Introduction
Hypovigilance in the drowsy state is now recognized as a major factor in road traffic and other accidents. Methods for monitoring drowsiness in such people as truck drivers have been proposed that measure the duration of their blinks and other eyelid closures (1). These methods are based on the self-evident fact that we cannot see when our eyelids are closed because light is blocked from entering the pupils. However, there is an associated premise, usually unstated, that eyelid closure is the only cause of lapses in performance in the drowsy state, ie it is assumed that we would be able to perform visual tasks when drowsy, albeit with reduced speed of reaction, if only our eyelids would stay open.
However, there is always active suppression of vision during saccadic eye movements, when the eyelids are open (2). This suppression, which begins before the saccade and ends soon after it, is central (neural) in nature. There is similar suppression of vision during blinks (3). In this case, vision is inhibited both by central suppression and by coverage of the pupils by the eyelids. We are never aware of the gaps in our perception caused by these episodes of visual suppression that may last for about 50 msec during saccades and 250 msec during blinks in alert subjects. Depending on the frequency of such events, it is normal for us not to be able to see for a total of about 3 seconds per minute. By contrast, we can easily detect a light going out for (say) 100 msec at other times.
Vision is also inhibited when we are drowsy and falling asleep. When we prepare to fall asleep purposely we close our eyes voluntarily and that blocks the entry of light into the eyes. However, vision is also inhibited centrally then, as can be demonstrated by taping open the eyelids and flashing a light in the subject’s eyes, to which there may be no response (4). By contrast, there is evidence that drowsy subjects who are trying to stay awake can keep their eyelids open voluntarily for some time, but they may not be able to see, at least intermittently(5), ie in the drowsy state, visual suppression may possibly occur without eyelid closure or saccadic eye movements.
