PROCEDIMIENTO DE CONFIGURACIÓN EN T2000

The purpose of this section is to discuss measures that may lead to detect driver drowsiness and their operational definition. In this thesis drowsiness and sleepiness are considered synonymous, but the term drowsiness will be used. Another concept commonly used is drowsiness, which is an extreme tiredness that results from physical or mental activity. Drowsiness can also be described by the grade of wakefulness or vigilance. Wakefulness is the same as alertness or state of sleep inability, whereas vigilance can be described as watchfulness or a state where one is prepared for something to happen (Leproult et al., 2003).

2.5.1.

Eyelid Closure

Eyelid closure is a very reliable predictor of driver drowsiness (Erwin, et al., 1980; Dinges, et al., 1985). Erwin et al, examined various measures to determine if they were predictive of sleep onsets, including plethysmography (a device for measuring and recording changes in the volume of the body or of a body part or organ), respiration rate, Electroencephalography (EEG), skin electrical characteristics, Electromyography (EMG), heart rate variability, and eyelid closure. It was found that eyelid closure was the most reliable predictor of the onset of sleep among the measures examined.

Eyelid closure is indicative of sleep onset and undoubtedly the cause of poor performance in visual tasks, especially tracking tasks such as driving. It seems quite obvious that if a driver’s eyelids are closed, the ability to operate a vehicle would be impaired. Skipper et al. (1984) examined the ability of sleep deprived drivers to

perform a one and one half hour driving task. Various disturbances were purposely input into the steering system of the driving simulator to mimic on-the-road conditions. It was found that performance measures such as lane deviation and steering velocity were highly correlated with eyelid closures. The apparatus used to capture eyelid closures in the studies by Skipper et al, (1984) was a low-light level camera. A linear potentiometer was used manually by an experimenter to track and record the eyelid movement of the participants.

2.5.2.

Eye Movements

There are two general methods used to record eye movements during sleep or before sleep. The first method is Rapid Eye Movements (REMs). The second is based on the onset of sleep in most participants being accompanied by slow, rolling eye movements (Carskadon, 1980). Slow, rolling eye movements may accompany the onset of sleep or are precursors of sleep onsets. This phenomenon also occurs with the transition to stage 1 sleep during the night. The characteristics of human eye movements change greatly with alertness level. Slow eye movements (SEMs) prove to be one of the most characteristic signs of the phase of transition between wakefulness and sleep (Planque et al., 1991). A completely awake individual can be observed as having quick eye movements. As participants become drowsy, their eyes move in a pendulum motion from left to right (Hiroshige and Niyata, 1990) and the number of quick, voluntary movements of the eyes begins to lessen. Several SEMs are detected during stage 1 sleep, but they also appear during the long period separating waking from sleep. Convergence of the eyes is also possible when a person becomes drowsy.

Electrooculography (EOG) Figure 2-3 (page 50) shows the measuring of eye movements via electrodes in contact with the skin surrounding the eyes. The process of measuring eye movements with EOG is quite simple due to the electrical nature of the human body. In the eyeball, there is a small electro-potential difference from the front to the back. The front (cornea) of the eye is positive with respect to the back (retina) of the eye. Before a certain point in a person’s awake but drowsy state, SEMs do not exist. However, after a particular moment in the onset of sleep, slow, rolling, lateral, ocular movements create sinusoidal activity in the EOG (Lairy and Salzarulo, 1974). On the EOG signal, the SEMs are translated by slow deflections lasting more than a second. It is likely that amplitudes of at least 100 microvolts will be seen (Torsvall and Akerstedt, 1988). The EOG waves that are normally observed are moderate in amplitude initially, but increase with the degree of drowsiness (Planque et al., 1991). These researchers found that after several minutes of driving only blinking and glances at simulator instrumentation were recorded. Approximately 30 minutes into the study deterioration of deliberate eye movement was observed. Planque et al. (1991) argue that, by analyzing the EOG, it is possible to follow clearly the deterioration of alertness.

2.5.3.

Muscle Activity

The Electromyogram (EMG), conventionally abbreviated as "EMG" is a record of the electrical activity which emanates from active muscles, especially in the facial muscles. It may also be recorded from electrodes on the skin surface overlying a muscle. In humans, the EMG is typically recorded from under the chin, since muscles in this area show very dramatic changes associated with the sleep stages (BOSB, 1997). Hauri (1982) demonstrates that EMG recorded on the chin steadily,

though not dramatically, decreases as a person nears sleep. Even when a person is totally relaxed, small muscle potentials will be seen. This is because every muscle is composed of many contractile fibres that are innervated by nerves. When a muscle fibre is activated through nerve innervations, a change in the electrical potential is seen. When the muscle is relaxed, fewer nerves discharge, thus a smaller EMG potential is recorded. EMG (figure 2-3) is used to predict drowsiness with electrical potential differences of facial muscle.

2.5.4.

Brain Wave Activity

The Electroencephalogram (EEG) is conventionally abbreviated as "EEG" and is popularly known as "brain waves." The EEG was discovered in 1929 by Hans Berger, a Swiss psychiatrist. He found that small changes in voltage between two electrodes occurred when they were placed in contact with the scalp. Voltage changes are amplified and examined for variations in duration. The exact physiologic basis of the voltage variations are not entirely known, but it is believed that they originate largely from changes in voltage of the membranes of nerve cells. Erwin, et al. (1980) found that there is no reliable alteration in background brain activity prior to eyelid closure. Upon eyelid closure, the researchers found that a very rapid shift in brain wave patterns takes place. This shift is identifiable as the early stage of sleep. However, Planque et al. (1991) argue that sharp changes in the frequency content of brain wave activity are observed during the crossing from alertness to a stage of hypoalertness, then to drowsiness, and finally to sleep. A slowdown of the brain activity in general, an increase in the percentage of alpha waves and, in turn, a decrease in the percentage of beta waves, is observed at the same time that a decline in performance is seen.

Figure 2-3: Positions of Physiological Measures (BOSB, 1997)

2.5.5.

Heart rate Variability

Heart beats interval variability has been found to correlate with drivers’ level of drowsiness (Wierwille and Muto, 1981). On the other hand, Volow and Erwin (1973) found no correlation between heart rate variability and sleep onset.

2.5.6.

Pupil Aperture Size Variability

Natural pupil movements in darkness in the normal awake individual have been described as reflecting “tiredness,” “drowsiness,” and “sleepiness”. The changes in pupillary stability and extent of oscillations have been consistently shown to occur in normal “tired” participants. The pupillary behaviour in individuals suggests that the actions of the pupil do reflect autonomic events, and that it is consequently an indirect but accurate indicator of sleepiness or arousal level (Sharon et al, 1996).

Table 2-1: Autonomic Nervous System Activity during Sleep (BOSB, 1997)

Table 2-1, shows how the pupil diameter changes during different stages of sleep. Therefore, recognition of pupil diameter can help to measure drowsiness.