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Pupillary Light Reflex (PLR)

The Pupillary Light Reflex (PLR) has been a well-known phenomenon for many years. In 1942, Lowenstein and Friedman demonstrated that the pupil contracts in response to light after a period of latency. They also found that the duration of this latency period, the amplitude of the response, and the speed of pupillary constriction are all dependent on the intensity of the stimulus used. These findings were later validated by Alpern et al. in 1963, Feinberg and Podolak in 1965, and Lowenstein and Loewenfeld in 1969.



The eyes receive innervation from both sympathetic and parasympathetic nerve fibers, as depicted in Figures 1 and 2. The preganglionic parasympathetic fibers originate in the Edinger-Westphal nucleus, which is a part of the third cranial nerve. These fibers then travel within the third nerve to the ciliary ganglion located behind the eye. At this point, the preganglionic axons connect with the postganglionic parasympathetic neurons, which subsequently send their fibers to the eyeball via the ciliary nerves. The ciliary nerves activate two main structures in the eye: 1) the ciliary muscle, which controls the focusing of the lens, and 2) the sphincter of the iris, which causes the pupil to constrict.

The sympathetic innervation of the eye arises from the intermediolateral horn cells located in the first thoracic segment of the spinal cord. From this origin, sympathetic fibers enter the sympathetic chain and travel toward the superior cervical ganglion, where they connect with postganglionic neurons. After this, the postganglionic sympathetic fibers move along the surface of the carotid artery until they reach the eye. Here, they innervate the radial fibers of the iris, which cause the pupil to dilate or open.


Fig. 1

The diameter of the pupil is controlled by two opposing mechanisms. Stimulation of the parasympathetic nerves triggers the activation of the sphincter pupillary muscle, causing a decrease in the size of the pupil, which is called miosis. On the other hand, stimulation of the sympathetic nerves excites the radial fibers of the iris, resulting in the dilation of the pupil, which is referred to as mydriasis.


Fig. 2

When light enters the eyes, the pupils naturally constrict as a reflex. The neural pathway responsible for this reflex is indicated by the black arrows shown in Figure 1. When light strikes the retina, a portion of the signal travels through the optic nerves to the pretectal region. From there, secondary impulses travel to the Edinger-Westphal nucleus and then return via the parasympathetic nerves to activate the iris sphincter, resulting in its contraction and subsequent decrease in pupil size. Conversely, in a dark environment, the reflex is inhibited, which leads to the dilation of the pupil.

The purpose of the Pupillary Light Reflex (PLR) is to assist the eyes in adapting rapidly to variations in light intensity. The diameter of the pupil has a minimum limit of about 1.5 mm and a maximum limit of about 8 mm. Since the brightness of light on the retina varies with the square of the pupillary diameter, the range of light and dark adaptation that can be achieved by the pupillary reflex is approximately 30 to 1. This implies that the PLR can accommodate up to a 30-fold shift in the amount of light that enters the eye, allowing the eyes to adjust to different lighting conditions more effectively.



The Pupillary Light Reflex (PLR) has proven to be a useful tool in assessing an individual's ability to perform certain tasks, such as driving a motor vehicle (Monticelli et al, 2009, 2015). This is because the PLR can be significantly impacted by factors such as sleep deprivation, alcohol consumption, and drug use. Therefore, monitoring changes in PLR can help identify impairment due to these factors, providing valuable information for evaluating an individual's fitness to perform certain tasks safely and effectively.


When evaluating an individual's ability to perform a task, it is important to consider the three different neural processes that contribute to reaction time: (1) information processing, (2) response programming, and (3) motor control of muscles. The brain's response to sensory stimulation, particularly visual stimulation, is influenced by the demands of perception, while the reaction time is influenced by the combined demands of perception and motor activity. By measuring the peak latency of sensory potentials and the reaction time separately, it may be possible to reach different conclusions about the individual's performance. However, when used together, these measurements can provide valuable information for distinguishing between different sources that may affect task performance.


Visually evoked cortical potentials provide a direct and precise measurement of brain activity with high temporal resolution (Hall, 2016). This can be achieved through the non-invasive measurement of the PLR, which indicates the activity of the system responsible for regulating pupillary reactions, located between the diencephalon and the mesencephalon. This method provides valuable information about brain activity and can be used to evaluate the function of this system in a non-invasive manner.


In a study conducted on 19 normal subjects using infrared pupillography, the pupillary light reflex (PLR) was analyzed in response to different stimulus intensities. Results showed that as the intensity of the stimulus increased, there was an increase in the amplitude and maximum rate of contraction and redilatation of PLR, and the latency of the response decreased. These findings led to the proposal of PLR analysis for clinical evaluation of pupillary function.

Monticelli et al. (2009) suggested using PLR as an objective measurement method for evaluating vehicle driving safety, which would allow for reproducible, reliable, and verifiable data collection. To achieve this, healthy individuals (n=41) and individuals under the influence of drugs and/or medications (n=105) were exposed to different light stimuli, and their PLR responses were evaluated. The primary goal of the study was to assess the applicability and value of pupillography as an objective measurement method for evaluating people with central nervous system disorders with regards to their driving safety and ability to drive vehicles.


The study showed significant differences in almost all parameters when comparing healthy individuals and those under the influence of drugs/medications, demonstrating that pupillography is an objective method to measure pupillary function and can be used in routine police control of vehicle drivers. This was confirmed by a follow-up study that demonstrated the reliability of PLR as an indicator of previous medication and/or drug use. Alcohol has also been shown to affect pupillary measures.


PLR response has also been studied in relation to fatigue and lack of sleep. Lowenstein and Lowefnfeld (1963) demonstrated the use of PLR analysis for objective evaluation of tiredness, while Morad, Lemberg, Yofe, and Dagan (2000) found significant differences in all pupillary parameters between alertness and fatigue.

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