In the blink of an eye
Tuesday, August 05, 2014 by Chris Hocking
Drowsiness significantly increases the risk of an accident and is believed to contribute to more than 30% of road crashes. The ability to identify the dangerous state of drowsiness can be achieved by characterising the dynamic changes in eyelid movements during blinks. While measures of blink duration, such as PERCLOS, can detect the very late stages of drowsiness, the effects are not the same for everyone. Rather, it is the early dynamic changes in blinks, particularly the speed of the eyelids, providing a reliable physiological marker of the fluctuating states of alertness and drowsiness.
Drowsiness can be defined as the intermediate state between alert wakefulness and sleep. It is an unstable, fluctuating state of reduced awareness and alertness that can result in significant performance decrement over relatively short periods of time. That makes drowsiness a dangerous state for drivers of motor vehicles or operators of industrial equipment, for whom the maintenance of vigilance can be a matter of life and death.
Most people are aware of the dangers of driving while intoxicated, but many don’t realise drowsiness impairs performance, judgement and reaction times in much the same way as alcohol and drugs. Studies show being awake for 17 hours results in an impairment equivalent to a blood alcohol concentration (BAC) of 0.05, the legal limit on Australian roads. Being awake for more than 20 hours is comparable to the impairment attributable to a BAC of 0.08, the legal limit in all American states. Being drowsy significantly increases the risk of an accident. Drowsiness contributes to an estimated 30% or more of road crashes and is a significant risk to occupational health and safety, particularly for the shift working population.
Changes in eye and eyelid movements, referred to as ‘ocular dynamics’, provide a direct link to fluctuations in human attention and alertness. Ocular movements, particularly eye blink parameters, are considered reliable physiological indicators of drowsiness levels. Blink characteristics during the eyelid closure and reopening show unique properties that can objectively quantify changes in drowsiness.
The typical eye blink lasts around 250-300 milliseconds (msec), comprising three distinct phases: (1) the closing phase, (2) full eye closure, (3) and reopening phase. Various ocular parameters can be derived from these phases which will be outlined below.
Measures of blink duration
In a well-rested state of alert wakefulness, the duration of typical closing and reopening phases of a blink are in the order of 100 and 150 msec respectively. Importantly, the separate durations of eyelids closing, remaining closed and reopening all tend to increase with drowsiness. However, correlations between the durations of the separate components of blinks are quite low, suggesting the reflex processes controlling such movements are partially independent of one another. However, substantial inter-individual differences are commonly associated with blink duration parameters and also blink rate, how frequently a person blinks, which makes these variables of limited value for quantifying drowsiness on their own.
A commonly used measure of eye closure duration is ‘PERCLOS’ – the percentage of time a person’s eyes are 80-100% closed over a period of time, typically five minutes. The PERCLOS method reflects slow eyelid closures, or ‘droops’, rather than blinks, and is best suited when averaged over longer time periods. As such, noted drawbacks of this method include; PERCLOS may only detect the very late stages of drowsiness which can have a significant impact on mitigating risk in operational settings; PERCLOS cannot detect the significant subset of drowsy individuals experiencing micro sleeps with eyes open. Like other measures of blink duration, the PERCLOS method alone cannot reliably characterise the fluctuating states constituting the alert-drowsy continuum, as it does not include any assessment of dynamic changes in eye and eyelid movement.
Importantly, how long a blink lasts depends on both the amplitude (size of movement) and the velocity (speed) of the eyelid movement, particularly the upper eyelid. While blink duration measures have previously been the most informative parameters for detecting drowsiness, albeit not consistent between people, parameters involving dynamic changes in the amplitude and velocity of eyelid movements have been shown to be far more sensitive.
The amplitude-velocity ratios for blinks
There are some ocular parameters that remain stable between different people and have been shown to be the most sensitive indicators of drowsiness. In alert people, the amplitude (size) of upper eyelid movements during each blink is known to be very closely related to its peak velocity (speed). The further the lids move, the higher their velocity.
In 2003, Professor Murray Johns introduced amplitude-velocity ratios (AVRs) for blinks as a measure of the relative velocity of their movements. The AVRs for eyelids closing during blinks are different from those for eyelids reopening, even though their amplitudes are usually the same. The AVRs have been shown to increase with drowsiness, particularly for the eyelids reopening, i.e., the upper eyelid moves more slowly when re-opening after a blink in people when drowsy than when alert. The eyelids may droop with drowsiness, which would decrease the amplitude of blinks, but that does not change their AVR. These ratios avoid the need for calibration of either the amplitude or velocity in absolute terms. The ratios are also very stable between people, which mean they do not have to be adjusted for individuals.
AVRs have been shown to remain stable in the alert state; however increasing levels of drowsiness delay the velocity of the eyelids, particularly for the eyelid reopening phase. It is the early physiological changes in blinks, particularly relative velocities of the eyelid closure and reopening phases that characterise the drowsy state.
The neuromuscular functions of skeletal muscles are influenced by sleep and wakefulness. This applies particularly to the eyelids which are open most of the time during wakefulness and closed continually during sleep. Drowsiness affects the neuromuscular functions of the eyelids in more than one way. It impairs the mechanisms controlling the activation and inhibition of muscles in each eyelid. In alert wakefulness, those mechanisms are highly integrated to produce typical spontaneous blinks. With increasing drowsiness, the degree of control and integration is reduced. The dynamics of eyelid movements provide a direct physiological link to changes in central nervous system function, especially during the transition from alert wakefulness to sleep.
A new scale for measuring drowsiness, the Johns Drowsiness Scale (JDS™), has been developed for use with Optalert technology. It is based on a combination of several ocular variables, particularly the relative velocities of eyelid movements during blinks. The development of the JDS™ and its validation are summarised in the accompanying whitepaper, Measuring drowsiness with the JDS™.