The Human Visual Cortex

      Our research on the human visual system is based on clinical experience with patients suffering from diseases that produce eye movements abnormalities, double vision, visual field deficits, optic nerve and retinal dysfunction. Through neuro-ophthalmological examination, neuroradiological imaging, electrophysiology, pathology, and laboratory studies we characterize deficits in function to arrive at a diagnosis. This process allows us to serve patients, and to further elucidate the natural history, pathogenesis, and treatment of neuro-ophthalmological diseases.

      In addition, we conduct studies of human brain tissues willed by patients to medical science. Through post-mortem examination of the occipital lobes we have been able to identify important functional elements of the human visual cortex. Figure 18 shows a montage of sections through layer 4c, processed for cytochrome oxidase, from a man who died several years after enucleation of one eye. A pattern of alternating light and dark columns are visible. The pale columns correspond to the ocular dominance columns of the missing eye:

Fig. 18) Montage of cytochrome oxidase sections through layer 4c cut through striate cortex along the medial face of the right occipital lobe, showing ocular dominance columns. They appear visible because loss of physiological activity in the missing eye’s columns causes a reduction in the levels of metabolic enzymes. From: Horton JC, Hedley-Whyte ET. Phil. Trans. R. Soc. (Lond.) B, 1984.

      In the superficial layers of striate cortex, dark and light rows of cytochrome oxidase patches were visible, aligned with the ocular dominance columns in layer 4c, just as in the macaque monkey (Fig. 19):

Fig. 19)(Top) Rows of dark and light cytochrome oxidase patches from layers 2/3, in a region of cortex corresponding to the ocular dominance columns illustrated in Figure 18. (Bottom) Borders of ocular dominance columns from Figure 18 have been transferred onto the pattern of patches, showing that dark and light rows of patches are in register with dark and light ocular dominance columns.

      In Figure 20 the pattern of ocular dominance columns along the medial face of the occipital lobe has been superimposed photographically on the brain surface:

Fig. 20) Pattern of ocular dominance columns along the medial face of the right human occipital lobe. The dashed line corresponds to the V1/V2 border. The portion of the column mosaic shown in Figure 18 is at the lower right. Human ocular dominance columns are similar to those in macaques, but wider.

      Most of human striate cortex is buried within the calcarine fissure, and therefore hidden from view in Figure 20. By flatmounting the entire striate cortex one can reconstruct the complete pattern of ocular dominance columns within V1(Fig. 21):

Fig. 21) Mosaic of ocular dominance columns in striate cortex revealed by processing the tissue for cytochrome oxidase in a patient who lost sight in one eye prior to his death. The image above shows the actual tissue montage; the image below is a sketch of the columns. Note the partial reconstruction of dark and pale cytochrome oxidase stripes in V2. BS = blind spot; MC = monocular crescent. From: Adams DL, Sincich LC, Horton JC. J Neuroscience, 2007.

      We have examined the ocular dominance columns in a patient with strabismic amblyopia and in a patient with anisometropic amblyopia (see: Horton JC, Stryker MP. Proc. Natl Acad. Sci, 1993 and Horton JC, Hocking DR. Visual Neuroscience, 1996). Neither case showed evidence of shrinkage of the amblyopic eye’s ocular dominance columns. From these findings, we conclude that amblyopia is not always associated with reduction in the size of ocular dominance columns. It is possible that when amblyopia begins at a later age it is not accompanied by a change in the dimensions of ocular dominance columns. Presumably abnormalities in intracortical wiring, yet to be revealed, are responsible for amblyopia in these cases.

      From our studies, it is apparent that many anatomical features of the macaque visual cortex are also present in the human visual cortex. Their similarity gives us confidence that our research findings in the macaque are applicable to the human. This is gratifying, because our ultimate goal is to understand how the human visual system functions. Continuing studies in the macaque, therefore, will allow us to make further advances in understanding the human visual system. In turn, human studies will continue to help shape our experiments in the macaque.