Description of the pupil response to light

Eye Diagram

Diagram of the human eye with a schematic enlargement of the retina, from webvision.med.utah.edu

The light enters the eye through the pupil and it is converted to nervous activity by the retina, a complex photosensitive structure at the back of the eye. The light signal travels via the optic nerve from the eye to the visual cortex. Some of the optic fibers go to a relatively small neuronal cluster for each eye, called the Edinger-Westphal nucleus, that sends signals to the pupil iris muscle, thus controlling the size of the pupil. The pupil contracts when the retinal illumination increases and dilates when the retinal illumination decreases or when the eye adapts to moderate ambient light. This mechanism protects the retinal photoreceptors from bright lights and keeps on average a constant light flux F. This light flux is given by all Fi received by the retinal ganglions, with Fi=IiAi (Ii is the light intensity received by, and Ai the receptive area of retinal ganglion i). The pathway described above is related mainly to the parasympathetic component of the reflex. There is also a very important sympathetic component, related to response to emotions, ambient temperature, pain, etc. The parasympathetic and sympathetic activities define the autonomic nervous system, and this makes the study of the pupil response to light very relevant to clinical neuroscience. It is important, for medical research, to be able to isolate the behaviour related to the parasympathetic and to the sympathetic system in the pupil light reflex. The noise and variation in the experimental data and nonlinearities are making this a difficult task.

There are some important details concerning the neuronal component of the reflex, that are yet not fully understood, like the interactions between neurons, the contribution of the sympathetic branch, the mechanism of the noise called pupillary hippus (fluctuations in pupil size at steady illumination), adaptation, the response to dark pulses or to the switch between black and white patterns of equal overall illuminance (the pupil contracts), etc. The iris muscle and the neurons involved in the reflex have different response characteristics, and this is widely used in measurements. Neurons involved in the pupil light reflex have a firing rate that can reach 60 to 100 Hz, while the iris muscle cannot follow oscillations in the light signal of a frequency much higer than 4 to 6 Hz. This indicates that high frequency components of the pupil response to light might be related to the neuronal behaviour. There have been studies of the pupil response to sinusoidally modulated light, to beats, and of the pupillary noise. The neurons have also a different response time delay, typically 50 to 80 milliseconds, compared to a larger time delay associated to the iris muscle (in the region of 200 milliseconds). Measuring the pupil response time delay is thus used to better understand the neural component. One way of doing this is examining pupil oscillations created by pupil-edge stimulation: the light stimulus is a narrow spot of light directed to a point near the edge of the pupil. When the pupil contracts, it becomes too small and the light spot is covered, then the pupil dilates and gets to the size where the light spot can enter the eye, so the pupil will contract again, and so on. The delay in the pupil response to the light being on or off determines the period of these oscillations. This pupil cycling is important in medical diagnosis, since some nerve deffects influence the way the pupil oscillates, and the range of the pupil threshold area (determined by the position of the light spot) for which it oscillates.