Jonathan Horton, MD, PhD

Structural Basis of Amblyopia and Strabismus

We are seeking to discover how visual perception occurs in the human brain. The function of the visual system is to guide our behavior by providing an efficient means for the rapid assimilation of information from the environment. As we navigate through our surroundings, a continuous stream of light images impinges on our eyes. In the back of each eye a light-sensitive tissue, the retina, converts patterns of light energy into electrical discharges known as action potentials. These signals are conveyed along the axons of retinal ganglion cells to the lateral geniculate body, a relay nucleus in the thalamus. Most of the output of the lateral geniculate body is relayed directly to the primary visual cortex (striate cortex, V1), and then to surrounding visual association areas. To understand the function of the visual pathways, our laboratory is focused on 5 major themes:

Strabismus and Visual Suppression: Binocular vision is such a natural experience that we take it for granted. In some children, however, the eyes fail to maintain normal alignment during development, leading to a condition called strabismus (crossed eyes). To avoid double vision, children with strabismus suppress perception through their deviated eye. This strategy eliminates the annoying experience of seeing two images, but it has the drawback of eliminating the incentive to fuse the eyes. We are studying how visual suppression occurs in the primate visual cortex. Understanding the mechanism of suppression may provide clues to the etiology of strabismus. 

Organization of Primary Visual Cortex: Within the primary visual cortex, visual information is processed by cells arranged within an elaborate system comprised of overlapping vertical columns and horizontal layers. Our goal is to elucidate the basic functional architecture of the visual cortex, to understand how groups of cells are organized in a modular fashion for information analysis. We are employing autoradiography, axon tracing, cytochrome oxidase histochemistry, functional gene expression, and electrophysiology to accomplish this aim. 

Mapping of Extrastriate Visual Cortex: After initial processing by the primary visual cortex, signals are transferred to a multitude of nearby visual areas (known collectively as extrastriate visual cortex), where further image analysis takes place. It is unclear why the primate brain has so many different visual areas beyond the primary visual cortex. The most popular theory is that various cortical areas are specialized for processing different aspects of vision, e.g., motion, color, form, etc. Our objective is to map the boundaries, layout, topography, functional architecture, and projections of visual areas in extrastriate cortex, to determine how they contribute to perception. 

Amblyopia and Visual Development: For centuries, philosophers have debated the relative contributions of innate factors (“nature”) versus environmental influences (“nurture”) in human development. A disease called amblyopia (“lazy eye”) can occur if a child is deprived of normal sight in one eye during infancy. We are seeking to define how visual deprivation affects the normal circuitry and function of the visual cortex. This research may lead to better treatments for amblyopia. 

The Human Visual Cortex: Our ultimate goal is to understand the human visual cortex. Whenever possible, guided by results obtained from animal experiments, we perform anatomical studies in specimens of human brain obtained at autopsy from subjects with a history of visual loss, amblyopia, or strabismus. These studies have confirmed the remarkable similarity of the monkey and human visual systems. 

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