Primary Visual Pathway - Introduction

Tutorial 15: Primary Visual Pathway

The primary visual pathway consists of the retina, optic nerve, lateral geniculate nucleus (LGN) of the thalamus, and the visual cortex of occipital lobe. Each of these structures function in sequence to transform the visual signal, leading to our visual perception of the external world. As described earlier, the retina is composed of cells that transduce, integrate, converge, and compress the visual signal. This signal acquires meaning in terms of form, color, brightness, contrast, and depth, however, only after processed by the LGN and primary visual cortex. The primary visual pathway is subdivided into multiple, specialized pathways that simultaneously encode the signal in parallel. Much of our knowledge concerning visual perception came from the investigations of David Hubel and Torsten Weisel. These experiments used microelectrodes to measure the responsiveness of single neurons in the cat to controlled stimulation. Hubel and Weisel won the Nobel Prize for Medicine in 1981 for the contributions they have made to our understanding of how information is encoded by the brain.

Figure 15 identifies the cerebral structures that compose the primary visual pathway and discusses the fundamentals of visual perception as contributed by these structures.

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As is the case for many of science's important discoveries, the responsiveness of neurons in visual cortex was discovered by serendipity (by chance). Hubel and Weisel, who began working together at Johns Hopkins University in Baltimore, Maryland in 1958, hoped to address this question by using microelectrodes to record the neuronal discharge of cells in visual cortex. They flashed spots of light on the visual receptive fields of cortical neurons, but rarely did the monitored cells respond. Not until a stimulus slide developed a crack (and therefore a visible line) did they learn of the importance of boundaries and movement in stimulating visual cortical neurons. When they discovered the "flawed" stimulus slide and removed it from the projector's carousel, the monitored neurons discharged at a wild rate! Thus, it was found that cortical cells are most responsive to stimuli of greater complexity than spots of light; they respond to lines and edges of particular size, orientation, contrast from background, and eye of origin.

Hubel and Weisel's successful collaboration has spanned many years. Their important discoveries include the selective pattern of responsiveness of visual cortical neurons, the binocular connections of visual cortical neurons, the anatomical arrangement of visual cortex, and the normal and abnormal development of visual cortex in early life.

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Tutorial 15: Primary Visual Pathway

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