Loss of Pupillary Light Reflex Resulting from Lesions in the Region of the Posterior Commissure

Abstract

Systematic exploration of the interior of the brain in the region of transition between midbrain and forebrain has enabled us to trace the central path for the pupillo-eonstrictor reflex in response to light. This was done with a fine needle electrode accurately placed with the aid of the Horsley-Clarke stereotaxic instrument and the points stimulated were checked by subsequent microscopic examination. These experiments, which have been fully described elsewhere, 1 showed that the impulses responsible for pupillary constriction travel from the optic tract through the superior quadrigeminal brachium and pretectal region but not through the superior colliculus. There appears to be a partial crossing in the posterior commissure and another ventral to the cerebral aqueduct.

Taking advantage of the precise information thus obtained we have been able to obliterate completely and permanently the pupillary light reflex in both eyes by placing small electrolytic lesions in the pretectal region on each side of the posterior commissure. This was done in cats with the aid of the stereotaxic instrument. In these cats both pupils remained for weeks widely dilated and entirely unresponsive to light. That the sphincter nucleus of the third nerve was not damaged was shown by the fact that pupillary constriction still accompanied winking, the well-known lid closure reflex.

Four of these cats with complete loss of the pupillary light reflex are being allowed to live for a longer period of time in order to make sure that the loss is permanent. Three others were sacrificed after periods ranging from 3 to 7 weeks. Microscopical sections of the brains of these 3 cats showed that the lesions had been accurately placed on both sides of the posterior commissure in the pretectal region and that except for a small area near the midline in the most rostral part of the tectum the superior colliculi were intact. The superior quadrigeminal brachia, the lateral geniculate bodies and optic tracts had not been damaged.

These experiments confirm the conclusions drawn from the stimulation experiments that the pupillary light reflex is mediated through the pretectal region and not through the superior colliculus. The slight amount of damage to the rostral part of the superior colliculus we regard as inconsequential. But in order to remove any possible objection on this ground other experiments are in progress in which the effort is being made to abolish the pupillary reflex with still smaller lesions which do not in any way involve the superior colliculi.