A Brain Stem Reflex in the Blink of an Eye

T here are at least three types of blinks, reflex, spontaneous, and voluntary, that can lower the lid as little as a few micrometers or completely close the eyes. Reflex blinks protect the eye, rapidly lowering the upper eyelid in response to an assortment of stimuli that could damage or signal damage to the eye. Because reflex blinks also occur in response to cornea1 drying, reflex blinks help maintain tear film continuity on the cornea by spreading tears. Like other reflexes, the size of reflex blinks depends on the magnitude of the stimulus. For example, a weak stimulus may evoke a reflex blink that lowers the upper eyelid as little as 100 pm, while a strong stimulus elicits full lid closure. Indeed, the nervous system adaptively regulates the relationship between stimulus magnitude and the amount of lid closure (see below). Primates also produce spontaneous blinks that occur in the absence of sensory stimuli, as if initiated by a central pattern generator. For example, humans make I O-20 spontaneous blinks a minute. This is a rate much higher than that required to keep the cornea moist. The size of spontaneous blinks is also quite variable. The modal size is -6 mm, which is about one-half the amplitude required for full lid closure (8, 14). Like blink amplitude, blink rate is not constant, changing dramatically with emotional state and cognitive tasks. Blink rate goes up when people are anxious. For example, rapid spontaneous blinking is obvious when someone is uncomfortable being in front of a television camera (e.g., Nixon in the Kennedy-Nixon de-