and the LP muscles contract again to elevate the upper eyelids. During most blinks in alert subjects the eyelids do not stay closed for more than a few msec, and the whole blink lasts about 100-300 msec.
In the drowsy state some blinks last longer than 500 msec because the eyelids stay closed for some time, presumably because tonic contraction of the LP muscles is inhibited for longer than normal. Closing and opening of the eyelids is also slower because the strength of contraction of LP and OO muscles is reduced.
In recent years, methods have been described (eg PERCLOS) for monitoring drowsiness from video-camera images of the face and eyes (2). Such methods measure the frequency and total duration of prolonged blinks and other eyelid closures per unit time, typically 1 to 6 min. However, there are other parameters of blinks that these methods do not measure. It is known that, in alert subjects, the peak closing velocity (PCV) of blinks is highly correlated with their amplitude (3). Blinks are not all the same, and the larger the blink the higher its velocity. However, with drowsiness, blinks become relatively slower for the same amplitude.
Absolute measurements of PCV, in mm or degrees/sec, and of A, in mm or degrees, are not easy to make because measurements of the linear or angular distances involved must be calibrated. However, the ratio A/PCV has the dimension of time, not of distance (amplitude) or of distance per unit time (velocity). So long as the measurements of A and PCV involve the same units of distance, their ratio can be derived from uncalibrated measurements.
