BACKGROUND: Lesions affecting the pretectum or the brachium of the superior colliculus (brachium) and sparing the optic tract cause a contralateral relative afferent pupil defect (RAPD) but no visual field loss. It has been assumed that the pupillomotor pathways within the brachium are a continuation of the pupillomotor pathways traveling in the optic tract. To investigate this assumption, we looked for hemihypokinesia by means of pupil perimetry. METHODS: Pupillary hemifield stimulation was performed in a 65-year-old woman with normal visual fields and an isolated left RAPD due to a cerebral hemorrhage affecting the right dorsal midbrain. The pupil responses from light stimulation of the nasal inferior, nasal superior, and temporal inferior and temporal superior quadrants of both eyes were recorded using computerized binocular infrared pupillography. Each stimulus was presented 5 times and the mean amplitude of the pupil response was calculated for each stimulus location. RESULTS: Pupil perimetry demonstrated a marked hemihypokinesia (reduced light reaction) in the hemifield contralateral to the site of the lesion. CONCLUSIONS: Our experiment suggests that the brachium is indeed a continuation of the afferent pupillary fibers traveling in the optic tract.
BACKGROUND: Lesions affecting the pretectum or the brachium of the superior colliculus (brachium) and sparing the optic tract cause a contralateral relative afferent pupil defect (RAPD) but no visual field loss. It has been assumed that the pupillomotor pathways within the brachium are a continuation of the pupillomotor pathways traveling in the optic tract. To investigate this assumption, we looked for hemihypokinesia by means of pupil perimetry. METHODS: Pupillary hemifield stimulation was performed in a 65-year-old woman with normal visual fields and an isolated left RAPD due to a cerebral hemorrhage affecting the right dorsal midbrain. The pupil responses from light stimulation of the nasal inferior, nasal superior, and temporal inferior and temporal superior quadrants of both eyes were recorded using computerized binocular infrared pupillography. Each stimulus was presented 5 times and the mean amplitude of the pupil response was calculated for each stimulus location. RESULTS: Pupil perimetry demonstrated a marked hemihypokinesia (reduced light reaction) in the hemifield contralateral to the site of the lesion. CONCLUSIONS: Our experiment suggests that the brachium is indeed a continuation of the afferent pupillary fibers traveling in the optic tract.