Fig. 1.

Illustration showing the principal factors that control pupil size. Light excitation of retinal melanopsin ganglion cells activates the neurons of the Edinger-Westphal nucleus. Inhibitory neurons can depress these neurons, and the pupil passively dilates. The preganglionic sympathetic neurons originate in the upper spinal cord, and the postganglionic sympathetic neurons excite the radially oriented muscle to dilate the pupil. However, the sympathetic component is also thought to traverse a pathway above the brainstem, and previous research indicates that consciousness is required for sympathetic tone to be present at the dilator muscle of the iris (asterisk). Our study was designed to confirm or reject experimental studies suggesting that sympathetic tone in the dilator is absent during unconscious states. CN cranial nerve