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Show Journal of Neuro- Ophlhalmology 20( 2): 89- 91, 2000. © 2000 Lippincott Williams & Wilkins, Inc, Philadelphia Photo Essay The Representation of the Horizontal Meridian in the Primary Visual Cortex Steven L. Galetta, MD, and Robert I. Grossman, MD The authors report the findings of two patients that confirm the location of the horizontal meridian in the human visual cortex. The first patient had an inferior quadrant defect with a band of horizontal meridian sparing. Magnetic resonance imaging showed a lesion concentrated along the medial striate cortex. The second patient had a homonymous horizontal defect that resulted from removal of an arteriovenous malformation located in the lateral striate cortex. The findings of these two patients demonstrate that the horizontal meridian is represented at the calcarine fissure base in the primary visual cortex. Key Words: Homonymous hemianopia- Meridian sparing- Occipital lobe- Striate cortex. Mapping of the striate cortex has been an arduous and often contentious process. As a result of examining injured soldiers, Inouye ( 1) and Holmes and Lister ( 2) were able to create the first modern maps of the striate cortex. They suggested that the vertical meridian was located most medially along the calcarine lips, while the horizontal meridian was found deeper within the calcarine banks at the fissure base. The clinical and radiologic findings of two patients whose cases confirm that the cortical representation of the horizontal meridian lies in the lateral striate cortex are reported. CASE REPORTS Case 1 A 56- year- old woman with a 40- year history of migraines developed her typical visual aura followed by an intense throbbing headache. She took sumatriptan 50 mg, followed several hours later by another 50 mg. However, the right lower quadrant visual field defect failed to clear. Manuscript received January 25, 2000; accepted February 10, 2000. From Departments of Ophthalmology ( SLG), Neurology ( SLG), and Radiology ( RIG), University of Pennsylvania Medical Center, Philadelphia, Pennsylvania. Address correspondence and reprint requests to Steven L. Galetta, MD, University of Pennsylvania Health Center, 3400 Spruce Street, 3 East Gates, Philadelphia, PA 19104. Examination revealed a visual acuity of 20/ 20 OU. Computerized visual fields demonstrated a right inferior quadrant field defect with 7° of sparing below the horizontal meridian ( Fig. 1). Color vision was normal. Pupils were 3 mm and briskly reactive to light stimulation. There was no afferent pupil defect. Ocular motility was full. Results of a funduscopic examination were normal. Results of the remainder of the neurologic examination were normal. Magnetic resonance imaging ( MRI) of the brain demonstrated high signal abnormality that was concentrated along the medial aspect of the upper bank of the striate cortex ( Fig. 2). Cervical angiography showed a branch of the left calcarine artery to be narrowed and irregular in appearance. The patient was started on warfarin therapy after her immunoglobulin G antiphospholipid titers were found to be markedly elevated. Case 2 A 33- year- old woman began to develop recurrent brief episodes of bilateral visual loss in a narrow band along the horizontal meridian. She was given the diagnosis of occipital lobe epilepsy and started on anticonvulsant therapy. Magnetic resonance imaging demonstrated an arteriovenous malformation concentrated in the posterior FIG. 1. Humphrey 30- 2 visual field examination demonstrating a right inferior quadrantanopia with approximately 7° of sparing below the horizontal meridian. The sparing extended about 15° along the horizontal meridian. 89 90 S. L. GALETTA ET AL. FIG. 2. ( A) Axial and ( B) coronal T2- weighted MR images ( TE = 98 ms; TR = 3,783 ms). High signal abnormality is observed along the medial calcarine cortex ( black arrows). There' is patchy sparing along the lateral striate cortex ( open arrows). Note that the occipital pole is not involved, which corresponds with the small amount of macular sparing also seen along the vertical meridian of the field. Z; . * ,< H . " ' ' • " • • • • " « . , : 4r\ t'- L.' » ' r » • SH; * u.; * • # * • , f M • ^ r: n. » " • * • ' • . » £ « ; t . ~ • FIG. 3. Axial T2- weighted MR images ( TE = 104 ms; TR = 4,000 ms). Multiple abnormal signal voids are seen in the left posterior and lateral left occipital lobe ( white arrows). The medial striate cortex is spared. and lateral aspects of the striate cortex ( Fig. 3). When the seizure frequency increased, she underwent elective removal of the vascular malformation. Postoperatively, she had a field defect along the horizontal meridian ( Fig. 4). The results of the remainder of the neuro- ophthalmologic examination were unremarkable, except for congenital hearing loss. A repeated cerebral angiogram showed no residual malformation. According to the patient, the field loss from her seizures was in the same location as the permanent field defect. A postoperative MRI scan showed surgical removal of the lateral portion of the left striate cortex. DISCUSSION Our understanding of the human visual cortex has been enhanced by correlating visual field defects with MRI findings ( 3,4). These studies have lead to a refinement of the retinotopic maps created by Inouye ( 1) and Holmes and Lister ( 2). In particular, the amount of striate cortex dedicated to the central 15° of visual field has FIG. 4. Humphrey 30- 2 visual field shows a narrow band of congruous, horizontal visual field loss in the right inferior quadrant. Lower vertical meridian Calcarine fissure Upper vertical meridian FIG. 5. Coronal section of the right occipital cortex viewed from behind, demonstrating the location of the cortical representations for the vertical and horizontal meridians. J Neuro- Ophthalmol, Vol. 20. No. 2, 2000 THE HORIZONTAL MERIDIAN IN THE PRIMARY VISUAL CORTEX 91 been expanded ( 3,4). While macular- sparing homonymous field defects are common, meridian- sparing defects are unusual. In primates and humans, the horizontal meridian has been mapped to the base of the calcarine cortex, and the vertical meridian has been represented along the medially located calcarine lips ( Fig. 5) ( 5,6) Horton and Hoyt ( 7) have also proposed that the horizontal meridian is secondarily represented in the perstriate cortex at the border zone of V2- V3. The MRI of our first patient showed a lesion concentrated along the medial border of the calcarine cortex. The base of the calcarine fissure was relatively spared, correlating with the horizontal meridian sparing observed. In contrast, our second patient had a horizontal wedge of homonymous field loss after removal of an arteriovenous malformation that was concentrated in the lateral striate cortex. The findings in our two patients confirm that the horizontal meridian is represented at the calcarine fissure base. REFERENCES 1. Glickstein M, Whitteridge D. Tatsuji Inouye and the mapping of the visual fields on the human cerebral cortex. Trends Neurosci 1987; 10: 350- 3. 2. Holmes GH, Lister WT. Disturbances of vision from cerebral lesions with special reference to the cortical representation of the macula. Brain 1916; 39: 34- 73. 3. Horton JC, Hoyt WF. The representation of the visual field in human striate cortex: a revision of the classic Holmes map. Arch Ophthalmol 1991; 109: 816- 24. 4. Wong AMF, Sharpe JA. Representation of the visual field in the human occipital cortex: a magnetic resonance imaging and perimetric correlation. Arch Ophthalmol 1999; 117: 208- 17. 5. Tootell RBH, Swifkes E, Silverman MS, Hamilton SL. Functional anatomy of macaque striate cortex. II. Retinopathic organization. J Neurosci 1988; 8: 1531- 68. 6. Gray LG, Galetta SL, Schatz NJ. Vertical and horizontal meridian sparing in occipital lobe homonymous hemianopias. Neurology 1998; 50: 1170- 3. 7. Horton JC, Hoyt WF. Quadrantic visual field defects: a hallmark of lesions in extrastriate ( V2/ V3) cortex. Brain 1991; 114: 1703- 18. J Neuro- Ophthalmol, Vol. 20, No. 2, 2000 |
