The localization of pupilloconstrictor function within the mid-brain of the cat.

1. Electrical stimulation techniques have been used to examine the localization of pupilloconstrictor function in the mid-brain of the cat.2. The results show the preganglionic pupilloconstrictor neurones to be localized to a discrete part of the small celled component of the oculomotor nucleus.3. The area described comprises both the Edinger-Westphal and anteromedian nuclei at the level where they adjoin and the caudal continuation of the anteromedian nucleus. It does not include the rostral anteromedian nucleus or the Edinger-Westphal nucleus caudal to the level where its cells are continuous with those of the anteromedian nucleus. These regions do not appear to relate to pupilloconstrictor function.4. The preganglionic nature of the pupilloconstrictor area within the small celled component was verified by an examination of the distribution of the antidromic evoked potential produced by stimulation of the preganglionic fibres at the ciliary ganglion.5. The distribution of pupilloconstrictor responses obtained from extranuclear structures suggested the presence of an excitatory tract from the region of the posterior commissure to the pupilloconstrictor nucleus.6. There was no indication of a lateral division of function within the pupilloconstrictor nucleus. All the responses obtained from nuclear or supranuclear structures were bilaterally equal.