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Pupillary Light Reflex (PLR)

The Pupillary Light Reflex (PLR) is a phenomenon widely known for many years.

Lowenstein and Friedman (1942) showed that, in response to light, the pupil contracts after a period of latency and that, the duration of this period, the amplitude of the response and the speed of pupillary constriction depend on the intensity of stimulus used. These findings were subsequently confirmed by Alpern et al. (1963), Feinberg and Podolak (1965), and Lowenstein and Loewenfeld (1969). 



Autonomic Nerves to the Eyes. The eye is innervated by both sympathetic and parasympathetic nerve fibers (Fig. 1 and 2). The preganglionic parasympathetic fibers arise in the Edinger-Westphal nucleus (third cranial nerve) and then pass in the third nerve to the ciliary ganglion, which lies behind the eye. At this point, the preganglionic axons synapse with the postganglionic parasympathetic neurons, which in turn project their fibers to the eyeball through the ciliary nerves. These nerves stimulate: 1) the ciliary muscle that controls focusing of the eye lens and 2) the sphincter of the iris that constricts the pupil.

The sympathetic innervation of the eye originates in the intermediolateral horn cells of the first thoracic segment of the spinal cord. From this point, sympathetic fibers enter the sympathetic chain and project towards the superior cervical ganglion, where they synapse with postganglionic neurons. Subsequently, postganglionic sympathetic fibers project along the surface of the carotid artery until reaching the eye where they innervate the radial fibers of the iris that open the pupil.


Fig. 1

Control of Pupillary Diameter. Stimulation of the parasympathetic nerves excites the sphincter pupillary muscle, thereby decreasing the pupillary aperture, phenomenon called miosis. Conversely, stimulation of the sympathetic nerves excites the radial fibers of the

iris inducing pupillary dilation, phenomenon called mydriasis.


Fig. 2

When light illuminates the eyes, the pupils constrict. The neuronal pathway responsible for this reflex is represented by the black arrows shown in Fig 1. When light strikes the retina, part of the signal is directed from the optic nerves to the pretectal region, from where the secondary impulses reach the Edinger-Westphal nucleus, returning through the parasympathetic nerves to contract the iris sphincter. Conversely, in a dark environment, the reflection is inhibited, which results in a dilation of the pupil.

The function of the PLR is to help the eye to adapt quickly to changing light conditions. The limits of pupillary diameter are about 1.5 mm on the small side and 8 mm on the large side. Therefore, because light brightness on the retina increases with the square of pupillary diameter, the range of light and dark adaptation that can be brought about by the pupillary reflex is about 30 to 1. In other word, up to as much as 30 times change in the amount of light entering the eye.



The PLR has been used successfully to evaluate the ability of an individual to perform a specific task, including motor vehicle driving (Monticelli et al, 2009, 2015). The reason is that the PLR can be severely affected by sleep deprivation and the consumption of alcohol and drugs.


In this context, it is essential to consider the existence of three different neural processes that determine the reaction time to perform a given task: (1) processing of information, (2) programming of the response and (3) motor control of the muscles. The cerebral potentials evoked by sensory stimulation (mainly by visual stimulation) are affected by the demands of perception, while the reaction time (RT) is affected by the sum of the perceptual and motor demands. Separate measurements, for example, the peak latency of the evoked sensory potentials and RT may or may not reach similar conclusions, however, together they can be used to discriminate between sources that influence task performance.


Visually evoked cortical potentials offer a direct measurement of brain activity with high temporal resolution (Hall, 2016). This possibility is offered by the registration of PLR, a non-invasive method that indicates the activity of a system, in this case, located between the diencephalon and the mesencephalon, and as previously mentioned, a system that regulates pupillary reactions.


In a pioneering trial, conducted in 19 normal subjects, the PLR was studied in response to different stimulus intensities, using infrared pupillography (Ellis, 1981). The results showed that the increase in the intensity of the stimulus was associated with an increase in the amplitude and the maximum rate of contraction and redilatation of PLR. Also, the latency of the stimulus at the beginning of the response decreased as the intensity of the stimulus increased. These results allowed to propose PLR analysis for clinical evaluation of pupillary function (Ellis, 1981). Monticelli et al. (2009) proposed to use the PLR to obtain objective measurement methods regarding vehicle driving safety that would allow the collection of reproducible, reliable and subsequently verifiable data. For this purpose, healthy individuals (n = 41), as well as individuals who were under the influence of drugs and/or medications (n = 105) were exposed to different light stimuli, and their PLR were evaluated. The primary objective of the study was to evaluate the applicability and the value of the pupillography as an objective measurement method to evaluate people with central nervous system disorders regarding their driving safety and their ability to drive vehicles.


The results showed highly significant differences for almost all the parameters evaluated when comparing the two groups, so that, on this basis, they showed that pupillography represented an objective method to measure the function of the pupil, and that, therefore, it could be established as part of a routine methodology for police control of vehicle drivers. This evidence was later confirmed by a second study, carried out by the same research group, which demonstrated the reliability of PLR as an indicator of previous medications and/or drug use (Monticelli et al, 2015). Similarly, to alterations produced by medication and/or drugs, it has been shown that alcohol also alters pupillary measures (Lobato-Rincón et al, 2013).

The effect of fatigue and lack of sleep on PLR response has also been studied extensively, Lowenstein, Lowefnfeld 1963 demonstrated the use of PLR analysis for the objetive evaluation of tiredness, Morad, Lemberg, Yofe, Dagan, 2000 verified that all pupillary parameters differed significantly between alertness and fatigue.

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