What does a direct PLR reaction involve?

A direct pupillary light reflex (PLR) involves the reaction of the illuminated eye to light. When a light source is directed at one eye, the normal response is for the pupil of that eye to constrict. This reaction is a result of the photoreceptor cells in the retina detecting light and signaling the neural pathways responsible for the pupillary response. The constriction helps to control the amount of light entering the eye and protects the retina from excessive illumination.

In this reflex, the primary focus is on the behavior of the pupil in the eye that is directly exposed to the light stimulus. The unequivocal contraction indicates that the neural pathways and muscles controlling pupil size are functioning correctly in response to light exposure.

While some reflex observations also consider the consensual reaction—where both pupils react simultaneously regardless of which eye is illuminated—that aspect is not what constitutes the direct PLR, emphasizing the specificity of the response in the illuminated eye. Therefore, the correct identification of the direct reflex is of paramount importance in assessing normal eye function and diagnosing potential neurological issues in a canine patient.